5-HT receptor
The 5-HT receptors, also known as serotonin receptors, are a group of membrane proteins that selectively bind the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) to transduce its signaling effects across the central and peripheral nervous systems.[1] There are seven distinct families (5-HT1 through 5-HT7) encompassing 14 subtypes, with 13 subtypes belonging to the G-protein-coupled receptor (GPCR) superfamily (including the non-functional human pseudogene 5-HT5B) and the 5-HT3 receptor functioning as a ligand-gated ion channel.[2] These receptors were first pharmacologically distinguished in the 1950s and molecularly cloned starting in the 1980s, enabling their classification based on structure, signaling, and ligand affinity.[3] In the central nervous system, 5-HT receptors modulate key processes such as mood regulation, cognition, anxiety, sleep-wake cycles, and pain perception, with subtypes like 5-HT1A and 5-HT2A playing prominent roles in psychiatric disorders including depression and schizophrenia.[2] Peripherally, they influence gastrointestinal motility (via 5-HT4), platelet aggregation and vasoconstriction (via 5-HT2A), and bronchoconstriction (via 5-HT2B), contributing to homeostasis in cardiovascular, digestive, and immune systems.[1] Dysregulation of these receptors is implicated in conditions ranging from migraine and irritable bowel syndrome to cancer proliferation and atherosclerosis.[1] Signaling through 5-HT receptors primarily occurs via G-protein coupling: 5-HT1 and 5-HT5 subtypes couple to Gi/o to inhibit adenylyl cyclase and modulate ion channels; 5-HT2 subtypes engage Gq/11 to activate phospholipase C and downstream pathways like PKC and calcium mobilization; while 5-HT4, 5-HT6, and 5-HT7 stimulate Gs to increase cAMP levels.[2] The 5-HT3 receptor, unique as a pentameric cation channel, permits rapid Na+ and Ca2+ influx, facilitating fast synaptic transmission in emesis and nociception.[2] Common downstream effectors across subtypes include MAPK/ERK, PI3K/AKT, and RhoA/ROCK pathways, which regulate cell proliferation, apoptosis, and migration.[1] Recent structural biology advances, including cryo-EM and X-ray crystallography since 2018, have resolved high-resolution structures for all 12 functional GPCR subtypes, revealing a conserved orthosteric binding site involving aspartate residue 3.32 and variable extracellular domains that dictate ligand selectivity and allosteric modulation by lipids like phosphatidylinositol 4-phosphate.[2] These insights have accelerated drug development, with therapies modulating the 5-HT system such as selective serotonin reuptake inhibitors (SSRIs) for depression (which increase serotonin availability to these receptors), direct receptor agonists like triptans for migraine (5-HT1B/1D), and 5-HT3 antagonists for chemotherapy-induced nausea proving clinically effective.[2] Ongoing research, including 2024–2025 studies on biased agonists and additional structures, emphasizes their therapeutic potential in neurodegenerative diseases, metabolic disorders, and cancer.[2][4][5]History and Discovery
Initial Identification
The discovery of serotonin, also known as 5-hydroxytryptamine (5-HT), occurred in 1948 when researchers at the Cleveland Clinic, including Maurice M. Rapport, Arda A. Green, and Irvine H. Page, isolated a vasoconstrictive substance from beef blood serum. This compound was identified through bioassays that demonstrated its potent contractile effects on smooth muscle tissues, such as the rat uterus and clam heart ventricle, distinguishing it from other serum factors and establishing its role in promoting vasoconstriction to aid blood clotting. Rapport and colleagues named the purified crystalline substance "serotonin" in 1949, deriving the term from its serum origin and tonic (contractile) properties on smooth muscle.[6] In the early 1950s, further experiments expanded understanding of serotonin's physiological roles, particularly its presence in the central nervous system. Betty M. Twarog, working in Page's laboratory, detected 5-HT in extracts of mammalian brain tissue in 1952 using fluorescence assays and confirmed its identity through parallel bioassays on smooth muscle preparations.[7] This finding, published in 1953, marked the first demonstration of serotonin in the brain and suggested its potential as a neurotransmitter, as the substance mimicked the effects of known transmitters in eliciting contractions in gut and vascular smooth muscle.[7] These pharmacological assays provided early indirect evidence for the existence of specific 5-HT receptors, as the consistent, dose-dependent responses in isolated tissues indicated mediation by dedicated binding sites rather than non-specific actions.[8] A pivotal theoretical advancement came in 1954 when D. Wayne Woolley and E. Shaw proposed that disruptions in serotonin function could underlie certain mental disorders, based on structural similarities between 5-HT and the hallucinogen lysergic acid diethylamide (LSD).[9] They hypothesized that LSD acts as an antagonist at serotonin receptors in the brain, blocking normal 5-HT signaling and thereby inducing psychotic-like states akin to schizophrenia, which linked peripheral receptor effects observed in smooth muscle to central nervous system implications.[9] This suggestion spurred further research into 5-HT receptor interactions and their therapeutic modulation.Classification Evolution
In 1957, John H. Gaddum and Zedenek Picarelli proposed the initial classification of peripheral 5-HT receptors based on pharmacological responses in guinea pig ileum, distinguishing two types: the D receptor, responsible for depressor effects blocked by morphine, and the M receptor, mediating musculotropic contractions resistant to morphine antagonism. This ad hoc nomenclature reflected early empirical observations of 5-HT's diverse actions but lacked molecular basis. During the 1970s, advances in radioligand binding techniques enabled a more systematic classification. In 1979, Stephen J. Peroutka and Solomon H. Snyder identified two major central 5-HT binding sites in rat brain using tritiated ligands: the 5-HT1 site, characterized by high affinity for 5-HT and low affinity for spiperone, and the 5-HT2 site, with low affinity for 5-HT but high affinity for spiperone and lysergic acid diethylamide. This binary framework expanded on Gaddum's model, incorporating high- and low-affinity distinctions and shifting focus to brain receptors, though it initially overlooked further heterogeneity. The 1980s and 1990s marked a pivotal era with molecular cloning, which refined and expanded the classification. The first 5-HT receptor cloned was the 5-HT1A subtype in 1988 by Fargin et al., revealing its G-protein-coupled receptor (GPCR) structure and sequence similarity to adrenergic receptors.[10] Subsequent cloning efforts, including the 5-HT3 receptor in 1991 by Maricq et al., demonstrated its divergence as a ligand-gated ion channel rather than a GPCR, prompting the abandonment of the catch-all "5-HT1-like" category for non-cloned sites and integrating structural data into nomenclature. By the mid-1990s, over a dozen subtypes had been sequenced, highlighting pharmacological and operational overlaps that necessitated unified standards.[11] In 1994, the International Union of Pharmacology (IUPHAR) Subcommittee on 5-HT Receptors, chaired by Daniel Hoyer, established a comprehensive nomenclature recognizing seven families—5-HT1 through 5-HT7—with 14 pharmacologically distinct subtypes based on integrated molecular, transductional, and operational criteria. This system formalized subtypes such as 5-HT1A–F, 5-HT2A–C, 5-HT3, 5-HT4, 5-HT5A, 5-HT6, and 5-HT7, while noting the unconfirmed status of the putative 5-HT1P receptor, an intestinal site lacking molecular identification. The classification emphasized GPCR dominance for most families, except 5-HT3. As of 2025, the IUPHAR framework remains unchanged with no new families added, reflecting stability in core classifications amid ongoing genomic refinements; notably, the human 5-HT5B gene is confirmed as a non-functional pseudogene due to disruptive stop codons, unlike its functional rodent ortholog.[12][13]Molecular Structure
General Architecture
The 5-HT receptors, also known as serotonin receptors, primarily belong to the G protein-coupled receptor (GPCR) superfamily, with six out of seven families (5-HT1, 5-HT2, 5-HT4, 5-HT5, 5-HT6, and 5-HT7) exhibiting a characteristic seven-transmembrane (7TM) architecture typical of Class A rhodopsin-like GPCRs.[14] These receptors feature seven α-helical transmembrane domains connected by three intracellular loops (ICL1–3) and three extracellular loops (ECL1–3), an extracellular N-terminus, and an intracellular C-terminus that facilitates interactions with signaling proteins.[14] The 7TM bundle forms a compact structure embedded in the cell membrane, with the N-terminus typically glycosylated to aid in proper folding, trafficking, and stability.[14] A hallmark of 5-HT GPCR structure is the presence of conserved motifs essential for ligand binding and receptor function. In transmembrane helix 3 (TM3), an aspartate residue (Asp3.32) forms a salt bridge with the positively charged amine group of serotonin (5-HT), anchoring the orthosteric binding site within the transmembrane bundle formed by TM3, TM5, TM6, and TM7, as well as contributions from ECL2.[14] Additionally, a conserved disulfide bridge between a cysteine in TM3 (Cys3.25) and another in ECL2 stabilizes the extracellular region, maintaining the integrity of the ligand-binding pocket across subtypes.[14] These receptors generally range from 300 to 500 amino acids in length, with variability arising from differences in loop lengths and the C-terminal tail.[14] The orthosteric site accommodates 5-HT in a deep pocket lined by aromatic residues from the TM helices, while some 5-HT receptor subtypes possess allosteric binding sites that modulate ligand affinity or efficacy without competing directly with 5-HT.[14] Advances in structural biology since 2018, primarily using cryo-electron microscopy (cryo-EM), have resolved high-resolution structures for all 12 5-HT GPCR subtypes, revealing conserved features like the Asp3.32 salt bridge and variable extracellular domains that influence ligand selectivity, as well as allosteric modulation by lipids such as phosphatidylinositol 4-phosphate (PtdIns4P).[2] For example, the 2013 X-ray crystal structure of the 5-HT1B receptor at 2.7 Å resolution in complex with ergotamine provided early insights into the amine-binding pocket with hydrogen bonds and π-π stacking interactions,[14] which have been corroborated and expanded by subsequent cryo-EM structures, including a 2025 study of 5-HT1A at ~2.7 Å resolution showing G-protein subtype selectivity influenced by TM6 shifts and allosteric lipids.[15] Family-specific variations, such as differences in ECL2 conformation, further refine these conserved elements but do not alter the core 7TM topology.[14]Family-Specific Variations
The 5-HT receptor family exhibits notable structural variations across its seven subfamilies, reflecting adaptations in transmembrane topology, loop lengths, and extracellular domains that influence protein interactions and ligand access. Most 5-HT receptors (5-HT1, 5-HT2, 5-HT4, 5-HT5, 5-HT6, and 5-HT7) belong to the G protein-coupled receptor (GPCR) superfamily, sharing a conserved seven-transmembrane (7TM) helical bundle, but diverge in intracellular and extracellular loop configurations. These GPCRs evolved from ancestral amine receptors approximately 600-700 million years ago, coinciding with the divergence of vertebrates from invertebrates.[16] In the 5-HT1 family (5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, 5-HT1F), intracellular loops (ICLs) are relatively compact, with ICL2 showing moderate length variations that support Gi/o coupling, while the extracellular loop 2 (ECL2) adopts a more rigid conformation to stabilize the orthosteric binding pocket for agonists like sumatriptan. The 5-HT1A gene (HTR1A) is located on chromosome 5q11.2-q13.[17][2] In contrast, the 5-HT2 family (5-HT2A, 5-HT2B, 5-HT2C) features a notably longer third intracellular loop (I3), often exceeding 150 residues, which forms a stabilized alpha-helical structure via hydrogen bonding networks to facilitate Gq/11 protein recruitment; for instance, in 5-HT2C, this extended I3 loop enhances interactions with effector proteins. ECL2 in 5-HT2 receptors is elongated (e.g., six additional residues in 5-HT2B compared to 5-HT2A), contributing to a side-extended binding cavity and increased flexibility that modulates ligand affinity for diverse psychedelics and antagonists.[2][18] The genes for this family are dispersed: HTR2A on chromosome 13q14.2, HTR2B on 2q37.1, and HTR2C on Xq24.[19][20][21] The 5-HT4, 5-HT6, and 5-HT7 subfamilies, primarily Gs-coupled, display ICL variations with shorter ICL3 in 5-HT4 (facilitating alternative splicing and Gi coupling in some variants) and more flexible ICL2 in 5-HT6/7, which accommodate broader effector diversity; ECL2 flexibility in these receptors is pronounced, enabling access to extended binding pockets for atypical antipsychotics. The 5-HT5 family (5-HT5A, 5-HT5B) stands out with a shorter C-terminal tail compared to other 5-HT GPCRs, limiting palmitoylation sites and potentially reducing Gi coupling efficiency through diminished interactions with regulatory proteins like arrestins.[2][22][2] Unlike the GPCR families, the 5-HT3 receptor forms a pentameric ligand-gated ion channel (LGIC) with each subunit comprising four transmembrane domains (M1-M4), where the M2 helix lines the central cation-permeable pore. A large intracellular loop between M3 and M4, including the MA helix, modulates channel gating and trafficking, while cryo-EM structures resolved since 2018 (e.g., apo-state at 3.5 Å resolution) have directly revealed the extracellular ligand-binding domain and its homology with nicotinic acetylcholine receptors.[23][24]Classification and Subtypes
Receptor Families
The 5-HT receptors, also known as serotonin receptors, are classified into seven major families based on structural, operational, and transductional characteristics, encompassing 14 confirmed subtypes across mammals.[25] A proposed 5-HT1P receptor has been suggested but remains unverified and is not included in standard classifications as of 2025.[25] With the exception of the 5-HT3 family, all belong to the G protein-coupled receptor (GPCR) superfamily, while the 5-HT3 family operates as ligand-gated ion channels.[26] The 5-HT1 family includes five subtypes coupled to Gi/Go proteins, which inhibit adenylyl cyclase and reduce cyclic AMP levels, leading to generally inhibitory effects on neuronal activity; these receptors primarily function postsynaptically in the brain to modulate neurotransmitter release.[25][26] The 5-HT2 family comprises three subtypes linked to Gq/11 proteins, activating phospholipase C to increase inositol trisphosphate and intracellular calcium, resulting in excitatory signaling; this family is broadly involved in vasoconstriction and hallucinatory phenomena.[25][26] Unlike the others, the 5-HT3 family forms a single cation-selective ligand-gated ion channel that permits rapid influx of sodium and potassium ions, mediating fast excitatory responses; it is particularly prominent in the enteric nervous system for coordinating gastrointestinal motility.[25][26] The 5-HT4 family features one subtype coupled to Gs proteins, stimulating adenylyl cyclase to elevate cyclic AMP and promote excitatory effects; it plays a key role in prokinetic actions within the gastrointestinal tract.[25][26] The 5-HT5 family has two subtypes associated with Gi/Go proteins, exerting inhibitory effects through adenylyl cyclase suppression, though functional data remain limited; these receptors show high expression in the cerebellum.[25][26] The 5-HT6 family consists of one Gs-coupled subtype that increases cyclic AMP levels, contributing to excitatory signaling with themes centered on cognition in the brain.[25][26] Finally, the 5-HT7 family includes one Gs-coupled subtype that elevates cyclic AMP, influencing excitatory pathways related to thermoregulation and circadian rhythms.[25][27][26]Subtype Characteristics
The 5-HT receptor family comprises 14 confirmed subtypes distributed across seven classes, each encoded by distinct genes and characterized by unique molecular and functional features. These subtypes were progressively identified through molecular cloning efforts from the late 1980s onward, revealing their structural diversity and specialized roles.[25][26]| Subtype | Gene | Cloning Year | Unique Properties |
|---|---|---|---|
| 5-HT1A | HTR1A | 1987 | Functions as an autoreceptor in raphe nuclei, contributing to anxiolytic effects through inhibitory Gi/o-coupled signaling. |
| 5-HT1B | HTR1B | 1991 | Mediates vascular constriction and serves as a target in migraine therapy via presynaptic inhibition of neurotransmitter release. |
| 5-HT1D | HTR1D | 1991 | Shares similarities with 5-HT1B, often co-expressed in the trigeminal system, and modulates vasoconstriction without prominent autoreceptor activity in rodents. |
| 5-HT1E | HTR1E | 1992 | Exhibits low expression levels in the brain, with largely unknown physiological functions despite coupling to Gi/o. |
| 5-HT1F | HTR1F | 1993 | Shows potential as an antimigraine target due to activation without inducing vasoconstriction, primarily expressed in trigeminal ganglia. |
| 5-HT2A | HTR2A | 1988 | Serves as a key target for psychedelics and antipsychotics, mediating hallucinogenic and therapeutic effects via Gq/11-coupled phosphoinositide hydrolysis. |
| 5-HT2B | HTR2B | 1992 | Associated with risks of cardiac valvulopathy upon prolonged agonist activation, involved in gastrointestinal and cardiac smooth muscle contraction. |
| 5-HT2C | HTR2C | 1987 (initially as 5-HT1C) | Features multiple RNA editing variants in its mRNA, which modulate Gq/11 coupling efficiency and receptor desensitization. |
| 5-HT3 | HTR3A/HTR3B | 1995 | Forms homopentameric (HTR3A) or heteropentameric (with HTR3B and additional subunits C, D, E cloned in 2003) ligand-gated ion channels, critical for emesis control and rapid excitatory signaling.[28] |
| 5-HT4 | HTR4 | 1995 | Exhibits splice variants with differing C-terminal tails, influencing trafficking and Gs-coupled adenylyl cyclase stimulation in the gut and brain. |
| 5-HT5A | HTR5A | 1994 | Displays low abundance in the central nervous system, potentially involved in neurodevelopmental processes via Gi/o coupling. |
| 5-HT5B | HTR5B | 1994 | Exists as a functional receptor in rodents but as a non-coding pseudogene in humans, limiting its role to preclinical studies. |
| 5-HT6 | HTR6 | 1993 | Enhances cognition in synergy with cholinesterase inhibitors, coupled to Gs and highly expressed in striatal regions. |
| 5-HT7 | HTR7 | 1993 | Produces splice isoforms affecting desensitization, regulates sleep-wake cycles through Gs-mediated adenylyl cyclase activation. |