-EIH profile in rats, can predict pain severity following nerve injury.
-EIH profile is associated with mechanical and thermal sensitivity in naïve rats.
-Low EIH is associated with contralateral effect following unilateral nerve injury.
The aim of this study was to investigate the predictive value of exercise-induced hypoalgesia (EIH) profile on pain intensity induced by nerve injury in a rat model. EIH was tested by evaluating the percentage of withdrawal responses to a train of 30 mechanical stimuli on the hind-paw before and after 180 seconds of exercise on a rotating rod. The rats were grouped into Low, Medium and High EIH based on their reduction in the percentage of withdrawal responses before and after exercise. Rats from each group then underwent left sciatic nerve constriction injury. Mechanical-allodynia, mechanical-hyperalgesia and heat-allodynia were assessed in the affected and contralateral hind-paw prior to, 3 and 7 days following the procedure.
The Low EIH rats demonstrated increased hypersensitivity at baseline and developed significantly more severe heat-allodynia, mechanical allodynia, and hyperalgesia 3 and 7 days following the injury compared to the Medium and High EIH rats. Moreover, the Low EIH rats developed contralateral heat allodynia following the injury.
A comparison between habituated and non-habituated rats’ EIH was made to study the role of stress on the hypoalgesic effect. No significant differences were found between the habituated and non-habituated rats at baseline, 1 and 5 minutes after the exercise.
EIH profile was found to be predictive of pain severity following nerve injury. It may suggest that selected patients with faulty pain modulation are at risk for developing chronic pain following injury or surgical procedures. EIH may represent a pre-operative means to detect this predisposition, and enable proactive management.