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
. Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs including barbiturates anaesthetics and benzodiazepines (BZs)
. The BZs are a diverse range of compounds including widely prescribed drugs such as librium and valium and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes. GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels
. Eight types of receptor subunit have been cloned with multiple subtypes within some classes: alpha 1-6 beta 1-4 gamma 1-4 delta epsilon pi rho 1-3 and theta
. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members the large cytoplasmic loop displays the most divergence in terms of primary structure the TM domains showing the highest level of sequence conservation
.Most GABAA receptors contain one type of alpha and beta subunit and a single gamma polypeptide in a ratio of 2:2:1
though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and hence sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity
. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity