Effects of mammillary bodies and mediodorsal thalamic lesions on the acquisition and retention of a learning set in mice: paradoxical effect of the intersession interval.

The effects of ibotenic acid lesions of either the mediodorsal thalamus (MD) or the mammillary bodies (MB) on the acquisition and retention of a reversal learning set in a T-maze were studied in mice. Both the MB and MD lesions disrupted performance in this task, but only MD lesions slowed down the rate of learning, MD-lesioned subjects requiring more trials than the other two groups to master the discrimination at the end of the learning phase. Surprisingly, increasing the intersession interval from 24 h to 10 days totally alleviated the deficit of MD-subjects and even improved their performance as compared to the last (8th day) session. The overall results of the study show that both MD and MB lesions do not induce important learning deficits nor anterograde amnesia in the reversal learning set task.