Temporary inactivation of the supramammillary area impairs spatial working memory and spatial reference memory retrieval.

The aim of our study was to examine the supramammillary (SuM) area involvement in spatial memory. Sprague-Dawley rats with chronically implanted cannula in the supramammillary area were trained in two spatial memory tasks with different memory demands: reference and working memory. In the spatial reference memory task, the rats received microinjections in the SuM area of tetrodotoxin (TTX) (0.5 ng diluted in 0.5 microL of saline) or saline (0.5 microL). The microinjections were administered 30 min before the spatial training (day 4) (to assess the effect on acquisition) and on the following two days (days 5 and 6) the training was conducted without microinjections (to study the effect on consolidation). On the last training day (day 7), in order to assess the retrieval of spatial information, the rats received the microinjections 30 min before the spatial training. The spatial working memory used was a delayed-matching-to-position (DMTP) task. Spatial training was performed for seven days. During the first three days of the spatial training, the rats achieved a good spatial knowledge and learnt the working memory rule necessary to solve the spatial task. On days 4 and 6, the rats received microinjections to study involvement of the SuM area in working memory. The results showed that temporary inactivation of SuM area impairs both the rat's ability to solve a spatial working memory task with DMTP demands and the recovery of spatial information in a spatial reference memory task. We suggest that SuM area is involved in the rearrangement of spatial information during spatial working memory tasks with DMTP memory demands.