Context memory formation requires activity-dependent protein degradation in the hippocampus.

Numerous studies have indicated that the consolidation of contextual fear memories supported by an aversive outcome like footshock requires de novo protein synthesis as well as protein degradation mediated by the ubiquitin-proteasome system (UPS). Context memory formed in the absence of an aversive stimulus by simple exposure to a novel environment requires de novo protein synthesis in both the dorsal (dHPC) and ventral (vHPC) hippocampus. However, the role of UPS-mediated protein degradation in the consolidation of context memory in the absence of a strong aversive stimulus has not been investigated. In the present study, we used the context preexposure facilitation effect (CPFE) procedure, which allows for the dissociation of context learning from context-shock learning, to investigate the role of activity-dependent protein degradation in the dHPC and vHPC during the formation of a context memory. We report that blocking protein degradation with the proteasome inhibitor clasto-lactacystin ß-lactone (ßLac) or blocking protein synthesis with anisomycin (ANI) immediately after context preexposure significantly impaired context memory formation. Additionally, we examined 20S proteasome activity at different time points following context exposure and saw that the activity of proteasomes in the dHPC increases immediately after stimulus exposure while the vHPC exhibits a biphasic pattern of proteolytic activity. Taken together, these data suggest that the requirement of increased proteolysis during memory consolidation is not driven by processes triggered by the strong aversive outcome (i.e., shock) normally used to support fear conditioning.

Dorsal hippocampus infusions of CNQX into the dentate gyrus disrupt expression of trace fear conditioning.

The hippocampus is essential for the consolidation of some explicit long-term memories, including trace conditioning. Lesions and pharmacological manipulations of the dorsal hippocampus (DH) have provided strong evidence for its involvement in the acquisition and expression of trace fear memories. However, no studies have specifically targeted DH subregions [CA1 and dentate gyrus (DG)] to determine their involvement in trace fear conditioning. In the present study, rats received bilateral cannulation targeting either the DG or CA1 of the DH. Following surgery, animals were trace fear conditioned. Forty-eight hours following training, rats received bilateral infusions of the AMPA/kainate glutamate receptor antagonist, CNQX, or vehicle. Following the infusion, rats were placed in a novel context for the tone test. Rats that received CNQX into the DG froze significantly less during the tone and trace interval as compared to controls. Rats that received CNQX into the DH CA1 showed no difference in freezing during the tone or trace interval as compared to controls. These data support a role for the DG in the expression of trace tone fear conditioning.