Prefrontal NMDA-receptor antagonism disrupts encoding or consolidation but not retrieval of incidental context learning.

The Context Preexposure Facilitation Effect (CPFE) is a variant of contextual fear conditioning in which learning about the context, acquiring a context-shock association, and retrieval of this association occur separately across three phases (context preexposure, immediate-shock training, and retention). We have shown that prefrontal inactivation or muscarinic-receptor antagonism prior to any phase disrupts retention test freezing during the CPFE in adolescent rats (Heroux et al., 2017; Robinson-Drummer et al., 2017). Furthermore, the medial prefrontal cortex (mPFC) is the only region in which robust learning-related expression of the immediate early genes c-Fos, Arc, Egr-1 and Npas4 is observed during immediate-shock training in the CPFE (Asok et al., 2013; Heroux et al., 2018; Schreiber et al., 2014). However, the role of prefrontal NMDA-receptor plasticity in supporting preexposure- and training-day processes of the CPFE is not known. Therefore, the current study examined the effects of intra-mPFC infusion of the NMDA-receptor antagonist MK-801 or saline vehicle prior to context preexposure (Experiment 1) or immediate-shock training (Experiment 2) in adolescent Long-Evans male and female rats. This infusion given prior to context preexposure but not training abolished retention test freezing, with no difference between MK-801-infused rats and non-associative controls preexposed to an alternative context (pooled across drug). These results demonstrate a role of prefrontal NMDA-receptor plasticity in the acquisition and/or consolidation of incidental context learning (i.e., encoded in the absence of reinforcement). In contrast, this plasticity is not required for context retrieval, or acquisition, expression, or consolidation of a context-shock association during immediate-shock training in the CPFE. These experiments add to a growing body of work implicating the mPFC in Pavlovian contextual fear conditioning processes in rodents.

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).

Medial prefrontal and ventral hippocampal contributions to incidental context learning and memory in adolescent rats.

The Context Preexposure Facilitation Effect (CPFE) is a contextual fear conditioning (CFC) paradigm in which context learning, context-shock learning, and retrieval of contextual fear occur in three distinct phases. The medial prefrontal cortex (mPFC), dorsal hippocampus (dHPC), and ventral hippocampus (vHPC) are required for the acquisition and/or consolidation of a context representation during incidental context exposure (Heroux et al., 2017; Robinson-Drummer et al., 2016; Rudy & Matus-Amat, 2006). This exposure also induces the expression of the immediate early genes (IEGs) c-Fos, Arc, Egr-1, and Npas4 in these regions (Heroux et al., 2018, 2019). Despite these studies, it is still unclear how mPFC and vHPC contribute to incidental context learning and memory. The current study examined whether prefrontal or ventral hippocampal inactivation during context preexposure interferes with long-term context memory and IEG activity in the mPFC, vHPC, dHPC and the ventral midline thalamus (VMT, a region connected to both the mPFC and HPC). Adolescent Long-Evans rats were given intra-mPFC (Experiment 1) or intra-vHPC (Experiment 2) infusions of the GABAA receptor agonist muscimol or PBS prior to context preexposure, and then were sacrificed 30 min later and whole mPFC, dHPC, vHPC, and VMT were collected and assayed for IEG mRNA expression via qPCR. Prefrontal or ventral hippocampal inactivation during context exposure abolished subsequent post-shock and retention test freezing in behaviorally-tested littermates of the sacrificed groups. In Experiment 1, prefrontal inactivation reduced expression of c-Fos, Arc, Egr-1, and Npas4 in the mPFC, c-Fos, Arc, and Npas4 in the vHPC, and c-Fos in the VMT, to the level of behaviorally-naïve home-cage controls. Prefrontal inactivation did not alter IEG expression in the dHPC during context exposure. In Experiment 2, ventral hippocampal inactivation impaired expression of all IEGs in the mPFC, dHPC, and vHPC, with no effect in the VMT. Taken together, these results suggest that context memory processes on the preexposure day of the CPFE may depend on mPFC-vHPC circuitry not typically emphasized in studies of incidental or configural learning and memory.

Cholinergic rescue of neurocognitive insult following third-trimester equivalent alcohol exposure in rats.

Neonatal ethanol exposure during the third trimester equivalent of human pregnancy in the rat significantly impairs hippocampal and prefrontal neurobehavioral functioning. Postnatal day [PD] 4-9 ethanol exposure in rats disrupts long-term context memory formation, resulting in abolished post-shock and retention test freezing in a variant of contextual fear conditioning called the Context Preexposure Facilitation Effect (CPFE). This behavioral impairment is accompanied by disrupted medial prefrontal, but not dorsal hippocampal expression of the immediate early genes (IEGs) c-Fos, Arc, Egr-1, and Npas4 (Heroux, Robinson-Drummer, Kawan, Rosen, & Stanton, 2019). The current experiment examined if systemic administration of the acetylcholinesterase inhibitor physostigmine (PHY) prior to context learning would rescue prefrontal IEG expression and freezing in the CPFE. From PD4-9, Long-Evans rats received oral intubation of ethanol (EtOH; 5.25 g/kg/day) or sham-intubation (SI). Rats received a systemic injection of saline (SAL) or PHY (0.01 mg/kg) prior to all three phases (Experiment 1) or just context exposure (Experiment 2) in the CPFE from PD31-33. A subset of rats were sacrificed 30 min after context learning to assay changes in IEG expression in the medial prefrontal cortex (mPFC), dorsal hippocampus (dHPC), and ventral hippocampus (vHPC). Administration of PHY prior to all three phases or just context learning rescued both post-shock and retention test freezing in the CPFE in EtOH rats without altering performance in SI rats. EtOH-SAL rats had significantly reduced mPFC but not dHPC expression of c-Fos, Arc, Egr-1, and Npas4. EtOH-PHY treatment rescued mPFC expression of c-Fos in ethanol-exposed rats and increased Arc and Npas4 regardless of dosing condition. While there was no effect of PHY on dHPC or vHPC expression of Arc, Egr-1, or Npas4, this treatment significantly boosted hippocampal expression of c-Fos regardless of ethanol treatment. These findings implicate impaired cholinergic and prefrontal function in cognitive deficits arising from 3rd-trimester equivalent alcohol exposure.