Which amino acid neurotransmitter is primarily known for inhibiting the firing of nerve cells?

Gamma-Aminobutyric Acid (GABA) is recognized as the primary inhibitory neurotransmitter in the central nervous system. It plays a vital role in reducing neuronal excitability throughout the nervous system. GABA achieves its inhibitory effects by binding to GABA receptors on neurons, which triggers the opening of channels that allow negatively charged ions to enter the cell or positively charged ions to leave. This process hyperpolarizes the neuron, making it less likely to fire an action potential.

In contrast, the other amino acids and neurotransmitters mentioned do not primarily function as inhibitory agents. Dopamine and norepinephrine are mainly involved in excitatory processes and modulation of mood and arousal, while serotonin is primarily associated with mood regulation, sleep, and gastrointestinal functions but does not serve as the main inhibitory neurotransmitter like GABA does. Thus, GABA's specificity and efficacy in inhibiting nerve cell firing is what makes it the correct answer in this context.