Effects of reversible inactivation of the median raphe nucleous on passive avoidance learning in rats

Abstract

Involvement of median raphe nucleous (MRN) in acquisition, consolidation and retrieval of passive avoidance response (PAR) was investigated with functional suppression of this area by lidocaine (LID). Rats carrying a chronically implanted cannula aimed at the MRN were trained on a stepthrough passive avoidance task and intra median raphe nucleous injection of LID or saline 5 min before acquisition trial or 5, 90 and 360 min after acquisition trial or 5 min before the retrieval test. LID injected 5 and 90 min after the acquisition trial significantly enhanced avoidance of the dark compartment in comparison with the control group injected with saline. PAR was not affected by LID injected 360 min after acquisition or 5 min before acquisition trial. PAR significantly increased in LID injected 5 min before retrieval test group comparison with the control group. Step-through latency of ndive rats was not affected by MRN blockade. The results indicate that the MRN contributes to PAR consolidation at least 90 min after acquisition and involve in PAR retention. It is concluded that functional ablation of the MRN may disrupt inhibitory actions of MRN projections to sub-cortical circuits participating in PAR learning.