Re-evaluation of the spatial memory deficits induced by hippocampal short lasting inactivation reveals the need for cortical co-operation

Evidence has accumulated that the rat hippocampus plays a central role in s patial memory. In complement to lesion studies, reversible lidocaline-induc ed inactivations have been used to investigate the time-course of the memor y processes mediated by the hippocampus. A number of studies suggest that, in some conditions, the hippocampus is not necessary for online acquisition of spatial information. To test this hypothesis, we examined the effects o f bilateral lidocaine-induced inactivations of the dorsal hippocampus in th e acquisition of new spatial information. After initial learning of a place navigation task in the water maze, rats were tested for acquisition of a n ew platform location and received injections of lidocaine in the hippocampu s prior to each daily four-trial block. The training blocks were separated by a 24-h period allowing the hippocampus to recover from inactivation. The results show that lidocaine-injected rats were able to learn the new platf orm location like controls. Inactivations, however, was found to induce a w ithin-block learning impairment. This suggests that the hippocampus can per form off-line processing and that another structure is able to handle spati al information during hippocampal inactivations. Parietal-lesioned rats tha t received an injection of lidocaine were still able to learn the new platf orm location suggesting that the parietal cortex does not sustain this role . Overall, our results suggest that the hippocampus is not necessary for al l stages of memory formation and co-operates with other brain, possibly cor tical, structures which remain to be determined. (C) 2001 Elsevier Science B.V. All rights reserved.