Paradoxical antagonism by bicuculline of the inhibition by baclofen of the electrically evoked release of [3H]GABA from rat cerebral cortex slices.

The presynaptic regulation of the electrically evoked release of [3H]GABA was studied in the rat cerebral cortex. Among the GABA receptor agonists tested (GABA, SL 75102, muscimol, THIP, isoguvacine, (+/-)-baclofen), only (+/-)-baclofen inhibited the stimulation-evoked release of [3H]GABA. This effect of baclofen was stereoselective in favor of the (-) enantiomer. The inhibition by (+/-)-baclofen of the electrically evoked release of [3H]GABA was antagonized by bicuculline and picrotoxin. Our results suggest that the release of [3H]GABA in vitro can be modulated by a receptor-mediated mechanism which is sensitive to baclofen, bicuculline and picrotoxin but not to GABA, muscimol or THIP.