Epileptiform activity induced by pilocarpine in the rat hippocampal-entorhinal slice preparation

An in vitro slice preparation of combined hippocampus and entorhinal cortex from adult rats was used to study the modalities of generation and propagation, as well as the pharmacological properties of the epileptiform activity induced by the muscarinic agonist pilocarpine (10 microM). Simultaneous field potentials recordings were made from the medial entorhinal cortex and from the dentate gyrus, CA3 and CA1 subfields. Pilocarpine application induced two types of interictal epileptiform discharges. The first occurred in the entorhinal cortex and consisted of bursts of population spikes lasting 408 +/- 135 ms (n = 20 slices) and repeating at a rate of 0.26 +/- 0.07 Hz (n = 20); this interictal activity propagated to the hippocampus via the perforant path. The second type was only observed in CA3 and CA1 subfields, had shorter duration (82 +/- 16 ms; n = 20) and occurred at a higher rate (1.42 +/- 0.7 Hz; n = 20) than the first type. Ictal epileptiform discharges (duration: 11.5 +/- 4.1 s; rate: 0.002 +/- 0.0009 Hz; n = 10) were also seen in the entorhinal cortex, from where they propagated to the dentate, CA3 and CA1 via the hippocampal trisynaptic loop as revealed by latency analysis and lesion experiments. Ictal and interictal discharges of entorhinal origin disappeared in the hippocampal sectors, but continued to occur in the entorhinal cortex following a cut of the perforant path (n = 5). Fast interictal discharges observed solely in the hippocampus originated in CA3, since sectioning the Schaffer collaterals made them disappear in CA1 (n = 7). All types of epileptiform activity disappeared during application of the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (10 microM; n = 7). By contrast, the N-methyl-D-aspartate receptor antagonist 3-3(2-carboxy-piperazine-4-yl) propyl-1-phosphonate (10 microM) abolished ictal discharges in the entorhinal cortex and reduced the duration of the interictal events recorded in this area (n = 7). Interictal discharges originating from CA3 continued to occur at a higher rate than in control during application of this N-methyl-D-aspartate receptor antagonist. Our study confirms that the combined hippocampal-entorhinal slice preparation represents a suitable model for understanding the modalities of origin and propagation of epileptiform activity within the limbic system. In this in vitro preparation, the entorhinal cortex is the site of origin for ictal discharges. Moreover, the different types of epileptiform activity induced by this muscarinic agonist have specific, structure-dependent pharmacological profiles. These results are discussed in relation to those obtained in vivo.