Selective loss of hilar neurons and impairment of initial learning in rats after repeated administration of electroconvulsive shock seizures.

Prolonged seizures induced by neurotoxins or intracranial electrical stimulation provoke death of hippocampal neurons, which results in conspicuous learning and memory deficits. We examined whether repeated brief seizures elicited by electroconvulsive shock (ECS) can also deteriorate hippocampal structure and function. Adult Wistar rats were administered six ECS seizures, the first five of which were 24 h apart, whilst the last two were spaced by a 2-h interval. Following a 2-month recovery period, the cognitive status of the animals was assessed using the water maze task. ECS-treated animals were incapable of learning the constant platform position version of this task during the first 4 days of training, but performed similarly to control rats throughout the rest of the acquisition period, on the probe trial, and on the variable platform position and visible platform tasks. The results of the morphological analysis showed that the total number of hippocampal pyramidal neurons and dentate gyrus granule cells were similar in control and ECS-treated rats. However, ECS treatment caused loss of approximately 17% of cells in the hilus of the dentate gyrus, which was accompanied by significant mossy fiber sprouting into the dentate inner molecular layer. In addition, we found that the ECS-induced decrease in the total number of hilar cells was not due to loss of inhibitory interneurons immunoreactive to somatostatin. These findings support the view that ECS-induced seizures can produce a number of morphological and functional changes in the rat hippocampal formation, which qualitatively resemble those previously described in other seizure models.