effects of lidocaine reversible inactivation of the dorsal raphe nucleus on passive avoidance learning in rats

ABSTRACT

Introduction: The role of serotonergic fibers in avoidance learning is controversial. Involvement of the dorsal raphe nucleus [DRN], the main source of hippocampal projecting serotonergic fibers in acquisition, consolidation and retrieval of passive avoidance [PA] learning, was investigated by functional suppression of this area. Materials and Methods: DRN functional inactivation was done by lidocaine [0.5µl, 2%] injection into the DRN, 5 min before training [n=10]; and 5 [n=9], 90 [n=10] and 360 min [n=9] after acquisition trial. In the last experiment, lidocaine was injected into the DRN 5 min before the retrieval test, which was 48 h after the training [n=10]. Results: Our results showed that PA learning was not impaired by DRN inactivation 5 min before training nor 5 and 360 min after training. Lidocaine injected 90 min after the acquisition trial significantly reduced avoidance of the dark compartment [P<0.001]. Intra-DRN injection of lidocaine before retrieval significantly increased PA retention [P<0.001]. Therefore, it seems that DRN has opposite effects on consolidation and retrieval of passive avoidance learning, but it has no effect on PA acquisition. Discussion: It is suggested that functional ablation of DRN may disrupt integrity of subcortical circuits participating in PA consolidation, but DRN inactivation by increasing brain awareness may affect PA retrieval in rats