Exploring combat stress exposure effects on burn pain in a female rodent model

In the military, constant physiological and psychological stress encountered by Soldiers can lead to development of the combat and operational stress reaction (COSR), which can effect pain management. Similar effects are seen in other populations subjected to high levels of stress. Using a model of COSR, our lab recently showed that four weeks of stress prior to an injury increases pain sensitivity in male rats. With the roles of women in the military expanding and recent studies indicating sex differences in stress and pain processing, this study sought to investigate how different amounts of prior stress exposure affects thermal injury-induced mechanosensitivity in a female rat model of COSR. Adult female Sprague Dawley rats were exposed to the unpredictable combat stress (UPCS) procedure for either 2 or 4 weeks. The UPCS procedure included exposure to one stressor each day for four days. The stressors include: (1) sound stress for 30 min, (2) restraint stress for 4 h, (3) cold stress for 4 h, and (4) forced swim stress for 15 min. The order of stressors was randomized weekly. Mechanical and thermal sensitivity was tested twice weekly. After the UPCS procedure, a sub-set of rats received a thermal injury while under anesthesia. The development of mechanical allodynia and thermal hyperalgesia was examined for 14 days post-burn. UPCS exposure increased mechanosensitivity after two weeks. Interestingly, with more stress exposure, females seemed to habituate to the stress, causing the stress-induced changes in mechanosensitivity to decrease by week three of UPCS. If thermal injury induction occurred during peak stress-induced mechanosensitivity, after two weeks, this resulted in increased mechanical allodynia in the injured hind paw compared to thermal injury alone. This data indicates a susceptibility to increased nociceptive sensitization when injury is sustained at peak stress reactivity. Additionally, this data indicates a sex difference in the timing of peak stress. Post-mortem examination of the prefrontal cortex (PFC) showed altered expression of p-TrkB in 4-week stressed animals given a thermal injury, suggesting a compensatory mechanism. Future work will examine treatment options for preventing stress-induced pain to maintain the effectiveness and readiness of the Warfighter.