Effects of inactivation of serotonergic neurons of the median raphe nucleus on learning and performance of contextual fear conditioning

Author(s)

Borelli‚ K.G. ; Gárgaro‚ A.C. ; Santos‚ J.M. ; Brandão‚ M.L.

• Document type: Journal article
• Source: Neuroscience Letters; v. 387, n. 2, p. 105-110, 2005.

Abstract

Several studies have shown that the median raphe nucleus (MRN) is involved in anxiety. However, no study assessed the role of 5-HT mechanisms of MRN in both freezing-and fear-potentiated startle (FPS) within a single form of conditioned learning. In this work we examined the effects of neurotoxic lesions of the MRN with NMDA on freezing and FPS of rats submitted to a contextual fear conditioning paradigm, in which they were tested in the same chamber where they received foot-shocks 24 h before. Compared to controls NMDA-injected rats showed a reduction of freezing and FPS in response to contextual cues. Next, we examined the effects of stimulation of 5-HT1A somatodendritic autoreceptors of the MRN with local injections of 8-OH-DPAT either before training or testing sessions conducted 2 or 24 h post-conditioning. Pre-training injections of 8-OH-DPAT intra-MRN reduced both freezing and FPS whereas post-training injections reduced only freezing to the aversive context without changing the FPS. Thus, freezing is easily disrupted by post-training MRN injections of 8-OH-DPAT while memory for FPS remained unchanged. It is proposed that the consolidation of contextual conditioned fear promoting freezing takes place through a slow mechanism of transference of information through 5-HT mechanisms of the MRN-hippocampus pathway. On the other hand, a rapid fear conditioning process operates for FPS, probably through other pathways. (C) 2005 Elsevier Ireland Ltd. All rights reserved. (AU)

• FAPESP's process: 02/03705-0 - Neurobiology of fear and stress
• Grantee: Marcus Lira Brandão
• Support Opportunities: Research Projects - Thematic Grants