Synchronous GABA-Mediated Potentials and Epileptiform Discharges in the Rat Limbic System<b><i>In Vitro</i></b>

Abstract

Application of 4-aminopyridine (4AP, 50 μ m ) to combined slices of adult rat hippocampus–entorhinal cortex-induced ictal and interictal epileptiform discharges, as well as slow field potentials that were abolished by the μ-opioid agonist [ d -Ala 2 ,N-Me-Phe 4 ,Gly-ol 5 ] enkephalin (DAGO, 10 μ m ) or the GABA A receptor antagonist bicuculline methiodide (BMI, 10 μ m ); hence, they represented synchronous GABA-mediated potentials. Ictal discharges originated in the entorhinal cortex and propagated to the hippocampus, whereas interictal activity of CA3 origin was usually recorded in the hippocampus. The GABA-mediated potentials had no fixed site of origin or modality of propagation; they closely preceded (0.2–5 sec) and thus appeared to initiate ictal discharges. Only ictal discharges were blocked by the antagonist of the NMDA receptor 3,3-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP, 10 μ m ), whereas the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 μ m ) abolished all epileptiform activities. The GABA-mediated potentials continued to occur synchronously in all regions even after concomitant application of CNQX and CPP. [K + ] o elevations were recorded in the entorhinal cortex during the ictal discharge (peak values = 13.9 ± 0.9 m m ) and the synchronous GABA-mediated potentials (peak values = 4.2 ± 0.1 m m ); the latter increases were presumably attributable to postsynaptic GABA A -receptor activation because they were abolished by DAGO or BMI. Their role in initiating ictal activity was demonstrated by using DAGO, which abolished both GABA-mediated synchronous potentials and ictal discharges. These data indicate that NMDA-mediated ictal discharges induced by 4AP originate in the entorhinal cortex; such a conclusion is in line with clinical evidence obtained in temporal lobe epilepsy patients. 4AP also induces GABA-mediated potentials that spread within the limbic system when excitatory transmission is blocked and may play a role in initiating ictal discharge by increasing [K + ] o .