{"gene":"HTR3A","run_date":"2026-06-10T01:55:22","timeline":{"discoveries":[{"year":2002,"finding":"A cation-π interaction between serotonin and Trp183 (loop B) of the 5-HT3A receptor was identified as critical for ligand binding, with an energetic contribution of ~4 kcal/mol, precisely locating the agonist-binding site.","method":"Unnatural amino acid mutagenesis with heterologous expression in Xenopus oocytes; electrophysiology","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — unnatural amino acid mutagenesis with quantitative energetic estimation, replicated across multiple tryptophan analogs in a rigorous single study","pmids":["12162741"],"is_preprint":false},{"year":2003,"finding":"Arginine 222 in the pre-transmembrane domain 1 (pre-TM1) of the 5-HT3A receptor links agonist binding to channel gating; R222A mutation increased agonist potency and efficacy, accelerated activation and desensitization kinetics, and converted a weak partial agonist/antagonist into a potent agonist.","method":"Site-directed mutagenesis, whole-cell patch clamp in HEK293 cells, kinetic analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro functional assay with mutagenesis and kinetic analysis in a single rigorous study","pmids":["12970351"],"is_preprint":false},{"year":2006,"finding":"Arginine 427 in the large cytoplasmic domain (LCD) of the 5-HT3A receptor contributes to receptor desensitization through electrostatic interactions with neighboring residues; deletion or point mutation of R427 slowed desensitization kinetics proportional to the polarity of the substituted residue, without altering single-channel conductance.","method":"Sequential deletion mutagenesis and site-directed mutagenesis, whole-cell patch clamp, single-channel recording in HEK293 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic mutagenesis with electrophysiology and single-channel analysis in a single rigorous study","pmids":["16754678"],"is_preprint":false},{"year":2000,"finding":"The 4′-lysine residue in the putative channel-lining M2 domain of the 5-HT3A receptor affects desensitization kinetics but is unlikely to be exposed to the channel lumen; mutations at this position slowed desensitization (rank: Arg > Gly > Ser > Gln) without altering single-channel conductance (~390 fS).","method":"Site-directed mutagenesis, whole-cell patch clamp, fluctuation analysis in HEK293 cells","journal":"The Journal of physiology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis with functional electrophysiology and fluctuation analysis, multiple mutations tested","pmids":["10639097"],"is_preprint":false},{"year":2001,"finding":"Homomeric 5-HT3A receptors require binding of at least three serotonin molecules to open and possess peak open probability >0.8; open probability varies with the number of bound agonist molecules, with reduced open probability for fully liganded receptors.","method":"Whole-cell and excised patch voltage clamp with rapid agonist application, macroscopic current modeling in HEK293 cells","journal":"The Journal of physiology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — quantitative state-model fitting to macroscopic currents at sub- and supra-maximal concentrations in a single rigorous study","pmids":["11533135"],"is_preprint":false},{"year":2004,"finding":"N-linked glycosylation at three sites (N109, N174, N190) in the extracellular N-terminus of the murine 5-HT3A receptor is required for receptor maturation and function: N109 is necessary for receptor assembly, while N174 and N190 are important for plasma membrane targeting and ligand binding; all three sites are necessary for 5-HT3A-mediated Ca2+ influx.","method":"Site-directed mutagenesis, tunicamycin treatment, radioligand binding, Ca2+ influx assay, plasma membrane expression analysis in transfected cells","journal":"Journal of neuroscience research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis of each site combined with multiple functional readouts in a single rigorous study","pmids":["15264219"],"is_preprint":false},{"year":2010,"finding":"RIC-3 directly interacts with 5-HT3A subunits and functions as a chaperone that preferentially enhances surface expression of homomeric 5-HT3A receptors; co-localization occurs in the endoplasmic reticulum, and RIC-3 increases Bmax (surface receptor density) without altering the 5-HT concentration-response relationship.","method":"Co-immunoprecipitation, immunocytochemistry (ER co-localization), flow cytometry (surface expression), Ca2+ influx assay, radioligand binding in HEK293 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal protein interaction with multiple orthogonal methods (co-IP, flow cytometry, binding, functional assay) in a single study","pmids":["20522555"],"is_preprint":false},{"year":2007,"finding":"Arg436 within the cytoplasmic portals (MA helix) of the 5-HT3A receptor acts as a rate-limiting determinant of single-channel conductance; dynamic modification of the charge at this residue via MTS reagents on substituted Cys436 reversibly modulates conductance, consistent with ion passage through cytoplasmic portals.","method":"Cysteine substitution mutagenesis, substituted cysteine accessibility method (SCAM) with MTS reagents, single-channel recording in outside-out patches","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — SCAM with multiple MTS reagents and single-channel conductance measurements, rigorous biophysical study","pmids":["17200121"],"is_preprint":false},{"year":2013,"finding":"Mutagenic scanning of the MA helix of the human 5-HT3A receptor identified residues at positions 435, 436, 439, and 440 as major determinants of single-channel conductance, providing a functional map of the cytoplasmic portals consistent with structural homology models.","method":"Alanine- and arginine-scanning mutagenesis, substituted cysteine accessibility method, single-channel recording","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic mutagenesis with SCAM and single-channel conductance measurements across the MA helix","pmids":["24030822"],"is_preprint":false},{"year":2011,"finding":"Two anionic residues in the extracellular vestibule of the 5-HT3A receptor, Asp113 and Asp127, markedly influence single-channel conductance, Ca2+ permeability (PCa/PCs), and suppression of conductance by extracellular Ca2+; charge neutralization or reversal at these positions strongly reduces inward conductance and Ca2+ selectivity.","method":"Site-directed mutagenesis, single-channel recording in outside-out patches, bi-ionic permeability measurements in high-conductance mutant 5-HT3A(QDA) receptors","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic mutagenesis with single-channel electrophysiology and ion permeability measurements","pmids":["21454663"],"is_preprint":false},{"year":2005,"finding":"Aspartate 298 (D298) in the TM2-TM3 extracellular loop of the mouse 5-HT3A receptor contributes to channel gating kinetics and extracellular Ca2+ modulation; charge-neutralizing (D→A) or charge-reversing (D→R) mutations reduce 5-HT potency, accelerate desensitization and deactivation, reduce partial agonist efficacy, and abolish Ca2+ modulation; charge-conserving D→E mutation preserves wild-type properties.","method":"Site-directed mutagenesis, whole-cell patch clamp with fast agonist application, Ca2+ modulation experiments in HEK293 cells","journal":"The Journal of physiology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic charge-altering mutagenesis with multiple functional readouts in a single rigorous study","pmids":["16096341"],"is_preprint":false},{"year":2005,"finding":"Loop C residues F226, I228, D229, and Y234 in the murine 5-HT3A receptor contribute to ligand binding and/or gating; I228 and D229 are specifically required for 5-HT (but not mCPBG) interactions, revealing differential ligand engagement with loop C; mutations at F226, I228, and Y234 alter relative agonist efficacies.","method":"Site-directed mutagenesis, radioligand binding, two-electrode voltage clamp, immunofluorescence, ligand docking modeling","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis combined with binding, electrophysiology, and structural modeling in a single study","pmids":["15966738"],"is_preprint":false},{"year":2006,"finding":"Granisetron binding to the 5-HT3A receptor involves loop B residues (H185, D189) and loop E residues (Y143, Y153); H185A mutation abolished granisetron binding and D189A reduced affinity 22-fold, while Y143F and Y153F decreased affinity equivalently to alanine substitutions, indicating the OH groups of these tyrosines are important.","method":"Homology modeling, site-directed mutagenesis, radioligand binding in Xenopus oocytes/HEK cells","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis with quantitative binding assays and structural modeling in a single study","pmids":["16430206"],"is_preprint":false},{"year":2006,"finding":"Ginsenoside Rg3 inhibits 5-HT3A receptor channel activity through interactions with residues V291, F292, and I295 in the TM2 gating pore; mutations at these positions attenuated or abolished Rg3-induced inhibition, and Rg3 acts at a site distinct from the TMB-8 and diltiazem open-channel blocker sites.","method":"Site-directed mutagenesis of TM2, two-electrode voltage clamp in Xenopus oocytes expressing wild-type or mutant receptors, occlusion experiments","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis with functional electrophysiology and pharmacological discrimination experiments","pmids":["17257631"],"is_preprint":false},{"year":2008,"finding":"L293 (L15′) in the second transmembrane domain (TM2) of the 5-HT3A receptor is a molecular determinant of allosteric modulation by 5-hydroxyindole (5-HI); L293C and L293S mutations abolished 5-HI potentiation and converted 5-HI to a partial agonist, while L293S enhanced dopamine efficacy; other TM2 mutations (D298A, T284F) did not alter 5-HI modulation.","method":"Site-directed mutagenesis, whole-cell patch clamp in neuroblastoma and HEK293 cells expressing native or recombinant 5-HT3A receptors","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — targeted mutagenesis with systematic electrophysiology in two cell systems","pmids":["18436267"],"is_preprint":false},{"year":2012,"finding":"A coupled array of noncovalent interactions governs 5-HT3A receptor activation: a cation-π interaction at W183 (loop B), a hydrogen bond at E129 (loop A), and backbone hydrogen bonds mediated by D124 (loop A) are tightly coupled and function as a unit; agonist mCPBG shows no cation-π interaction at W183 and extreme sensitivity to E129 positioning, revealing agonist-specific differences in binding-site interactions.","method":"Mutant cycle analysis using conventional and unnatural amino acid mutagenesis with two-electrode voltage clamp in Xenopus oocytes","journal":"ACS chemical neuroscience","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutant cycle analysis with unnatural amino acids, multiple orthogonal probes in a single rigorous study","pmids":["23077719"],"is_preprint":false},{"year":2004,"finding":"The S2′C substitution in the cytoplasmic selectivity filter of the 5-HT3A receptor showed high-affinity Cd2+ inhibition in both open and closed states, indicating that the cytoplasmic selectivity filter maintains a narrow pore during channel gating with minimal structural rearrangement.","method":"Systematic cysteine substitution scanning of M1-M2 loop and M2 domain, Cd2+ inhibition in outside-out and whole-cell patches, voltage-dependent recovery experiments","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — SCAM with metal-ion probing in defined open and closed states, multiple control experiments","pmids":["15131114"],"is_preprint":false},{"year":2009,"finding":"Hydrophobic photoaffinity labeling with [125I]TID identified Ser451 in the M4 transmembrane segment of the 5-HT3A receptor as exposed to lipid at the lipid-protein interface, with ~60% of photolabel incorporated into the M4-containing fragment and ~40% into the M1-M3 fragment.","method":"Affinity purification of alphaBgTx-tagged 5-HT3A receptors, hydrophobic photoaffinity labeling ([125I]TID), proteolytic mapping, sequencing","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct photoaffinity labeling with amino acid-level mapping on purified receptor in lipid environment","pmids":["19715355"],"is_preprint":false},{"year":2004,"finding":"V13′S mutation in the M2 channel-lining domain of the 5-HT3A receptor renders the channel 70-fold more sensitive to serotonin and produces constitutive activity when co-expressed with 5-HT3B subunit, causing excitotoxic neuronal cell death in the urinary bladder and functional bladder deficits in vivo.","method":"Targeted exon replacement knock-in mouse model, whole-cell electrophysiology in sympathetic ganglion cells, bladder functional assays, immunohistochemistry","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Moderate — knock-in mouse combined with electrophysiology and in vivo organ function assays","pmids":["15201326"],"is_preprint":false},{"year":2000,"finding":"Human 5-HT3A receptor has two alternatively spliced isoforms (short/truncated h5-HT3AT, 238 aa with single TM domain; long h5-HT3AL with 32 additional aa in M2-M3 extracellular loop); neither forms functional homomeric receptors alone, but co-expression with 5-HT3A modifies function: h5-HT3AT slows desensitization and increases cation flux, while h5-HT3AL reduces cation flux.","method":"Molecular cloning and sequencing, heterologous expression in HEK293 cells, patch clamp electrophysiology","journal":"Naunyn-Schmiedeberg's archives of pharmacology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — functional characterization of splice variants with patch-clamp in transfected cells showing distinct biophysical effects","pmids":["11111833"],"is_preprint":false},{"year":2007,"finding":"Naturally occurring SNPs A33T, S253N, and M257I in the human 5-HT3A receptor reduce 5-HT-induced maximal responses to 3-64% of wild-type; A33T, M257I, and R344H reduce surface expression 2-4-fold without altering total expression levels, indicating impaired receptor biogenesis/trafficking; co-expression with wild-type 5-HT3A or 5-HT3B cannot rescue these defects.","method":"Fluorescence-based Ca2+ influx assay, radioligand binding ([3H]GR65630), flow cytometry for surface expression in transfected HEK cells","journal":"Pharmacogenetics and genomics","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (functional assay, binding, surface expression) for five SNP variants in a single study","pmids":["17496724"],"is_preprint":false},{"year":2004,"finding":"5-HT3A subunit is localized to axonal profiles (presynaptic vesicle membranes and extrasynaptic plasma membranes), somatodendritic profiles (near excitatory-type synapses), and glial processes in the rat medial NTS, indicating roles in both presynaptic modulation of transmitter release and postsynaptic signaling.","method":"Electron microscopic immunocytochemistry with anti-5-HT3A antibody and anti-SERT antibody in rat brain sections","journal":"Brain research","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct subcellular localization by electron microscopy but single lab, single method, no functional manipulation","pmids":["15527741"],"is_preprint":false},{"year":2004,"finding":"YFP-tagged 5-HT3A receptor is targeted to the plasma membrane and dendritic spines of hippocampal neurons and to micropodia of HEK293 cells, with predominant surface expression; this contrasts with predominantly intracellular retention of alpha3beta4-nAChRs, and the surface targeting correlates with large serotonin-evoked currents.","method":"Fluorescent protein tagging (YFP), live fluorescence microscopy, extracellular probe labeling, whole-cell voltage clamp in HEK293 cells and primary hippocampal neurons","journal":"The European journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — live imaging with functional correlation, two orthogonal methods, but single lab","pmids":["15009132"],"is_preprint":false},{"year":2012,"finding":"The M3-M4 intracellular loop of the 5-HT3A receptor is not required for receptor assembly or function but its length and amino acid composition influence channel expression and desensitization rate; replacing the 115-residue loop with 3 or 5 alanines dramatically accelerated desensitization (>10-fold), while 2, 4, 6, or 7 alanines preserved near-wild-type desensitization.","method":"Loop replacement mutagenesis, two-electrode voltage clamp in Xenopus oocytes","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — systematic mutagenesis with functional electrophysiology but single lab, single method","pmids":["22539982"],"is_preprint":false},{"year":2008,"finding":"The C-terminal residue (Ala455) of the human 5-HT3A subunit is critical for receptor folding, assembly, and plasma membrane expression; deletion of the three C-terminal residues (Gln453-Tyr454-Ala455) prevents specific radioligand binding and membrane expression, while ΔAla455 alone reduces both; growth at 27°C partially rescues these defects, suggesting the C-terminus stabilizes subunit folding.","method":"C-terminal deletion and point mutagenesis, radioligand binding, cell membrane expression assays in HEK293 cells at 27°C and 37°C","journal":"Neuropharmacology","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — mutagenesis with binding and expression assays, single lab","pmids":["18786552"],"is_preprint":false},{"year":2003,"finding":"The 5-HT3B subunit confers reduced sensitivity to picrotoxin (100-fold reduction in inhibitory effect) when co-expressed with 5-HT3A, establishing picrotoxin as a selective probe that can distinguish homomeric 5-HT3A from heteromeric 5-HT3A/3B receptors.","method":"Whole-cell patch clamp recordings comparing homomeric mouse 5-HT3A vs. heteromeric 5-HT3A/3B receptors","journal":"Brain research. Molecular brain research","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct functional comparison of receptor compositions with electrophysiology, single lab","pmids":["14625088"],"is_preprint":false},{"year":2007,"finding":"Dopamine acts as a low-efficacy agonist at human 5-HT3A receptors with kinetics distinct from serotonin: dopamine activation rates are concentration-independent, deactivation is faster than desensitization (opposite to serotonin), and recovery from desensitization is faster and non-sigmoidal; an allosteric kinetic model indicates these differences arise from much slower rates of channel opening and faster dissociation from open/desensitized states for dopamine.","method":"Rapid solution exchange electrophysiology, whole-cell patch clamp in HEK293 cells, kinetic modeling","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1 / Moderate — quantitative kinetic modeling with multiple agonist concentrations and rigorous electrophysiology in a single comprehensive study","pmids":["18045909"],"is_preprint":false},{"year":2014,"finding":"Partial agonism of tryptamine at 5-HT3A receptors results from reduced priming (fewer/slower transitions through preopen closed states) rather than reduced channel blockade; 2-Me-5HT is not a genuine partial agonist because priming is intact and low apparent efficacy is primarily due to channel blockade within the activating concentration range.","method":"Single-channel recording using high-conductance 5-HT3A mutant receptors, full activation scheme modeling, molecular docking","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1 / Moderate — single-channel analysis with comprehensive kinetic modeling and structural docking in a single rigorous study","pmids":["25505338"],"is_preprint":false},{"year":2013,"finding":"Deletion of the 5-HT3A receptor (Htr3a KO) in mice abolishes fear extinction for both contextual and tone-cued fear without affecting fear acquisition or retention, establishing the 5-HT3A receptor as essential for fear extinction.","method":"Htr3a knockout mouse model, fear conditioning and extinction paradigms","journal":"Learning & memory","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — clean knockout with specific behavioral readout, single lab","pmids":["24344177"],"is_preprint":false},{"year":2014,"finding":"5-HT3A receptor deletion in mice abolishes NMDA receptor-dependent long-term depression (LTD) induced by low-frequency stimulation in hippocampal CA1 and inhibits AMPA receptor internalization, without affecting mGluR-dependent LTD, basal AMPAR surface levels, synapse number, or spine morphology.","method":"Htr3a knockout mouse model, hippocampal slice electrophysiology (LFS-LTD), AMPAR internalization assay","journal":"Neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KO with specific synaptic plasticity readout and receptor internalization assay, two orthogonal methods, single lab","pmids":["25130560"],"is_preprint":false},{"year":2021,"finding":"Htr3a knockout mice exhibit autistic-like behaviors, impaired memory, and reduced seizure susceptibility; mechanistically, loss of HTR3A leads to upregulation of NMDAR in parvalbumin-positive interneurons, increased NMDAR-mediated excitability of these interneurons, enhanced GABAergic transmission onto pyramidal neurons, and decreased E/I ratio; NMDAR antagonist memantine rescues both the GABAergic phenotype and autistic-like behaviors.","method":"TALEN-generated Htr3a KO mice, behavioral testing, transcriptome sequencing, patch-clamp electrophysiology in pyramidal neurons and PV+ interneurons, immunoblotting, immunofluorescence, immunoprecipitation, qRT-PCR, memantine rescue experiment","journal":"Theranostics","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (electrophysiology, transcriptomics, pharmacological rescue) in a single comprehensive study with specific mechanistic pathway placement","pmids":["34646371"],"is_preprint":false},{"year":2024,"finding":"Peripheral nerve injury upregulates ALKBH5 (m6A demethylase) in trigeminal ganglion neurons via a FOXD3-H3K27ac-dependent transcriptional mechanism; ALKBH5 removes m6A from Htr3a mRNA, preventing YTHDF2-mediated degradation and thereby stabilizing Htr3a mRNA, increasing 5-HT3A protein expression and 5-HT3 channel currents to promote neuropathic pain.","method":"Nerve injury rat model, ALKBH5 knockdown/overexpression in TG, m6A sequencing, RIP assay, electrophysiology, histone ChIP, behavioral pain assays, 5-HT3A knockdown rescue","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (m6A mapping, RIP, electrophysiology, behavior, genetic rescue) establishing the ALKBH5-m6A-YTHDF2-Htr3a axis in a single comprehensive study","pmids":["38285939"],"is_preprint":false},{"year":2012,"finding":"Ethanol consumption in mice induces tissue-specific DNA methylation changes at the Htr3a promoter; in the dorsomedial striatum, ethanol drinking reduces CpG methylation at the Htr3a promoter and increases Htr3a mRNA expression (1.43-fold), suggesting an inverse correlation between promoter methylation and Htr3a expression.","method":"Sequenom MassARRAY methylation quantification of 8-9 CpGs across 9 brain regions and blood, qRT-PCR for Htr3a mRNA in CD-1 mice","journal":"Alcoholism, clinical and experimental research","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct methylation quantification correlated with expression in a defined brain region, single lab, single paradigm","pmids":["22834954"],"is_preprint":false},{"year":2012,"finding":"Chronic ethanol exposure in rat prefrontal cortex increases histone H3K9 acetylation at the htr3a promoter region and upregulates htr3a mRNA; histone deacetylase inhibitor sodium butyrate potentiated both effects and enhanced ethanol-induced conditioned place preference.","method":"Chromatin immunoprecipitation (ChIP) for H3K9ac at htr3a promoter, qRT-PCR for htr3a mRNA, conditioned place preference behavioral assay in rats","journal":"Neural regeneration research","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — ChIP with functional behavioral assay, single lab, single method for each readout","pmids":["25722691"],"is_preprint":false},{"year":2022,"finding":"SETDB1 (histone H3K9 methyltransferase) represses Htr3a transcription in embryonic GABAergic interneuron progenitors via the endogenous retroviral enhancer element RMER21B, which forms distal chromatin interaction with the Htr3a promoter; Setdb1 knockout increases Htr3a expression, increases the number and excitability of Htr3a+ cortical interneurons, and causes anxiety/depressive-like behaviors reversible by 5-HT3 receptor antagonist.","method":"Conditional Setdb1 knockout mice, RNA-seq, ATAC-seq, ChIP-seq, luciferase assay, chromatin conformation capture (3C), CRISPR/dCas9, in situ hybridization, whole-cell recording, behavioral testing","journal":"Biological psychiatry","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal epigenomic, transcriptomic, electrophysiological, and behavioral methods establishing the SETDB1-RMER21B-Htr3a regulatory axis","pmids":["36335068"],"is_preprint":false},{"year":2001,"finding":"A C178T variant in the 5′ UTR upstream open reading frame of HTR3A drives 245% increased luciferase reporter expression compared to wild-type, establishing this polymorphism as a functional variant that increases translational-level gene expression.","method":"Luciferase reporter gene assay comparing C178T (Pro16Ser) allele vs. wild-type and compound variant constructs in transfected cells","journal":"Pharmacogenetics","confidence":"Medium","confidence_rationale":"Tier 3 / Weak — single reporter assay in a single study establishing functional effect of the variant at the expression level","pmids":["11505217"],"is_preprint":false},{"year":2020,"finding":"HTR3A knockdown in lung adenocarcinoma cells attenuates proliferation by reducing ERK phosphorylation, identifying ERK signaling as a downstream effector of HTR3A in cancer cell proliferation.","method":"siRNA knockdown of HTR3A in lung adenocarcinoma cell lines, proliferation assay, Western blot for ERK phosphorylation","journal":"Cancer science","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — loss-of-function with specific molecular readout (ERK phosphorylation), single lab, single method","pmids":["32736413"],"is_preprint":false},{"year":2017,"finding":"5-HT3A receptor is expressed in fetal mouse pelvic ganglia and loss of Htr3a in male mice causes increased urinary voiding frequency, decreased voiding efficiency, and a transient imbalance of autonomic neuronal subtypes in fetal pelvic ganglia; adult Htr3a KO mice show higher density of autonomic and sensory nerve fibers in bladder smooth muscle.","method":"Htr3a KO mouse model, anesthetized cystometry, immunohistochemistry for autonomic neuron markers (TH, ChAT) in pelvic ganglia and bladder","journal":"Frontiers in neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO mouse with functional voiding assay and neuroanatomical markers, two orthogonal readouts, single lab","pmids":["29311772"],"is_preprint":false},{"year":2017,"finding":"5-HT3AR is expressed in GABAergic interneurons containing somatostatin or calretinin in the mouse brain, with strong expression in olfactory bulb, cerebral cortex, hippocampus, and amygdala, and no expression in cerebellum; confirmed by double-immunostaining with neural markers in 5-HT3AR-GFP transgenic mice.","method":"Immunofluorescent staining of 5-HT3AR-GFP transgenic mice, double-immunostaining with GAD67, somatostatin, calretinin markers","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct cell-type identification by double-labeling in transgenic reporter mice, single lab","pmids":["28276429"],"is_preprint":false},{"year":2009,"finding":"Proline P303 in the M2-M3 linker of human 5-HT3A receptor is important for receptor function but trans-cis isomerization at this position is not required for channel gating; P303H and P303W mutations do not abolish agonist-mediated currents but accelerate desensitization 10-fold and alter Ca2+ dependence.","method":"Site-directed mutagenesis (natural amino acids), two-electrode voltage clamp in Xenopus oocytes, Ca2+ concentration-response experiments","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — mutagenesis with functional electrophysiology, single lab, directly challenges prior claim about isomerization mechanism","pmids":["19457066"],"is_preprint":false},{"year":2004,"finding":"5-HT3A and CB1 cannabinoid receptors are co-expressed in GABAergic interneurons throughout the rat telencephalon (cortex, hippocampus, amygdala), with 37-53% of 5-HT3A-expressing neurons co-expressing CB1; this co-expression pattern was confirmed by combining double in situ hybridization with GABA immunohistochemistry.","method":"Double in situ hybridization, combination of in situ hybridization and immunohistochemistry for GABA in rat brain sections","journal":"The Journal of comparative neurology","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct cell-type and co-expression mapping with two complementary methods, single lab","pmids":["14648680"],"is_preprint":false}],"current_model":"HTR3A encodes the obligatory subunit of the homomeric 5-HT3A receptor, a pentameric Cys-loop ligand-gated cation channel that requires binding of ≥3 serotonin molecules for activation; serotonin engages the binding site through a cation-π interaction with W183 and coupled hydrogen bonds at E129/D124 in the extracellular domain, while channel gating involves the pre-TM1 residue R222, the TM2-TM3 linker residue D298, and M2 residues (including L293 for allosteric modulation); desensitization is governed by the 4′-lysine in M2 and cytoplasmic domain residues (R427, MA-helix residues 435-440); single-channel conductance is rate-limited by cytoplasmic portal residues (R436, R440) and extracellular vestibule charges (D113, D127); surface expression requires N-linked glycosylation (N109, N174, N190) and the C-terminus (A455), and is enhanced by the chaperone RIC-3; the receptor is expressed predominantly in GABAergic interneurons in limbic/cortical regions and is required for fear extinction, NMDA-dependent hippocampal LTD, and AMPAR internalization, while its transcription is epigenetically regulated by SETDB1-mediated H3K9 methylation and by m6A modification of its mRNA through the ALKBH5/YTHDF2 axis."},"narrative":{"mechanistic_narrative":"HTR3A encodes the obligatory subunit of the homomeric 5-HT3A receptor, a pentameric serotonin-gated cation channel that requires the concerted binding of at least three serotonin molecules to open with high peak open probability [PMID:11533135]. Agonist recognition occurs in the extracellular domain through a cation-π interaction between serotonin and Trp183 in loop B, energetically coupled to a hydrogen-bond network involving E129 and D124 in loop A that operates as a single functional unit and discriminates among agonists [PMID:12162741, PMID:23077719]; loop C residues (I228, D229) further confer agonist-specific engagement [PMID:15966738]. Binding is mechanically relayed to the pore: the pre-TM1 residue R222 couples agonist occupancy to gating [PMID:12970351], while the TM2-TM3 linker residue D298 and the M2-M3 proline P303 set gating kinetics and extracellular Ca2+ modulation [PMID:16096341, PMID:19457066], and the M2 residue L293 mediates allosteric potentiation [PMID:18436267]. Single-channel conductance and Ca2+ permeability are rate-limited not at the membrane but at peripheral charges—extracellular vestibule aspartates D113/D127 and the cytoplasmic portal arginines R436/R440 of the MA helix [PMID:21454663, PMID:17200121, PMID:24030822]—while desensitization is governed by the M2 4'-lysine, the cytoplasmic R427, and the length/composition of the M3-M4 intracellular loop [PMID:10639097, PMID:16754678, PMID:22539982]. Receptor biogenesis and surface delivery depend on N-linked glycosylation at N109/N174/N190, on the extreme C-terminus (A455), and is enhanced by the ER chaperone RIC-3, which binds the subunit directly [PMID:15264219, PMID:18786552, PMID:20522555]. In the brain the receptor is expressed in GABAergic interneurons of limbic and cortical regions [PMID:28276429], where it is required for fear extinction, NMDA-dependent hippocampal LTD with AMPAR internalization, and proper excitatory/inhibitory balance, such that its loss elevates NMDAR-driven interneuron excitability and produces autistic-like behavior reversible by memantine [PMID:24344177, PMID:25130560, PMID:34646371]. HTR3A transcription and mRNA stability are epigenetically controlled—repressed by SETDB1-mediated H3K9 methylation through an endogenous retroviral enhancer [PMID:36335068] and stabilized after nerve injury by ALKBH5-mediated m6A demethylation that blocks YTHDF2-driven decay to promote neuropathic pain [PMID:38285939].","teleology":[{"year":2000,"claim":"Establishing which residues set desensitization addressed how the channel terminates its response; the M2 4'-lysine was shown to shape desensitization kinetics without lining the conducting pore.","evidence":"Site-directed mutagenesis with whole-cell patch clamp and fluctuation analysis in HEK293 cells","pmids":["10639097"],"confidence":"High","gaps":["Did not resolve the structural pathway linking 4'-lysine to the desensitized state","Conductance was unaffected, leaving the conductance determinants unidentified"]},{"year":2001,"claim":"Defining the activation stoichiometry answered how many agonist molecules are needed to open the channel, showing ≥3 serotonin molecules with high open probability.","evidence":"Voltage clamp with rapid agonist application and macroscopic state-model fitting in HEK293 cells","pmids":["11533135"],"confidence":"High","gaps":["Single-channel transitions not directly resolved here","Did not map binding sites to specific residues"]},{"year":2002,"claim":"Locating the agonist-binding site answered where serotonin docks, identifying a cation-π interaction with Trp183 worth ~4 kcal/mol.","evidence":"Unnatural amino acid mutagenesis with electrophysiology in Xenopus oocytes","pmids":["12162741"],"confidence":"High","gaps":["Did not establish coupling to gating","Single binding-site residue, not the full network"]},{"year":2003,"claim":"Linking binding to gating addressed how occupancy is transduced to channel opening, identifying pre-TM1 R222 as a binding-gating coupling element.","evidence":"Mutagenesis with whole-cell patch clamp and kinetic analysis in HEK293 cells","pmids":["12970351"],"confidence":"High","gaps":["Structural basis of coupling not resolved","Interaction partners of R222 not identified"]},{"year":2004,"claim":"Multiple studies dissected biogenesis, pore architecture, splice diversity, and trafficking: glycosylation sites required for assembly and surface targeting, a narrow cytoplasmic selectivity filter maintained during gating, alternatively spliced isoforms that modify function, and predominant surface expression in neurons.","evidence":"Glycosylation-site mutagenesis with Ca2+ influx and binding assays; cysteine-scanning with Cd2+ probing; cloning and patch clamp of splice variants; YFP imaging with electrophysiology","pmids":["15264219","15131114","11111833","15009132"],"confidence":"High","gaps":["Trafficking machinery beyond glycosylation not defined","Physiological role of splice isoforms in vivo unknown"]},{"year":2004,"claim":"Cellular and subcellular localization studies addressed where the receptor acts, placing it in GABAergic interneurons co-expressing CB1 and at both pre- and postsynaptic profiles.","evidence":"Electron microscopic immunocytochemistry and double in situ hybridization with GABA immunohistochemistry in rat brain","pmids":["15527741","14648680"],"confidence":"Medium","gaps":["Single-method, single-lab localization without functional manipulation","Functional consequence of presynaptic localization not tested"]},{"year":2005,"claim":"Mapping the gating-pore interface answered how the TM2-TM3 region and loop C contribute to gating and Ca2+ modulation, identifying D298 and loop C residues with agonist-specific roles.","evidence":"Charge-altering and conventional mutagenesis with patch clamp, radioligand binding, and docking","pmids":["16096341","15966738"],"confidence":"High","gaps":["Coupling mechanism between binding site and TM2-TM3 not structurally resolved"]},{"year":2006,"claim":"Defining cytoplasmic desensitization and conductance elements and antagonist binding addressed how the large cytoplasmic domain and binding loops shape function: R427 governs desensitization, MA-helix R436 rate-limits conductance, and granisetron engages loop B/E residues.","evidence":"Deletion and SCAM/MTS mutagenesis with single-channel recording; homology modeling with radioligand binding","pmids":["16754678","17200121","16430206"],"confidence":"High","gaps":["Did not resolve full set of portal residues","Structural model of cytoplasmic portals inferred, not solved"]},{"year":2007,"claim":"Naturally occurring HTR3A SNPs and dopamine pharmacology addressed how genetic variation and alternative agonists affect the receptor, revealing trafficking-impairing variants and a slow-opening low-efficacy dopamine response.","evidence":"Ca2+ influx, binding, and surface-expression assays for SNP variants; rapid-exchange electrophysiology with kinetic modeling","pmids":["17496724","18045909"],"confidence":"High","gaps":["In vivo phenotypic consequence of SNPs not established","Physiological relevance of dopamine agonism unclear"]},{"year":2008,"claim":"Identifying allosteric and C-terminal determinants answered how 5-HI potentiation and folding/assembly are controlled, implicating M2 L293 and the C-terminal A455.","evidence":"Mutagenesis with patch clamp in two cell systems; C-terminal deletion with binding/expression assays and temperature rescue","pmids":["18436267","18786552"],"confidence":"High","gaps":["Mechanism of C-terminal stabilization of folding not defined"]},{"year":2010,"claim":"Identifying a chaperone addressed how surface receptor density is set, showing RIC-3 binds the subunit and enhances homomeric surface expression from the ER.","evidence":"Co-immunoprecipitation, ER co-localization, flow cytometry, binding, and Ca2+ influx assays in HEK293 cells","pmids":["20522555"],"confidence":"High","gaps":["Structural basis of RIC-3 binding not defined","Whether RIC-3 acts on assembly versus export not fully resolved"]},{"year":2013,"claim":"Systematic MA-helix scanning answered which cytoplasmic portal residues set conductance, mapping positions 435, 436, 439, and 440 as major determinants.","evidence":"Alanine/arginine scanning with SCAM and single-channel recording","pmids":["24030822"],"confidence":"High","gaps":["Functional map relied on homology models, not an experimental structure"]},{"year":2014,"claim":"Single-channel dissection of partial agonism answered why some ligands are weak agonists, showing tryptamine reduces priming whereas 2-Me-5HT's low efficacy is channel block.","evidence":"Single-channel recording in high-conductance mutants with activation-scheme modeling and docking","pmids":["25505338"],"confidence":"High","gaps":["Generalizability of priming model to other partial agonists untested"]},{"year":2014,"claim":"Loss-of-function studies established the receptor's synaptic role, showing it is required for fear extinction and for NMDAR-dependent LTD with AMPAR internalization.","evidence":"Htr3a knockout mice with fear paradigms and hippocampal slice electrophysiology/internalization assays","pmids":["24344177","25130560"],"confidence":"Medium","gaps":["Single-lab behavioral readouts","Cellular circuit linking the receptor to LTD not fully resolved"]},{"year":2021,"claim":"Defining the circuit consequence of HTR3A loss answered how the receptor maintains E/I balance, placing it upstream of NMDAR levels in PV+ interneurons with autistic-like behavior rescued by memantine.","evidence":"TALEN Htr3a KO mice with behavior, transcriptomics, cell-type-specific electrophysiology, and pharmacological rescue","pmids":["34646371"],"confidence":"High","gaps":["Molecular link between HTR3A and NMDAR upregulation not defined","Relevance to human autism not established"]},{"year":2024,"claim":"Epigenetic and post-transcriptional control studies answered how HTR3A expression is regulated, identifying SETDB1/H3K9-RMER21B repression and an ALKBH5-m6A-YTHDF2 axis stabilizing Htr3a mRNA to drive neuropathic pain; methylation/acetylation changes link ethanol exposure to expression.","evidence":"Conditional Setdb1 KO with epigenomics and 3C; nerve-injury model with m6A-seq, RIP, electrophysiology and genetic rescue; promoter methylation/ChIP with mRNA quantification","pmids":["36335068","38285939","22834954","25722691","11505217"],"confidence":"High","gaps":["Whether these regulatory axes converge in the same neurons unknown","Causal contribution to human disease not established"]},{"year":null,"claim":"How the extracellular binding-site network mechanically couples to the cytoplasmic portals to set both gating and conductance in a single experimentally resolved structure remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No experimental full-length structure cited in the corpus","Integration of binding, gating, conductance, and desensitization determinants into one mechanistic model incomplete"]}],"mechanism_profile":{"molecular_activity":[],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[5,22,21]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[6,24]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[28,29,30,38]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[4,9,7]}],"complexes":["homomeric 5-HT3A receptor","heteromeric 5-HT3A/5-HT3B receptor"],"partners":["RIC-3","HTR3B"],"other_free_text":["Transporter/molecular_activity slot: serotonin-gated cation channel; controlled MF id GO:0005230 (ligand-gated ion channel) not in allowed list, recorded here [#0,#4,#9]"]}},"prefetch_data":{"uniprot":{"accession":"P46098","full_name":"5-hydroxytryptamine receptor 3A","aliases":["5-hydroxytryptamine receptor 3","5-HT-3","5-HT3R","Serotonin receptor 3A","Serotonin-gated ion channel 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HTR3E","url":"https://www.omim.org/entry/610123"},{"mim_id":"610122","title":"5-@HYDROXYTRYPTAMINE RECEPTOR 3D; HTR3D","url":"https://www.omim.org/entry/610122"},{"mim_id":"610121","title":"5-@HYDROXYTRYPTAMINE RECEPTOR 3C; HTR3C","url":"https://www.omim.org/entry/610121"},{"mim_id":"604654","title":"5-@HYDROXYTRYPTAMINE RECEPTOR 3B; HTR3B","url":"https://www.omim.org/entry/604654"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"brain","ntpm":2.3},{"tissue":"intestine","ntpm":2.0},{"tissue":"lymphoid tissue","ntpm":7.5},{"tissue":"pancreas","ntpm":2.7},{"tissue":"salivary 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\"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — unnatural amino acid mutagenesis with quantitative energetic estimation, replicated across multiple tryptophan analogs in a rigorous single study\",\n      \"pmids\": [\"12162741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Arginine 222 in the pre-transmembrane domain 1 (pre-TM1) of the 5-HT3A receptor links agonist binding to channel gating; R222A mutation increased agonist potency and efficacy, accelerated activation and desensitization kinetics, and converted a weak partial agonist/antagonist into a potent agonist.\",\n      \"method\": \"Site-directed mutagenesis, whole-cell patch clamp in HEK293 cells, kinetic analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro functional assay with mutagenesis and kinetic analysis in a single rigorous study\",\n      \"pmids\": [\"12970351\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Arginine 427 in the large cytoplasmic domain (LCD) of the 5-HT3A receptor contributes to receptor desensitization through electrostatic interactions with neighboring residues; deletion or point mutation of R427 slowed desensitization kinetics proportional to the polarity of the substituted residue, without altering single-channel conductance.\",\n      \"method\": \"Sequential deletion mutagenesis and site-directed mutagenesis, whole-cell patch clamp, single-channel recording in HEK293 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic mutagenesis with electrophysiology and single-channel analysis in a single rigorous study\",\n      \"pmids\": [\"16754678\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The 4′-lysine residue in the putative channel-lining M2 domain of the 5-HT3A receptor affects desensitization kinetics but is unlikely to be exposed to the channel lumen; mutations at this position slowed desensitization (rank: Arg > Gly > Ser > Gln) without altering single-channel conductance (~390 fS).\",\n      \"method\": \"Site-directed mutagenesis, whole-cell patch clamp, fluctuation analysis in HEK293 cells\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis with functional electrophysiology and fluctuation analysis, multiple mutations tested\",\n      \"pmids\": [\"10639097\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Homomeric 5-HT3A receptors require binding of at least three serotonin molecules to open and possess peak open probability >0.8; open probability varies with the number of bound agonist molecules, with reduced open probability for fully liganded receptors.\",\n      \"method\": \"Whole-cell and excised patch voltage clamp with rapid agonist application, macroscopic current modeling in HEK293 cells\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — quantitative state-model fitting to macroscopic currents at sub- and supra-maximal concentrations in a single rigorous study\",\n      \"pmids\": [\"11533135\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"N-linked glycosylation at three sites (N109, N174, N190) in the extracellular N-terminus of the murine 5-HT3A receptor is required for receptor maturation and function: N109 is necessary for receptor assembly, while N174 and N190 are important for plasma membrane targeting and ligand binding; all three sites are necessary for 5-HT3A-mediated Ca2+ influx.\",\n      \"method\": \"Site-directed mutagenesis, tunicamycin treatment, radioligand binding, Ca2+ influx assay, plasma membrane expression analysis in transfected cells\",\n      \"journal\": \"Journal of neuroscience research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis of each site combined with multiple functional readouts in a single rigorous study\",\n      \"pmids\": [\"15264219\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"RIC-3 directly interacts with 5-HT3A subunits and functions as a chaperone that preferentially enhances surface expression of homomeric 5-HT3A receptors; co-localization occurs in the endoplasmic reticulum, and RIC-3 increases Bmax (surface receptor density) without altering the 5-HT concentration-response relationship.\",\n      \"method\": \"Co-immunoprecipitation, immunocytochemistry (ER co-localization), flow cytometry (surface expression), Ca2+ influx assay, radioligand binding in HEK293 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal protein interaction with multiple orthogonal methods (co-IP, flow cytometry, binding, functional assay) in a single study\",\n      \"pmids\": [\"20522555\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Arg436 within the cytoplasmic portals (MA helix) of the 5-HT3A receptor acts as a rate-limiting determinant of single-channel conductance; dynamic modification of the charge at this residue via MTS reagents on substituted Cys436 reversibly modulates conductance, consistent with ion passage through cytoplasmic portals.\",\n      \"method\": \"Cysteine substitution mutagenesis, substituted cysteine accessibility method (SCAM) with MTS reagents, single-channel recording in outside-out patches\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — SCAM with multiple MTS reagents and single-channel conductance measurements, rigorous biophysical study\",\n      \"pmids\": [\"17200121\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Mutagenic scanning of the MA helix of the human 5-HT3A receptor identified residues at positions 435, 436, 439, and 440 as major determinants of single-channel conductance, providing a functional map of the cytoplasmic portals consistent with structural homology models.\",\n      \"method\": \"Alanine- and arginine-scanning mutagenesis, substituted cysteine accessibility method, single-channel recording\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic mutagenesis with SCAM and single-channel conductance measurements across the MA helix\",\n      \"pmids\": [\"24030822\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Two anionic residues in the extracellular vestibule of the 5-HT3A receptor, Asp113 and Asp127, markedly influence single-channel conductance, Ca2+ permeability (PCa/PCs), and suppression of conductance by extracellular Ca2+; charge neutralization or reversal at these positions strongly reduces inward conductance and Ca2+ selectivity.\",\n      \"method\": \"Site-directed mutagenesis, single-channel recording in outside-out patches, bi-ionic permeability measurements in high-conductance mutant 5-HT3A(QDA) receptors\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic mutagenesis with single-channel electrophysiology and ion permeability measurements\",\n      \"pmids\": [\"21454663\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Aspartate 298 (D298) in the TM2-TM3 extracellular loop of the mouse 5-HT3A receptor contributes to channel gating kinetics and extracellular Ca2+ modulation; charge-neutralizing (D→A) or charge-reversing (D→R) mutations reduce 5-HT potency, accelerate desensitization and deactivation, reduce partial agonist efficacy, and abolish Ca2+ modulation; charge-conserving D→E mutation preserves wild-type properties.\",\n      \"method\": \"Site-directed mutagenesis, whole-cell patch clamp with fast agonist application, Ca2+ modulation experiments in HEK293 cells\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic charge-altering mutagenesis with multiple functional readouts in a single rigorous study\",\n      \"pmids\": [\"16096341\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Loop C residues F226, I228, D229, and Y234 in the murine 5-HT3A receptor contribute to ligand binding and/or gating; I228 and D229 are specifically required for 5-HT (but not mCPBG) interactions, revealing differential ligand engagement with loop C; mutations at F226, I228, and Y234 alter relative agonist efficacies.\",\n      \"method\": \"Site-directed mutagenesis, radioligand binding, two-electrode voltage clamp, immunofluorescence, ligand docking modeling\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis combined with binding, electrophysiology, and structural modeling in a single study\",\n      \"pmids\": [\"15966738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Granisetron binding to the 5-HT3A receptor involves loop B residues (H185, D189) and loop E residues (Y143, Y153); H185A mutation abolished granisetron binding and D189A reduced affinity 22-fold, while Y143F and Y153F decreased affinity equivalently to alanine substitutions, indicating the OH groups of these tyrosines are important.\",\n      \"method\": \"Homology modeling, site-directed mutagenesis, radioligand binding in Xenopus oocytes/HEK cells\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis with quantitative binding assays and structural modeling in a single study\",\n      \"pmids\": [\"16430206\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Ginsenoside Rg3 inhibits 5-HT3A receptor channel activity through interactions with residues V291, F292, and I295 in the TM2 gating pore; mutations at these positions attenuated or abolished Rg3-induced inhibition, and Rg3 acts at a site distinct from the TMB-8 and diltiazem open-channel blocker sites.\",\n      \"method\": \"Site-directed mutagenesis of TM2, two-electrode voltage clamp in Xenopus oocytes expressing wild-type or mutant receptors, occlusion experiments\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis with functional electrophysiology and pharmacological discrimination experiments\",\n      \"pmids\": [\"17257631\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"L293 (L15′) in the second transmembrane domain (TM2) of the 5-HT3A receptor is a molecular determinant of allosteric modulation by 5-hydroxyindole (5-HI); L293C and L293S mutations abolished 5-HI potentiation and converted 5-HI to a partial agonist, while L293S enhanced dopamine efficacy; other TM2 mutations (D298A, T284F) did not alter 5-HI modulation.\",\n      \"method\": \"Site-directed mutagenesis, whole-cell patch clamp in neuroblastoma and HEK293 cells expressing native or recombinant 5-HT3A receptors\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — targeted mutagenesis with systematic electrophysiology in two cell systems\",\n      \"pmids\": [\"18436267\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A coupled array of noncovalent interactions governs 5-HT3A receptor activation: a cation-π interaction at W183 (loop B), a hydrogen bond at E129 (loop A), and backbone hydrogen bonds mediated by D124 (loop A) are tightly coupled and function as a unit; agonist mCPBG shows no cation-π interaction at W183 and extreme sensitivity to E129 positioning, revealing agonist-specific differences in binding-site interactions.\",\n      \"method\": \"Mutant cycle analysis using conventional and unnatural amino acid mutagenesis with two-electrode voltage clamp in Xenopus oocytes\",\n      \"journal\": \"ACS chemical neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutant cycle analysis with unnatural amino acids, multiple orthogonal probes in a single rigorous study\",\n      \"pmids\": [\"23077719\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The S2′C substitution in the cytoplasmic selectivity filter of the 5-HT3A receptor showed high-affinity Cd2+ inhibition in both open and closed states, indicating that the cytoplasmic selectivity filter maintains a narrow pore during channel gating with minimal structural rearrangement.\",\n      \"method\": \"Systematic cysteine substitution scanning of M1-M2 loop and M2 domain, Cd2+ inhibition in outside-out and whole-cell patches, voltage-dependent recovery experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — SCAM with metal-ion probing in defined open and closed states, multiple control experiments\",\n      \"pmids\": [\"15131114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Hydrophobic photoaffinity labeling with [125I]TID identified Ser451 in the M4 transmembrane segment of the 5-HT3A receptor as exposed to lipid at the lipid-protein interface, with ~60% of photolabel incorporated into the M4-containing fragment and ~40% into the M1-M3 fragment.\",\n      \"method\": \"Affinity purification of alphaBgTx-tagged 5-HT3A receptors, hydrophobic photoaffinity labeling ([125I]TID), proteolytic mapping, sequencing\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct photoaffinity labeling with amino acid-level mapping on purified receptor in lipid environment\",\n      \"pmids\": [\"19715355\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"V13′S mutation in the M2 channel-lining domain of the 5-HT3A receptor renders the channel 70-fold more sensitive to serotonin and produces constitutive activity when co-expressed with 5-HT3B subunit, causing excitotoxic neuronal cell death in the urinary bladder and functional bladder deficits in vivo.\",\n      \"method\": \"Targeted exon replacement knock-in mouse model, whole-cell electrophysiology in sympathetic ganglion cells, bladder functional assays, immunohistochemistry\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knock-in mouse combined with electrophysiology and in vivo organ function assays\",\n      \"pmids\": [\"15201326\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Human 5-HT3A receptor has two alternatively spliced isoforms (short/truncated h5-HT3AT, 238 aa with single TM domain; long h5-HT3AL with 32 additional aa in M2-M3 extracellular loop); neither forms functional homomeric receptors alone, but co-expression with 5-HT3A modifies function: h5-HT3AT slows desensitization and increases cation flux, while h5-HT3AL reduces cation flux.\",\n      \"method\": \"Molecular cloning and sequencing, heterologous expression in HEK293 cells, patch clamp electrophysiology\",\n      \"journal\": \"Naunyn-Schmiedeberg's archives of pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — functional characterization of splice variants with patch-clamp in transfected cells showing distinct biophysical effects\",\n      \"pmids\": [\"11111833\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Naturally occurring SNPs A33T, S253N, and M257I in the human 5-HT3A receptor reduce 5-HT-induced maximal responses to 3-64% of wild-type; A33T, M257I, and R344H reduce surface expression 2-4-fold without altering total expression levels, indicating impaired receptor biogenesis/trafficking; co-expression with wild-type 5-HT3A or 5-HT3B cannot rescue these defects.\",\n      \"method\": \"Fluorescence-based Ca2+ influx assay, radioligand binding ([3H]GR65630), flow cytometry for surface expression in transfected HEK cells\",\n      \"journal\": \"Pharmacogenetics and genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (functional assay, binding, surface expression) for five SNP variants in a single study\",\n      \"pmids\": [\"17496724\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"5-HT3A subunit is localized to axonal profiles (presynaptic vesicle membranes and extrasynaptic plasma membranes), somatodendritic profiles (near excitatory-type synapses), and glial processes in the rat medial NTS, indicating roles in both presynaptic modulation of transmitter release and postsynaptic signaling.\",\n      \"method\": \"Electron microscopic immunocytochemistry with anti-5-HT3A antibody and anti-SERT antibody in rat brain sections\",\n      \"journal\": \"Brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct subcellular localization by electron microscopy but single lab, single method, no functional manipulation\",\n      \"pmids\": [\"15527741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"YFP-tagged 5-HT3A receptor is targeted to the plasma membrane and dendritic spines of hippocampal neurons and to micropodia of HEK293 cells, with predominant surface expression; this contrasts with predominantly intracellular retention of alpha3beta4-nAChRs, and the surface targeting correlates with large serotonin-evoked currents.\",\n      \"method\": \"Fluorescent protein tagging (YFP), live fluorescence microscopy, extracellular probe labeling, whole-cell voltage clamp in HEK293 cells and primary hippocampal neurons\",\n      \"journal\": \"The European journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — live imaging with functional correlation, two orthogonal methods, but single lab\",\n      \"pmids\": [\"15009132\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The M3-M4 intracellular loop of the 5-HT3A receptor is not required for receptor assembly or function but its length and amino acid composition influence channel expression and desensitization rate; replacing the 115-residue loop with 3 or 5 alanines dramatically accelerated desensitization (>10-fold), while 2, 4, 6, or 7 alanines preserved near-wild-type desensitization.\",\n      \"method\": \"Loop replacement mutagenesis, two-electrode voltage clamp in Xenopus oocytes\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — systematic mutagenesis with functional electrophysiology but single lab, single method\",\n      \"pmids\": [\"22539982\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The C-terminal residue (Ala455) of the human 5-HT3A subunit is critical for receptor folding, assembly, and plasma membrane expression; deletion of the three C-terminal residues (Gln453-Tyr454-Ala455) prevents specific radioligand binding and membrane expression, while ΔAla455 alone reduces both; growth at 27°C partially rescues these defects, suggesting the C-terminus stabilizes subunit folding.\",\n      \"method\": \"C-terminal deletion and point mutagenesis, radioligand binding, cell membrane expression assays in HEK293 cells at 27°C and 37°C\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — mutagenesis with binding and expression assays, single lab\",\n      \"pmids\": [\"18786552\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The 5-HT3B subunit confers reduced sensitivity to picrotoxin (100-fold reduction in inhibitory effect) when co-expressed with 5-HT3A, establishing picrotoxin as a selective probe that can distinguish homomeric 5-HT3A from heteromeric 5-HT3A/3B receptors.\",\n      \"method\": \"Whole-cell patch clamp recordings comparing homomeric mouse 5-HT3A vs. heteromeric 5-HT3A/3B receptors\",\n      \"journal\": \"Brain research. Molecular brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct functional comparison of receptor compositions with electrophysiology, single lab\",\n      \"pmids\": [\"14625088\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Dopamine acts as a low-efficacy agonist at human 5-HT3A receptors with kinetics distinct from serotonin: dopamine activation rates are concentration-independent, deactivation is faster than desensitization (opposite to serotonin), and recovery from desensitization is faster and non-sigmoidal; an allosteric kinetic model indicates these differences arise from much slower rates of channel opening and faster dissociation from open/desensitized states for dopamine.\",\n      \"method\": \"Rapid solution exchange electrophysiology, whole-cell patch clamp in HEK293 cells, kinetic modeling\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — quantitative kinetic modeling with multiple agonist concentrations and rigorous electrophysiology in a single comprehensive study\",\n      \"pmids\": [\"18045909\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Partial agonism of tryptamine at 5-HT3A receptors results from reduced priming (fewer/slower transitions through preopen closed states) rather than reduced channel blockade; 2-Me-5HT is not a genuine partial agonist because priming is intact and low apparent efficacy is primarily due to channel blockade within the activating concentration range.\",\n      \"method\": \"Single-channel recording using high-conductance 5-HT3A mutant receptors, full activation scheme modeling, molecular docking\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — single-channel analysis with comprehensive kinetic modeling and structural docking in a single rigorous study\",\n      \"pmids\": [\"25505338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Deletion of the 5-HT3A receptor (Htr3a KO) in mice abolishes fear extinction for both contextual and tone-cued fear without affecting fear acquisition or retention, establishing the 5-HT3A receptor as essential for fear extinction.\",\n      \"method\": \"Htr3a knockout mouse model, fear conditioning and extinction paradigms\",\n      \"journal\": \"Learning & memory\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — clean knockout with specific behavioral readout, single lab\",\n      \"pmids\": [\"24344177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"5-HT3A receptor deletion in mice abolishes NMDA receptor-dependent long-term depression (LTD) induced by low-frequency stimulation in hippocampal CA1 and inhibits AMPA receptor internalization, without affecting mGluR-dependent LTD, basal AMPAR surface levels, synapse number, or spine morphology.\",\n      \"method\": \"Htr3a knockout mouse model, hippocampal slice electrophysiology (LFS-LTD), AMPAR internalization assay\",\n      \"journal\": \"Neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KO with specific synaptic plasticity readout and receptor internalization assay, two orthogonal methods, single lab\",\n      \"pmids\": [\"25130560\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Htr3a knockout mice exhibit autistic-like behaviors, impaired memory, and reduced seizure susceptibility; mechanistically, loss of HTR3A leads to upregulation of NMDAR in parvalbumin-positive interneurons, increased NMDAR-mediated excitability of these interneurons, enhanced GABAergic transmission onto pyramidal neurons, and decreased E/I ratio; NMDAR antagonist memantine rescues both the GABAergic phenotype and autistic-like behaviors.\",\n      \"method\": \"TALEN-generated Htr3a KO mice, behavioral testing, transcriptome sequencing, patch-clamp electrophysiology in pyramidal neurons and PV+ interneurons, immunoblotting, immunofluorescence, immunoprecipitation, qRT-PCR, memantine rescue experiment\",\n      \"journal\": \"Theranostics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (electrophysiology, transcriptomics, pharmacological rescue) in a single comprehensive study with specific mechanistic pathway placement\",\n      \"pmids\": [\"34646371\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Peripheral nerve injury upregulates ALKBH5 (m6A demethylase) in trigeminal ganglion neurons via a FOXD3-H3K27ac-dependent transcriptional mechanism; ALKBH5 removes m6A from Htr3a mRNA, preventing YTHDF2-mediated degradation and thereby stabilizing Htr3a mRNA, increasing 5-HT3A protein expression and 5-HT3 channel currents to promote neuropathic pain.\",\n      \"method\": \"Nerve injury rat model, ALKBH5 knockdown/overexpression in TG, m6A sequencing, RIP assay, electrophysiology, histone ChIP, behavioral pain assays, 5-HT3A knockdown rescue\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (m6A mapping, RIP, electrophysiology, behavior, genetic rescue) establishing the ALKBH5-m6A-YTHDF2-Htr3a axis in a single comprehensive study\",\n      \"pmids\": [\"38285939\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Ethanol consumption in mice induces tissue-specific DNA methylation changes at the Htr3a promoter; in the dorsomedial striatum, ethanol drinking reduces CpG methylation at the Htr3a promoter and increases Htr3a mRNA expression (1.43-fold), suggesting an inverse correlation between promoter methylation and Htr3a expression.\",\n      \"method\": \"Sequenom MassARRAY methylation quantification of 8-9 CpGs across 9 brain regions and blood, qRT-PCR for Htr3a mRNA in CD-1 mice\",\n      \"journal\": \"Alcoholism, clinical and experimental research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct methylation quantification correlated with expression in a defined brain region, single lab, single paradigm\",\n      \"pmids\": [\"22834954\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Chronic ethanol exposure in rat prefrontal cortex increases histone H3K9 acetylation at the htr3a promoter region and upregulates htr3a mRNA; histone deacetylase inhibitor sodium butyrate potentiated both effects and enhanced ethanol-induced conditioned place preference.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP) for H3K9ac at htr3a promoter, qRT-PCR for htr3a mRNA, conditioned place preference behavioral assay in rats\",\n      \"journal\": \"Neural regeneration research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — ChIP with functional behavioral assay, single lab, single method for each readout\",\n      \"pmids\": [\"25722691\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SETDB1 (histone H3K9 methyltransferase) represses Htr3a transcription in embryonic GABAergic interneuron progenitors via the endogenous retroviral enhancer element RMER21B, which forms distal chromatin interaction with the Htr3a promoter; Setdb1 knockout increases Htr3a expression, increases the number and excitability of Htr3a+ cortical interneurons, and causes anxiety/depressive-like behaviors reversible by 5-HT3 receptor antagonist.\",\n      \"method\": \"Conditional Setdb1 knockout mice, RNA-seq, ATAC-seq, ChIP-seq, luciferase assay, chromatin conformation capture (3C), CRISPR/dCas9, in situ hybridization, whole-cell recording, behavioral testing\",\n      \"journal\": \"Biological psychiatry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal epigenomic, transcriptomic, electrophysiological, and behavioral methods establishing the SETDB1-RMER21B-Htr3a regulatory axis\",\n      \"pmids\": [\"36335068\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"A C178T variant in the 5′ UTR upstream open reading frame of HTR3A drives 245% increased luciferase reporter expression compared to wild-type, establishing this polymorphism as a functional variant that increases translational-level gene expression.\",\n      \"method\": \"Luciferase reporter gene assay comparing C178T (Pro16Ser) allele vs. wild-type and compound variant constructs in transfected cells\",\n      \"journal\": \"Pharmacogenetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single reporter assay in a single study establishing functional effect of the variant at the expression level\",\n      \"pmids\": [\"11505217\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"HTR3A knockdown in lung adenocarcinoma cells attenuates proliferation by reducing ERK phosphorylation, identifying ERK signaling as a downstream effector of HTR3A in cancer cell proliferation.\",\n      \"method\": \"siRNA knockdown of HTR3A in lung adenocarcinoma cell lines, proliferation assay, Western blot for ERK phosphorylation\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — loss-of-function with specific molecular readout (ERK phosphorylation), single lab, single method\",\n      \"pmids\": [\"32736413\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"5-HT3A receptor is expressed in fetal mouse pelvic ganglia and loss of Htr3a in male mice causes increased urinary voiding frequency, decreased voiding efficiency, and a transient imbalance of autonomic neuronal subtypes in fetal pelvic ganglia; adult Htr3a KO mice show higher density of autonomic and sensory nerve fibers in bladder smooth muscle.\",\n      \"method\": \"Htr3a KO mouse model, anesthetized cystometry, immunohistochemistry for autonomic neuron markers (TH, ChAT) in pelvic ganglia and bladder\",\n      \"journal\": \"Frontiers in neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO mouse with functional voiding assay and neuroanatomical markers, two orthogonal readouts, single lab\",\n      \"pmids\": [\"29311772\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"5-HT3AR is expressed in GABAergic interneurons containing somatostatin or calretinin in the mouse brain, with strong expression in olfactory bulb, cerebral cortex, hippocampus, and amygdala, and no expression in cerebellum; confirmed by double-immunostaining with neural markers in 5-HT3AR-GFP transgenic mice.\",\n      \"method\": \"Immunofluorescent staining of 5-HT3AR-GFP transgenic mice, double-immunostaining with GAD67, somatostatin, calretinin markers\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct cell-type identification by double-labeling in transgenic reporter mice, single lab\",\n      \"pmids\": [\"28276429\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Proline P303 in the M2-M3 linker of human 5-HT3A receptor is important for receptor function but trans-cis isomerization at this position is not required for channel gating; P303H and P303W mutations do not abolish agonist-mediated currents but accelerate desensitization 10-fold and alter Ca2+ dependence.\",\n      \"method\": \"Site-directed mutagenesis (natural amino acids), two-electrode voltage clamp in Xenopus oocytes, Ca2+ concentration-response experiments\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — mutagenesis with functional electrophysiology, single lab, directly challenges prior claim about isomerization mechanism\",\n      \"pmids\": [\"19457066\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"5-HT3A and CB1 cannabinoid receptors are co-expressed in GABAergic interneurons throughout the rat telencephalon (cortex, hippocampus, amygdala), with 37-53% of 5-HT3A-expressing neurons co-expressing CB1; this co-expression pattern was confirmed by combining double in situ hybridization with GABA immunohistochemistry.\",\n      \"method\": \"Double in situ hybridization, combination of in situ hybridization and immunohistochemistry for GABA in rat brain sections\",\n      \"journal\": \"The Journal of comparative neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct cell-type and co-expression mapping with two complementary methods, single lab\",\n      \"pmids\": [\"14648680\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"HTR3A encodes the obligatory subunit of the homomeric 5-HT3A receptor, a pentameric Cys-loop ligand-gated cation channel that requires binding of ≥3 serotonin molecules for activation; serotonin engages the binding site through a cation-π interaction with W183 and coupled hydrogen bonds at E129/D124 in the extracellular domain, while channel gating involves the pre-TM1 residue R222, the TM2-TM3 linker residue D298, and M2 residues (including L293 for allosteric modulation); desensitization is governed by the 4′-lysine in M2 and cytoplasmic domain residues (R427, MA-helix residues 435-440); single-channel conductance is rate-limited by cytoplasmic portal residues (R436, R440) and extracellular vestibule charges (D113, D127); surface expression requires N-linked glycosylation (N109, N174, N190) and the C-terminus (A455), and is enhanced by the chaperone RIC-3; the receptor is expressed predominantly in GABAergic interneurons in limbic/cortical regions and is required for fear extinction, NMDA-dependent hippocampal LTD, and AMPAR internalization, while its transcription is epigenetically regulated by SETDB1-mediated H3K9 methylation and by m6A modification of its mRNA through the ALKBH5/YTHDF2 axis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"HTR3A encodes the obligatory subunit of the homomeric 5-HT3A receptor, a pentameric serotonin-gated cation channel that requires the concerted binding of at least three serotonin molecules to open with high peak open probability [#4]. Agonist recognition occurs in the extracellular domain through a cation-\\u03c0 interaction between serotonin and Trp183 in loop B, energetically coupled to a hydrogen-bond network involving E129 and D124 in loop A that operates as a single functional unit and discriminates among agonists [#0, #15]; loop C residues (I228, D229) further confer agonist-specific engagement [#11]. Binding is mechanically relayed to the pore: the pre-TM1 residue R222 couples agonist occupancy to gating [#1], while the TM2-TM3 linker residue D298 and the M2-M3 proline P303 set gating kinetics and extracellular Ca2+ modulation [#10, #39], and the M2 residue L293 mediates allosteric potentiation [#14]. Single-channel conductance and Ca2+ permeability are rate-limited not at the membrane but at peripheral charges\\u2014extracellular vestibule aspartates D113/D127 and the cytoplasmic portal arginines R436/R440 of the MA helix [#9, #7, #8]\\u2014while desensitization is governed by the M2 4'-lysine, the cytoplasmic R427, and the length/composition of the M3-M4 intracellular loop [#3, #2, #23]. Receptor biogenesis and surface delivery depend on N-linked glycosylation at N109/N174/N190, on the extreme C-terminus (A455), and is enhanced by the ER chaperone RIC-3, which binds the subunit directly [#5, #24, #6]. In the brain the receptor is expressed in GABAergic interneurons of limbic and cortical regions [#38], where it is required for fear extinction, NMDA-dependent hippocampal LTD with AMPAR internalization, and proper excitatory/inhibitory balance, such that its loss elevates NMDAR-driven interneuron excitability and produces autistic-like behavior reversible by memantine [#28, #29, #30]. HTR3A transcription and mRNA stability are epigenetically controlled\\u2014repressed by SETDB1-mediated H3K9 methylation through an endogenous retroviral enhancer [#34] and stabilized after nerve injury by ALKBH5-mediated m6A demethylation that blocks YTHDF2-driven decay to promote neuropathic pain [#31].\"\n  ,\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Establishing which residues set desensitization addressed how the channel terminates its response; the M2 4'-lysine was shown to shape desensitization kinetics without lining the conducting pore.\",\n      \"evidence\": \"Site-directed mutagenesis with whole-cell patch clamp and fluctuation analysis in HEK293 cells\",\n      \"pmids\": [\"10639097\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the structural pathway linking 4'-lysine to the desensitized state\", \"Conductance was unaffected, leaving the conductance determinants unidentified\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Defining the activation stoichiometry answered how many agonist molecules are needed to open the channel, showing \\u22653 serotonin molecules with high open probability.\",\n      \"evidence\": \"Voltage clamp with rapid agonist application and macroscopic state-model fitting in HEK293 cells\",\n      \"pmids\": [\"11533135\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single-channel transitions not directly resolved here\", \"Did not map binding sites to specific residues\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Locating the agonist-binding site answered where serotonin docks, identifying a cation-\\u03c0 interaction with Trp183 worth ~4 kcal/mol.\",\n      \"evidence\": \"Unnatural amino acid mutagenesis with electrophysiology in Xenopus oocytes\",\n      \"pmids\": [\"12162741\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish coupling to gating\", \"Single binding-site residue, not the full network\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Linking binding to gating addressed how occupancy is transduced to channel opening, identifying pre-TM1 R222 as a binding-gating coupling element.\",\n      \"evidence\": \"Mutagenesis with whole-cell patch clamp and kinetic analysis in HEK293 cells\",\n      \"pmids\": [\"12970351\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of coupling not resolved\", \"Interaction partners of R222 not identified\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Multiple studies dissected biogenesis, pore architecture, splice diversity, and trafficking: glycosylation sites required for assembly and surface targeting, a narrow cytoplasmic selectivity filter maintained during gating, alternatively spliced isoforms that modify function, and predominant surface expression in neurons.\",\n      \"evidence\": \"Glycosylation-site mutagenesis with Ca2+ influx and binding assays; cysteine-scanning with Cd2+ probing; cloning and patch clamp of splice variants; YFP imaging with electrophysiology\",\n      \"pmids\": [\"15264219\", \"15131114\", \"11111833\", \"15009132\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trafficking machinery beyond glycosylation not defined\", \"Physiological role of splice isoforms in vivo unknown\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Cellular and subcellular localization studies addressed where the receptor acts, placing it in GABAergic interneurons co-expressing CB1 and at both pre- and postsynaptic profiles.\",\n      \"evidence\": \"Electron microscopic immunocytochemistry and double in situ hybridization with GABA immunohistochemistry in rat brain\",\n      \"pmids\": [\"15527741\", \"14648680\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-method, single-lab localization without functional manipulation\", \"Functional consequence of presynaptic localization not tested\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Mapping the gating-pore interface answered how the TM2-TM3 region and loop C contribute to gating and Ca2+ modulation, identifying D298 and loop C residues with agonist-specific roles.\",\n      \"evidence\": \"Charge-altering and conventional mutagenesis with patch clamp, radioligand binding, and docking\",\n      \"pmids\": [\"16096341\", \"15966738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Coupling mechanism between binding site and TM2-TM3 not structurally resolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Defining cytoplasmic desensitization and conductance elements and antagonist binding addressed how the large cytoplasmic domain and binding loops shape function: R427 governs desensitization, MA-helix R436 rate-limits conductance, and granisetron engages loop B/E residues.\",\n      \"evidence\": \"Deletion and SCAM/MTS mutagenesis with single-channel recording; homology modeling with radioligand binding\",\n      \"pmids\": [\"16754678\", \"17200121\", \"16430206\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve full set of portal residues\", \"Structural model of cytoplasmic portals inferred, not solved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Naturally occurring HTR3A SNPs and dopamine pharmacology addressed how genetic variation and alternative agonists affect the receptor, revealing trafficking-impairing variants and a slow-opening low-efficacy dopamine response.\",\n      \"evidence\": \"Ca2+ influx, binding, and surface-expression assays for SNP variants; rapid-exchange electrophysiology with kinetic modeling\",\n      \"pmids\": [\"17496724\", \"18045909\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo phenotypic consequence of SNPs not established\", \"Physiological relevance of dopamine agonism unclear\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identifying allosteric and C-terminal determinants answered how 5-HI potentiation and folding/assembly are controlled, implicating M2 L293 and the C-terminal A455.\",\n      \"evidence\": \"Mutagenesis with patch clamp in two cell systems; C-terminal deletion with binding/expression assays and temperature rescue\",\n      \"pmids\": [\"18436267\", \"18786552\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of C-terminal stabilization of folding not defined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identifying a chaperone addressed how surface receptor density is set, showing RIC-3 binds the subunit and enhances homomeric surface expression from the ER.\",\n      \"evidence\": \"Co-immunoprecipitation, ER co-localization, flow cytometry, binding, and Ca2+ influx assays in HEK293 cells\",\n      \"pmids\": [\"20522555\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of RIC-3 binding not defined\", \"Whether RIC-3 acts on assembly versus export not fully resolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Systematic MA-helix scanning answered which cytoplasmic portal residues set conductance, mapping positions 435, 436, 439, and 440 as major determinants.\",\n      \"evidence\": \"Alanine/arginine scanning with SCAM and single-channel recording\",\n      \"pmids\": [\"24030822\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional map relied on homology models, not an experimental structure\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Single-channel dissection of partial agonism answered why some ligands are weak agonists, showing tryptamine reduces priming whereas 2-Me-5HT's low efficacy is channel block.\",\n      \"evidence\": \"Single-channel recording in high-conductance mutants with activation-scheme modeling and docking\",\n      \"pmids\": [\"25505338\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Generalizability of priming model to other partial agonists untested\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Loss-of-function studies established the receptor's synaptic role, showing it is required for fear extinction and for NMDAR-dependent LTD with AMPAR internalization.\",\n      \"evidence\": \"Htr3a knockout mice with fear paradigms and hippocampal slice electrophysiology/internalization assays\",\n      \"pmids\": [\"24344177\", \"25130560\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab behavioral readouts\", \"Cellular circuit linking the receptor to LTD not fully resolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defining the circuit consequence of HTR3A loss answered how the receptor maintains E/I balance, placing it upstream of NMDAR levels in PV+ interneurons with autistic-like behavior rescued by memantine.\",\n      \"evidence\": \"TALEN Htr3a KO mice with behavior, transcriptomics, cell-type-specific electrophysiology, and pharmacological rescue\",\n      \"pmids\": [\"34646371\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular link between HTR3A and NMDAR upregulation not defined\", \"Relevance to human autism not established\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Epigenetic and post-transcriptional control studies answered how HTR3A expression is regulated, identifying SETDB1/H3K9-RMER21B repression and an ALKBH5-m6A-YTHDF2 axis stabilizing Htr3a mRNA to drive neuropathic pain; methylation/acetylation changes link ethanol exposure to expression.\",\n      \"evidence\": \"Conditional Setdb1 KO with epigenomics and 3C; nerve-injury model with m6A-seq, RIP, electrophysiology and genetic rescue; promoter methylation/ChIP with mRNA quantification\",\n      \"pmids\": [\"36335068\", \"38285939\", \"22834954\", \"25722691\", \"11505217\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether these regulatory axes converge in the same neurons unknown\", \"Causal contribution to human disease not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the extracellular binding-site network mechanically couples to the cytoplasmic portals to set both gating and conductance in a single experimentally resolved structure remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No experimental full-length structure cited in the corpus\", \"Integration of binding, gating, conductance, and desensitization determinants into one mechanistic model incomplete\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005230\", \"supporting_discovery_ids\": []}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [5, 22, 21]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [6, 24]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [28, 29, 30, 38]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [4, 9, 7]}\n    ],\n    \"complexes\": [\n      \"homomeric 5-HT3A receptor\",\n      \"heteromeric 5-HT3A/5-HT3B receptor\"\n    ],\n    \"partners\": [\n      \"RIC-3\",\n      \"HTR3B\"\n    ],\n    \"other_free_text\": [\n      \"Transporter/molecular_activity slot: serotonin-gated cation channel; controlled MF id GO:0005230 (ligand-gated ion channel) not in allowed list, recorded here [#0,#4,#9]\"\n    ]\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":7,"faith_total":7,"faith_pct":100.0}}