Epileptiform activity, which appears to be endogenous, has been recorded in the granule cells of the dentate gyrus before the onset of synchronized seizure activity and has been termed cellular bursts. It has been postulated that an increase in input to the dentate gyrus causes a local increase in extracellular K+ concentration ([K+]o) and a decrease in [Ca2+]o that results in this cellular bursting. The first test of this hypothesis is to determine whether the cellular bursts appear in ionic conditions that occur in vivo before the onset of synchronized epileptic activity. This hypothesis was tested in vitro by varying the ionic concentrations in the perfusing solution and recording changes in the granule cells of the dentate gyrus. Intra- and extracellular recordings were made in the dentate gyri of hippocampal slices prepared from anesthetized adult Sprague-Dawley rats. Increasing the extracellular potassium or decreasing the extracellular calcium of the perfusing solution caused three forms of spontaneous activity to appear: depolarizing potentials, action potentials, and cellular bursts. Increasing potassium or decreasing calcium also caused the granule cells to depolarize and reduced their input resistance. No synchronized extracellular field activity was detected. Simultaneously increasing potassium and decreasing calcium caused cellular bursts to appear at concentrations recorded in vivo before the onset of synchronized reverberatory seizure activity.
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