[The role of the periaqueductal gray in behavioral selection under threat]

Fear responses and defensive behaviors are essential for survival, allowing animals to detect threatening stimuli and classify them as aversive. Neural circuits governing fear dynamically select adaptive defensive strategies, such as freezing or flight, depending on threat imminence and escape availability. The lateral/ventrolateral periaqueductal gray (l/vlPAG) plays a central role in these behaviors, and activation of its excitatory neurons elicits diverse fear responses based on subregion, projection target, and stimulation intensity. A subset of l/vlPAG neurons projects to the magnocellular nucleus of the medulla (Mc), yet the contribution of this pathway to intensity-dependent defensive behaviors and negative emotional valence remained unclear. Using in vivo calcium imaging, Mc-projecting l/vlPAG neurons were shown to be strongly activated by aversive stimuli. Optogenetic stimulation at two intensities revealed that strong activation induced flight behavior and place aversion, whereas weak activation evoked freezing. These results indicate that strong activation of the l/vlPAG-Mc pathway drives active defensive behaviors coupled with negative valence, while weak activation mediates passive defensive responses. Overall, these findings suggest that adaptive switching between defensive strategies is achieved through modulation of activation levels within l/vlPAG output pathways, allowing rapid responses to varying threat levels and environmental context. The l/vlPAG functions as a hub integrating diverse inputs and selectively recruiting neuronal subpopulations, and dysfunction in this circuit may contribute to fear- and anxiety-related psychiatric disorders. Understanding the l/vlPAG and its input-output pathways is therefore critical for identifying potential therapeutic targets.