Name the major inhibitory neurotransmitter in the CNS that hyperpolarizes postsynaptic neurons.

• A. Glycine
• B. Glutamate
• C. Dopamine
• D. GABA

Answer: (D)

Inhibitory signaling works by making the postsynaptic neuron less likely to fire, which happens when the membrane potential becomes more negative (hyperpolarization). The major inhibitory neurotransmitter in the CNS is GABA. It produces hyperpolarization mainly through GABA_A receptors, which are ligand-gated chloride channels that let Cl− enter the neuron, and through GABA_B receptors, which are metabotropic and open potassium channels to drive K+ out. This fast-acting and widespread inhibition helps keep neural activity in check and prevents excessive excitation. Glycine is inhibitory as well but is most influential in the spinal cord and brainstem, not as the principal CNS transmitter. Glutamate is excitatory, and dopamine modulates activity in various pathways rather than serving as the primary inhibitory transmitter.