GABAA receptors are responsible for the majority of neuronal inhibition in the mammalian CNS. Agonist activation results in the opening of their integral anion channel, generally leading to hyperpolarization of the cell membrane and thus inhibition. Electron microscopic studies of the native receptors have shown that they are composed of five subunits arranged pseudosymmetrically around the ion channel, which passes through the cell membrane. Viewed from the cell exterior, the receptor appears as a 'doughnut' with an external diameter of around 8 nm; the 3 nm central cavity representing the channel vestibule opening.
GABAA receptors are hetero-oligomers whose subunits are selected from four principle families named α, β, γ and δ, although others, including ρ, π, θ and ε, have been identified. In the human brain, molecular cloning studies have so far isolated six α, three β and three γ subunit isoforms while only a single δ subunit is currently known. A single gene encodes each of the subunit isoforms, although additional heterogeneity is introduced by alternative splicing in a number of cases. This plethora of subunits may suggest that there exist a vast array of GABAA receptor subtypes, but preferred assemblies clearly exist with most estimates proposing the presence of tens rather than hundreds of receptor subtypes. It is currently believed that 70-80% of GABAA receptors contain a benzodiazepine binding site and are composed of β, γ2 and either an α1, α2, α3 or α5 subunit, the most abundant of which is the α1 subtype.
Although the precise subunit composition of the receptor subtype determines its pharmacological and biophysical characteristics, additional functional diversity is introduced by a number of additional factors, including its cellular location and phosphorylation status. Certain receptor subtypes appear to be localized to the subsynaptic membrane where they are exposed, for brief periods, to high concentrations of released GABA, producing the phasic transmission associated with inhibitory postsynaptic currents. However, it is becoming increasingly clear that other receptor subtypes are found extrasynaptically; these are sensitive to significantly lower concentrations of the transmitter present at these sites where they mediate a tonic, slowly desensitizing current the importance of which is now becoming recognized.
The GABAA receptor family is the target for a number of psychoactive drugs, notably benzodiazepines, barbiturates, neurosteroids and general anesthetics, each class interacting with unique allosteric sites on the receptor. Agents with positive efficacy facilitate agonist-induced receptor activation that may produce sedation/hypnosis, anxiolysis, anticonvulsant activity, muscle relaxation, anterograde amnesia and loss of consciousness. However, inverse agonists exhibit diametrically opposed effects, decreasing the effects of agonist activation; subtype-selective inverse agonists may have potential as promnesic agents.
Each GABAA receptor subunit isoform exhibits a distinct topographical distribution in the brain, suggesting that they mediate specific physiological functions, a conclusion that has gained support from advances with the 'knock-in' technology. The subunit isoform distribution pattern is not static, however, and changes not only developmentally, but also as a consequence of normal physiological cycles and pharmacological intervention with agents that are known to produce their effects by interaction with these receptors. Indeed, aberrant expression of certain receptor subtypes may be of pathophysiological importance.
Significant progress has been made in the identification of specific amino acids, within the subunit sequences, which underpin the recognition properties of the distinct GABAA receptor subtypes. This, together with the realization that ligands may exhibit distinct intrinsic efficacies at individual subtypes, has led to a renewed interest in the potential of the GABAA receptor family for the development of new therapeutics with a more limited pharmacodynamic profile. These receptors of the mammalian CNS have proved to be an important drug target over many years and recent developments hold much promise for the future.
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