Which of the following substances is classified as inhibitory?

GABA, or gamma-aminobutyric acid, is classified as an inhibitory neurotransmitter in the central nervous system. It plays a crucial role in reducing neuronal excitability throughout the nervous system. When GABA binds to its receptors on neurons, it typically causes an influx of chloride ions into the cell or an efflux of potassium ions, leading to hyperpolarization of the neuron. This makes it less likely to fire an action potential, effectively inhibiting the activity of the neuron.

Understanding the functions of neurotransmitters is key in neurobiology. For instance, glutamate is considered the main excitatory neurotransmitter, facilitating synaptic transmission and enhancing neuronal activity. Acetylcholine can be both excitatory and inhibitory, depending on its receptor type and location in the nervous system. Dopamine is primarily known for its roles in reward, motivation, and motor control, functioning mainly as a modulator rather than a straight excitatory or inhibitory agent.

Thus, GABA serves a unique and critical function in maintaining the balance between excitation and inhibition in the brain, helping to regulate various physiological processes, including anxiety, sleep, and overall cortical excitability.