Identification and Quantification of Transient Receptor Potential Vanilloid 1 (TRPV1) in Equine Articular Tissue

SIMPLE SUMMARY: Osteoarthritis is affecting several species including the horse. Transient receptor potential vanilloid 1 (TRPV1)—also known as the ‘chili receptor’—is currently being investigated as a target for treating osteoarthritis in humans. To evaluate whether it could be a potential target in treating osteoarthritis in horses, we collected synovial membrane samples from healthy horse joints and horse joints with joint disease to investigate the expression of the TRPV1. Laboratory analysis showed that TRPV1 is present in horse joints, and that the levels might be elevated in diseased joints, even at a chronic state of disease. This means that TRPV1 could be used as a potential target for osteoarthritis treatment in horses. Future studies in this area will not only be beneficial to horses and their owners, but more knowledge about TRPV1, the mode of action, and possible side effects will also benefit translation into human osteoarthritis research. ABSTRACT: Joint pain and osteoarthritis (OA) are some of the most common causes of lameness in horses, and most of the available treatments focus on symptomatic relief without a disease-modifying effect. TRPV1 is a potential target for treating joint diseases, including OA, and the present study aims to investigate if the TRPV1 receptor is present in equine articular tissue and determine whether the number of receptors is upregulated in joint inflammation. Metacarpo/metatarsophalangeal (MCP/MTP) joints from 15 horses euthanised for reasons unrelated to this study were included. Based on synovial fluid analysis, macroscopic evaluation, and magnetic resonance imaging (MRI), joints were divided into two groups: healthy joints and joints with pathology. ELISA analysis was performed on synovial tissue harvested from all joints. TPRV1 was found in all joints. The mean concentration of TRPV1 compared to total protein in healthy joints (8.4 × 10(−7) ng/mL) and joints with pathology (12.9 × 10(−7) ng/mL) differed significantly (p = 0.01, t-test with Welch correction). Quantitative real-time reverse transcriptase PCR analysis was performed on RNA isolates from synovial tissue from all joints. TRPV1 mRNA expression ratio normalized to glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in healthy joints (0.16 (SD: 0.19)) and joints with pathology (0.24 (SD: 0.14)) did not differ significantly (p = 0.43, t-test with Welch correction). mRNA expression of interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) was very low for both groups. In conclusion, TRPV1 was detected both on mRNA and the protein level, with a higher expression of TRPV1 in samples from joints with pathology. Future studies will determine the clinical potential of equine TRPV1 as a target in the management of joint pain and inflammation.