a substance that exerts an antagonistic (inhibitive) effect at gamma-aminobutyric acid (GABA) receptor sites (see GABAA receptorgabaa-receptor; GABAB receptorgabab-receptor) or on the action of GABA. Like GABA agonists, GABA antagonists can be direct or indirect. Direct GABA antagonists block the GABA receptor; the best known of these is bicuculline, which acts as a competitive antagonist for GABA at its receptor site. Indirect GABA antagonists include picrotoxin, which is a noncompetitive antagonist at the GABAA receptor complex and blocks the effects of GABA on the receptor complex.