Project Summary Historically, osteoarthritis (OA) pathology is defined by the breakdown of articular cartilage. While cartilage loss remains a hallmark of OA, the pathology of OA is now widely recognized to be a disease of the entire joint, including cartilage, bone, ligaments, menisci, and synovium. This definition of OA pathology provides a whole joint perspective; however, there is another problem with this definition of OA – the discordance between OA pathology and symptoms. Simply put, more degeneration of the joint does not necessarily mean more pain. This issue markedly complicates the development of pain-relieving therapies for OA, as the collection of all people with joint degeneration is far larger than the people with OA pathology and painful symptoms. Then, within the collection of people with OA pathology and symptoms, the etiology of OA is typically unknown and joint pathology tends to be a poor predictor of symptomatic progression. Thus, when focusing on the development of therapies for OA pain, the question becomes: Is a model of OA pathology sufficient to model the heterogeneity and complexity of OA pain? Clearly, disease models are needed for drug discovery, mechanistic testing, and the translation of new therapies from the laboratory to the clinic. For OA pain, a good model should also reflect the heterogeneity of the clinical OA pain experience described above. Thus, we propose that the best model of OA pain is a veterinary clinical population that replicates the breadth of OA cases and heterogeneity of OA symptoms. In fact, equine OA patients capture the years-long cascade of OA disease progression, model the heterogeneity of human OA pain reports, and provide unique behavioral parallels for the deep phenotyping assays currently being used in humans. However, a critical technological gap needs to be closed for studies in equine OA populations. While detailed behavioral protocols to evaluate pain-related experiences have been developed and validated for human patient populations, these assessments of behavioral assay reliability and validity have never been conducted for the horse. This is not to say that pain has not been studied in horses; gait analysis, sensory function, activity, and heart rate monitoring (among other assays) have all been conducted in horses with OA. However, pain-related behaviors are typically studied in isolation, and these assays are rarely assessed for their reliability or validity across studies. A goal of RFA-NS-22-070 is to recapitulate the behavioral aspects of human pain disorders in large animal models, and thereby provide well-validated measures that facilitate the development of non-opioid analgesic therapies with little or no addiction liability in the future. In response to this program, we propose to rigorously test the reliability and validity of pain-related behavioral metrics in the horse through psychometric analyses, including assessments of quantitative sensory tests (Aim 1), locomot...