Role of dorsal and ventral hippocampal muscarinic receptor activity in acquisition and retention of contextual fear conditioning.

The current study further examined the effect of the muscarinic acetylcholine antagonist, scopolamine, on the Context Preexposure Facilitation Effect (CPFE; Robinson-Drummer, Dokovna, Heroux, & Stanton, 2016). In the CPFE, context representations formed during the preexposure phase are retrieved and associated with immediate shock during the training phase and expressed as freezing during a 24-hr retention phase. Scopolamine abolished postshock and retention freezing when administered systemically prior to preexposure (Experiment 1A) or immediate-shock training (Experiment 1B). Pretraining infusion of scopolamine into dorsal hippocampus (dHPC) disrupted both postshock and retention freezing (Experiments 2A) and retention freezing when the postshock freezing test was omitted (Experiment 2B) but did not alter expression of freezing behavior to an auditory fear stimulus (Experiment 2C). Finally, pretraining scopolamine infusion into ventral hippocampus (vHPC) also abolished postshock and retention test freezing (Experiment 3). These findings suggest similar roles for muscarinic receptor activity in both the dHPC and vHPC in the CPFE. This study advances understanding of the neurobiology of the CPFE by showing that context-shock associations are not learned following disruption of the cholinergic and/or hippocampal function on either the preexposure or training day. Existing theories of the CPFE (Rudy, 2009) have inferred this effect based on impaired 24-hr retention observed in previous studies (Matus-Amat, Higgins, Barrientos, & Rudy, 2004; Robinson-Drummer et al., 2016). However, the present study is the first to demonstrate it directly by including a postshock freezing measure. Further, this study is the first to identify vHPC as another important region necessary for context-shock learning during the CPFE paradigm. (PsycInfo Database Record (c) 2020 APA, all rights reserved).