{"gene":"HTR2C","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":1995,"finding":"5-HT2C receptor knockout mice develop obesity due to abnormal control of feeding behavior and are prone to spontaneous seizures, establishing roles for this receptor in serotonergic control of appetite and tonic inhibition of neuronal network excitability.","method":"Targeted gene knockout in mice with behavioral and physiological phenotypic readouts","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple defined cellular/behavioral phenotypes, highly cited foundational study","pmids":["7700379"],"is_preprint":false},{"year":1997,"finding":"5-HT2C receptor null mutant mice are extremely susceptible to audiogenic seizures with complete penetrance in adults, and immediate early gene expression indicates seizures are subcortical phenomena in auditory circuits, implicating 5-HT2C receptors in serotoninergic control of epilepsy.","method":"Knockout mouse model, audiogenic seizure testing, immediate early gene expression (c-fos)","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — KO model with defined phenotype and circuit-level mechanistic data, replicated finding from same gene","pmids":["9241279"],"is_preprint":false},{"year":1997,"finding":"Site-directed mutagenesis of serine 312 to phenylalanine or lysine in the 5-HT2C receptor produces constitutively active receptors that spontaneously couple to G proteins and stimulate phosphatidylinositol (PI) hydrolysis in the absence of agonist; inverse agonists mianserin and mesulergine decrease basal PI hydrolysis of the S312K mutant.","method":"Site-directed mutagenesis, radioligand binding, PI hydrolysis assay in COS-7 cells","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro mutagenesis with functional reconstitution and multiple assays","pmids":["9282936"],"is_preprint":false},{"year":1995,"finding":"5-HT2C receptor activation in choroid plexus stimulates cyclic GMP (cGMP) formation in addition to phosphoinositide turnover; cGMP formation is calcium-, phospholipase A2-, and lipoxygenase-dependent and not inhibited by pertussis toxin, indicating coupling through a Gi-independent pathway.","method":"Choroid plexus tissue slice assay, cGMP measurement, pharmacological antagonism, pertussis toxin pretreatment, enzyme inhibitors","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro functional assay with multiple pharmacological tools and biochemical inhibitors","pmids":["7798914"],"is_preprint":false},{"year":1995,"finding":"The rat 5-HT2C receptor is a glycoprotein containing N-linked oligosaccharides; N-linked sugars contribute approximately 30% of the relative mass, and sialic acid residues associate with 5-HT2C receptors in the choroid plexus but not in a recombinant cell line.","method":"Western blotting with anti-5-HT2C antibodies, tunicamycin metabolic inhibition, N-glycosidase F and neuraminidase enzymatic treatments","journal":"Brain research. Molecular brain research","confidence":"High","confidence_rationale":"Tier 1 — reconstituted biochemical analysis with multiple enzymatic and metabolic approaches","pmids":["8750891"],"is_preprint":false},{"year":1998,"finding":"5-HT2C receptor mutant mice show selective impairment of long-term potentiation at medial perforant path-dentate gyrus synapses and exhibit abnormal Morris water maze performance and reduced aversion to novel environments, establishing a role for 5-HT2C receptors in dentate gyrus synaptic plasticity.","method":"Knockout mouse model, LTP electrophysiology at four hippocampal regions, Morris water maze, behavioral assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — KO model with electrophysiological and behavioral phenotypic readouts, multiple orthogonal methods","pmids":["9844009"],"is_preprint":false},{"year":1999,"finding":"Inverse agonists at 5-HT2C receptors expressed at physiological density do not reduce basal inositol phosphate (IP) accumulation but selectively enhance 5-HT2C-mediated IP accumulation (not arachidonic acid release) after prolonged treatment; this enhancement may involve increased expression of Gαq/11 and possibly Gα12 and Gα13 proteins.","method":"IP accumulation assay, arachidonic acid release assay, pharmacological characterization in CHO cells, G protein expression analysis","journal":"Molecular pharmacology","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro functional assays; single lab study","pmids":["10220565"],"is_preprint":false},{"year":1999,"finding":"Agonist-directed signaling (functional selectivity) at 5-HT2C receptors: LSD and serotonin differ in their activation of phosphatidylinositol signaling; RNA editing of the 5-HT2C receptor dramatically alters the ability of LSD to stimulate PI signaling.","method":"Phosphatidylinositol signaling assay, comparison of edited vs. non-edited receptor isoforms","journal":"Neuropsychopharmacology","confidence":"Medium","confidence_rationale":"Tier 1 — functional in vitro assay; single lab, limited mechanistic detail in abstract","pmids":["10432492"],"is_preprint":false},{"year":2006,"finding":"PTEN physically interacts with the third intracellular loop (3L4F region) of the 5-HT2C receptor and limits agonist-induced phosphorylation of the receptor through its protein phosphatase activity; disruption of this PTEN:5-HT2cR complex by a Tat-3L4F interfering peptide suppresses VTA dopaminergic neuron firing and blocks conditioned place preference for THC and nicotine.","method":"Co-immunoprecipitation in cell cultures, interfering peptide (Tat-3L4F), electrophysiology of VTA neurons, conditioned place preference behavioral tests","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, functional peptide disruption, in vivo electrophysiology, and behavioral assays","pmids":["16474401"],"is_preprint":false},{"year":2006,"finding":"5-HT2C receptor homodimerization/oligomerization begins in the endoplasmic reticulum and persists through Golgi and plasma membrane, as demonstrated by real-time FRET between CFP- and YFP-tagged receptors in intact living cells, suggesting dimer formation is a step in receptor maturation.","method":"Confocal fluorescence resonance energy transfer (FRET), acceptor photobleaching, live-cell time-lapse confocal imaging","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — FRET in living cells with compartment-specific analysis and rigorous controls","pmids":["16857671"],"is_preprint":false},{"year":2007,"finding":"5-HT2C receptors are co-localized with GABA neurons (GAD-positive) and dopamine neurons (tyrosine hydroxylase-positive) in the VTA, establishing the anatomical basis for 5-HT2C-mediated inhibitory control of both GABAergic and dopaminergic output in the mesolimbic system.","method":"Double-label immunofluorescence in rat VTA tissue","journal":"Neuroscience","confidence":"Medium","confidence_rationale":"Tier 3 — direct localization experiment with functional implication, single lab","pmids":["17367945"],"is_preprint":false},{"year":2007,"finding":"5-HT2C receptors are coexpressed with proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus; melanocortin 4 receptor (MC4R) knockout mice are unresponsive to 5-HT2CR agonist-induced hypophagia, establishing MC4R as a requisite downstream pathway for 5-HT2CR agonist control of food intake.","method":"Immunofluorescence co-localization, MC4R knockout mice, pharmacological feeding studies","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis via knockout rescue plus anatomical co-localization","pmids":["18039773"],"is_preprint":false},{"year":2010,"finding":"After spinal cord injury, post-transcriptional RNA editing of 5-HT2C receptor mRNA increases constitutively active receptor isoforms in motoneurons; this constitutive activity restores persistent calcium currents and enables locomotor recovery but also underlies muscle spasms; blocking constitutively active 5-HT2C receptors with SB206553 or cyproheptadine eliminates these currents and spasms.","method":"In vivo spinal transection model, mRNA editing analysis, patch-clamp electrophysiology in rats, pharmacological blockade in rats and humans","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (RNA analysis, electrophysiology, pharmacology) in animals and humans, replicated","pmids":["20512126"],"is_preprint":false},{"year":2009,"finding":"Loss of the imprinted snoRNA mbii-52 leads to increased editing (but not alternative splicing) of 5-HT2C receptor pre-mRNA in mice, resulting in altered 5-HT2CR-related behaviors including impulsive responding, locomotor activity, and palatable food reactivity, confirming mbii-52 as a negative regulator of 5-HT2C receptor RNA editing in vivo.","method":"PWS-IC+/- mouse model (mbii-52 knockout), RNA editing analysis, behavioral testing with pharmacological validation","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — genetic model with molecular (RNA editing) and behavioral readouts plus pharmacological specificity confirmation","pmids":["19304781"],"is_preprint":false},{"year":2012,"finding":"5-HT2C receptor activation suppresses the M-current (KCNQ channel-mediated) in hypothalamic POMC neurons via a mechanism requiring 5-HT2C receptors (blocked by RS-102221), thereby increasing POMC neuronal excitability to reduce food intake.","method":"Visualized patch-clamp recording in POMC-EGFP transgenic mice neurons, pharmacological agonists and selective antagonist RS-102221","journal":"American journal of physiology. Endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 1 — in vitro electrophysiology with selective receptor antagonist validation in identified neurons","pmids":["22436698"],"is_preprint":false},{"year":2015,"finding":"Native 5-HT2C receptors in choroid plexus epithelial cells exist as homodimers on the apical surface; fluorescence correlation spectroscopy with photon counting histogram molecular brightness analysis demonstrated homodimeric structure; agonist binding to one protomer activates G protein signaling, but maximal stimulation requires occupancy of both protomers.","method":"Fluorescence correlation spectroscopy (FCS) with photon counting histogram (PCH), anti-5-HT2C Fab protein, molecular brightness analysis, wash-resistant agonist occupancy experiments in HEK293 cells","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 1 — biophysical measurement in native tissue plus functional reconstitution with mutagenesis-equivalent occupancy approach","pmids":["25609374"],"is_preprint":false},{"year":2003,"finding":"PNU-69176E is a positive allosteric modulator of the human 5-HT2C receptor that enhances [3H]5-HT binding by increasing agonist affinity and renders receptors constitutively active (as measured by GTPγS binding, IP3 release, and IP accumulation) without affecting antagonist binding; this action is selective for 5-HT2C over related receptors (5-HT2A, 5-HT2B, 5-HT6, 5-HT7, D2, D3).","method":"Radioligand binding, GTPγS binding, IP3 assay, inositol phosphate accumulation in multiple cell lines expressing human 5-HT2C","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 1 — multiple functional assays with receptor selectivity panel and mechanistic characterization","pmids":["12815163"],"is_preprint":false},{"year":1996,"finding":"5-HT2C receptor stimulation in the subthalamic nucleus is sufficient to elicit orofacial dyskinesia; bilateral subthalamic infusion of selective 5-HT2C antagonists blocks systemically-induced oral movements, establishing the subthalamic nucleus as a key anatomical site for 5-HT2C-mediated oral dyskinesia.","method":"Local stereotaxic infusion into subthalamic nucleus, systemic drug administration, pharmacological antagonism in rats","journal":"Neuroscience","confidence":"High","confidence_rationale":"Tier 2 — bilateral infusion with selective antagonists establishes anatomical specificity and receptor involvement","pmids":["8730711"],"is_preprint":false},{"year":2010,"finding":"5-HT2C receptor antagonism in the orbitofrontal cortex (OFC), but not the medial prefrontal cortex or nucleus accumbens, improves spatial reversal learning by reducing perseverative errors, establishing the OFC as the neuroanatomical locus for 5-HT2C-mediated cognitive flexibility.","method":"Targeted intracerebral microinfusions into OFC, mPFC, and NAc in rats; spatial reversal learning task","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — anatomically targeted pharmacological intervention with region-specific behavioral dissociation","pmids":["20089901"],"is_preprint":false},{"year":2010,"finding":"5-HT2C receptors on alveolar macrophages mediate serotonin-induced calcium influx and CCL2 (MCP-1) production; these responses are absent in alveolar macrophages from 5-HT2C-deficient mice, demonstrating a functional role for 5-HT2C in macrophage immune activation.","method":"qPCR, calcium imaging, ELISA/dot-blot for CCL2, microarray, 5-HT2C knockout mouse alveolar macrophages, selective inhibitor RS-102221","journal":"American journal of physiology. Lung cellular and molecular physiology","confidence":"High","confidence_rationale":"Tier 2 — KO mouse model with multiple orthogonal methods confirming receptor-specific functional effects","pmids":["20495077"],"is_preprint":false},{"year":2009,"finding":"Overexpression of 5-HT2C receptors in forebrain (cortex and limbic regions) under CaMKIIα promoter in transgenic mice causes elevated anxiety-like behavior and hypoactivity without exogenous ligand stimulation, demonstrating that increased 5-HT2C receptor signaling is causative for anxiety phenotypes.","method":"Transgenic mouse overexpression, elevated plus-maze, wheel-running, novel environment activity testing","journal":"The European journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — gain-of-function transgenic with defined behavioral phenotype","pmids":["19614978"],"is_preprint":false},{"year":2019,"finding":"5-HT2C receptor signaling in the lateral habenula (LHb) contributes to alcohol withdrawal-induced anxiety; LHb 5-HT2CR protein and serotonin turnover are elevated in post-ethanol rats; intra-LHb 5-HT2CR antagonism alleviates anxiety-like behaviors and increases KCNQ2/3 membrane protein expression, indicating 5-HT2CR interacts with M-channels to regulate LHb excitability.","method":"Intra-LHb microinfusion of selective agonist/antagonist, elevated plus-maze, open-field, marble burying, c-Fos immunostaining, Western blotting for KCNQ2/3, operant alcohol self-administration","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 2 — anatomically targeted pharmacology with multiple behavioral readouts and molecular validation","pmids":["31778691"],"is_preprint":false},{"year":2005,"finding":"5-HT2C receptor mRNA is expressed in GABAergic neurons (glutamic acid decarboxylase-positive) but not in serotonergic neurons of the dorsal and median raphe nuclei, providing anatomical support for a negative feedback loop in which 5-HT2C-expressing GABAergic interneurons suppress 5-HT cell firing.","method":"Double in situ hybridization in rat brain sections","journal":"Journal of chemical neuroanatomy","confidence":"Medium","confidence_rationale":"Tier 3 — anatomical localization experiment with functional implication; single method","pmids":["15652696"],"is_preprint":false},{"year":2022,"finding":"iPSC-derived GABAergic interneurons from MDD patients with suicidal behavior show decreased 5-HT2C receptor expression, increased neural firing, and weakened calcium signaling; pharmacological activation or genetic restoration of 5-HT2C receptor rescues neuronal activity deficits, establishing a causal role for reduced 5-HT2C signaling in abnormal interneuron activity in depression.","method":"iPSC differentiation to GABAergic interneurons and ventral forebrain organoids, electrophysiology, calcium imaging, transcriptomic sequencing, 5-HT2C agonist and genetic rescue","journal":"EMBO molecular medicine","confidence":"High","confidence_rationale":"Tier 2 — patient-derived human cellular model with multiple orthogonal methods and rescue experiments","pmids":["36373384"],"is_preprint":false},{"year":1996,"finding":"Fluoxetine, norfluoxetine, and citalopram act as antagonists (not inverse agonists) at the 5-HT2C receptor in radioligand binding and phosphoinositide hydrolysis assays, with 10- to 23-fold selectivity for 5-HT2C over 5-HT2A receptors in vitro.","method":"Radioligand binding assays, phosphoinositide hydrolysis assay in clonal cell line and rat choroid plexus","journal":"Psychopharmacology","confidence":"High","confidence_rationale":"Tier 1 — in vitro functional and binding assays with multiple drugs and receptor subtypes","pmids":["8876023"],"is_preprint":false},{"year":2016,"finding":"Loss of Snord115 (the mouse mbii-52 ortholog) increases levels of truncated/alternatively spliced hypothalamic 5-HT2C receptor pre-mRNA, reduces POMC mRNA in the arcuate nucleus, and renders mice unresponsive to 5-HT2C agonist-induced anorexia with attenuated activation of POMC neurons, demonstrating that Snord115-regulated 5-HT2C mRNA processing is required for normal 5-HT2C-POMC appetite control.","method":"PWS-IC mouse model, RT-PCR for splicing isoforms, in situ hybridization for POMC mRNA, c-Fos immunostaining of POMC neurons, pharmacological feeding studies","journal":"Molecular brain","confidence":"High","confidence_rationale":"Tier 2 — genetic model with molecular and circuit-level validation using multiple methods","pmids":["27931246"],"is_preprint":false},{"year":2006,"finding":"WAY-161503 is a functional agonist at 5-HT2C receptors stimulating both phospholipase C (IP formation, calcium mobilization) and phospholipase A2 (arachidonic acid release) pathways, with higher potency and efficacy for PLC than PLA2, demonstrating biased signaling at the 5-HT2C receptor.","method":"Radioligand binding, inositol phosphate accumulation assay, calcium mobilization assay, arachidonic acid release assay in CHO cells","journal":"Brain research","confidence":"High","confidence_rationale":"Tier 1 — multiple parallel second messenger assays in reconstituted cell system","pmids":["16430874"],"is_preprint":false}],"current_model":"HTR2C encodes a G protein-coupled receptor (primarily Gq) that, upon activation, stimulates phospholipase C (phosphoinositide turnover), cyclic GMP formation (via a calcium/PLA2/lipoxygenase pathway), and inhibition of the M-current in POMC neurons; it forms constitutive homodimers beginning in the endoplasmic reticulum, is subject to extensive RNA editing that reduces G protein coupling efficacy, physically interacts with PTEN (which limits receptor phosphorylation), is expressed on GABAergic interneurons in the raphe and VTA (where it tonically inhibits dopaminergic output) and on hypothalamic POMC neurons (through which it suppresses appetite via downstream melanocortin-4 receptor signaling), and mediates tonic inhibition of neuronal network excitability as demonstrated by seizures and altered dentate gyrus LTP in knockout mice."},"narrative":{"teleology":[{"year":1995,"claim":"Knockout studies established that HTR2C is required for normal feeding behavior control and tonic suppression of neuronal excitability, answering whether the receptor has non-redundant physiological roles in vivo.","evidence":"Targeted gene knockout in mice showing obesity and spontaneous seizures","pmids":["7700379"],"confidence":"High","gaps":["Circuit identity of neurons mediating seizure susceptibility was not resolved","Whether obesity reflects central appetite regulation or peripheral metabolic effects was unclear","Downstream signaling pathways driving feeding phenotype uncharacterized"]},{"year":1995,"claim":"Identification of the cGMP signaling pathway downstream of HTR2C revealed that this receptor couples to effectors beyond canonical PLC/IP3, establishing multimodal second messenger engagement.","evidence":"Choroid plexus tissue slice assays with pharmacological inhibitors showing Ca²⁺/PLA2/lipoxygenase-dependent cGMP formation independent of Gi","pmids":["7798914"],"confidence":"High","gaps":["Physiological relevance of cGMP signaling in neurons (vs. choroid plexus) not established","G protein identity for the cGMP arm not determined"]},{"year":1997,"claim":"Demonstration that constitutive activity arises from specific transmembrane mutations and that inverse agonists suppress basal signaling resolved how receptor conformation governs ligand-independent G protein coupling.","evidence":"Site-directed mutagenesis (S312F/K) with PI hydrolysis and radioligand binding in COS-7 cells","pmids":["9282936"],"confidence":"High","gaps":["Whether naturally edited isoforms exhibit comparable constitutive activity was not tested","Structural basis for constitutive activation unknown"]},{"year":1997,"claim":"Audiogenic seizure susceptibility in knockout mice with subcortical c-fos mapping pinpointed HTR2C as a tonic brake on auditory circuit excitability, extending the seizure phenotype to a defined neural circuit.","evidence":"Knockout mouse audiogenic seizure testing with immediate early gene expression mapping","pmids":["9241279"],"confidence":"High","gaps":["Cell-type-specific rescue not performed","Whether GABAergic interneuron loss of 5-HT2C is the causal mechanism remained untested"]},{"year":1998,"claim":"Selective impairment of dentate gyrus LTP in knockout mice established that HTR2C modulates synaptic plasticity at a specific hippocampal synapse, linking receptor function to spatial learning.","evidence":"Electrophysiology at four hippocampal pathways and Morris water maze in 5-HT2C null mice","pmids":["9844009"],"confidence":"High","gaps":["Molecular mechanism of LTP modulation (pre- vs. postsynaptic) not determined","Whether effect is cell-autonomous in dentate neurons unknown"]},{"year":1999,"claim":"Discovery that RNA editing alters agonist-directed signaling (functional selectivity) at HTR2C established that post-transcriptional modification tunes receptor pharmacology in a ligand-dependent manner.","evidence":"Comparison of PI signaling by LSD and serotonin across edited and non-edited receptor isoforms","pmids":["10432492"],"confidence":"Medium","gaps":["Which specific edited isoforms are present in vivo was not mapped","Structural basis for editing-induced signaling change unknown","Limited to PI readout; effects on other effectors not tested"]},{"year":2005,"claim":"Localization of HTR2C mRNA to GABAergic but not serotonergic neurons of the raphe nuclei provided the anatomical basis for a tonic inhibitory feedback loop controlling serotonin release.","evidence":"Double in situ hybridization for 5-HT2C and GAD mRNAs in rat raphe","pmids":["15652696"],"confidence":"Medium","gaps":["Functional confirmation by cell-type-specific deletion not performed","Single-method anatomical study"]},{"year":2006,"claim":"FRET-based demonstration that HTR2C forms homodimers from the ER through the plasma membrane established dimerization as a constitutive biosynthetic feature rather than a ligand-induced event.","evidence":"Live-cell confocal FRET with CFP/YFP-tagged receptors and compartment-specific analysis","pmids":["16857671"],"confidence":"High","gaps":["Functional consequence of preventing dimerization not tested","Whether heterodimers with other 5-HT2 subtypes form in vivo not addressed"]},{"year":2006,"claim":"Identification of PTEN as a physical interactor that dephosphorylates HTR2C and controls VTA dopaminergic output linked receptor phosphorylation status to drug reward behavior.","evidence":"Co-immunoprecipitation, Tat-3L4F interfering peptide, VTA electrophysiology, conditioned place preference","pmids":["16474401"],"confidence":"High","gaps":["Whether PTEN acts via lipid or protein phosphatase activity at the receptor was not fully resolved","In vivo stoichiometry of the PTEN–5-HT2C complex unknown"]},{"year":2007,"claim":"Co-localization of HTR2C with POMC neurons and epistasis with MC4R knockout established that the anorexigenic effect of 5-HT2C agonism operates through the melanocortin pathway, resolving the downstream circuit for appetite suppression.","evidence":"Immunofluorescence co-localization in arcuate nucleus; MC4R KO mice unresponsive to 5-HT2C agonist hypophagia","pmids":["18039773"],"confidence":"High","gaps":["Direct electrophysiological demonstration in POMC neurons came later","Contribution of non-POMC neuronal populations not excluded"]},{"year":2009,"claim":"Genetic ablation of snoRNA MBII-52 increased 5-HT2C RNA editing in vivo and altered impulsivity-related behaviors, establishing snoRNA-guided regulation as a physiologically relevant control mechanism for receptor function.","evidence":"PWS-IC mouse model with RNA editing quantification and behavioral pharmacology","pmids":["19304781"],"confidence":"High","gaps":["Mechanism by which snoRNA inhibits editing enzyme access not resolved","Relative contribution of editing vs. splicing changes debated"]},{"year":2010,"claim":"Post-injury RNA editing shifts in spinal motoneurons produce constitutively active 5-HT2C isoforms that restore persistent calcium currents but cause spasticity, revealing a pathophysiological consequence of editing dysregulation.","evidence":"Spinal transection model with mRNA editing analysis, patch-clamp, and pharmacological blockade in rats and humans","pmids":["20512126"],"confidence":"High","gaps":["Editing enzyme identity driving post-injury changes not identified","Long-term therapeutic safety of inverse agonist treatment not assessed"]},{"year":2012,"claim":"Electrophysiological recording in identified POMC neurons showed that HTR2C activation suppresses M-current (KCNQ), providing the ionic mechanism for 5-HT2C-mediated increase in POMC neuron excitability and feeding suppression.","evidence":"Visualized patch-clamp in POMC-EGFP transgenic mouse neurons with selective antagonist RS-102221","pmids":["22436698"],"confidence":"High","gaps":["Signaling intermediates between Gq and KCNQ suppression not mapped","Whether M-current suppression also occurs in other HTR2C-expressing neurons not tested"]},{"year":2015,"claim":"Biophysical measurement confirmed native homodimeric stoichiometry and showed that maximal G protein activation requires dual-protomer agonist occupancy, establishing a functional consequence of dimerization.","evidence":"Fluorescence correlation spectroscopy with photon counting histogram analysis on native choroid plexus receptors and HEK293 cells","pmids":["25609374"],"confidence":"High","gaps":["Whether dimerization interface residues are conserved across species not tested","Crystal or cryo-EM structure of the dimer not available"]},{"year":2022,"claim":"Patient-derived iPSC-GABAergic interneurons with reduced HTR2C expression showed increased firing and impaired calcium signaling, and genetic/pharmacological rescue reversed these deficits, causally linking diminished 5-HT2C signaling to interneuron dysfunction in major depression.","evidence":"iPSC differentiation to GABAergic interneurons from MDD patients, electrophysiology, calcium imaging, genetic rescue","pmids":["36373384"],"confidence":"High","gaps":["Whether reduced HTR2C expression is a cause or consequence of disease epigenetic changes unclear","In vivo circuit-level validation in patients lacking"]},{"year":null,"claim":"Key unresolved questions include the high-resolution structure of the HTR2C homodimer, the precise signaling intermediates from Gq to KCNQ channel suppression, cell-type-specific contributions to seizure phenotypes, and the enzymatic mechanism by which editing enzymes are regulated by Snord115.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No cryo-EM or crystal structure of HTR2C dimer","Gq-to-KCNQ signaling cascade intermediates unmapped","Cell-type-specific conditional knockout studies across brain regions incomplete"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[2,3,7,16,26]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[14,21]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[9,15]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[9]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,3,6,7,16,26]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[0,1,5,14,17,18,21]}],"complexes":["HTR2C homodimer"],"partners":["PTEN","MC4R","KCNQ2","KCNQ3","SNORD115"],"other_free_text":[]},"mechanistic_narrative":"HTR2C encodes a Gq-coupled serotonin receptor that functions as a central integrator of serotonergic signaling in the brain, governing feeding behavior, neuronal excitability, anxiety, cognitive flexibility, and dopaminergic tone. Upon serotonin binding, HTR2C activates phospholipase C to stimulate phosphoinositide hydrolysis and also engages a calcium/PLA2/lipoxygenase-dependent cGMP pathway; in hypothalamic POMC neurons it suppresses the M-current (KCNQ channels) to increase excitability, driving anorexia through downstream melanocortin-4 receptor signaling [PMID:7798914, PMID:22436698, PMID:18039773]. The receptor forms constitutive homodimers beginning in the endoplasmic reticulum, undergoes extensive pre-mRNA editing regulated by snoRNA Snord115/MBII-52 that alters G protein coupling efficacy and constitutive activity, and is modulated by a physical interaction with PTEN that limits agonist-induced phosphorylation and thereby controls VTA dopaminergic output [PMID:16857671, PMID:19304781, PMID:16474401]. Knockout mice exhibit obesity, spontaneous and audiogenic seizures, impaired dentate gyrus LTP, and loss of tonic inhibition of neuronal networks, while forebrain overexpression causes anxiety, collectively demonstrating that HTR2C calibrates excitability in GABAergic interneuron circuits across cortical, limbic, and subcortical regions [PMID:7700379, PMID:9241279, PMID:9844009, PMID:19614978]."},"prefetch_data":{"uniprot":{"accession":"P28335","full_name":"5-hydroxytryptamine receptor 2C","aliases":["5-hydroxytryptamine receptor 1C","5-HT-1C","5-HT1C","Serotonin receptor 2C"],"length_aa":458,"mass_kda":51.8,"function":"G-protein coupled receptor for 5-hydroxytryptamine (serotonin) (PubMed:12970106, PubMed:18703043, PubMed:19057895, PubMed:29398112, PubMed:7895773). Also functions as a receptor for various drugs and psychoactive substances, including ergot alkaloid derivatives, 1-2,5,-dimethoxy-4-iodophenyl-2-aminopropane (DOI) and lysergic acid diethylamide (LSD) (PubMed:19057895, PubMed:29398112). Ligand binding causes a conformation change that triggers signaling via guanine nucleotide-binding proteins (G proteins) and modulates the activity of downstream effectors (PubMed:18703043, PubMed:29398112). HTR2C is coupled to G(q)/G(11) G alpha proteins and activates phospholipase C-beta, releasing diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3) second messengers that modulate the activity of phosphatidylinositol 3-kinase and promote the release of Ca(2+) ions from intracellular stores, respectively (PubMed:18703043, PubMed:29398112). Beta-arrestin family members inhibit signaling via G proteins and mediate activation of alternative signaling pathways (PubMed:29398112). Regulates neuronal activity via the activation of short transient receptor potential calcium channels in the brain, and thereby modulates the activation of pro-opiomelanocortin neurons and the release of CRH that then regulates the release of corticosterone (By similarity). Plays a role in the regulation of appetite and eating behavior, responses to anxiogenic stimuli and stress (By similarity). 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Molecular brain research","url":"https://pubmed.ncbi.nlm.nih.gov/8750891","citation_count":32,"is_preprint":false},{"pmid":"9543230","id":"PMC_9543230","title":"Meta-chlorophenylpiperazine induced changes in locomotor activity are mediated by 5-HT1 as well as 5-HT2C receptors in mice.","date":"1998","source":"European journal of pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/9543230","citation_count":32,"is_preprint":false},{"pmid":"20010450","id":"PMC_20010450","title":"A genetic variant of HTR2C may play a role in the manifestation of Tourette syndrome.","date":"2010","source":"Psychiatric genetics","url":"https://pubmed.ncbi.nlm.nih.gov/20010450","citation_count":32,"is_preprint":false},{"pmid":"21185157","id":"PMC_21185157","title":"Association between HTR2C polymorphisms and metabolic syndrome in patients with schizophrenia treated with atypical antipsychotics.","date":"2010","source":"Schizophrenia research","url":"https://pubmed.ncbi.nlm.nih.gov/21185157","citation_count":31,"is_preprint":false},{"pmid":"8831107","id":"PMC_8831107","title":"Effects of the 5-HT2C/2B receptor agonist 1-(3-chlorophenyl) piperazine on plasma glucose levels of rats.","date":"1996","source":"European journal of pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/8831107","citation_count":30,"is_preprint":false},{"pmid":"16021472","id":"PMC_16021472","title":"Complex HTR2C linkage disequilibrium and promoter associations with body mass index and serum leptin.","date":"2005","source":"Human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/16021472","citation_count":28,"is_preprint":false},{"pmid":"16146672","id":"PMC_16146672","title":"Serotonin2C receptors (5-HT2C R) control expression of cocaine-induced conditioned hyperactivity.","date":"2005","source":"Drug and alcohol dependence","url":"https://pubmed.ncbi.nlm.nih.gov/16146672","citation_count":28,"is_preprint":false},{"pmid":"24196946","id":"PMC_24196946","title":"Stimulation of 5-HT2C receptors improves cognitive deficits induced by human tryptophan hydroxylase 2 loss of function mutation.","date":"2013","source":"Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/24196946","citation_count":28,"is_preprint":false},{"pmid":"10994642","id":"PMC_10994642","title":"Sex differences in allelic frequencies of the 5-HT2C Cys23Ser polymorphism in psychiatric patients and healthy volunteers: findings from an association study.","date":"2000","source":"Psychiatric genetics","url":"https://pubmed.ncbi.nlm.nih.gov/10994642","citation_count":28,"is_preprint":false},{"pmid":"21510767","id":"PMC_21510767","title":"Polymorphisms of the LEP, LEPR and HTR2C gene: obesity and BMI change in patients using antipsychotic medication in a naturalistic setting.","date":"2011","source":"Pharmacogenomics","url":"https://pubmed.ncbi.nlm.nih.gov/21510767","citation_count":28,"is_preprint":false},{"pmid":"15912142","id":"PMC_15912142","title":"Pharmacological properties of the Cys23Ser single nucleotide polymorphism in human 5-HT2C receptor isoforms.","date":"2005","source":"The pharmacogenomics journal","url":"https://pubmed.ncbi.nlm.nih.gov/15912142","citation_count":27,"is_preprint":false},{"pmid":"17141810","id":"PMC_17141810","title":"Activation of the brain 5-HT2C receptors causes hypolocomotion without anxiogenic-like cardiovascular adjustments in mice.","date":"2006","source":"Neuropharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/17141810","citation_count":27,"is_preprint":false},{"pmid":"34367066","id":"PMC_34367066","title":"Central 5-HTR2C in the Control of Metabolic Homeostasis.","date":"2021","source":"Frontiers in endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/34367066","citation_count":26,"is_preprint":false},{"pmid":"22448217","id":"PMC_22448217","title":"Dynamic 5-HT2C receptor editing in a mouse model of obesity.","date":"2012","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/22448217","citation_count":26,"is_preprint":false},{"pmid":"19496825","id":"PMC_19496825","title":"Promoter and functional polymorphisms of HTR2C and suicide victims.","date":"2009","source":"Genes, brain, and behavior","url":"https://pubmed.ncbi.nlm.nih.gov/19496825","citation_count":26,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":51616,"output_tokens":6205,"usd":0.123962},"stage2":{"model":"claude-opus-4-6","input_tokens":9837,"output_tokens":3519,"usd":0.20574},"total_usd":0.329702,"stage1_batch_id":"msgbatch_01DzrsgVgYngY1hvM3DU1yF2","stage2_batch_id":"msgbatch_01KA9b7iRzKFdhoBUT7vCvSe","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1995,\n      \"finding\": \"5-HT2C receptor knockout mice develop obesity due to abnormal control of feeding behavior and are prone to spontaneous seizures, establishing roles for this receptor in serotonergic control of appetite and tonic inhibition of neuronal network excitability.\",\n      \"method\": \"Targeted gene knockout in mice with behavioral and physiological phenotypic readouts\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple defined cellular/behavioral phenotypes, highly cited foundational study\",\n      \"pmids\": [\"7700379\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"5-HT2C receptor null mutant mice are extremely susceptible to audiogenic seizures with complete penetrance in adults, and immediate early gene expression indicates seizures are subcortical phenomena in auditory circuits, implicating 5-HT2C receptors in serotoninergic control of epilepsy.\",\n      \"method\": \"Knockout mouse model, audiogenic seizure testing, immediate early gene expression (c-fos)\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO model with defined phenotype and circuit-level mechanistic data, replicated finding from same gene\",\n      \"pmids\": [\"9241279\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Site-directed mutagenesis of serine 312 to phenylalanine or lysine in the 5-HT2C receptor produces constitutively active receptors that spontaneously couple to G proteins and stimulate phosphatidylinositol (PI) hydrolysis in the absence of agonist; inverse agonists mianserin and mesulergine decrease basal PI hydrolysis of the S312K mutant.\",\n      \"method\": \"Site-directed mutagenesis, radioligand binding, PI hydrolysis assay in COS-7 cells\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro mutagenesis with functional reconstitution and multiple assays\",\n      \"pmids\": [\"9282936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"5-HT2C receptor activation in choroid plexus stimulates cyclic GMP (cGMP) formation in addition to phosphoinositide turnover; cGMP formation is calcium-, phospholipase A2-, and lipoxygenase-dependent and not inhibited by pertussis toxin, indicating coupling through a Gi-independent pathway.\",\n      \"method\": \"Choroid plexus tissue slice assay, cGMP measurement, pharmacological antagonism, pertussis toxin pretreatment, enzyme inhibitors\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro functional assay with multiple pharmacological tools and biochemical inhibitors\",\n      \"pmids\": [\"7798914\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"The rat 5-HT2C receptor is a glycoprotein containing N-linked oligosaccharides; N-linked sugars contribute approximately 30% of the relative mass, and sialic acid residues associate with 5-HT2C receptors in the choroid plexus but not in a recombinant cell line.\",\n      \"method\": \"Western blotting with anti-5-HT2C antibodies, tunicamycin metabolic inhibition, N-glycosidase F and neuraminidase enzymatic treatments\",\n      \"journal\": \"Brain research. Molecular brain research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted biochemical analysis with multiple enzymatic and metabolic approaches\",\n      \"pmids\": [\"8750891\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"5-HT2C receptor mutant mice show selective impairment of long-term potentiation at medial perforant path-dentate gyrus synapses and exhibit abnormal Morris water maze performance and reduced aversion to novel environments, establishing a role for 5-HT2C receptors in dentate gyrus synaptic plasticity.\",\n      \"method\": \"Knockout mouse model, LTP electrophysiology at four hippocampal regions, Morris water maze, behavioral assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO model with electrophysiological and behavioral phenotypic readouts, multiple orthogonal methods\",\n      \"pmids\": [\"9844009\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Inverse agonists at 5-HT2C receptors expressed at physiological density do not reduce basal inositol phosphate (IP) accumulation but selectively enhance 5-HT2C-mediated IP accumulation (not arachidonic acid release) after prolonged treatment; this enhancement may involve increased expression of Gαq/11 and possibly Gα12 and Gα13 proteins.\",\n      \"method\": \"IP accumulation assay, arachidonic acid release assay, pharmacological characterization in CHO cells, G protein expression analysis\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro functional assays; single lab study\",\n      \"pmids\": [\"10220565\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Agonist-directed signaling (functional selectivity) at 5-HT2C receptors: LSD and serotonin differ in their activation of phosphatidylinositol signaling; RNA editing of the 5-HT2C receptor dramatically alters the ability of LSD to stimulate PI signaling.\",\n      \"method\": \"Phosphatidylinositol signaling assay, comparison of edited vs. non-edited receptor isoforms\",\n      \"journal\": \"Neuropsychopharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — functional in vitro assay; single lab, limited mechanistic detail in abstract\",\n      \"pmids\": [\"10432492\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"PTEN physically interacts with the third intracellular loop (3L4F region) of the 5-HT2C receptor and limits agonist-induced phosphorylation of the receptor through its protein phosphatase activity; disruption of this PTEN:5-HT2cR complex by a Tat-3L4F interfering peptide suppresses VTA dopaminergic neuron firing and blocks conditioned place preference for THC and nicotine.\",\n      \"method\": \"Co-immunoprecipitation in cell cultures, interfering peptide (Tat-3L4F), electrophysiology of VTA neurons, conditioned place preference behavioral tests\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, functional peptide disruption, in vivo electrophysiology, and behavioral assays\",\n      \"pmids\": [\"16474401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"5-HT2C receptor homodimerization/oligomerization begins in the endoplasmic reticulum and persists through Golgi and plasma membrane, as demonstrated by real-time FRET between CFP- and YFP-tagged receptors in intact living cells, suggesting dimer formation is a step in receptor maturation.\",\n      \"method\": \"Confocal fluorescence resonance energy transfer (FRET), acceptor photobleaching, live-cell time-lapse confocal imaging\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — FRET in living cells with compartment-specific analysis and rigorous controls\",\n      \"pmids\": [\"16857671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"5-HT2C receptors are co-localized with GABA neurons (GAD-positive) and dopamine neurons (tyrosine hydroxylase-positive) in the VTA, establishing the anatomical basis for 5-HT2C-mediated inhibitory control of both GABAergic and dopaminergic output in the mesolimbic system.\",\n      \"method\": \"Double-label immunofluorescence in rat VTA tissue\",\n      \"journal\": \"Neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — direct localization experiment with functional implication, single lab\",\n      \"pmids\": [\"17367945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"5-HT2C receptors are coexpressed with proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus; melanocortin 4 receptor (MC4R) knockout mice are unresponsive to 5-HT2CR agonist-induced hypophagia, establishing MC4R as a requisite downstream pathway for 5-HT2CR agonist control of food intake.\",\n      \"method\": \"Immunofluorescence co-localization, MC4R knockout mice, pharmacological feeding studies\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis via knockout rescue plus anatomical co-localization\",\n      \"pmids\": [\"18039773\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"After spinal cord injury, post-transcriptional RNA editing of 5-HT2C receptor mRNA increases constitutively active receptor isoforms in motoneurons; this constitutive activity restores persistent calcium currents and enables locomotor recovery but also underlies muscle spasms; blocking constitutively active 5-HT2C receptors with SB206553 or cyproheptadine eliminates these currents and spasms.\",\n      \"method\": \"In vivo spinal transection model, mRNA editing analysis, patch-clamp electrophysiology in rats, pharmacological blockade in rats and humans\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (RNA analysis, electrophysiology, pharmacology) in animals and humans, replicated\",\n      \"pmids\": [\"20512126\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Loss of the imprinted snoRNA mbii-52 leads to increased editing (but not alternative splicing) of 5-HT2C receptor pre-mRNA in mice, resulting in altered 5-HT2CR-related behaviors including impulsive responding, locomotor activity, and palatable food reactivity, confirming mbii-52 as a negative regulator of 5-HT2C receptor RNA editing in vivo.\",\n      \"method\": \"PWS-IC+/- mouse model (mbii-52 knockout), RNA editing analysis, behavioral testing with pharmacological validation\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic model with molecular (RNA editing) and behavioral readouts plus pharmacological specificity confirmation\",\n      \"pmids\": [\"19304781\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"5-HT2C receptor activation suppresses the M-current (KCNQ channel-mediated) in hypothalamic POMC neurons via a mechanism requiring 5-HT2C receptors (blocked by RS-102221), thereby increasing POMC neuronal excitability to reduce food intake.\",\n      \"method\": \"Visualized patch-clamp recording in POMC-EGFP transgenic mice neurons, pharmacological agonists and selective antagonist RS-102221\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro electrophysiology with selective receptor antagonist validation in identified neurons\",\n      \"pmids\": [\"22436698\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Native 5-HT2C receptors in choroid plexus epithelial cells exist as homodimers on the apical surface; fluorescence correlation spectroscopy with photon counting histogram molecular brightness analysis demonstrated homodimeric structure; agonist binding to one protomer activates G protein signaling, but maximal stimulation requires occupancy of both protomers.\",\n      \"method\": \"Fluorescence correlation spectroscopy (FCS) with photon counting histogram (PCH), anti-5-HT2C Fab protein, molecular brightness analysis, wash-resistant agonist occupancy experiments in HEK293 cells\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — biophysical measurement in native tissue plus functional reconstitution with mutagenesis-equivalent occupancy approach\",\n      \"pmids\": [\"25609374\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"PNU-69176E is a positive allosteric modulator of the human 5-HT2C receptor that enhances [3H]5-HT binding by increasing agonist affinity and renders receptors constitutively active (as measured by GTPγS binding, IP3 release, and IP accumulation) without affecting antagonist binding; this action is selective for 5-HT2C over related receptors (5-HT2A, 5-HT2B, 5-HT6, 5-HT7, D2, D3).\",\n      \"method\": \"Radioligand binding, GTPγS binding, IP3 assay, inositol phosphate accumulation in multiple cell lines expressing human 5-HT2C\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple functional assays with receptor selectivity panel and mechanistic characterization\",\n      \"pmids\": [\"12815163\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"5-HT2C receptor stimulation in the subthalamic nucleus is sufficient to elicit orofacial dyskinesia; bilateral subthalamic infusion of selective 5-HT2C antagonists blocks systemically-induced oral movements, establishing the subthalamic nucleus as a key anatomical site for 5-HT2C-mediated oral dyskinesia.\",\n      \"method\": \"Local stereotaxic infusion into subthalamic nucleus, systemic drug administration, pharmacological antagonism in rats\",\n      \"journal\": \"Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — bilateral infusion with selective antagonists establishes anatomical specificity and receptor involvement\",\n      \"pmids\": [\"8730711\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"5-HT2C receptor antagonism in the orbitofrontal cortex (OFC), but not the medial prefrontal cortex or nucleus accumbens, improves spatial reversal learning by reducing perseverative errors, establishing the OFC as the neuroanatomical locus for 5-HT2C-mediated cognitive flexibility.\",\n      \"method\": \"Targeted intracerebral microinfusions into OFC, mPFC, and NAc in rats; spatial reversal learning task\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — anatomically targeted pharmacological intervention with region-specific behavioral dissociation\",\n      \"pmids\": [\"20089901\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"5-HT2C receptors on alveolar macrophages mediate serotonin-induced calcium influx and CCL2 (MCP-1) production; these responses are absent in alveolar macrophages from 5-HT2C-deficient mice, demonstrating a functional role for 5-HT2C in macrophage immune activation.\",\n      \"method\": \"qPCR, calcium imaging, ELISA/dot-blot for CCL2, microarray, 5-HT2C knockout mouse alveolar macrophages, selective inhibitor RS-102221\",\n      \"journal\": \"American journal of physiology. Lung cellular and molecular physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO mouse model with multiple orthogonal methods confirming receptor-specific functional effects\",\n      \"pmids\": [\"20495077\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Overexpression of 5-HT2C receptors in forebrain (cortex and limbic regions) under CaMKIIα promoter in transgenic mice causes elevated anxiety-like behavior and hypoactivity without exogenous ligand stimulation, demonstrating that increased 5-HT2C receptor signaling is causative for anxiety phenotypes.\",\n      \"method\": \"Transgenic mouse overexpression, elevated plus-maze, wheel-running, novel environment activity testing\",\n      \"journal\": \"The European journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — gain-of-function transgenic with defined behavioral phenotype\",\n      \"pmids\": [\"19614978\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"5-HT2C receptor signaling in the lateral habenula (LHb) contributes to alcohol withdrawal-induced anxiety; LHb 5-HT2CR protein and serotonin turnover are elevated in post-ethanol rats; intra-LHb 5-HT2CR antagonism alleviates anxiety-like behaviors and increases KCNQ2/3 membrane protein expression, indicating 5-HT2CR interacts with M-channels to regulate LHb excitability.\",\n      \"method\": \"Intra-LHb microinfusion of selective agonist/antagonist, elevated plus-maze, open-field, marble burying, c-Fos immunostaining, Western blotting for KCNQ2/3, operant alcohol self-administration\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — anatomically targeted pharmacology with multiple behavioral readouts and molecular validation\",\n      \"pmids\": [\"31778691\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"5-HT2C receptor mRNA is expressed in GABAergic neurons (glutamic acid decarboxylase-positive) but not in serotonergic neurons of the dorsal and median raphe nuclei, providing anatomical support for a negative feedback loop in which 5-HT2C-expressing GABAergic interneurons suppress 5-HT cell firing.\",\n      \"method\": \"Double in situ hybridization in rat brain sections\",\n      \"journal\": \"Journal of chemical neuroanatomy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — anatomical localization experiment with functional implication; single method\",\n      \"pmids\": [\"15652696\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"iPSC-derived GABAergic interneurons from MDD patients with suicidal behavior show decreased 5-HT2C receptor expression, increased neural firing, and weakened calcium signaling; pharmacological activation or genetic restoration of 5-HT2C receptor rescues neuronal activity deficits, establishing a causal role for reduced 5-HT2C signaling in abnormal interneuron activity in depression.\",\n      \"method\": \"iPSC differentiation to GABAergic interneurons and ventral forebrain organoids, electrophysiology, calcium imaging, transcriptomic sequencing, 5-HT2C agonist and genetic rescue\",\n      \"journal\": \"EMBO molecular medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — patient-derived human cellular model with multiple orthogonal methods and rescue experiments\",\n      \"pmids\": [\"36373384\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Fluoxetine, norfluoxetine, and citalopram act as antagonists (not inverse agonists) at the 5-HT2C receptor in radioligand binding and phosphoinositide hydrolysis assays, with 10- to 23-fold selectivity for 5-HT2C over 5-HT2A receptors in vitro.\",\n      \"method\": \"Radioligand binding assays, phosphoinositide hydrolysis assay in clonal cell line and rat choroid plexus\",\n      \"journal\": \"Psychopharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro functional and binding assays with multiple drugs and receptor subtypes\",\n      \"pmids\": [\"8876023\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Loss of Snord115 (the mouse mbii-52 ortholog) increases levels of truncated/alternatively spliced hypothalamic 5-HT2C receptor pre-mRNA, reduces POMC mRNA in the arcuate nucleus, and renders mice unresponsive to 5-HT2C agonist-induced anorexia with attenuated activation of POMC neurons, demonstrating that Snord115-regulated 5-HT2C mRNA processing is required for normal 5-HT2C-POMC appetite control.\",\n      \"method\": \"PWS-IC mouse model, RT-PCR for splicing isoforms, in situ hybridization for POMC mRNA, c-Fos immunostaining of POMC neurons, pharmacological feeding studies\",\n      \"journal\": \"Molecular brain\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic model with molecular and circuit-level validation using multiple methods\",\n      \"pmids\": [\"27931246\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"WAY-161503 is a functional agonist at 5-HT2C receptors stimulating both phospholipase C (IP formation, calcium mobilization) and phospholipase A2 (arachidonic acid release) pathways, with higher potency and efficacy for PLC than PLA2, demonstrating biased signaling at the 5-HT2C receptor.\",\n      \"method\": \"Radioligand binding, inositol phosphate accumulation assay, calcium mobilization assay, arachidonic acid release assay in CHO cells\",\n      \"journal\": \"Brain research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple parallel second messenger assays in reconstituted cell system\",\n      \"pmids\": [\"16430874\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"HTR2C encodes a G protein-coupled receptor (primarily Gq) that, upon activation, stimulates phospholipase C (phosphoinositide turnover), cyclic GMP formation (via a calcium/PLA2/lipoxygenase pathway), and inhibition of the M-current in POMC neurons; it forms constitutive homodimers beginning in the endoplasmic reticulum, is subject to extensive RNA editing that reduces G protein coupling efficacy, physically interacts with PTEN (which limits receptor phosphorylation), is expressed on GABAergic interneurons in the raphe and VTA (where it tonically inhibits dopaminergic output) and on hypothalamic POMC neurons (through which it suppresses appetite via downstream melanocortin-4 receptor signaling), and mediates tonic inhibition of neuronal network excitability as demonstrated by seizures and altered dentate gyrus LTP in knockout mice.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"HTR2C encodes a Gq-coupled serotonin receptor that functions as a central integrator of serotonergic signaling in the brain, governing feeding behavior, neuronal excitability, anxiety, cognitive flexibility, and dopaminergic tone. Upon serotonin binding, HTR2C activates phospholipase C to stimulate phosphoinositide hydrolysis and also engages a calcium/PLA2/lipoxygenase-dependent cGMP pathway; in hypothalamic POMC neurons it suppresses the M-current (KCNQ channels) to increase excitability, driving anorexia through downstream melanocortin-4 receptor signaling [PMID:7798914, PMID:22436698, PMID:18039773]. The receptor forms constitutive homodimers beginning in the endoplasmic reticulum, undergoes extensive pre-mRNA editing regulated by snoRNA Snord115/MBII-52 that alters G protein coupling efficacy and constitutive activity, and is modulated by a physical interaction with PTEN that limits agonist-induced phosphorylation and thereby controls VTA dopaminergic output [PMID:16857671, PMID:19304781, PMID:16474401]. Knockout mice exhibit obesity, spontaneous and audiogenic seizures, impaired dentate gyrus LTP, and loss of tonic inhibition of neuronal networks, while forebrain overexpression causes anxiety, collectively demonstrating that HTR2C calibrates excitability in GABAergic interneuron circuits across cortical, limbic, and subcortical regions [PMID:7700379, PMID:9241279, PMID:9844009, PMID:19614978].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Knockout studies established that HTR2C is required for normal feeding behavior control and tonic suppression of neuronal excitability, answering whether the receptor has non-redundant physiological roles in vivo.\",\n      \"evidence\": \"Targeted gene knockout in mice showing obesity and spontaneous seizures\",\n      \"pmids\": [\"7700379\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Circuit identity of neurons mediating seizure susceptibility was not resolved\",\n        \"Whether obesity reflects central appetite regulation or peripheral metabolic effects was unclear\",\n        \"Downstream signaling pathways driving feeding phenotype uncharacterized\"\n      ]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Identification of the cGMP signaling pathway downstream of HTR2C revealed that this receptor couples to effectors beyond canonical PLC/IP3, establishing multimodal second messenger engagement.\",\n      \"evidence\": \"Choroid plexus tissue slice assays with pharmacological inhibitors showing Ca²⁺/PLA2/lipoxygenase-dependent cGMP formation independent of Gi\",\n      \"pmids\": [\"7798914\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Physiological relevance of cGMP signaling in neurons (vs. choroid plexus) not established\",\n        \"G protein identity for the cGMP arm not determined\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Demonstration that constitutive activity arises from specific transmembrane mutations and that inverse agonists suppress basal signaling resolved how receptor conformation governs ligand-independent G protein coupling.\",\n      \"evidence\": \"Site-directed mutagenesis (S312F/K) with PI hydrolysis and radioligand binding in COS-7 cells\",\n      \"pmids\": [\"9282936\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether naturally edited isoforms exhibit comparable constitutive activity was not tested\",\n        \"Structural basis for constitutive activation unknown\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Audiogenic seizure susceptibility in knockout mice with subcortical c-fos mapping pinpointed HTR2C as a tonic brake on auditory circuit excitability, extending the seizure phenotype to a defined neural circuit.\",\n      \"evidence\": \"Knockout mouse audiogenic seizure testing with immediate early gene expression mapping\",\n      \"pmids\": [\"9241279\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Cell-type-specific rescue not performed\",\n        \"Whether GABAergic interneuron loss of 5-HT2C is the causal mechanism remained untested\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Selective impairment of dentate gyrus LTP in knockout mice established that HTR2C modulates synaptic plasticity at a specific hippocampal synapse, linking receptor function to spatial learning.\",\n      \"evidence\": \"Electrophysiology at four hippocampal pathways and Morris water maze in 5-HT2C null mice\",\n      \"pmids\": [\"9844009\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Molecular mechanism of LTP modulation (pre- vs. postsynaptic) not determined\",\n        \"Whether effect is cell-autonomous in dentate neurons unknown\"\n      ]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Discovery that RNA editing alters agonist-directed signaling (functional selectivity) at HTR2C established that post-transcriptional modification tunes receptor pharmacology in a ligand-dependent manner.\",\n      \"evidence\": \"Comparison of PI signaling by LSD and serotonin across edited and non-edited receptor isoforms\",\n      \"pmids\": [\"10432492\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Which specific edited isoforms are present in vivo was not mapped\",\n        \"Structural basis for editing-induced signaling change unknown\",\n        \"Limited to PI readout; effects on other effectors not tested\"\n      ]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Localization of HTR2C mRNA to GABAergic but not serotonergic neurons of the raphe nuclei provided the anatomical basis for a tonic inhibitory feedback loop controlling serotonin release.\",\n      \"evidence\": \"Double in situ hybridization for 5-HT2C and GAD mRNAs in rat raphe\",\n      \"pmids\": [\"15652696\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Functional confirmation by cell-type-specific deletion not performed\",\n        \"Single-method anatomical study\"\n      ]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"FRET-based demonstration that HTR2C forms homodimers from the ER through the plasma membrane established dimerization as a constitutive biosynthetic feature rather than a ligand-induced event.\",\n      \"evidence\": \"Live-cell confocal FRET with CFP/YFP-tagged receptors and compartment-specific analysis\",\n      \"pmids\": [\"16857671\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Functional consequence of preventing dimerization not tested\",\n        \"Whether heterodimers with other 5-HT2 subtypes form in vivo not addressed\"\n      ]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identification of PTEN as a physical interactor that dephosphorylates HTR2C and controls VTA dopaminergic output linked receptor phosphorylation status to drug reward behavior.\",\n      \"evidence\": \"Co-immunoprecipitation, Tat-3L4F interfering peptide, VTA electrophysiology, conditioned place preference\",\n      \"pmids\": [\"16474401\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether PTEN acts via lipid or protein phosphatase activity at the receptor was not fully resolved\",\n        \"In vivo stoichiometry of the PTEN–5-HT2C complex unknown\"\n      ]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Co-localization of HTR2C with POMC neurons and epistasis with MC4R knockout established that the anorexigenic effect of 5-HT2C agonism operates through the melanocortin pathway, resolving the downstream circuit for appetite suppression.\",\n      \"evidence\": \"Immunofluorescence co-localization in arcuate nucleus; MC4R KO mice unresponsive to 5-HT2C agonist hypophagia\",\n      \"pmids\": [\"18039773\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Direct electrophysiological demonstration in POMC neurons came later\",\n        \"Contribution of non-POMC neuronal populations not excluded\"\n      ]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Genetic ablation of snoRNA MBII-52 increased 5-HT2C RNA editing in vivo and altered impulsivity-related behaviors, establishing snoRNA-guided regulation as a physiologically relevant control mechanism for receptor function.\",\n      \"evidence\": \"PWS-IC mouse model with RNA editing quantification and behavioral pharmacology\",\n      \"pmids\": [\"19304781\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Mechanism by which snoRNA inhibits editing enzyme access not resolved\",\n        \"Relative contribution of editing vs. splicing changes debated\"\n      ]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Post-injury RNA editing shifts in spinal motoneurons produce constitutively active 5-HT2C isoforms that restore persistent calcium currents but cause spasticity, revealing a pathophysiological consequence of editing dysregulation.\",\n      \"evidence\": \"Spinal transection model with mRNA editing analysis, patch-clamp, and pharmacological blockade in rats and humans\",\n      \"pmids\": [\"20512126\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Editing enzyme identity driving post-injury changes not identified\",\n        \"Long-term therapeutic safety of inverse agonist treatment not assessed\"\n      ]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Electrophysiological recording in identified POMC neurons showed that HTR2C activation suppresses M-current (KCNQ), providing the ionic mechanism for 5-HT2C-mediated increase in POMC neuron excitability and feeding suppression.\",\n      \"evidence\": \"Visualized patch-clamp in POMC-EGFP transgenic mouse neurons with selective antagonist RS-102221\",\n      \"pmids\": [\"22436698\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Signaling intermediates between Gq and KCNQ suppression not mapped\",\n        \"Whether M-current suppression also occurs in other HTR2C-expressing neurons not tested\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Biophysical measurement confirmed native homodimeric stoichiometry and showed that maximal G protein activation requires dual-protomer agonist occupancy, establishing a functional consequence of dimerization.\",\n      \"evidence\": \"Fluorescence correlation spectroscopy with photon counting histogram analysis on native choroid plexus receptors and HEK293 cells\",\n      \"pmids\": [\"25609374\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether dimerization interface residues are conserved across species not tested\",\n        \"Crystal or cryo-EM structure of the dimer not available\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Patient-derived iPSC-GABAergic interneurons with reduced HTR2C expression showed increased firing and impaired calcium signaling, and genetic/pharmacological rescue reversed these deficits, causally linking diminished 5-HT2C signaling to interneuron dysfunction in major depression.\",\n      \"evidence\": \"iPSC differentiation to GABAergic interneurons from MDD patients, electrophysiology, calcium imaging, genetic rescue\",\n      \"pmids\": [\"36373384\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether reduced HTR2C expression is a cause or consequence of disease epigenetic changes unclear\",\n        \"In vivo circuit-level validation in patients lacking\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the high-resolution structure of the HTR2C homodimer, the precise signaling intermediates from Gq to KCNQ channel suppression, cell-type-specific contributions to seizure phenotypes, and the enzymatic mechanism by which editing enzymes are regulated by Snord115.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No cryo-EM or crystal structure of HTR2C dimer\",\n        \"Gq-to-KCNQ signaling cascade intermediates unmapped\",\n        \"Cell-type-specific conditional knockout studies across brain regions incomplete\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [2, 3, 7, 16, 26]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [14, 21]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [9, 15]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 3, 6, 7, 16, 26]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [0, 1, 5, 14, 17, 18, 21]}\n    ],\n    \"complexes\": [\n      \"HTR2C homodimer\"\n    ],\n    \"partners\": [\n      \"PTEN\",\n      \"MC4R\",\n      \"KCNQ2\",\n      \"KCNQ3\",\n      \"SNORD115\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}