DISSOCIATION OF MOTOR HYPERACTIVITY AND SPATIAL MEMORY DEFICITS BY SELECTIVE HIPPOCAMPAL-LESIONS IN THE NEONATAL RAT

Hippocampal lesions can disrupt the acquisition of new memories and te nd to increase motor activity. Although hyperactivity may affect explo ration, it is unclear how these performance variables contribute to th e learning deficit and it is also not known which brain structures are involved. The present study provides evidence for a dissociation betw een activity and memory Following unilateral or bilateral electrolytic lesions of the hippocampus in neonatal rats, we assessed open field b ehavior and performance of discrete trials alternation in a T-maze. Wh en tested 6 and 20 weeks postoperatively, rats subjected to bilateral lesions were hyperactive. Their performance in the discrete trials alt ernation task was impaired. In contrast, rats subjected to unilateral lesions did not display an increase in motor activity, but were still deficient in performance on the T-maze. To define whether these behavi oral changes were accompanied by secondary changes in structures that project to the hippocampus, we studied the function of the septum and locus coeruleus after the lesions. Septal choline acetyltransferase (C AT, the acetylcholine-synthesizing enzyme) activity was reduced and ty rosine hydroxylase (TH, the rate-limiting enzyme in catecholamine bios ynthesis) activity in noradrenergic Ic neurons was increased after bot h the unilateral and bilateral lesion. Therefore, these changes map co ntribute to the memory impairment but are not necessarily related to m otor hyperactivity. We conclude that a deficit in spatial memory is no t attributable to altered performance variables such as activity. Furt hermore, spatial memory deficits in both the unilateral and bilateral lesion paradigms may be associated with changes in septal and Ic funct ion.