Entorhinal cortex entrains epileptiform activity in CA1 in pilocarpine-treated rats

Layer III neurons of the medial entorhinal cortex (mEC) project to CA1 via the temporoammonic pathway and exert a powerful feed-forward inhibition of CA1 pyramidal neurons. The present study evaluates the hypothesis that disrupted inhibition of CA1 pyramidal neurons causes an eased propagation of entorhinal seizures to the hippocampus via the temporoammonic pathway. Using a method to induce a confined epileptic focus in brain slices, we investigated the spread of epileptiform activity from the disinhibited mEC to CA1 in control and pilocarpine-treated rats that had displayed status epilepticus and spontaneous recurrent seizures. In pilocarpine-treated rats, the mEC showed a moderate layer III cell loss and an enhanced susceptibility to epileptiform discharges compared to control animals. Entorhinal discharges propagated to CA1 in pilocarpine-treated rats but not in controls. Disconnecting CA3 from CA1 did not affect the spread of epileptiform activity to CA1 excluding its propagation via the trisynaptic hippocampal loop. Mimicking the invasion of epileptiform discharges by repetitive stimulation of the temporoammonic pathway caused a facilitation of field potentials in CA1 that were contaminated by population spikes and afterdischarges in pilocarpine-treated but not control rats. Single cell recordings of CA1 pyramidal neurons revealed a dramatic loss of feed-forward inhibition and the occurrence of strong postsynaptic excitatory potentials in pilocarpine-treated rats. Excitatory responses in CA1 were characterized by multiple NMDA receptor-mediated afterdischarges and a strong paired-pulse facilitation in response to activation of the temporoammonic pathway. Our results suggest that, irrespective of the enhanced seizure-susceptibility of the mEC in epileptic rats, the loss of feed-forward inhibition and the enhanced NMDA receptor-mediated excitability CA1 pyramidal cells ease the spread of epileptiform activity from the mEC to CA1 via the temporoammonic pathway bypassing the classical trisynaptic hippocampal loop.