Characterisation of novel preclinical models of wound-related pain: Role of the endocannabinoid system

Wound-related pain is a significant unmet clinical need. Up to 80% of individuals with lived experience of chronic wounds experience persistent, debilitating pain. Current wound management therapies typically do not focus on specific management strategies for pain. Current therapeutic approaches for the management of wound-related pain, such as topical morphine, may impede wound healing. Consultation with individuals with lived experience of wounds revealed that wound-related pain management was the area in which they wished to see new research. The endocannabinoid system has a role in wound healing, skin homeostasis, and modulation of the pain experience. Several case reports and open-label trials report that administration of cannabis-based therapeutics provides benefit for the management of wound-related pain. However, there is a paucity of preclinical research investigating the potential of the endocannabinoid system for the management of wound-related pain. In fact, to my knowledge, there is no validated, characterised model for the study of wound-related pain in vivo, representing a major barrier to broadening our understanding of the wound-related pain state. The work presented in this thesis tested the hypothesis that current rat models of wound healing, and the aetiologies underlying chronic wound formation, are suitable for the study of wound-related pain, and provide a solid framework for the investigation of endocannabinoid system modulation on wound-related pain. The overarching aims of the work presented herein were to 1) characterise pain-related behaviour, and the endocannabinoid system in a rodent incisional wound model, and a rodent model of ischemia-reperfusion injury, 2) provide evidence for the predictive validity of the incisional wound model as a model for the study of wound-related pain via mu-opioid receptor agonist administration and 3) investigate the effects of endocannabinoid system modulation on pain-related behaviour following incisional wound. Unilateral hind limb ischemia-reperfusion injury led to robust mechanical hypersensitivity and more transient cold hypersensitivity in male Sprague Dawley rats, with a less pronounced phenotype in females. Investigation into the endocannabinoid system revealed that 30 days following ischemia-reperfusion injury, there were increased levels of N-palmitoylethanolamide (PEA) in the thalamus of female rats vs their female sham counterparts, and vs their male ischemia-reperfusion counterparts. Following the creation of an incisional wound on the dorsum, there was robust mechanical hypersensitivity at the dorsum for up to 14 days post-incision in male Sprague Dawley rats, and up to 7 days post-incision in females. Secondary mechanical hypersensitivity was observed in the hind paws. There were significantly lower levels of the gene encoding the cannabinoid CB1 receptor in the rostral ventromedial medulla (RVM) of male incision rats vs sham on post-incision day 35. We sought to pharmacologically validate the phenotype observed post-incision via the administration of a mu-opioid receptor agonist, whilst the pain phenotype was robustly present. Morphine (3 mg/kg s.c.) significantly attenuated mechanical hypersensitivity in the dorsum and hind paws at post-incision day 8. Morphine (3 mg/kg s.c.) significantly increased levels of PEA in the plasma of sham and incision rats, and increased levels of N-oleoylethanolamide (OEA) in the plasma of sham rats. Finally, an investigation into the effects of inhibiting the endocannabinoid-degrading enzymes, fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MGL) on mechanical hypersensitivity post-incision was completed. Pharmacological inhibition of MGL or FAAH had differential effects on mechanical hypersensitivity post-incision. MGL inhibition attenuated mechanical hypersensitivity at the dorsum and hind paws, whereas FAAH inhibition attenuated mechanical hypersensitivity in the hind paws only. MGL inhibition increased levels of 2-AG in the spinal cord, and key brain regions related to nociception. FAAH inhibition increased levels of AEA, OEA and PEA in the plasma, spinal cord and key brain regions related to nociception. In conclusion, the data presented in this thesis provides evidence to support the use of the dorsum incision model, and the hind limb ischemia-reperfusion model, as models for the study of wound-related pain. Moreover, there is evidence to suggest that modulation of the endocannabinoid system attenuates wound-related pain behaviour following incisional wound creation.

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University of Galway

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Attribution-NonCommercial-NoDerivatives 4.0 International

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University of Galway Theses (PhD Theses)