Fetal hippocampal transplants restore conditioned blocking in rats with dorsal hippocampal lesions: effect of age.

Previous studies have shown that conditioned blocking of taste aversion learning in 3-month-old Wistar rats depends on the hippocampal system integrity. Thus, the aim was to demonstrate that enough connectivity would develop after a graft to support the attention mechanism required for conditioned blocking. In the first experiment, bilateral homotopic grafts of 16-17 day-old hippocampal fetal tissue applied to 3-month-old male Wistar rats after electrolytical lesions of the dorsal hippocampus reinstated conditioned blocking tested 5 months after the transplantation. Unexpectedly, an early age-dependent impairment of conditioned blocking, similar to that induced by hippocampal lesions, was found in the 8-month-old control group. This finding was further supported by the results of the second experiment. Non-operated 3-month-old but not 8-month-old Wistar rats showed conditioned blocking. The results are discussed in terms of early hippocampal vulnerability, prevented by fetal grafts.