Functional analysis of periaqueductal gray neurons projecting to the medulla in active and passive defensive behaviors

Fear responses and defensive behaviors are essential for survival. Fear responses are mediated by neural circuits that detect threatening stimuli and classify them as "unpleasant" or "aversive". These circuits coordinate the selection of defensive behaviors, such as freezing or flight, depending on the perceived imminence of the threat. Activation of excitatory neurons in the lateral/ventrolateral periaqueductal gray (l/vlPAG) induces diverse fear responses, depending on the subregions, projection targets, and stimulation intensity. Some l/vlPAG neurons project to the magnocellular nucleus of the medulla (Mc). However, it remains unclear which of the diverse behaviors arising from different intensities of l/vlPAG stimulation are mediated via the l/vlPAG-Mc pathway. Additionally, the role of the l/vlPAG-Mc pathway activation in driving negative or positive valence-related behavior is unknown. To address these questions, we used optogenetics to stimulate Mc-projecting l/vlPAG neurons at two levels of light intensity and analyzed the resulting behavioral changes. Strong stimulation of Mc-projecting l/vlPAG neurons induced flight behavior, whereas weak stimulation elicited a freezing response. Real-time and conditioned place aversion tests indicated that strong stimulation was aversive. These findings suggest that strong stimulation of Mc-projecting l/vlPAG neurons induces active defensive behavior and behavioral aversion, while weak stimulation induces passive defensive behavior.