Exercise-Induced Hypoalgesia Profile in a Rat Neuropathic Pain Model Predicts Pain Severity Following Infraorbital Nerve Injury and Is Associated with Local Cytokine Levels, Systemic Endocannabinoids, and Endogenous Opioids.

Aims: To investigate the role of exercise-induced hypoalgesia (EIH) in the development of neuropathic pain (NP) following infraorbital nerve (ION) injury and to explore possible underlying mechanisms defining the differences between rats with high and low EIH.

Methods: EIH was evaluated by measuring the percentage of withdrawal responses to a series of 30 mechanical stimuli applied to the hind paw before and after 180 seconds of exercise on a rotating rod. The rats were assigned to low- and high-EIH groups based on reduction in the percent of withdrawal responses following exercise. NP was induced in high- and low-EIH rats via ION constriction injury. Rats were tested with graded nylon monofilaments to establish the withdrawal threshold. Increasingly stiff monofilaments were applied to the ION territory until there was a clear withdrawal by the rat. This was repeated a total of three times. A decreased withdrawal threshold indicates allodynia. Testing was performed at baseline and at 3, 10, and 17 days following the injury. On day 17 postinjury, IONs were harvested for the assessment of interleukin (IL)-6, IL-1β, and IL-10 levels. Samples from high-EIH and low-EIH surgically naïve rats served as control for the cytokines study. In this second part of the study, the effects of cannabinoid 1 (CB1) and cannabinoid 2 (CB2) antagonists and naltrexone on EIH profiles and on the withdrawal thresholds to mechanical stimulation were measured. EIH and withdrawal thresholds in high- and low-EIH rats were measured before and after administration of antagonists.

Results: Low-EIH rats developed significantly more pronounced allodynia in the ION territory following injury compared to high-EIH rats. At 17 days postinjury, ION IL-1β levels were higher in low-EIH rats, and IL-10 levels were higher in high-EIH rats. CB1 antagonist blocked the analgesic effect induced by exercise in high- but not in low-EIH rats. The CB2 antagonist had no significant effect on high- or low-EIH rats. Naltrexone blocked the effects of EIH in both high- and low-EIH rats. Exercise induced a significant analgesic effect in high-EIH but not in low-EIH rats. CB1 or CB2 antagonist administration had no effect on pre-exercise responses to mechanical stimulation, while naltrexone administration resulted in significant allodynia in both low- and high-EIH rats.

Conclusion: This study demonstrated substantial differences between rats with high and low EIH. The results suggest that following ION injury, high-EIH rats may have a more prominent or activated endocannabinoids system and that their inflammatory response is moderated, with higher levels of IL-10 and lower levels of IL-1β.