TRPV1 has been implicated in OA pain, both in animal models and by the finding that TRPV1 genetic variants are associated with the risk of symptomatic knee OA in humans: S Kelly, R J Chapman, S Woodhams, D R Sagar, J Turner, J J Burston, C Bullock, K Paton, J Huang, A Wong, D F McWilliams, B N Okine, D A Barrett, G J Hathway, D A Walsh, V Chapman. Increased function of pronociceptive TRPV1 at the level of the joint in a rat model of osteoarthritis pain. Ann Rheum Dis doi:10.1136/annrheumdis-2013-203413.
Methods: Rat spinal cord sections from MIA- and saline-treated rats (n=5/group) (see online supplemental methods) were incubated with a polyclonal guinea pig anti-TRPV1 antibody (1 : 500, Neuromics, Edina, Minnesota, USA catalogue number GP14100) and then with Alexa 568-conjugated goat anti-guinea pig secondary antibody (1:300, Molecular Probes). TRPV1 immunostaining was visualised with a Leica DMRB/DM4000 B fluorescence microscope and images were acquired using Openlab software (PerkinElmer)...
Images: Transient receptor potential vanilloid 1 (TRPV1) immunoreactivity in the spinal cord. TRPV1 immunofluorescence detected in superficial dorsal horn (10× magnification) in rat lumbar (L3–L5) spinal cord at day 28 post-intra-articular injection of saline (A) or mono-iodoacetate (MIA) (B). Minimum and maximum brightness values were altered (32.01 min and 90.14 max) using Image J so as to highlight the area of TRPV1 positive staining. (C) Quantification of TRPV1 immunofluorescence in superficial dorsal horn of spinal cord taken from rats at 14 or 28 days following intra-articular injection of MIA and at day 28 following intra-articular injection of saline. Data are expressed as mean and SEM (n=5 per group).
Clinical trials of oral TRPV1 antagonists have been limited by on-target-induced hyperthermia. Here experimental evidence for increased functional role of TRPV1 at the level of the joint in a model of OA pain and the demonstration that blockade of joint TRPV1 ablates sensory afferent sensitization and pain behaviour support future targeted site-specific investigations of the therapeutic potential of TRPV1 for OA pain associated with synovitis. This could be good news for OA sufferers.