Facilitation of fear extinction by novelty is modulated by β-adrenergic and 5-HT serotoninergic receptors in hippocampus.

Extinction is the learned inhibition of retrieval of a previously acquired memory and is a major component of exposure therapy, which has attracted much attention because of the use in the treatment of drug addiction, phobias and particularly fear disorders such as post-traumatic stress disorder (PTSD). Exposure to a novel environment before or after extinction training can enhance the extinction of contextual fear conditioning, however the cellular and molecular substrates are still unclear. Here, we investigated the participation of H2-histaminergic, β-adrenergic and 5-HT-serotonergic receptors of the hippocampus on the enhancement of extinction memory caused by novelty. The infusion into the CA1 region of the serotonin 5-HT-receptor agonist, 8-OH-DPAT and the β-adrenergic blocker, Timolol, after the exposure to the novelty hindered the enhancement of extinction by novelty, while Timolol also hindered the extinction consolidation when infused post-extinction. These impairments were abolished by the coinfusion of 8-OH-DPAT plus the 5-HT receptor antagonist, NAN-190 and Timolol plus β-adrenergic agonist, Isoproterenol. However, Dimaprit and Ranitidine blocked the retrieval of CFC, but did not prevented the extinction learning. Here we elucidated some of the molecular mechanisms that are involved on the enhancement of extinction by novelty, demonstrating that the β-adrenoreceptors and 5-HT serotonergic receptors participate on this process alongside with dopaminergic D1 receptors previously described, while histamine H2 receptors, so ubiquitous in learning-related functions in hippocampus are not involved.