Late low magnesium-induced epileptiform activity in rat entorhinal cortex slices becomes insensitive to the anticonvulsant valproic acid.

We investigated time-dependent changes in low magnesium-induced epileptiform activity in combined rat entorhinal cortex/hippocampal slices with extracellular recording techniques. While in area CA3 short interictal discharges are generated without any major changes in activity during prolonged recording periods, initial tonic clonic ictaform events in the entorhinal cortex may change with time. We observed often a transition into a state of recurrent tonic activity without any clonic afterdischarges. Alternatively, seizures could stay in the clonic discharge mode for the rest of the experiment. These different seizure states were not equally affected by the anticonvulsant valproic acid. While the early clonic tonic discharges in the entorhinal cortex and the interictal like activity in area CA3 were effectively suppressed by valproic acid (VPA) the late recurrent tonic seizure discharge state was unaffected by the drug. It was, however, still sensitive to the N-methyl-D-aspartate (NMDA) receptor antagonist 2-aminophosphonovalerate. These findings point to seizure-induced changes in neuronal interaction in rat entorhinal cortex.