Hippocampus and remote spatial memory in rats - PubMed (original) (raw)

Hippocampus and remote spatial memory in rats

Robert E Clark et al. Hippocampus. 2005.

Abstract

Damage to the hippocampus typically produces temporally graded retrograde amnesia, whereby memories acquired recently are impaired more than memories acquired remotely. This phenomenon has been demonstrated repeatedly in a variety of species and tasks. It has also figured prominently in theoretical treatments of memory and hippocampal function. Yet temporally graded retrograde amnesia has not been demonstrated following hippocampal damage in spatial tasks like the water maze. We have assessed recent and remote spatial memory following hippocampal lesions in three different tests of spatial memory: (1) the standard water maze; (2) the Oasis maze, a dry-land version of the water maze; and (3) the annular water maze, where training and testing occur within a circular corridor. Training protocols were developed for each task such that retention of spatial memory could be expressed after very long retention intervals. In addition, retention in each task was assessed with single probe trials so that the assessment of remote memory did not depend on the ability to relearn across multiple trials. The findings were consistent across the three tasks. In the standard water maze (Experiment 1), spatial memory was impaired after training-surgery intervals of 1 day, 8 weeks, or 14 weeks. Similarly, in the Oasis maze (Experiment 2), spatial memory was impaired after training-surgery intervals of 1 day and 9 weeks. Finally, in the annular water maze (Experiment 3), spatial memory was impaired after training-surgery intervals of 9 weeks and 14 weeks. Dorsal hippocampal lesions impaired performance to the same extent as complete lesions. The impairment in remote spatial memory could reflect disruption of previously acquired spatial information. Alternatively, it is possible that in these tasks hippocampal lesions might produce an impairment in performance that prevents the expression of an otherwise intact spatial memory.

(c) 2004 Wiley-Liss, Inc.

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Figures

FIGURE 1

FIGURE 1

Top view of the three spatial tasks. All three tasks require the use of distal spatial cues (e.g., posters located on testing room walls). A: Water maze (Experiment 1). Rats were trained to find an escape platform hidden just below the surface of the water. B: Oasis maze (Experiment 2). Water-deprived rats were trained to find a single well containing a 0.3 ml drop of water among 426 wells. C: Annular water maze (Experiment 3). Rats were trained to find an escape platform hidden just below the surface of the water. The swim path was restricted by clear walls that formed an annulus, such that rats necessarily encountered the platform location in the course of swimming. Recognition of the platform location during probe trials was indicated by a slowing of swim speed in the vicinity of the platform. Scale bar = 1 m for all three mazes.

FIGURE 2

FIGURE 2

Reconstructions of coronal sections showing the largest (striped) and smallest (black) area of damage for rats with large hippocampal lesions (H-RF) and dorsal hippocampal lesions (DH-RF). Reconstructions from rats in Experiment 1(A), Experiment 2 (B), and Experiment 3 (C). Each series of sections progresses (top to bottom) from anterior to posterior levels. Numbers represent the distance in millimeters posterior to bregma.

FIGURE 3

FIGURE 3

Water maze performance (Experiment 1). Percentage time in the training quadrant during a retention probe given 14 days after surgery. Control rats (CON), rats with complete hippocampal lesions (H-RF), and rats with dorsal hippocampal lesions (DH-RF) were tested for retention after a 1-day (CON, n = 8; H-RF, n = 8), 8-week (CON, n = 16; H-RF, n = 9; DH-RF, n = 8), or 14-week (CON, n = 12; H-RF, n = 12) training–surgery interval. None of the lesion groups performed above chance (25%). The 1-day and 8-week control groups performed above chance and better than the corresponding lesion group. Error bars = SEM.

FIGURE 4

FIGURE 4

Performance on the Oasis maze (Experiment 2). Percentage of time in the training circle during a retention probe trial that was given 14 days after surgery. Control rats (CON) and rats with complete hippocampal lesions (H-RF) or dorsal hippocampal lesions (DH-RF) were tested for retention after a 1-day (CON, n = 16; H-RF, n = 16) or 9-week (CON, n = 24; H-RF, n = 22; DH-RF, n = 8) training–surgery interval. None of the lesion groups performed better than rats that were presented on each trial with random water locations (RANDOM, n = 12). Mean performance of the RANDOM group is represented by the filled square. The 1-day and 9-week control groups performed better than the corresponding lesion groups and better than the RANDOM group. Error bars = SEM.

FIGURE 5

FIGURE 5

Performance on the annular water maze (Experiment 3). A: For scoring, twelve 30° arc zones were constructed around the training (platform location) zone. Chance performance is 8.3% (100% divided by 12 zones). B: Percentage time spent by each group in the 12 zones during the retention probe trial that was given 14 days after surgery. Memory of the platform location is indicated as increased percentage time in the training zone. C: Percentage time in the training zone replotted as a function of training–surgery interval. For rats with complete hippocampal lesions (H-RF), surgery occurred 9 weeks or 14 weeks after training (n = 7 for all groups). None of the lesion groups performed above chance (8.3%). Both the 9-week CON and the 14-week CON groups performed better than the corresponding lesion groups. Error bars = SEM.

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