Anterior hypothalamic parvalbumin neurons are glutamatergic and promote escape behavior

The anterior hypothalamic area (AHA) is a critical structure for defensive responding. Here, we identified a cluster of parvalbumin-expressing neurons in the AHA (AHA^PV) that are glutamatergic with fast-spiking properties and send axonal projections to the dorsal premammillary nucleus (PMD). Using in vivo functional imaging, optogenetics, and behavioral assays, we determined the role of these AHA^PV neurons in regulating behaviors essential for survival. We observed that AHA^PV neuronal activity significantly increases when mice are exposed to a predator, and in a real-time place preference assay, we found that AHA^PV neuron photoactivation is aversive. Moreover, activation of both AHA^PV neurons and the AHA^PV → PMD pathway triggers escape responding during a predator-looming test. Furthermore, escape responding is impaired after AHA^PV neuron ablation, and anxiety-like behavior as measured by the open field and elevated plus maze assays does not seem to be affected by AHA^PV neuron ablation. Finally, whole-brain metabolic mapping using positron emission tomography combined with AHA^PV neuron photoactivation revealed discrete activation of downstream areas involved in arousal, affective, and defensive behaviors including the amygdala and the substantia nigra. Our results indicate that AHA^PV neurons are a functional glutamatergic circuit element mediating defensive behaviors, thus expanding the identity of genetically defined neurons orchestrating fight-or-flight responses. Together, our work will serve as a foundation for understanding neuropsychiatric disorders triggered by escape such as post-traumatic stress disorder (PTSD).