Reversible inactivation of the median raphe nucleus enhances consolidationand retrieval but not acquisition of passive avoidance learning in rats

Involvement of median raphe nucleus (MRN) in acquisition, consolidation and retrieval of passive avoidance (PA) was investigated with functional suppr ession of this area by lidocaine. Rats carrying a chronically implanted can nula aimed at the MRN were trained on a step-through passive avoidance task and received intra-MRN injection of lidocaine or saline 5 min before train ing or 5, 90 and 360 min after acquisition trial or 5 min before the retrie val test. Lidocaine MRN inactivation had no effect on PA learning. Lidocain e injected 5 and 90 min after the acquisition trial significantly enhanced avoidance of the dark compartment in comparison with the control group inje cted with saline. But PA retention was not affected by lidocaine injected 3 60 min after acquisition or 5 min before training. Retention latency signif icantly increased, when lidocaine injected 5 min before retrieval test. Ste p-through latency of naive rats was not affected by MRN blockade, Furthermo re, reversible inactivation of MRN did not have a significant effect on loc omotor activity. Our results indicate that the MRN contributes to PA consol idation at least until 90 min after acquisition and involves in PA retrieva l. It is concluded that functional ablation of the MRN may disrupt the inhi bitory actions of MRN projections to sub-cortical circuits participating in PA memorization and retrieval, (C) 1999 Elsevier Science B.V. All rights r eserved.