Final answer:
The most common subunit composition for the GABAA receptor in the adult brain is α1β2γ2, which plays a critical role in the fast inhibitory effects of GABA neurotransmission.
Step-by-step explanation:
The most common sub-synaptic GABAA receptor configuration in the adult brain is composed of α1β2γ2 subunits. GABA, the main inhibitory neurotransmitter in the central nervous system (CNS), primarily binds to GABAA receptors at synaptic junctions to exert phasic inhibition. This type of inhibition is characterized by a transient increase in chloride conductance, leading to hyperpolarization of the neuron and a decrease in neuronal excitability. The α1β2γ2 configuration is particularly important for producing these fast inhibitory effects that are crucial for balancing neural circuit activity and preventing excessive excitation that could lead to conditions such as epilepsy.