Dynamic tripartite construct of interregional engram circuits underlies forgetting of extinction memory.

Fear extinction allows for adaptive control of learned fear responses but often fails, resulting in a renewal or spontaneous recovery of the extinguished fear, i.e., forgetting of the extinction memory readily occurs. Using an activity-dependent neuronal labeling strategy, we demonstrate that engram neurons for fear extinction memory are dynamically positioned in the medial prefrontal cortex (mPFC), basolateral amygdala (BLA), and ventral hippocampus (vHPC), which constitute an engram construct in the term of directional engram synaptic connectivity from the BLA or vHPC to mPFC, but not that in the opposite direction, for retrieval of extinction memory. Fear renewal or spontaneous recovery switches the extinction engram construct from an accessible to inaccessible state, whereas additional extinction learning or optogenetic induction of long-term potentiation restores the directional engram connectivity and prevents the return of fear. Thus, the plasticity of engram construct underlies forgetting of extinction memory.

Insular cortical circuits as an executive gateway to decipher threat or extinction memory via distinct subcortical pathways.

Threat and extinction memories are crucial for organisms' survival in changing environments. These memories are believed to be encoded by separate ensembles of neurons in the brain, but their whereabouts remain elusive. Using an auditory fear-conditioning and extinction paradigm in male mice, here we discovered that two distinct projection neuron subpopulations in physical proximity within the insular cortex (IC), targeting the central amygdala (CeA) and nucleus accumbens (NAc), respectively, to encode fear and extinction memories. Reciprocal intracortical inhibition of these two IC subpopulations gates the emergence of either fear or extinction memory. Using rabies-virus-assisted tracing, we found IC-NAc projection neurons to be preferentially innervated by intercortical inputs from the orbitofrontal cortex (OFC), specifically enhancing extinction to override fear memory. These results demonstrate that IC serves as an operation node harboring distinct projection neurons that decipher fear or extinction memory under the top-down executive control from OFC.