Sex differences in pain-related behaviour and the endocannabinoid system following hind limb ischemia-reperfusion injury

Ischemia-reperfusion (I/R) injury causes tissue damage in organ transplantation, stroke and myocardial infarction, and underlies chronic wound formation. The endocannabinoid system (ECS) is involved in pain modulation, maintenance of skin homeostasis, and ischemic preconditioning, an endogenous protective mechanism against I/R-mediated damage. The rodent hind limb I/R model is a model of chronic post-ischemia pain, complex regional pain syndrome-type 1 (CRPS-1), and a potential model of wound-related pain. Research into sex differences in pain-related behaviour and the ECS following hind limb I/R injury is limited. Herein, we aimed to characterise the pain-related behavioural phenotype and ECS in male and female Sprague-Dawley rats following I/R injury. Mechanical, cold and heat hypersensitivity were quantified in both hind paws at baseline and up to 29 days post-unilateral hind limb I/R injury (or sham control). Endocannabinoid and N-acylethanolamine levels, and expression of ECS-related genes, were quantified in pain-related brain regions and spinal cord 30 days post-I/R injury. Persistent mechanical and transient cold hypersensitivity were observed in male rats following I/R, but were not robustly present in females. In female rats, I/R induced significantly higher levels of N-palmitoylethanolamide in the thalamus, compared with sham and male I/R counterparts, and significantly lower levels of 2-AG in the contralateral amygdala, compared with sham counterparts. There were no I/R-induced alterations in expression of ECS genes at 30 days post-IR. These results indicate sex differences in pain-related behaviour and in the ECS following I/R injury, and provide a solid foundation for future studies targeting the endocannabinoid system for therapeutic benefit. PERSPECTIVE: This article presents an investigation into sex differences following hind limb I/R injury, and associated alterations in the endocannabinoid system. This research could inform future research into novel endocannabinoid system-targeting treatment strategies for the management of pain in conditions affected by I/R injury including chronic regional pain syndrome type-1, and chronic wounds.