Cannabinoid Receptors Are Expressed in Equine Synovium and Upregulated with Synovitis. Miagkoff L, Girard C, Richard H, Beauchamp G, Laverty S. Université de Montréal, Saint-Hyacinthe, QC, Canada.
Osteoarthritis (OA) causes joint pain and there is a need for long-term therapy with minimal side effects. Medical cannabis is an emerging topic of interest in human orthopedics as cannabinoids are promising for the treatment of joint pain. The endocannabinoid system has not been investigated in the equine joint. The objectives of the study were to assess the presence and expression pattern of cannabinoid receptors 1 (CB1) and 2 (CB2) in the synovium of healthy and OA joints and their relationship to disease severity. Twenty-five metacarpophalangeal joints were dissected and scored for gross cartilage OA lesions. Synovial membrane specimens (n = 45) were processed for histology and scored for synovitis. Site-matched sections were immunostained with CB1 and CB2 antibodies. Digitalized synovial images were segmented into regions of interest (ROIs) for cellular immunoreactivity grading. Five ROIs per section were scored by two blinded observers. Interrater agreement was calculated with an intraclass correlation coefficient (ICC) and the relationship between the scores was interrogated with mixed linear models. The ICCs were 84.6% (CB1) and 92.9% (CB2). CB1 was expressed in all specimens and CB2 in 95%. CB1 and CB2 scores were positively correlated with the synovitis score (P = 0.038 and P = 0.027 respectively). In contrast, the CB1 score was negatively correlated with the OA score (P = 0.03). Equine synovial intimal cells constitutively express cannabinoid receptors that are upregulated with synovitis revealing a functional endocannabinoid system in the joint. Further studies are required to identify specific synovial cell receptor expression and to determine their potential role for OA pain management.