AbstractProgressive structural changes in osteoarthritis (OA) involve synovial inflammation and angiogenesis, as well as activation of the proinflammatory cytokines tumor necrosis factor alpha (TNF-α) and interleukin (IL)-8, and the angiogenic factor vascular endothelial growth factor (VEGF). The endogenous hormone melatonin (N-acetyl-5-methoxytryptamine) is involved in antioxidative and anti-inflammatory activities, but how it antagonizes OA progression via its specific receptors is unclear. Here, we demonstrate that the MT1melatonin receptor, but not the MT2receptor, is highly expressed in normal tissue and only minimally in OA tissue. By targeting the MT1receptor, melatonin reversed OA-induced pathology and effectively reduced levels of TNF-α, IL-8, and VEGF expression in OA synovial fibroblasts and synovium from rats with severe OA. Interestingly, we found that the anabolic activities of melatonin involved the MT1receptor, which upregulated microRNA-185a through the PI3K/Akt and ERK signaling pathways in OA synovial fibroblasts. Our investigation confirms the role of the MT1receptor in melatonin-induced anti-catabolic effects in OA disease.