CHARACTERISING THE NEUROPHYSIOLOGY OF POST-TRAUMATIC OSTEOARTHRITIS IN RATS AND THE INVOLVEMENT OF PROTEASE ACTIVATED RECEPTOR-4
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Osteoarthritis (OA) affects 1 in 10 Canadians and is more prevalent in women than men. Remarkably, 60% of OA patients cite inadequate pain relief with current treatments. This may in part be due to multiple OA phenotypes and obscure mechanisms of OA pain. To investigate these phonemena, the neurophysiology of post-traumatic OA (PTOA) was assessed in both sexes and the contribution of pain subtypes was explored. The role of algogenic proteases was examined as a potential mechanism of OA pain. In the medial meniscal transection (MMT) model of PTOA, similar levels of joint damage were observed in male and female animals; however, marked sex-differences in joint neurobiology were detected. At endstage disease, only male MMT animals exhibited secondary allodynia, weight bearing deficits, and joint nociceptor sensitisation. Saphenous nerve damage was observed in male but not female MMT animals and the neuropathic analgesic amitriptyline reduced pain in males suggesting that peripheral neuropathy may be contributing to PTOA pain in male animals. The lack of nociception in female MMT animals may have been afforded by the endogenous opioid beta-endorphin whose levels were elevated in female animals. Serine proteases are elevated in the joints of OA patients and contribute to catabolic destruction of joint tissue as well as joint pain and inflammation. Through activation of protease activated receptor 4 (PAR4), some serine proteases cause pain in inflamed joints but in a neuropathic state the actions of PAR4 are unknown. In this thesis, PAR4 antagonism with pepducin P4pal10 attenuated pain behaviour and joint afferent firing in the early inflammatory phases of the MMT and monoiodoacetate (MIA) models of OA. During endstage OA, however, PAR4 antagonism was not effective, suggesting that the role of PAR4 in neuropathic pain may be minimal. In other experiments, human arthritic synovial fluid was found to sensitise rodent joint nociceptors, increasing both evoked and spontaneous firing. Proteomic analysis of the synovial fluid revealed a diverse protein composition including PAR4-cleaving serine proteases. Pretreatment with pepducin P4pal10 prevented the sensitising effects of arthritic synovial fluid suggesting that PAR4 cleaving proteases are, in part, responsible for the pain of arthritis. In conclusion, MMT is a viable model of PTOA to study pain in male but not female rats. The reason for this discrepancy is due to joint neuropathy in males and a hyperactive endogenous opioid system in females. Furthermore, antagonist studies demonstrate that blockade of PAR4 represents a viable target for the treatment of OA pain.