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1- Department of Biology, Islamic Azad University, Damghan Branch, Damghan, Iran.
2- Student Research Committee, Faculty of Medicine, Semnan University of Medical Sciences, Semnan, Iran.
3- Laboratory of Learning and Memory, Research Center of Physiology, Semnan University of Medical Sciences, Semnan, Iran.
Purpose of the study: Post-training administration of glucocorticoids enhance memory consolidation of inhibitory avoidance learning. Given the involvement of 5-HT6 receptors in memory processing and that glucocorticoids interact with brain serotonergic system in regulating memory processing, we examined whether glucocorticoid influences on the consolidation of emotional memory depend on hippocampal 5-HT6 receptors.
Methods: Rats were trained in an inhibitory avoidance task and immediately received systemic injections of corticosterone (CORT) and intra-hippocampal injections of 5-HT receptors agonist or antagonist. 48 hours after training, the memory retention test was done and immediately after the behavioral test the animals were sacrificed and the hippocampi (left and right) rapidly dissected out for molecular studies.
Results: Post-training injections of different doses of CORT (1.25, 2.5. 5, and 10 mg/kg) enhanced memory retention in a dose-dependent manner. The CORT-induced enhancement of memory consolidation was blocked by bilateral intra-hippocampal injections of 5-HT6 receptor antagonist SB271046 (5 or 10 ng/per side), but not agonist EMD386088 (5 or 10 ng/per side). Furthermore, systemic CORT reduced 5-HT6 receptor mRNA and protein expression in the hippocampus. Both doses of 5-HT6 receptor agonist and antagonist significantly enhanced and reduced the expression of 5-HT6 receptor, respectively, and both ligands at the higher dose (10 ng) enhanced memory consolidation.  Moreover, corticosterone injection attenuated and enhanced, respectively, the effects of agonist and antagonist on 5-HT6 receptor expression.
Conclusions: These behavioral and molecular findings indicated an interaction between glucocorticoids and hippocampal 5-HT6 receptors in the consolidation of emotionally arousing experiences.
Type of Study: Original | Subject: Behavioral Neuroscience
Received: 2018/08/17 | Accepted: 2018/08/20 | Published: 2018/03/15

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