5-HT2B receptor
The 5-HT2B receptor, encoded by the HTR2B gene on human chromosome 2q37.1, is a class A G protein-coupled receptor (GPCR) that selectively binds the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) with high affinity (pKi 7.9–8.4).[1][2] It consists of 481 amino acids forming seven transmembrane helices, three extracellular and three intracellular loops, and N- and C-terminal domains, with a molecular weight of approximately 54.3 kDa.[3][1] As a member of the 5-HT2 receptor subfamily, it plays critical roles in mediating serotonin's effects on cellular proliferation, contraction, and signaling across multiple physiological systems.[1][2] Upon activation, the 5-HT2B receptor primarily couples to Gq/11 proteins, triggering phospholipase C-β (PLC-β) activation, which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3).[3][1] This cascade elevates intracellular calcium levels and activates protein kinase C (PKC), leading to downstream effects such as ERK1/2 phosphorylation and gene transcription modulation.[3] It also exhibits biased agonism, particularly through β-arrestin pathways, which can influence non-canonical signaling like Src kinase activation and TGF-β-mediated fibrosis.[3] Crystal structures, such as those bound to ergotamine (PDB ID: 4IB4 at 2.7 Å resolution), have elucidated orthosteric and extended binding pockets that accommodate diverse ligands.[3][1] The receptor is widely expressed in human tissues, with highest levels in the uterus, trachea, small intestine, liver, heart, and ovaries, as detected by RT-PCR, while in rodents it predominates in the stomach fundus and vascular smooth muscle.[1][2] Physiologically, it regulates cardiovascular development and homeostasis, including heart valve formation and endothelial relaxation; gastrointestinal motility via smooth muscle contraction; pulmonary function in arterial hypertension; and central nervous system processes like dopamine release, pain perception, and impulsivity.[3][1] In immune cells such as eosinophils and macrophages, it contributes to inflammatory responses.[1] Dysregulation is implicated in pathologies including cardiac valvulopathy, pulmonary arterial hypertension, organ fibrosis (e.g., liver and lung), irritable bowel syndrome, and certain cancers like hepatocellular carcinoma.[3] Pharmacologically, the 5-HT2B receptor is targeted by selective antagonists like RS 127445 (pKi 9.0–9.5) and PRX-08066 (Ki ~3.4 nM), which have shown promise in preclinical models for fibrosis and hypertension, while agonists such as lysergic acid diethylamide (LSD; pKi 9.0) and methylergonovine (pKi 9.3) are associated with adverse effects like valvular heart disease, prompting off-target screening in drug development.[3][1] Genetic studies, including knockout mice exhibiting lethal heart defects, underscore its essential role in embryogenesis and adult tissue maintenance.[1][3]Genetics and molecular structure
Gene characteristics
The HTR2B gene, which encodes the 5-HT2B receptor, is located on the long arm of human chromosome 2 at the cytogenetic band 2q37.1, spanning genomic positions 231,108,230 to 231,125,042 on the reverse strand in the GRCh38.p14 assembly.[4][5][2] The gene comprises 4 exons, with the canonical transcript (ENST00000258400.4) producing an mRNA of 2,170 nucleotides that encodes a 481-amino acid protein precursor.[6][7] Alternative splicing yields additional transcripts, though the canonical isoform predominates in most tissues. Evolutionary conservation of HTR2B is evident across mammals, with the human protein sharing approximately 82% sequence identity with the mouse ortholog and 79% with the rat ortholog, reflecting conserved functional domains despite species-specific variations.[8] A notable genetic variant is the Q20* stop codon mutation (c.58C>T), which truncates the protein and has been associated with severe impulsivity and alcohol-related risk behaviors, particularly in Finnish populations where it reaches a minor allele frequency of about 2%.[9][10] Transcriptional regulation of HTR2B occurs through promoter elements approximately 1.5 kb upstream of the transcription start site, which contain binding sites responsive to transcription factors including Sp1, which activates expression in microglial cells, and nuclear factor I (NFI), which positively regulates transcription in uveal melanoma cells.[11][12]Protein structure
The 5-HT2B receptor belongs to the rhodopsin-like subfamily (family A) of G protein-coupled receptors (GPCRs), which are characterized by a conserved architecture consisting of seven transmembrane α-helices (TM1–TM7) connected by three extracellular loops (ECL1–ECL3) and three intracellular loops (ICL1–ICL3), with an extracellular N-terminus and an intracellular C-terminus.[13] This topology positions the orthosteric ligand-binding site within the transmembrane bundle, accessible from the extracellular space.[14] The first high-resolution structure of the 5-HT2B receptor was obtained in 2013 via X-ray crystallography of a thermostabilized chimeric construct bound to the agonist ergotamine, resolved at 2.7 Å (PDB ID: 4IB4).[15] In this active-like conformation, the orthosteric binding pocket is a narrow cavity formed by residues from TM3, TM5, TM6, and the second extracellular loop (ECL2), with ergotamine's ergoline core engaging hydrophobic interactions and its basic amine forming a salt bridge with the conserved Asp3.32 (Asp129) in TM3.[16] Key structural motifs include the DRY sequence (Asp-Arg-Tyr) at the TM3–ICL2 junction, which stabilizes the inactive state through an ionic lock but rearranges upon activation to facilitate G protein coupling, and the conserved Asp3.32 in TM3, essential for coordinating the positively charged amine of endogenous ligands like serotonin.[17][14] Recent cryo-electron microscopy (cryo-EM) structures, determined in 2022, have captured the 5-HT2B receptor in complex with lysergic acid diethylamide (LSD) in transducer-free (2.7 Å), Gq-coupled (2.9 Å), and β-arrestin-1-coupled (3.3 Å) states (PDB IDs: 7SRQ, 7SRR, 7SRS), revealing conformational shifts that underlie biased signaling, including outward movement of TM6 and rearrangements in ICL2 that accommodate different transducers.[18] These structures highlight ligand-specific pocket expansions and intracellular rearrangements promoting biased agonism towards Gq and β-arrestin pathways.[19] Post-translational modifications of the 5-HT2B receptor include N-linked glycosylation, with consensus sites in the N-terminus (e.g., Asn7) and ECL2 that influence receptor trafficking and membrane expression; glycosylation at ECL2 has been resolved in structural studies and contributes to stabilizing extracellular domains.[20][21]Tissue distribution
Central nervous system expression
The 5-HT2B receptor is expressed in various regions of the central nervous system, with notable levels in key brain areas involved in emotion, cognition, and motor control. High mRNA and protein expression has been reported in the frontal cortex and basal ganglia, while moderate expression occurs in the hypothalamus; lower levels are observed in the cerebellum.[22][23] In the hippocampus, expression is moderate, contributing to its potential role in limbic functions.[22] At the cellular level, the 5-HT2B receptor is predominantly localized to neurons, including subpopulations of serotonergic neurons in the dorsal raphe nuclei and dopaminergic neurons in the mesoaccumbens pathway.[24] It is also expressed on microglia, the primary immune cells of the brain, where it influences neuroinflammatory responses.[25] Immunohistochemical studies confirm its distribution on both presynaptic and postsynaptic sites in cortical and striatal neurons, supporting bidirectional modulation of neurotransmission.[26] Quantitative assessments using reverse transcription polymerase chain reaction (RT-PCR) indicate that 5-HT2B mRNA levels are substantially higher in the cerebral cortex compared to the striatum, with fold differences ranging from 5- to 10-fold in rodent models under baseline conditions.[27] These patterns highlight region-specific roles in serotonergic signaling. During development, 5-HT2B receptor expression is upregulated in the early embryonic brain, peaking around embryonic days 8-9 in mice, particularly in neural structures including the midbrain raphe nuclei that give rise to serotonergic projections.[27][20] This temporal profile suggests involvement in neurogenesis and circuit formation.Peripheral tissue expression
The 5-HT2B receptor exhibits prominent expression in several peripheral tissues, particularly within the cardiovascular system, where it is found in valvular interstitial cells of heart valves and the endothelium of the pulmonary artery. High expression is also observed in the uterus, trachea, small intestine, ovaries, and other reproductive and respiratory tissues. Radioligand binding assays quantify receptor density in human cardiac valves at approximately 28.8 fmol/mg protein across aortic, mitral, tricuspid, and pulmonary valves.[28][3] In the gastrointestinal tract, the receptor is expressed in smooth muscle cells, with notable levels in the colon.[3] Hepatic expression occurs primarily in hepatocytes and hepatic stellate cells, contributing to its role in liver physiology.[29] mRNA distribution studies via RT-PCR and RNA-seq reveal high levels of 5-HT2B expression in human uterus, small intestine, trachea, liver, heart, and ovaries, with moderate levels in kidney, lung, pancreas, and spleen; protein confirmation via Western blot supports these patterns in analogous rodent tissues like stomach and heart.[1][7][30] Expression is also evident in immune-associated cells, with elevated levels on fibroblasts and myofibroblasts, especially in fibrotic contexts such as lung and dermal tissues.[3] These cells show receptor presence via immunohistochemical and mRNA analyses in conditions like idiopathic pulmonary fibrosis and systemic sclerosis. Species differences influence peripheral expression profiles: in humans, 5-HT2B mRNA is abundant in uterus, small intestine, and trachea among others, whereas in rodents, it predominates in the fundic stomach, affecting the suitability of translational models for peripheral receptor studies.[31] This human-rodent disparity highlights broader peripheral distribution in humans compared to the more restricted rodent pattern.[32]Function and signaling
Signal transduction mechanisms
The 5-HT2B receptor, a G protein-coupled receptor (GPCR), primarily couples to Gq/11 proteins upon activation by serotonin (5-HT), initiating a canonical signaling cascade. This coupling activates phospholipase C-β (PLC-β), which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 mobilizes intracellular calcium stores, while DAG activates protein kinase C (PKC), amplifying downstream effects such as gene expression and cellular proliferation.[33][18] In addition to Gq/11-mediated pathways, the 5-HT2B receptor engages β-arrestin recruitment, which scaffolds and activates the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway, promoting cell proliferation and differentiation independently of G protein signaling.[18][34] Receptor desensitization occurs through phosphorylation of the C-terminal tail by G protein-coupled receptor kinases (GRKs), particularly GRK2, and PKC, which uncouples the receptor from Gq/11 and facilitates β-arrestin binding. This phosphorylation promotes clathrin-mediated internalization, terminating signaling and enabling receptor recycling or degradation.[18][20]Physiological roles
The 5-HT2B receptor contributes to cardiovascular homeostasis, particularly in the maintenance of mitral valve integrity. In valvular interstitial cells, activation of this receptor supports cellular responses that preserve tissue structure under hemodynamic stress, ensuring proper valve function without excessive remodeling.[35] Additionally, the receptor modulates pulmonary vasoconstriction, where serotonin signaling through 5-HT2B facilitates acute responses to hypoxia in pulmonary artery smooth muscle cells, aiding in the regulation of pulmonary vascular tone during normal physiological conditions.[36] In the gastrointestinal tract, the 5-HT2B receptor mediates excitatory effects on smooth muscle, promoting colonic motility through contraction and coordination of peristaltic activity. This role is evident in human colon preparations, where receptor activation enhances serotonin-induced contractions that support digestive transit.[37] Within the central nervous system, the 5-HT2B receptor modulates dopamine release in the nucleus accumbens and striatum, influencing reward processing and motor control.[38] It also contributes to pain perception, with activation mediating mechanical hyperalgesia through regulation of transient receptor potential vanilloid 1 in sensory neurons.[39] Genetic variants in the HTR2B gene, such as a stop codon polymorphism, are associated with increased impulsivity, particularly in the context of alcohol consumption, as demonstrated in human and mouse studies.[9] Beyond these systems, the 5-HT2B receptor promotes fibroblast proliferation during wound healing, where it enhances cell migration and extracellular matrix remodeling to support tissue repair. In injury models, serotonin release activates the receptor on fibroblasts, driving their expansion without leading to fibrosis in controlled physiological contexts.[40] In liver regeneration following partial hepatectomy, the receptor modulates hepatocyte proliferation, acting as a regulatory brake on regenerative growth to prevent over-expansion while allowing restoration of liver mass.[41]Pharmacology
Agonists
The primary endogenous ligand for the 5-HT2B receptor is serotonin (5-HT), which serves as the orthosteric agonist and activates the receptor with an EC50 ranging from approximately 4 to 20 nM in functional assays measuring phosphoinositide hydrolysis or calcium mobilization.[42][43] This potency underscores serotonin's central role in mediating 5-HT2B-dependent signaling across various tissues. Among selective synthetic agonists, BW 723C86 stands out as a high-affinity, orally active compound with preferential activity at the 5-HT2B receptor (Ki ≈ 4 nM) and notable use in preclinical studies of peripheral gastrointestinal function.[44] Similarly, 6-APB, a benzofuran derivative classified as an entactogen, functions as a partial agonist at the 5-HT2B receptor, displaying nanomolar potency in functional assays while also interacting with other serotonin receptors and monoamine transporters.[45][46] Non-selective agonists include metabolites of fenfluramine, such as norfenfluramine, which bind with high affinity (Ki 10–50 nM) and act as full agonists at the 5-HT2B receptor in contractile and binding assays.[47] Ergoline derivatives like pergolide also exhibit potent full agonism (pEC50 8.42) at this receptor, contributing to their broader pharmacological profile across dopamine and serotonin systems.[48]| Compound | Type | Affinity/Potency | Key Notes | Source |
|---|---|---|---|---|
| Serotonin (5-HT) | Endogenous | EC50 4–20 nM | Orthosteric full agonist | Pharmacological evidence for a functional serotonin-2B receptor... |
| BW 723C86 | Selective synthetic | Ki ≈ 4 nM | Peripheral GI focus | BW 723C86, a 5-HT2B receptor agonist... |
| 6-APB | Selective synthetic (partial) | Nanomolar EC50 | Entactogen | Pharmacological profile of novel psychoactive benzofurans |
| Norfenfluramine | Non-selective (fenfluramine metabolite) | Ki 10–50 nM | Full agonist | Evidence for Possible Involvement of 5-HT2B Receptors... |
| Pergolide | Non-selective (ergoline) | pEC50 8.42 | Full agonist | Agonism at 5-HT2B receptors is not a class effect... |
Antagonists
Antagonists of the 5-HT2B receptor inhibit the binding of serotonin to this G protein-coupled receptor, thereby blocking downstream signaling pathways such as phospholipase C activation and calcium mobilization. These compounds are valuable tools for dissecting the receptor's role in physiological processes and have been characterized through radioligand binding assays, often using [3H]-LSD or [3H]-5-HT as tracers to measure displacement and determine inhibition constants (Ki) or half-maximal inhibitory concentrations (IC50). Selectivity is typically assessed against related 5-HT2A and 5-HT2C subtypes, with high-affinity antagonists exhibiting nanomolar potencies and substantial fold-selectivity to minimize off-target effects.[50] Selective 5-HT2B antagonists include RS-127445, a high-affinity compound with a pKi of 9.5 at human recombinant 5-HT2B receptors, demonstrating approximately 1000-fold selectivity relative to 5-HT2A and 5-HT2C subtypes in radioligand binding assays using [3H]-LSD displacement in CHO cell membranes. This orally bioavailable agent has been widely used in preclinical studies to probe 5-HT2B-mediated responses without significant interference at other serotonin receptors. Similarly, SB-204741 serves as a potent selective antagonist with a pA2 of 7.95 (equivalent to Ki ≈11 nM) at 5-HT2B, showing at least 135-fold selectivity over 5-HT2C and greater than 1000-fold over 5-HT2A, particularly emphasizing its utility in peripheral tissues where 5-HT2B expression is prominent.[51][52][53] LY-266,097 represents an early selective antagonist with a pKi of 9.3 (Ki ≈ 0.5 nM) for 5-HT2B, displaying over 100-fold selectivity against 5-HT2A and 5-HT2C in binding assays on human embryonic kidney (HEK293) cell membranes expressing the receptor; its tetrahydro-β-carboline structure contributes to a mixed pharmacological profile in functional assays, though it remains a standard tool for 5-HT2B blockade. Non-selective antagonists, such as the ergot derivative methysergide, exhibit broader activity across 5-HT2 receptor subtypes, with a Ki of approximately 9 nM at 5-HT2B determined via [3H]-LSD binding in human receptor assays, alongside comparable affinities at 5-HT2A (Ki ≈ 3 nM) and 5-HT2C (Ki ≈ 1 nM), making it useful for migraine prophylaxis but less ideal for subtype-specific investigations.[54][34][47] Among peripherally selective agents, MARY1, discovered in 2025, is a novel high-affinity 5-HT2B antagonist with an IC50 of 380 nM and Ki of 764 nM in human 5-HT2B-expressing cells using [3H]-5-HT displacement assays, showing subtype selectivity and preferential activity in renal tissues where it promotes mitochondrial biogenesis through receptor blockade without notable central nervous system penetration. The following table summarizes representative binding affinities for these antagonists from key radioligand studies:| Antagonist | 5-HT2B Ki/IC50 (nM) | Selectivity (fold over 5-HT2A/C) | Assay Type | Source |
|---|---|---|---|---|
| RS-127445 | pKi = 9.5 (Ki ≈0.3) | ≈1000 | [3H]-LSD displacement | Knight et al., 1997 |
| SB-204741 | pA2 = 7.95 (Ki ≈11) | ≥135 over 2C; >1000 over 2A | [3H]-5-HT binding | Kennett et al., 1995 |
| LY-266,097 | pKi = 9.3 (Ki ≈0.5) | >100 | [3H]-LSD displacement | Knight et al., 2004 |
| Methysergide | Ki ≈ 9 | Low (≈1-2) | [3H]-LSD binding (human) | Fitzgerald et al., 2000 |
| MARY1 | Ki = 764; IC50=380 | Subtype-selective (peripheral) | [3H]-5-HT displacement | Victor et al., 2025 |