Laminar Organization of Epileptiform Discharges in the Rat Entorhinal Cortex in Vitro

1. Interictal and ictal epileptiform discharges induced by 4-aminopyridine (4AP, 50 microM) were studied in the rat lateral entorhinal cortex with field potential and intracellular recordings in an in vitro slice preparation. Both types of discharge disappeared in layer II, but continued to occur in layers IV-VI after a knife cut separation was made at approximately 600 micro(m) from the pia (n = 4 slices). 2. Interictal depolarizations recorded in layer IV-VI cells (amplitude, 29.4 +/- 8.6 mV; duration, 386 +/- 177.4 ms, means +/- s.d.; n = 17) were capped by action potential bursts, while smaller interictal depolarizations in layer II cells (amplitude, 11.7 +/- 5.8 mV; duration, 192.6 +/- 47.9 ms; n = 10) were associated with single action potentials and were terminated by a hyperpolarization. Ictal discharges were initiated by an interictal discharge; they were characterized by a depolarization of 31.5 +/- 6.2 mV (n = 12) in layer IV-VI and 11.6 +/- 3.5 mV (n = 7) in layer II neurones. 3. Slow, presumptive Ca2+-mediated spikes occurred in layer II (n = 4) and IV-VI (n = 6) cells loaded with the Na+ channel blocker QX-314 (50 mM). These events were synchronized with population spikes during interictal and ictal discharges, and were abolished by Ni2+ (1 mM, n = 4 cells) along with the 4AP-induced synchronous activity. 4. The N-methyl-D-aspartate (NMDA) receptor antagonist 3, 3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate (CPP, 10 microM) abolished ictal discharges and reduced interictal depolarizations in layer IV-VI neurones (n = 4). The non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) abolished both interictal and ictal activity (n = 4 cells). 5. These findings provide evidence for a role played by NMDA-mediated mechanisms in the generation of epileptiform discharges in the entorhinal cortex. Lack of an NMDA-mediated component along with presence of inhibition in layer II neurones results in attenuation of epileptiform activity at this site. Moreover Ca2+-mediated spikes may contribute to the appearance of epileptiform discharges in this model.