It has been reported that stress-related activation of the noradrenergic system strengthens the formation of aversive memories and that beta-adrenergic receptors seem to be involved in this emotional memory processing. In this study, the effects of beta-adrenergic compounds on the extinction of contextual conditioned fear responses were evaluated. Rats were trained with footshock in a conditioning box. In the 3 days following the training, the animals were re-exposed to the apparatus and received either a single or repeated intraperitoneal injections of the beta-adrenergic antagonist propranolol, the beta-adrenergic agonist isoproterenol, or saline 30 min before (acquisition of extinction) or immediately after (consolidation of extinction) the extinction sessions. A drug-free session was performed on the last day. While repeated isoproterenol treatment facilitated the consolidation of contextual fear extinction, repeated propranolol administration impaired the acquisition and the consolidation of this process. Further, the role of ventromedial prefrontal cortex (vmPFC) in the extinction of contextual conditioned fear was tested with an immunohistochemistry assay. Our results show a reduction in Fos-protein expression between the first and the last extinction session. In a follow-up experiment, intra-vmPFC microinjection of isoproterenol before the first extinction session facilitated the extinction of contextual fear. This facilitation was antagonized by pre-treatment with atenolol, suggesting that this change is mediated by beta-1-adrenergic activity. Our results reinforce the role of the vmPFC in fear extinction mechanisms, suggesting that vmPFC-beta-1-adrenergic receptor activation underlies part of the facilitation of the fear extinction processes.
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