As memories age, their processing increasingly relies upon cortical rather than hippocampal circuits, but the adaptive significance and mechanisms of this shift are not fully understood. Here we investigated the behavioral features and cortical mechanisms underlying extinction of remotely versus recently acquired context fear in mice. Behaviorally, extinction and reinstatement were similar, but reextinction of remote fear was significantly faster, suggesting time-dependent engagement of mechanisms specific for processing remote memory. Using pharmacological manipulations ofNMDAreceptors and associated signaling pathways in the in the retrosplenial cortex, we demonstrated that extinction of remote fear uniquely required NR2B-mediated downregulation of the cAMP-dependent protein kinase (PKA)/cAMP response element-binding protein pathway. Interestingly, NR2B/PKA interactions weakened independently of the age of the memory, but the functional significance of this molecular change was evident only asmemoryretrieval became PKA-dependent over time. Thus, corticalPKAsignalingmayprovide a molecular signature of when amemoryhas become "remote," and inhibition of this pathway may open the door for modulation of remote memories.
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