GABA receptor control of parasympathetic outflow to heart: characterization and brainstem localization.

Blockade of gamma-aminobutyric acid (GABA) receptor function by direct microinjection of the GABA receptor antagonist bicuculline into the nucleus ambiguus of the brainstem produced a marked, dose-related depression of heart rate and blood pressure which was mediated by the vagus nerve. This effect was not obtained in other regions of the brainstem and was reversed by the GABA receptor agonist muscimol. These data indicate that the nucleus ambiguus may be the site of a GABA receptor-mediated inhibition of vagal outflow.