Evolution of hippocampal epileptic activity during the development of hippocampal sclerosis in a mouse model of temporal lobe epilepsy.

Unilateral intrahippocampal injection of kainic acid in adult mice reproduces most of the morphological characteristics of hippocampal sclerosis (neuronal loss, gliosis, reorganization of neurotransmitter receptors, mossy fiber sprouting, granule cell dispersion) observed in patients with temporal lobe epilepsy. Whereas some neuronal loss is observed immediately after the initial status epilepticus induced by kainate treatment, most reorganization processes develop progressively over a period of several weeks. The aim of this study was to characterize the evolution of seizure activity in this model and to assess its pharmacological reactivity to classical antiepileptic drugs. Intrahippocampal electroencephalographic recordings showed three distinct phases of paroxystic activity following unilateral injection of kainic acid (1 nmol in 50 nl) into the dorsal hippocampus of adult mice: (i) a non-convulsive status epilepticus, (ii) a latent phase lasting approximately 2 weeks, during which no organized activity was recorded, and (iii) a phase of chronic seizure activity with recurrent hippocampal paroxysmal discharges characterized by high amplitude sharp wave onset. These recurrent seizures were first seen about 2 weeks post-injection. They were limited to the injected area and were not observed in the cerebral cortex, contralateral hippocampus or ipsilateral amygdala. Secondary propagation to the contralateral hippocampus and to the cerebral cortex was rare. In addition hippocampal paroxysmal discharges were not responsive to acute carbamazepine, phenytoin, or valproate treatment, but could be suppressed by diazepam. Our data further validate intrahippocampal injection of kainate in mice as a model of temporal lobe epilepsy and suggest that synaptic reorganization in the lesioned hippocampus is necessary for the development of organized recurrent seizures.