Comparison of penicillin epileptogenesis in rat somatosensory and motor cortex

The relative sensitivities of somatosensory and motor areas of the cerebral cortex in penicillin epileptogenesis were compared in urethane-anaesthetized rats. Penicillin was applied electrophoretically from a fluid-filled micro-electrode. Spontaneous focal interictal epileptiform discharges were detected by a nearby recording electrode. In motor cortex, every cortical layer was less sensitive in penicillin epileptogenesis than the corresponding layer in somatosensory cortex; epileptic spikes occurred later, were of lower amplitude and were less frequent. In motor cortex, the sensitive depth extended from the deep part of layer III to the upper part of layer V. It seemed possible that penicillin applied to motor cortex might be producing its effects by diffusing back to the sensitive somatosensory area. This was excluded by applying penicillin to motor cortex whilst recording from both somatosensory and motor areas and demonstrating that the spikes were found in motor but not in somatosensory cortex.