The 5-HT3 receptor is a member of the superfamily of ligand-gated ion channels, a superfamily that also includes the neuronal nicotinic acetylcholine receptors, and the inhibitory neurotransmitter receptors for GABA (both GABAA and GABAA-ρ receptors) and glycine. The 5-HT3 receptor is most closely related by homology to the nicotinic acetylcholine receptor.
The 5-HT3 receptor consists of 5 subunits arranged around a central ion conducting pore, which is permeable to sodium, potassium, and calcium ions. Binding of the neurotransmitter 5-hydroxytryptamine (serotonin) to the 5-HT3 receptor opens the channel, which, in turn, leads to an excitatory response in neurons. The 5-HT3 receptor differs markedly in structure and mechanism from the other 5-HT receptor subtypes, which are all G-protein-coupled.
CHEMICAL STRUCTURE AND IMAGES
Protein Aminoacids Percentage (Width 700 px)
5-HT3 is involved in extracellular ligand-gated ion channel activity. This is one of the several different receptors for 5- hydroxytryptamine (serotonin), a biogenic hormone that functions as a neurotransmitter, a hormone, and a mitogen. This receptor is a ligand-gated ion channel, which when activated causes fast, depolarizing responses in neurons. It is a cation-specific, but otherwise relatively nonselective, ion channel. (http://www.hmdb.ca/proteins/)
1. Cell junction
3. postsynaptic cell membrane
4. Multi-pass membrane protein
5. Cell membrane
6. Multi-pass membrane protein
Homomeric 5-HT3A receptors mediate rapidly activating and desensitizing inward currents, which are carried primarily by Na+ and K+ ions. The receptors are also permeable to Ca2+ and other small organic cations.
The functional 5-HT3 receptor (Fig1), like other Cys loop receptors, is a pentameric assembly of five identical or non-identical subunits that surround, in a pseudo-symmetric manner, a water-filled ion channel. Each subunit has a large extracellular domain (ECD) that forms the ligand-binding site, a transmembrane domain (TMD) consisting of four membrane-spanning alpha-helices (M1-M4) that enable ions to cross the membrane, and an intracellular domain (ICD) formed by the large M3-M4 intracellular loop, which is responsible for receptor modulation, sorting, and trafficking, and which contains portals (openings) that influence ion conductance. The presence of portals has been deduced from nACh receptor data, but they are likely to exist, as these receptors are highly homologous. The structural and functional similarity of these two receptors is such that chimeric receptors consisting of the ECD of the a7-nACh receptor and the TMD of the 5-HT3A receptor can be activated by ACh and have the channel properties of the 5-HT3A receptor.
Fig.1 Homology models of the 5-HT3 receptor". A, a single receptor subunit based on the nACh receptor structure (Protein Data Bank code 2BG9) showing the mostly β-sheet-containing ECD (blue), the four transmembrane α-helices (purple), and the α-helix that forms part of the ICD (orange). The structure of the remainder of this domain is not yet known. B, models of the 5-HT3 receptor ECD based on AChBP (blue; code 1UV6) and the nACh receptor (cyan; code 2BG9), with 5-HT docked into the intersubunit binding pocket.
5-HT3 RECEPTOR SUBUNITS
Five distinct 5-HT3 receptor subunits (A–E) have been identified so far, which is relatively few for a Cys loop receptor, although the repertoire is increased by a number of different isoforms (24, 37, 38). There are, for example, a long and short form of the human 5-HT3A subunit that differ by 32 amino acids, three translational variants of the human 5-HT3B subunit, and five isoforms of the 5-HT3E subunit. The stoichiometry of heteromeric receptors is still not clear, although it has been established that only 5-HT3A subunits can form functional homomeric 5-HT3 receptors, and the presence of at least one 5-HT3A subunit appears to be obligatory in heteromeric receptors.
Localization of the subunits reveals considerable overlap. Distribution of 5-HT3A receptor mRNA and protein is widespread and has been observed in many regions of the CNS (where it correlates well with radiolabeled antagonist binding studies), in peripheral and sensory ganglia, and in a wide range of other tissues, including the gastrointestinal tract.
5-HT3B subunit mRNA and protein were originally shown to be located in the spleen, colon, small intestine, and kidney, with some controversy as to their presence in the brain. Later studies showing that there are several 5-HT3B receptor isoforms provided an answer to this conundrum: different tissue preferences of the different subunits. The longer B subunit variant is broadly expressed in many tissues, including the kidney, liver, brain, and gastrointestinal tract, whereas the shorter variant has a brain-specific expression pattern.
5-HT3 receptor C–E subunits were first identified in humans, and genes for these proteins have now been shown to exist in a range of species, although not in rodents. A recent study suggested that they all have a relatively widespread distribution, although initial studies suggested that the D and E subunits had a very restricted expression in the gastrointestinal tract. Studies examining protein levels have lagged behind the genetic work, but expression of the C–E subunits at the protein level in the gastrointestinal tract has recently been demonstrated. 5-HT receptors, 2012
5-HT3 RECEPTOR BINDING POCKET
The agonist-binding site lies at the interface of two adjacent subunits in the extracellular N-terminal domain and is formed by three loops (A–C) from one (the principal) subunit and three ß-strands (referred to as loops D–F) from the adjacent or complementary subunit, as in other Cys loop receptors. Only a few residues within each loop face into the binding pocket, with other residues having roles in maintaining the structure of the pocket and/or contributing to the conformational changes that result in channel opening. Evidence from AChBP structures suggests that the binding pocket contracts around agonists, which may initiate the conformational change that ultimately leads to channel opening, whereas antagonists tend to have little effect or may cause binding site expansion.
Key residues that contribute to the 5-HT3 receptor ligand-binding site include one or more from each of the six binding loops.
Acute tryptophan depletion (ATD) studies indicate that low serotonin can lower mood and also increase aggression, although results vary somewhat between studies with similar participants. Lowering of mood after ATD is related to the susceptibility of the study participants to clinical depression, and some participants show no effect on mood. This indicates that low serotonin can contribute to lowered mood, but cannot-by itself-cause lowered mood, unless other unknown systems interact with serotonin to lower mood. Studies using tryptophan supplementation demonstrate that increased serotonin can decrease quarrelsomeness and increase agreeableness in everyday life. Social interactions that are more agreeable and less quarrelsome are associated with better mood. Thus, serotonin may have direct effects on mood, but may also be able to influence mood through changes in social behaviour. The increased agreeableness and decreased quarrelsomeness resulting from increases in serotonin will help foster congenial relations with others and should help to increase social support. As social support and social isolation have an important relationship with both physical and mental health, more research is needed on the implications of the ability of serotonin to modulate social behaviour for the regulation of mood, and for future physical and mental health.
The effect of raising and lowering tryptophan levels on human mood and social behaviour. 2013
There are currently a range of 5-HT3 antagonists available for clinical use in Europe, including tropisetron (Navaban®), ondansetron (Zofran®, Emetron®), granisetron (Kytril®), dolasetron (Anzemet®), and palonosetron (Aloxi®). These drugs have revolutionized the treatment of nausea and vomiting in cancer patients receiving chemotherapy or radiation therapy, which is their largest therapeutic use. The efficiency of these drugs may depend on the particular variants of 5-HT3 receptors expressed by the patient; , for example, one isoform of the 5-HT3B receptor has a promoter deletion that is associated with reduced efficacy of tropisetron and ondansetron.
Irritable bowel syndrome IBS is a common gastrointestinal disorder affecting 10–15% of adults and is another major therapeutic area for 5-HT3 receptor-selective compounds, perhaps not surprisingly as these receptors have roles in gastrointestinal motility, sensation, and secretion. Alosetron (Lotronex®, Lotronox®), a 5-HT3 receptor antagonist, has been approved for the treatment of IBS, but there have been problems with constipation and, more rarely, ischemic colitis, and it is now less frequently used (primarily in female patients suffering from IBS with diarrhea). A range of other 5-HT3-selective compounds, including some partial agonists, may prove more successful and are currently being explored.
An important consideration is that the 5-HT3 receptor-related actions of all drugs now on the market have been determined using homomeric 5-HT3A receptors. This may not prove to be the most useful testing protocol given that other subunits may play important roles. Emerging studies suggest that alterations in a number of 5-HT3 receptor subunits contribute to a range of disorders. Thus, mutations in the A, B, D, and E subunits have been associated with bipolar disorder, depression, anxiety, IBS, and anorexia. A greater understanding of the roles of C–E subunit-containing heteromeric receptors may therefore allow a wide range of other diseases to be treated with 5-HT3 receptor-selective drugs, potentially including addiction, pruritis, emesis, fibromyalgia, migraine, chronic heart pain, bulimia, and neurological phenomena such as anxiety, psychosis, nociception, and cognitive function.
The 5-HT3 receptor is a widely expressed, cation-selective member of the Cys loop receptor family. A range of studies, in particular heterologous expression and molecular modeling, have revealed many molecular details of its distribution, structure, function, and pharmacology. Nevertheless, information on receptor stoichiometry and the roles of these receptors in the CNS and PNS is still limited, suggesting there is much potential for therapeutic intervention in areas beyond those for which 5-HT3 receptor-specific drugs are proving highly successful.