State-dependent slow outlasting activities following neocortical kindling in cats

Some forms of electrographic seizures are generated at the level of the cortical network. Neocortical kindling exhibits a resistance to produce generalized convulsive seizures, and therefore, it was rather difficult to use it to study the cortical epileptogenesis. Here, using supra-threshold cortical kindling, we report electrophysiological patterns of field-potential synchronization and intracellular activities in chronically implanted non-anesthetized cats, during different states of vigilance, and during acute seizures elicited by prolonged (20-60 s) electrical stimulation. Acute seizures were easily elicited during transition from slow-wave sleep (SWS) to waking state. The seizures were mainly clonic accompanied with tonic components followed by prolonged postictal depression. Delayed rhythmic outlasting activities (OA) at approximately 1.5 Hz, first time reported here, followed the postictal depression, and lasted up to 2 h. These activities were clear during waking state, slightly reduced during SWS and completely absent during rapid-eye movement sleep. They started focally and following daily stimulations generalized over the entire cortical surface. Extra- and intracellular neuronal recordings during OA displayed spike-doublets, built on the summation of successive excitatory postsynaptic potentials and fast-prepotentials, entailing an increased dendritic excitation. Our results suggest that such rhythmic long-lasting oscillatory activity outlasting seizures are the key factor of epileptogenesis, leading to epilepsy.