The effect of reversible inactivation of the supramammillary nucleus on passive avoidance learning in rats

Previous studies have shown that the presence of hippocampal theta activity is important for learning and memory, and that the medial supramammillary nucleus (mSuM) is involved in the control of the frequency of theta rhythm. It has also been shown that the depression of mSuM activity by chlordiazepoxide causes modest impairment of spatial learning. On the other hand, the lateral supramammillary nucleus (lSum) increases long-term potentiation (LTP) of hippocampal population spikes. However, to our knowledge, no reports exist concerning the role of the supramammillary area (SuM) in passive avoidance (PA) learning. In the present study, rats were chronically implanted with a cannula aimed at SuM and were trained on a step-through PA task. They received intra-SuM injection of lidocaine or saline at the following intervals: 5 min before training, 5, 90, and 360 min after the acquisition trial, or 5 min before the retrieval test. When lidocaine was injected 5 min before training there was no effect on acquisition of PA but retrieval was significantly poorer than the control group injected with saline. Lidocaine injection 5 min after the acquisition trial impaired PA retention, but reversible inactivation of SuM at 90 and 360 min after training and 5 min before the retrieval test showed no significant effect on PA retention. It can be concluded that SuM contributes to PA consolidation at least 5 min after the acquisition trial and that this effect may be accomplished through SuM projections to the septal and/or hippocampal areas participating in the PA memorization processes.