Glutamatergic and gabaergic neuronal populations in the dorsal Periacqueductual Gray have different functional roles in aversive conditioning.

The dorsal periacqueductal gray (dPAG) is a midbrain structure having an essential role in coordinating defensive behaviors in response to aversive stimulation. However, the question of whether dPAG neurons can respond to aversive conditioning and retrieval, properties involved in emergence of negative emotional state, is still under debate. Here we used calcium imaging by fiber photometry to record the activity of dPAGVGluT2+ and dPAGGAD2+ neuronal populations during unconditioned and conditioned aversive stimulation. Then, following an unconditioned stimulation we performed a retrieval experiment to quantify memory-like responses of dPAG neurons. This shown that whilst both dPAGVGluT2+ and dPAGGAD2+ neuronal populations respond to direct US stimulation, and to CS stimulation during conditioning, only the dPAGVGluT2+ population persisted in responding to the CS stimulation during retrieval. Finally to better understand these divergences in dPAGVGluT2+ and dPAGGAD2+ responses, we investigated their respective connectivity patterns by performing a cell specific monosynaptic retrograde rabies virus tracing experiment. This revealed that different patterns of fibers projects to dPAGVGluT2+ and dPAGGAD2+, which could explain part of their response specificities. This may indicate that glutamatergic subpopulation is a main contributor of aversive memories in dPAG.