Text Explanation

Glutamate.: This answer choice is incorrect, because glutamate is the primary excitatory, not inhibitory neurotransmitter of the central nervous system.

Acetylcholine.: This answer choice is incorrect, because acetylcholine is both a central and peripheral neurotransmitter and can be either excitatory or inhibitory. For the MCAT, acetylcholine is usually tested in reference to its function at the neuromuscular junctions of skeletal muscles, where it acts as an excitatory neurotransmitter.

Serotonin.: This answer choice is incorrect, because while serotonin can act as both an excitatory and inhibitory CNS neurotransmitter (depending on the receptors), it is not accurate to characterize serotonin as the primary inhibitory neurotransmitter of the CNS. Serotonin is usually tested on the MCAT in reference to how it helps regulate mood, appetite, and sleep. Drugs that increase the serotonin levels within the synaptic cleft are used clinically as antidepressants.

GABA (gamma-aminobutyric acid).: This answer choice is correct. For the MCAT, the only way to answer this question is to have the role of the major neurotransmitters committed to memory.

GABA is the major inhibitory neurotransmitter in the central nervous system. Alcohol and tranquilizers stimulate the release of GABA, which lowers the firing rate of post-synaptic neurons that have GABA receptors. This is why alcohol is considered a CNS depressant.