DECIPHERING THE HETEROGENEOUS ANTERIOR HYPOTHALAMIC NUCLEUS GLUTAMATERGIC NEURONS IN MEDIATING FREEZING AND FLIGHT DEFENSIVE BEHAVIORS

Defensive behaviors are innate responses exhibited by organisms upon encountering predators, varying dynamically in accordance with spatiotemporal threat proximity based on threat-imminence continuum theory framework. However, the neural basis underlying how the brain responds to different threat stimuli proximity and subsequently executes context-appropriate defensive strategies, such as flight or freezing remain poorly understood. Here, we demonstrate that glutamatergic neurons in the anterior hypothalamic nucleus (AHN) are involved in both visual looming-induced flight and freezing defensive behaviors. Utilizing neuroanatomical tracing, optogenetic manipulation and fiber photometry recording, we identified two distinct subpopulations of AHN glutamatergic neurons, which exhibit heterogeneity in spatial distribution, structural connectivity, and function, alongside distinct temporal firing patterns during consecutive defensive action spectrum. Specially, the caudal subpopulation projects to the periaqueductal gray matter (PAG), which is activated during risk assessment in the “pre-encounter” phase and mediates freezing responses in low proximity context of the “circa-strike” phase. In contrast, the rostral subpopulation projects to the dorsal premammillary nucleus (PMD), which is activated upon perception of external visual predator stimuli during the “post-encounter” phase and mediates flight responses in high proximity context of the “circa-strike” phase. Notably, optogenetic activation of PAG-projecting and PMD-projecting AHN glutamatergic neurons specifically reverses the stereotypical defensive strategies in looming-induced flight and freezing paradigm, respectively. Overall, our study elucidates the neural circuit basis by which distinct AHN glutamatergic subpopulations respond to varying levels of threat imminence and mediate innate defensive behaviors through two separate circuit pathways.