**Question:** GABA transmission is facilitated by:

**Core Concept:** GABA (gamma-aminobutyric acid) is a major inhibitory neurotransmitter in the central nervous system (CNS) and peripheral nervous system (PNS). It plays a crucial role in regulating neuronal excitability, controlling the pace of neuronal firing, and maintaining homeostasis in the brain.

**Why the Correct Answer is Right:** GABA A receptors are ligand-gated ion channels that open upon binding of the neurotransmitter GABA, allowing chloride ions to flow into the neuron, hyperpolarizing the membrane and inhibiting neuronal activity. This process contributes to the overall inhibitory response in the CNS and PNS.

**Why Each Wrong Option is Incorrect:**

A. GABA B receptors are also involved in modulating neuronal activity, but they are coupled to G-proteins and regulate ion channels (such as potassium channels) rather than ligand-gated ion channels.

B. Glutamate is the primary excitatory neurotransmitter in the CNS, not involved in facilitating GABA transmission.

C. Serotonin is a neurotransmitter involved in regulating mood, appetite, and sleep, not directly facilitating GABA transmission.

D. Dopamine is primarily involved in modulating movement, reward, and motivation, not directly facilitating GABA transmission.

**Clinical Pearl / High-Yield Fact:** GABA's role in inhibiting neuronal activity is essential for maintaining homeostasis, regulating neuronal excitability, and controlling the pace of neuronal firing. Understanding GABA's role in the brain helps medical students and practitioners appreciate the intricacies of neurotransmission and its implications in various neurological disorders, such as epilepsy, anxiety disorders, and alcohol addiction.

**Correct Answer:** C. Serotonin (5-HT) is involved in regulating mood, appetite, and sleep, but not directly facilitating GABA transmission. The correct answer is serotonin because it modulates GABAergic neurons indirectly, by influencing the release, reuptake, or degradation of GABA neurotransmitter.