Remote spatial memory and the hippocampus: effect of early and extensive training in the radial maze.

In a previous study we showed a temporally graded retrograde amnesia after hippocampal lesions when rats learned a spatial reference memory task in which two types of signals simultaneously indicated the goal arm (shape of the experimental room and extramaze landmarks). To investigate the effect that the navigational demands of the task have on remote memory expression, the same task was used in the present study as in our previous report, but on this occasion the shape of the surroundings was not predictive, which resulted in a highly demanding spatial task. Additionally, animals received extensive training in an early phase to ensure that the task was well learned. Results indicated a profound retrograde amnesia when dorsal hippocampal lesions were made 1 or 70 d after the end of the training (experiments 1 and 2). Using a long period of retraining, however, lesioned animals in the 70-d groups showed progressively more spared memory than the lesioned rats of the 1-d group. Experiments 3 and 4 showed that rats did not learn the above spatial task through an S-R association. Specifically, when animals acquired the task using a single cue (intra- or extramaze), hippocampal lesions did not produce retrograde amnesia. These findings support the possibility that in a highly demanding spatial task, hippocampal lesions produce a performance/navigational impairment that could interfere with the expression of spared remote spatial memory. The long period of retraining, however, seems to partially compensate for this deficit, but only when a long learning-surgery interval is employed.