Acute Restraint Stress and Pain Modulation Depend on the Interaction between the Periaqueductal Gray and the Lateral Septum.

Acute restraint stress (RS) causes analgesia in humans and laboratory animals, but the underlying mechanisms are unknown. Recently, we have shown that a multinodal circuitry between the dorsal lateral septum (dLS), lateral hypothalamic area (LHA), and rostral ventromedial medulla (RVM) plays an instructive role in RS-induced analgesia (SIA). We found that the dLS neurons are activated when mice struggle to escape the restraint, and we wondered about the origin of the escape signals. Hence, we performed retrograde viral labeling from the dLS and found that the ventrolateral periaqueductal gray (vlPAG), a known anatomical substrate for escape behaviors, provides inputs to the dLS. Through anatomical, behavioral, and in vivo fiber photometry, we show that the PAG and dLS neurons are synaptically connected; activation of either PAG or the postsynaptic dLS neurons is sufficient to cause analgesia and, when inhibited, cause hyperalgesia. Moreover, we found that the dLS neurons that receive inputs from PAG send axonal projections to the LHA. Together, our data indicate that the vlPAG neurons encoding nociceptive and escape behaviors provide synaptic inputs to the dLS-LHA-RVM circuitry to mediate acute restraint SIA in both male and female mice.