γ-Aminobutyric acid (GABA) is used as an inhibitory neurotransmitter by 20-50% of all synapses in the brain. Most of the actions of GABA are mediated by GABAA receptors. GABAA receptors are chloride ion channels that can be opened by GABA and are the site of action of benzodiazepines, barbiturates, steroids, anesthetics and convulsants. They modulate anxiety, excitability of the brain, motor activity, circadian rhythms, sleep, vigilance, learning and memory.
GABAA receptors are composed of five protein subunits that can belong to different subunit classes. The individual subunits have a distinct but overlapping regional distribution in the brain. The occurrence of many different subunits in the same neurons gives rise to the formation of a large variety of different GABAA receptor subtypes with distinct subunit composition and pharmacological properties.