Seizure-induced structural and functional changes in the rat hippocampal formation: comparison between brief seizures and status epilepticus

Prolonged seizures produce death of hippocampal neurons, which is thought to initiate epileptogenesis and cause a disruption of hippocampally mediated behaviors. This study aimed to evaluate behavioral and neuroanatomical changes induced by brief seizures and to compare them with changes induced by prolonged seizures. Adult rats were administered 6 brief seizures, elicited by electroshock (ECS). Prolonged seizures (status epilepticus, SE) were induced by pilocarpine. Two months later, the rats' behavior was tested using the Morris water maze, passive avoidance and active avoidance tests. The number of neurons in the hippocampal formation was estimated using stereological methods. ECS seizures produced loss of neurons, ranging between 14% and 26%, in the dentate hilus, subiculum, presubiculum, parasubiculum, and entorhinal layers III and V/VI. However, the neuron loss caused by SE in the same structures, as well as in the hippocampal CA3 and CA1 fields, ranged between 34% and 50%. SE additionally killed many neurons in the dentate granular layer, postsubiculum and entorhinal layer II. ECS treatment caused mild impairments in spatial learning and passive avoidance, but it was not associated with spontaneous motor seizures. In contrast, SE produced a severe disruption of spatial learning, passive and active avoidance, and led to the development of spontaneous seizures. These data show that both prolonged seizure activity and brief seizures result in structural and functional alterations in the temporal lobe circuits, but those caused by prolonged seizures are considerably more severe. Hippocampal damage elicited by brief seizures does not necessarily lead to spontaneous motor seizures.