GABA antagonists potentiate the cardioinhibitory reflex induced by capsaicin in the cat

The GABA antagonists picrotoxin (PX) and bicuculline (BIC) were given intravenously (i.v.) or applied topically to the region of the nucleus of the solitary tract (NTS) in intact or in precollicularly decerebrate cats under chloralose-urethane anaesthesia, and their effects on capsaicin (CAP)-induced reflex bradycardia were studied. The administration of PX or BIC was found to result in a decrease of the resting heart rate and a significant increase of the cardioinhibitory reflex evoked by CAP. The maximum effects of these GABA antagonists developed within 3-10 min and lasted for about 40-60 min. Neither the resting heart rate nor the response to CAP was affected by strychnine. It is concluded that the CAP-sensitive unmyelinated barosensory afferents and/or the interneurons in the NTS are under a tonically active GABA-ergic inhibitory control originating in the brain stem.