Spontaneous paroxysmal activity induced by zero magnesium and bicuculline: suppression by NMDA antagonists and GABA...

Slices of rat cerebral cortex developed spontaneous paroxysmal discharges when superfused with Krebs medium containing zero Mg2+ or 50 microM bicuculline. In both situations, the N-methyl-D-aspartate (NMDA) antagonists APV, 100 microM, and ketamine, 100 microM substantially reduced the frequency of the paroxysmal events, the reduction being greater in zero Mg2+. gamma-Aminobutyric acid (GABA) 1 mM, the GABA-A agonist muscimol 2 microM and the GABA-B receptor agonist baclofen 10 microM, each reduced the frequency of events in zero Mg2+ while muscimol and GABA also reduced the amplitude of the events. GABA and baclofen were similarly effective against bicuculline-induced events but the muscimol concentration required was 5-10-fold higher. These results suggest that, under our vitro conditions, neocortical cells are normally restrained from paroxysmal discharges by Mg2+. Inhibition by GABA through GABA-A receptors and inhibition by GABA through GABA-B receptors, may also contribute to this restraint.