Hippocampal, granule cell and CA3-4 lesions impair formation of a place learning-set in the rat and induce reflex epilepsy.

Rats were pretrained on a place learning-set task, in which a platform, submerged in a swimming pool filled with opaque water, was moved to a new location each day. Then they received either: suction removal of the dorsal hippocampus, intrahippocampal microinjections of colchicine to remove dentate gyrus granule cells, kainic acid to remove CA3-4 cells of the hippocampus proper, suction removal of parietal cortex overlying the hippocampus, or no surgery. Performance was then evaluated for 48 days. All lesion groups were chronically impaired with respect to the control group, but the rats with parietal cortex lesions retained the ability to solve the task, whereas rats with hippocampal damage did not. Training frequently induced task-related behavioural seizures in the rats with granule cell or CA3-4 lesions. The results show that the hippocampus, including granule cell and CA3-4 cell populations, is essential for the rapid acquisition of place responses in the swimming pool task. The finding that training on the task induced reflex epilepsy in granule cell and CA3-4-damaged rats, but not those with aspirative removals, suggests that residual portions of the hippocampus are activated by training and are involved in production of the epileptic attacks.