DESYNCHRONIZATION OF EPILEPTIFORM ACTIVITY BY EXTRACELLULAR CURRENT PULSES IN RAT HIPPOCAMPAL SLICES

1. A single cathodic current pulse applied in the somatic CA1 region o f the hippocampus was found to induce a large decrease in the amplitud e of the population spike. 2. Intracellular recordings showed intense cellular firing suggesting the amplitude decrease could not be attribu ted to a decrease in neuronal firing. 3. Simultaneous intracellular an d extracellular potentials were recorded to analyse the synchronizatio n of neuronal firing in the CA1 region. Action potentials were synchro nized with the first population spike but this synchronization decreas ed with subsequent spikes. Histograms of the phase of the action poten tials displayed a normal distribution. 4. Histograms of the phases of the action potentials following the application of the 'singular stimu lus' (one producing a singular response) revealed a uniform distributi on of the phases suggesting that the neuronal population was desynchro nized. 5. This desynchronization effect of the singular stimulus was v erified by double intracellular recordings. The simultaneous firing of two neurons could be desynchronized by the application of the singula r stimulus. 6. These findings indicate that it is possible to desynchr onize a neuronal pool with the application of a single current pulse. In addition, the results show that it is possible for a neuronal popul ation to fire a large number of action potentials with no resulting ev oked potentials in the extracellular space.