Epileptiform discharges and a synchronous GABAergic potential induced by 4-aminopyridine in the rat immature hippocampus.

Extracellular field potential recordings were performed in the CA3 subfield of hippocampal slices that were obtained from 10- to 30-day-old rats. During perfusion with medium containing the convulsant drug 4-aminopyridine (4-AP, 50 microM) 3 main types of spontaneously occurring potentials were observed. The first one was a short-lasting (duration: 400-1100 ms) potential with a frequency of occurrence that ranged between 0.6 and 1.3 Hz. Thus it resembled an epileptiform interictal event. The second type was reminiscent of an ictal epileptiform discharge, lasted 10-35 s and recurred every 40-100 s. The third one was of opposite polarity as compared with the other two types, occurred every 10-100 s and was often followed by the ictal discharge. When recorded in isolation this potential lasted 1.2-2 s. The interictal and ictal discharges were blocked by the non-N-methyl-D-aspartate (non-NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM), while the potential of opposite polarity was not affected by this pharmacological procedure. It was, however, blocked in a reversible way by the GABAA receptor antagonist bicuculline methiodide (5 microM). These results indicate that in addition to epileptiform activity of interictal and ictal type, 4-AP also induces in the immature rat hippocampus a synchronous event that is due to the activation of the GABAA receptor.