Acute restraint stress and pain modulation depend on the interaction between the periaqueductal gray and the lateral septum

Acute restraint stress causes analgesia in humans and laboratory animals, but the underlying mechanisms are unknown. Recently, we have shown that a multi-nodal circuitry between the dorsal lateral septum (dLS), lateral hypothalamic area (LHA), and rostral ventromedial medulla (RVM) plays an instructive role in restraint stress-induced analgesia (SIA). We found that the LS 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 LS and found that the ventrolateral periaqueductal gray (vlPAG), a known anatomical substrate for escape behaviors, provides inputs to the LS. Through anatomical, behavioral, and in vivo fiber photometry, we show that the PAG and LS neurons are synaptically connected; activation of either PAG or the post-synaptic LS neurons is sufficient to cause analgesia and, when inhibited, cause hyperalgesia. Moreover, we found that the LS neurons that receive inputs from PAG send axonal projections to the LHA. Together, we found that the vlPAG neurons encoding nociceptive and escape behaviors provide synaptic inputs to the dLS-LHA-RVM circuitry to mediate acute restraint stress-induced analgesia in both male and female mice. Significance statement Forced restraint causes stress, and this paradigm has been used often in the laboratory to study the physiological effects of stress, including pain modulation. The lateral septum (LS) has been recently shown to drive restraint-mediated stress and analgesia. However, it is unknown what signals encoded in the LS allow it to instruct stress and pain. A novel neural pathway between the periaqueductal gray (PAG) and LS is activated when mice try to escape the restraint, and persistent activity in the PAG-LS circuitry due to the inescapability of the restraint can cause stress and analgesia.