Effect of daily treadmill running exercise on masseter muscle nociception associated with social defeat stress in mice.

We investigated the effects of social defeat stress (SDS) and treadmill running on masseter muscle nociception, which was quantified by the orofacial formalin test and c-Fos and FosB immunoreactivities in the upper cervical spinal cord (C1/C2) region in male mice. After daily SDS or non-SDS conditioning for 10 days, SDS-conditioned mice were categorized into SDS-susceptible versus resilient mice. Several mice, including non-SDS-conditioned, SDS-susceptible, and resilient mice, were selected to assess masseter muscle nociception on Day 11. SDS conditioning for 10 days increased masseter muscle-evoked nocifensive behaviors and c-Fos and FosB expression in SDS-susceptible compared to non-SDS and resilient mice. The remaining SDS-susceptible and non-SDS mice were subjected to an additional 10 days of SDS plus treadmill running or sedentary sessions before assessing masseter muscle nociception on Day 21. Daily treadmill running sessions reduced enhanced masseter muscle nociception in SDS-susceptible mice but not in non-SDS mice. The preventive effects of daily treadmill running immediately after each SDS conditioning for 10 days on orofacial nocifensive behaviors were assessed on Day 11. Treadmill running conducted immediately after daily SDS inhibited enhanced orofacial nocifensive behaviors. These findings indicate that repeated SDS increases masseter muscle nociception, which could be prevented by daily treadmill running exercise.