GABA(A)-MEDIATED INHIBITION AND IN-VITRO EPILEPTOGENESIS IN THE HUMANNEOCORTEX

1. We made intracellular and extracellular field potential recordings and ion-selective measurements of extracellular Ca2+ concentration ([C a2+](o)) and extracellular K+ concentration ([K+](o)) in human neocort ical slices that were obtained in the course of epilepsy surgery. Slic es were maintained in vitro at 34-35 degrees C and were perfused with Mg2+-free artificial cerebrospinal fluid (ACSF). 2. Spontaneous field potential epileptiform discharges (duration = 2.5-80 s) occurred in mo st of the slices studied (similar to 60%) after 1.5-2 h of perfusion w ith Mg2+-free ACSF. Intracellular recordings from regular-spiking neoc ortical neurons showed that epileptiform events consisted of large-amp litude( 15-30 mV) de polarizing shifts that were capped by bursts of f ast action potentials. A decrease in [Ca2+](o) (change in [Ca2+](o) = 0.02-0.17 mM, 0.07 +/- 0.046 mM, mean +/- SD, from a baseline of 1.8 m M, n = 10 slices) and an increase in [K+](o) (change in [K+](o) = 0.5- 3.8 mM, 1.6 +/- 1.24 mM, from a baseline of 3.25 mM, n = 10) were asso ciated with each epileptiform discharge. 3. The epileptiform activity induced by Mg2+-free ACSF uas abolished by bath application of antagon ists of the: N-methyl-D-aspnrtate (NMDA) receptor. This procedure also blocked the appearance of spreading depression-like episodes. By cont rast, the rate of occurrence of epileptiform discharges was not signif icantly modified by antagonizing non-NMDA receptors. 4. We also observ ed spontaneous, rhythmic potentials of positive polarity during perfus ion of Mg2+-free ACSF; the potentials became hyperpolarizing when the neuron membrane was made less negative than -75 mV with intracellular injection of depolarizing current, and they were decreased or abolishe d during application of the gamma-aminobutyric acid-A (GABA(A)) recept or antagonist bicuculline methiodide (BMI). The rate of occurrence and /or the amplitude of these presumably GABA(A)-mediated events decrease d similar to 2 s before the onset of each epileptiform discharge. 5. A pplication of BMI prolonged the epileptiform discharges while decreasi ng their rate of occurrence. These changes were also accompanied by an increase in the amplitude of the epileptiform field potential DC shif t, whereas the concomitant decreases in [Ca2+](o) and increases in [K](o) became more pronounced than in control Mg2+-free medium (31.2% an d 42.8%, respectively, n = 10 slices). 6. Intracellular analysis of re gular-spiking neurons in slices that did not generate spontaneous epil eptiform discharges after >2 h of perfusion with Mg2+-free ACSF showed all-or-none, variable latency epileptiform bursts that were induced b y high-strength focal extracellular stimuli. Low-frequency (0.5-5 Hz) extracellular focal stimuli could also elicit epileptiform afterdischa rges and longlasting potentiation of postsynaptic responses. 7. In the se slices repetitive trains of focal stimuli caused spontaneous epilep tiform discharges to occur. This phenomenon was accompanied by a decre ase of the GABA(A)-mediated spontaneous rhythmic intracellular potenti als whose rate of occurrence between one discharge and the next remain ed lower than under pretrain conditions (42-65%, n = 3). 8. These find ings indicate that inhibitory potentials caused by the activation of t he GABAA receptor are operant in the human neocortex maintained in vit ro at the lime when Mg2+-free epileptiform discharges are recorded. A transient decrease in such GABA(A)-mediated mechanism might be involve d in the onset of epileptiform discharges and might play a role in con trolling the changes in [Ca2+](o) and [K+](o) that accompany each epil eptiform event. 9. We also propose that in slices that did not generat e spontaneous epileptiform activity, a decrease of GABA(A)-mediated in hibition can be induced by trains of focal stimuli. Such a change migh t lead to the appearance of spontaneous epileptiform activity.