Unit activity in experimental epileptic foci during focal cortical hypothermia

Abstract

The change in neuronal firing frequency was measured by extracellular microelectrode recordings during focal cooling of several experimental epileptic foci, using a Peltier cooling device. The experimental epileptic foci studied included foci in cat sensory motor cortex secondary to tungstic acid, strychnine, and freezing, and foci in monkey sensory motor cortex secondary to alumina. Cooling of tungstic acid and strychnine foci was associated with changes in neuron firing that were similar to those seen in normal cortex similarly cooled. By contrast, cooling of alumina foci exhibiting spontaneous seizures was invariably associated with a single seizure after only a 1–3 C drop in cortical temperature followed by electrical silence persisting until rewarming. Initial cooling of alumina foci without spontaneous seizures was similar to normal cortex; however, repeated cooling cycles were associated with changes similar to those seen in alumina foci exhibiting spontaneous seizures. Frozen foci showed variable changes with cooling; occasional cells responding in a manner similar to alumina foci exhibiting spontaneous seizures, others more like normal cortex. These findings were interpreted as suggesting that there are pathophysiologic differences between the various experimental epileptic foci under normothermic conditions. Membrane potentials of neurons in tungstic acid and strychnine foci are likely normal. Neurons in alumina foci may be partially depolarized under normothermic conditions as may some but not all units in foci secondary to freezing.