The Effects of Lidocaine Reversible Inactivation of the Dorsal Raphe Nucleus on Passive Avoidance Learning in Rats

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. 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). 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. 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.