Daily rhythms in fear extinction memory reflect structural and functional changes in a prefrontal-amygdala circuit.

Circadian rhythms exert widespread influences on learning and memory processes across diverse brain regions. The extinction of emotional memories is enhanced when training and testing occur during the circadian active phase in both rodents and humans, yet the underlying neural mechanisms remain poorly understood. Here, we examined time-of-day differences in conditioned fear extinction memory and associated neural activity within the ventromedial prefrontal cortex (vmPFC), including within a vmPFC-to-basolateral/basomedial amygdala (BLA/BMA) pathway in rats. Using an intersectional viral strategy to label vmPFC to BLA/BMA projection neurons, we found that female rats trained and tested during their active phase exhibited superior extinction recall compared to rats trained and tested during their inactive phase. The superior extinction recall was associated with greater FOS expression throughout the vmPFC, including within vmPFC to BLA/BMA projection neurons. In a follow-up experiment, we examined dendritic spine morphology in untrained female and male rats. We observed greater dendritic spine density in vmPFC to BLA/BMA neurons during the active phase compared to inactive phase, with this difference driven primarily by increased thin and stubby spine subtypes. These findings demonstrate that superior active phase extinction memory is associated with enhanced vmPFC circuit activity and corresponds to time-of-day differences in dendritic spine density within extinction-relevant neural pathways. Our results reveal a potential structural plasticity mechanism by which circadian rhythms modulate fear extinction learning and memory.