POPULATION CHARACTERISTICS AND FUNCTIONAL MECHANISMS OF NEURONAL ENSEMBLES IN THE PREFRONTAL CORTEX INVOLVED IN THE RETRIEVAL OF FEAR EXTINCTION MEMORY

Fear extinction is a new learning process stored in the ventromedial prefrontal cortex (vmPFC), yet how vmPFC populations retrieve extinction memories and suppress fear remains unclear. This study aimed to define vmPFC population dynamics during extinction retrieval and identify the neural mechanisms terminating conditioned freezing.We combined in vivo fiber photometry, dual-site local field potential (LFP) recordings, single-cell microscopic imaging, activity-dependent engram labeling, optogenetics, genetic manipulation, and immunohistochemistry in mice during auditory-cue fear extinction retrieval.We found freezing behavior was dynamically associated with theta rhythms: ~4 Hz oscillations accompanied freezing initiation, whereas ~8 Hz oscillations marked freezing termination. Dual LFP recordings revealed that 4 Hz activity originated in the basolateral amygdala (BLA), while 8 Hz rhythms were generated in the vmPFC and depended on BLA input. Extinction engram cells in vmPFC were not activated by the cue itself but showed rapid activity increases immediately before freezing cessation. Silencing these engram cells reduced vmPFC 8 Hz oscillations and increased freezing. Single-cell imaging identified two antagonistic vmPFC populations synchronized to either 4 Hz or 8 Hz rhythms. The 8 Hz–related neurons predicted freezing termination and overlapped strongly with extinction engram cells. Mimicking vmPFC 8 Hz activity via optogenetic stimulation enhanced extinction retrieval and reduced freezing.Fear extinction retrieval is driven by vmPFC-generated 8 Hz oscillations that antagonize BLA-derived 4 Hz rhythms to terminate fear behavior. Extinction engram cells are key drivers of this process. These findings reveal a population-level mechanism for suppressing conditioned fear and provide circuit-level insight relevant to treating anxiety and PTSD.