Though much attention has been given to the neural structures that underlie the long-term consolidation of contextual memories, little is known about the mechanisms responsible for the maintenance of memory precision. Here, we demonstrate a rapid time-dependent decline in memory precision in GABA B(1a) receptor knockout mice. First, we show that GABA B(1a) receptors are required for the maintenance, but not encoding, of a precise fear memory. We then demonstrate that GABA B(1a) receptors are required for the maintenance, but not encoding, of spatial memories. Our findings suggest that GABA-mediated presynaptic inhibition regulates the maintenance of memory precision as a function of memory age.Individuals suffering from post-traumatic stress disorder (PTSD) often come to re-experience the traumatic event in the presence of stimuli that were not present at the time of the trauma but bear some similarity to that event. Furthermore, this generalization of fear to neutral cues increases over time, so that a multiplicity of stimuli serves as a reminder for the original trauma (Riccio et al. 1984;Zhou and Riccio 1996;Wiltgen and Silva 2007;Lissek et al. 2008;Jasnow et al. 2012). In short, the fear memory becomes imprecise and is reactivated by cues not present at the time of learning. Using contextual fear conditioning and novel object training in rodents, the current study directly investigated the precision of both a fear and a nonfear memory in order to gain an understanding of how memory precision develops and is maintained over time.Contextual fear conditioning involves pairing a novel context (conditioned stimulus) with footshock (unconditioned stimulus) that serves to condition fear (assessed as freezing behavior) to that context. Many studies have demonstrated that shifting contextual cues shortly after contextual fear conditioning results in reduced freezing (i.e., the context shift effect). At early time points, rodents exhibit a contextually precise memory and can discriminate between the training and a neutral context. However, as the retention interval between training and testing increases, fear memories become less precise with animals exhibiting fear responses to neutral contexts (context specificity is lost 14-to 36-d post-training) (Zhou and Riccio 1996;Wiltgen and Silva 2007;Lissek et al. 2008;Jasnow et al. 2012). Though a significant amount of neurobehavioral research has investigated the mechanisms underlying the long-term consolidation of a contextual fear memory, very little is known about what underlies this loss in memory precision, or how memory precision is maintained over time.Recent evidence suggests that GABA B receptor-mediated presynaptic inhibition may play a role in stimulus discrimination. GABA B receptors are G protein-coupled receptors that exist as heterodimers containing two subunits, GABA B1 and GABA B2 (Gassmann and Bettler 2012). The GABA B1 receptor exists in two isoforms, GABA B(1a) and GABA B(1b) , with the isoforms being localized to presynaptic and postsynaptic termi...

A common symptom of anxiety disorders is the overgeneralization of fear across a broad range of contextual cues. We previously found that the ACC and ventral hippocampus (vHPC) regulate generalized fear. Here, we investigate the functional projections from the ACC and vHPC to the amygdala and their role in governing generalized fear in a preclinical rodent model. A chemogenetic approach (designer receptor exclusively activated by designer drugs) was used to inhibit glutamatergic projections from the ACC or vHPC that terminate within the BLA at recent (1 d) or remote (28 d) time points after contextually fear conditioning male mice. Inactivating ACC or vHPC projections to the BLA significantly reduced generalized fear to a novel, nonthreatening context but had no effect on fear to the training context. Further, our data indicate that the ACC-BLA circuit supports generalization in a time-independent manner. We also identified, for the first time, a strictly time-dependent role of the vHPC-BLA circuit in supporting remote generalized contextual fear. Dysfunctional signaling to the amygdala from the ACC or the HPC could underlie overgeneralized fear responses that are associated with anxiety disorders. Our findings demonstrate that the ACC and vHPC regulate fear expressed in novel, nonthreatening environments via projections to the BLA but do so as a result of training intensity or time, respectively.

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Significance Statement 47Anxiety disorders are characterized by a common symptom that promotes 48 overgeneralization of fear in non-threatening environments. Dysregulation of the 49 amygdala, anterior cingulate cortex (ACC), or hippocampus has been hypothesized to 50 contribute to increased fear associated with anxiety disorders. Our findings show that 51 the ACC and HPC projections to the basolateral amygdala regulate generalized fear in 52 non-threatening, environments. However, descending ACC projections control fear 53 generalization independent of time, whereas HPC projections play a strictly time-54 dependent role in regulating generalized fear. Thus, dysfunctional ACC/HPC signaling 55 to the BLA may be a predominant underlying mechanism of non-specific fear associated 56 with anxiety disorders. Our data have important implications for predictions made by 57 theories about aging memories and interactions between the hippocampus and cortical 58 regions. 59 Surgical Procedures 126Mice were anesthetized with a subcutaneous injection of a Ketamine (75 mg/kg) 127 + Xylazine (10 mg/kg) + Acepromazine (2 mg/kg) cocktail. Following administration of 128 anesthesia, mice were mounted on a stereotaxic apparatus (David Kopf Instruments, 129 Tujunga, CA). The scalp of each mouse was retracted; the skull was adjusted so that 130 bregma and lambda were on the same horizontal plane (within .05mm of each other). 131Two 0.33 gauge infusion needles were guided to the appropriate coordinates relative to 132 bregma and small bilateral burr holes were drilled. Coordinates for the following brain 133 regions were ACC: .08 mm AP, ±.07 mm ML, -3.6 mm DV from bregma at a 14° angle; 134 vHPC: -3.2 mm AP, ±3.3 mm ML, -4.25 mm DV from bregma. AAV8-CaMKIIa-135 hM4D(Gi)-mCherry virus (hM4D) (Addgene) or a control virus under the same promoter, 136 AAV8-CamKIIa-EGFP (EGFP) (Addgene) was bilaterally infused at 0.1µL/minute to a