Effect of Circadian Phase on Performance of Rats in the Morris Water Maze Task

The authors examined spatial working memory in the Morris water maze during the activity and rest periods of Wistar rats. Wheel-running activity was measured continuously as a marker of circadian phase. To minimize possible masking effects on performance, animals were placed in constant dim light the day before testing and tested in similar light conditions. Three experiments were run, each of them using animals varying in their previous experience in the water maze. Half of the animals of each experiment were tested 2 to 3 h after activity onset (active group), and the other half were tested 14 to 15 h after activity onset (inactive group). In the three experiments, a significant phase effect was observed in the animals' performance in the water maze; animals tested in the active phase showed steeper acquisition curves. These phase effects on performance are due to the animals' search pattern and not to a better acquisition and maintenance of spatial information; rats tested in the inactive phase found the platform faster on the first trial of the test, when the information on the location of the platform had not been presented to the animals. This effect vanished as the amount of training in the pool increased. Finally, swimming speed also showed a temporal effect, suggesting the existence of a phase effect for motivation to escape from the water; rats tested during their inactive phase tended to swim faster. All together, the data suggest a modulating effect of the biological clock on performance in the water maze, particularly when the animals are less experienced.