5-hydroxytryptamine 3 receptor, A subunit (5HT3_rcpt_A)
Type: Family

Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses . Five of these ion channel receptor families have been shown to form a sequence-related superfamily:

Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits .

Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits .

Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known .

Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) .

Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) .

These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence .

Serotonin (5-hydroxytryptamine, 5-HT) is widely distributed in both the central and peripheral nervous system, where it acts as a neurotransmitter and neuromodulator . It has been implicated in several aspects of brain function, including regulation of affective states, ingestive behavior and addiction. 5-HT can activate a number of different receptor subtypes that produce diverse neuronal responses, principally through activation of G-protein-mediated signalling pathways. Signalling through the 5-HT3 receptor (5-HT3R) differs, since this subtype belongs to the ligand-gated ion channel (LGIC) superfamily, which also includes the inhibitory gamma-aminobutyric acid type A and glycine receptors, and excitatory nicotinic acetylcholine receptors (nAChR) . 5-HT3 receptor function has been implicated in a variety of neural processes, including pain perception, emesis, anxiety and drug abuse.

Like the other members of the LGIC superfamily, the 5HT3R exhibits a high degree of sequence similarity, and therefore putative structural similarity, with nAChRs . Thus, functional 5HT3Rs comprise a pentamer: the ion channel is formed at the centre of a rosette formed between five homologous subunits. Two classes of 5-HT3R subunit are currently known, termed 5-HT3A and 5-HT3B. Whilst homomeric pentamers of 5-HT3A form functional receptors, heteromeric assemblies display channel conductances, cation permeabilities and current-voltage relationships typical of characterised neuronal 5-HT3 channels .

The proposed topology of 5-HT3R subunits comprises four putative transmembrane (TM) domains (designated M1-4); a large extracellular N-terminal region (~200 amino acids); and a variable cytoplasmic loop between M3 and M4. The M2 domains from each subunit are thought to form the channel pore. The agonist binding site is formed by the N terminus, which, on binding, induces a conformational change in the channel pore, a process often referred to as "gating" . Opening of the pore allows cation flux through the neuronal membrane and depolarises the membrane potential. Thus, 5-HT3Rs may be thought of as excitatory receptors .

Cloning of the 5-HT3A subunit from a neuroblastoma expression library was reported in 1991 . Whilst recombinant expression of 5-HT3A yields functional receptors, the channel conductance and permeability to cations are different from that observed for native receptors . Alternative exon splicing gives rise to two isoforms of 5-HT3A, termed 5-HT3AS and 5-HT3AL for short and long variants, respectively. The 5-HT3RA subunit is widely expressed throughout the peripheral and central nervous systems, including several regions of the brain.