Immunohistochemical localization of cannabinoid receptor type I in the feline synovial membrane with and without degenerative lesions

This study aimed to evaluate cannabinoid receptor type I (CB1) expression in femoropatellar synovial membranes of adult cats with and without degenerative lesions. 

Synovial tissues were collected post-mortem from twenty-seven joints. Samples were fixed, embedded in paraffin, and sectioned for histology and immunostained with anti-CB1 antibody (Ab23703 1%; antigen retrieval with EDTA buffer). Antibody specificity was tested on feline synovium samples employing western blot analysis. A board-certified pathologist graded the hematoxylin-eosin-stained sections according to patterns of osteoarthritis (OA) - associated synoviopathy including absence (0) or presence (1) of: synovial membrane proliferation, inflammatory infiltrate, villous hypertrophy, blood vessels proliferation and cartilage/bone detritus. The scores were summed to provide the final synovial sample grade (0-5). The localization of CB1 receptors within the synovium was assessed by immunohistochemistry. 

Samples were represented as follow: grade (0) 18.5 %, (1) 48.1 %, (2) 29.7 %, (3) 0 %, (4) 0 % and (5) 3.7 %. Western blot analysis confirmed antibody specificity in the synovium (single band at the expected molecular weight). Positive cytoplasmic staining of CB1 was remarkable in the lining layer of the synovium and interstitial sublining layer areas of grade 5, but not grade 0 tissues. Grade 1 and 2 had milder positive staining than grade 5 tissues, mainly around blood vessels. 

This study suggests that CB1 could be a relevant target for cannabinoid therapy and chronic pain management in osteoarthritic cats with high grading synovitis. Quantitative methods are warranted to evaluate the correlation between microscopic grading and synovial CB1 expression.