Prefrontal long-range somatostatin inhibitory projections modulate fear expression.

Memory extends beyond the simple encoding of learned associations; it also requires the flexible and context-dependent expression of associative memories to appropriately guide behavior. Auditory fear conditioning provides a powerful model to study how neutral cues become threat predictors, relying on coordinated activity across distributed cortical and subcortical circuits. Among these circuits, the medial prefrontal cortex (mPFC) and the periaqueductal gray (PAG) are recognized as key structures involved in both the formation and the expression of fear memories. However, the mechanisms through which these two regions interact to regulate defensive memory expression remain largely unknown. Here, we identify a previously uncharacterized long-range inhibitory projection from somatostatin-expressing (SST) neurons in the mPFC to the ventrolateral PAG (vlPAG) that regulates the temporal maintenance of freezing during fear expression. Through a combination of anatomical tracing and slice electrophysiology, we demonstrate that these SST neurons form direct, monosynaptic GABAergic connections onto vlPAG neurons. Furthermore, optogenetic inhibition of mPFC SST terminals during conditioned stimulus (CS+) presentations markedly shortened the duration of freezing episodes. Together, these results extend the classical view of prefrontal-midbrain communication by revealing that inhibitory mPFC → PAG projections play a pivotal role in shaping the temporal dynamics of fear expression. Within the broader framework of SST neurons plasticity, our findings align with the role of mPFC long-range inhibitory SST projections exerting fine control over cue-specific freezing behavior and the flexible expression of fear-related memories.