Voluntary running-induced activation of ventral hippocampal GABAergic interneurons contributes to exercise-induced hypoalgesia in neuropathic pain model mice

The exact mechanism of exercise-induced hypoalgesia (EIH) in exercise therapy to improve chronic pain has not been fully clarified. Recent studies have suggested the importance of the ventral hippocampus (vHPC) in inducing chronic pain. We investigated the effects of voluntary running (VR) on FosB(+) cells and GABAergic interneurons (parvalbumin-positive [PV(+)] and somatostatin-positive [SOM(+)]) in the vHPC-CA1 in neuropathic pain (NPP) model mice. VR significantly improved thermal hyperalgesia in the NPP model. The number of the FosB(+) cells was significantly higher in partial sciatic nerve ligation-sedentary mice than in Sham and Naive mice, whereas VR significantly suppressed the FosB(+) cells in the vHPC-CA1. Furthermore, VR significantly increased the proportion of activated PV(+) and SOM(+) interneurons in the vHPC-CA1, and tracer experiments indicated that approximately 24% of neurons projecting from the vHPC-CA1 to the basolateral nucleus of amygdala were activated in NPP mice. These results indicate that feedforward suppression of the activated neurons via VR-induced activation of GABAergic interneurons in the vHPC-CA1 may be a mechanism to produce EIH effects, and suggested that disappearance of negative emotions such as fear and anxiety by VR may play a critical role in improving chronic pain.