Glossary

apoenzyme An enzyme without its obligate cofactor, which is therefore inactive.

carrier-mediated release Nonsynaptic release (usually of a neurotransmitter) by a plasma membrane transporter.

GABA g-Aminobutyric acid; the major inhibitory neurotransmitter of the central nervous system; also found in extraneuronal tissues (e.g., the pancreas and reproductive tracts).

GABA receptor A protein that binds GABA and initiates downstream effects.

GABA-T GABA-transaminase; enzyme that degrades GABA.

GABA transporter A protein that moves GABA across either the plasma membrane or the vesicular membrane.

GAD Glutamic acid decarboxylase; enzyme that converts glutamate to GABA.

holoenzyme An enzyme bound to its obligate cofactor, which is therefore active.

inhibitory neurotransmitter A neurotransmitter that, when it activates its postsynaptic receptor, decreases the probability that the postsynaptic cell will fire.

phasic inhibition Short-term (milliseconds) decrease in excitability of a cell, usually synaptically mediated.

pyridoxal phosphate Vitamin B6; a cofactor required for GAD activity.

receptor agonist A substance that binds to a receptor and mimics or enhances the receptor response.

receptor antagonist A substance that binds to a receptor and blocks or decreases the receptor response.

tonic inhibition Long-lasting decrease in the overall excitability of a cell or cells, usually extrasynaptically mediated.

GABA (g-aminobutyric acid; 4-aminobutyric acid) is an inhibitory amino acid neurotransmitter. Three separate groups published the first reports of GABA in the brain in 1950, although its function as a neurotransmitter was not recognized until later. Eugene Roberts, the author of one of the first reports, identified GABA in the course of chromatographic studies on the amino acid profiles of murine neuroblastomas. GABA from potatoes was used as a standard and comigrated with an unknown substance isolated from the tumors. Roberts rigorously pursued his studies of GABA synthesis and degradation in the brain and GABA is now recognized as the most prominent inhibitory neurotransmitter in the central nervous system (CNS).

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