Effects of selective granular retrosplenial cortex lesions on spatial working memory in rats

The rat retrosplenial cortex comprises two major subregions (granular and dysgranular) that differ in morphology and connectivity. Although the effects of selective dysgranular retrosplenial cortex (area 30) lesions and the effects of selective lesions within separate sub-areas of the granular retrosplenial cortex have been described, the effects of complete granular lesions (area 29) remain unknown. The present study, therefore, contrasted excitotoxic lesions of the total granular retrosplenial cortex with complete retrosplenial cortex lesions (dysgranular plus granular) using two spatial working memory tasks variably sensitive to complete retrosplenial damage. The granular retrosplenial and complete retrosplenial lesion groups were comparably impaired throughout most of radial-arm maze acquisition, including when subsequently challenged by having the maze rotated mid-trial or being tested in the dark. The other test, reinforced spatial alternation in a T-maze, provided a slightly different result as it was the rats with selective granular cortex lesions that were most impaired when the rats were tested in two, parallel mazes (one for the sample run, the other for the test run). These findings reveal the importance of the granular retrosplenial cortex for learning across a variety of different spatial tasks. Combining these findings with the results of previous functional and anatomical studies suggests that the granular and dysgranular retrosplenial subregions function in close conjunction to support spatial learning.