Within-day variation in spatial memory deficit induced by scopolamine

We studied the effect of scopolamine on radial maze performance during both light and dark periods in sham and suprachiasmatic nucleus (SCN)-lesioned rats. Scopolamine dose-dependently caused a significant increase in the number of total, reference memory and working memory errors during both light and dark periods. The extent of memory deficit induced by scopolamine in the light period was significantly higher than in the dark period. The difference in the scopolamine effect between light and dark periods disappeared completely after SCN lesioning. These results clearly indicate that within-day variation in the effect of scopolamine was controlled by circadian rhythm.