Effect of reversible inactivation of the supramammillary nucleus on spatial learning and memory in rats

Memory includes processes such as acquisition, consolidation and retrieval. Reference memory (RM) and working memory (WM) are two kinds of memory that can be assessed in rodents using spatial tasks, especially using the Morris water maze. The Morris water maze is particularly sensitive to hippocampal lesions. The supramammillary nucleus (SuM) has strong links with the hippocampus and septum. The role of the SuM on spatial learning is controversial. In the present study, involvement of SuM in the different steps of spatial RM and WM was investigated in the Morris water maze using reversible inactivation of SuM with lidocaine. Lidocaine (0.5 microl, 4%) was injected into the SuM through a guide cannula implanted above the SuM. The rats were trained on RM and WM versions of the Morris water maze. SuM was inactivated before training or immediately after training or before the probe trial of retrieval tests. Reversible inactivation of the SuM impaired consolidation of RM, and of consolidation and retrieval of WM. Therefore, it seems that activity of SuM neurons plays a role in spatial RM and WM learning and memory in the rat.