5-HT3 receptor
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 (nAChRs), and the inhibitory neurotransmitter receptors for GABA (both GABAA and GABAA- 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.

# Structure
As with other ligand gated ion channels, the 5-HT3 receptor is composed of five subunits pseudo symmetrically arranged about a central ion conducting pore.   These subunits are proteins encoded by the HTR3A, HTR3B, HTR3C, HTR3D, and/or HTR3E genes.
A functional channel may be compossed of five identical 5-HT3A subunits (homopentameric) or a mixture of 5-HT3A and one of the other four 5-HT3B, 5-HT3C, 5-HT3D, or 5-HT3E subunits (heteropentameric). It appears that only the 5-HT3A subunits form functional homopentameric channels.  All other subunit subtypes must heteropentamerize with 5-HT3A subunits to form functional channels.

# Tissue distribution
The 5-HT3 receptor is expressed throughout the central and peripheral nervous systems and mediates a variety of physiological functions.  On a cellular level, it has been shown that postsynaptic 5-HT3 receptors mediate fast excitatory synaptic transmission in rat neocortical interneurons and amygdala, and in ferret visual cortex.  5-HT3 receptors are also present on presynaptic nerve terminals, where they are thought to mediate or modulate neurotransmitter release.

# Effects
When the receptor is activated to open the ion channel by agonists, the following effects are observed:
- CNS: anxiety
- PNS: neuronal excitation (in autonomic, nociceptive neurons), emesis

# Agonists
Agonists for the receptor include:
- 2-methyl-5-HT
- varenicline
- Chlorophenylbiguanide
- Quipazine
- RS-56812: potent and selective 5-HT3 partial agonist, 1000x selectivity over other serotonin receptors
- SR-57227
- YM-31636

# Antagonists
Antagonists for the receptor (sorted by their respective therapeutic application) include:
- Antiemetics
AS-8112
Granisetron
Ondansetron
Tropisetron
- AS-8112
- Granisetron
- Ondansetron
- Tropisetron
- Gastroprokinetics
Alosetron
Batanopride
Metoclopramide (high doses)
Renzapride
Zacopride
- Alosetron
- Batanopride
- Metoclopramide (high doses)
- Renzapride
- Zacopride
- Antidepressants
Mianserin
Mirtazapine
- Mianserin
- Mirtazapine
- Antipsychotics
Clozapine
Olanzapine
Quetiapine
- Clozapine
- Olanzapine
- Quetiapine
- Others
Memantine (Alzheimer's disease medication)
- Memantine (Alzheimer's disease medication)

# Discovery
Identification of the 5-HT3 receptor did not take place until 1986 because of a lack of selective pharmacological tool. However, with the discovery that the 5-HT3 receptor plays a prominent role in chemotherapy- and radiotherapy-induced vomiting, and the concomitant development of selective 5-HT3 receptor antagonists to suppress these side effects aroused intense interest from the pharmaceutical industry and therefore the identification of 5-HT3 receptors in cell lines and native tissues quickly followed.