Unexpected finding of a marked non-neuronal cholinergic system in human knee joint synovial tissue

The cholinergic anti-inflammatory pathway is a newly discovered pathway. Another recent concept is the existence of a non-neuronal cholinergic system that has, so far, been defined for human tendons, intestine, airways and urinary bladder. The existence of such a system in joint synovial tissue is yet to be described. We therefore aimed to investigate the expression of choline acetyltransferase (ChAT) at both the protein and mRNA level using immunohistochemistry and in situ hybridisation, in human knee synovial tissue from rheumatoid arthritis (RA) and osteoarthritis (OA) patients. The biopsy samples were collected from patients undergoing knee prosthetic surgery. Our results show that both ChAT protein and mRNA is expressed in fibroblast-like and mononuclear-like cells, and to some extent in blood vessel walls in the synovial tissue. The mononuclear-like cells showing ChAT expression were scattered throughout the synovial tissue or located in association with lymphoid aggregates. Thus, we present the first evidence of the existence of a marked non-neuronal cholinergic system in human synovial tissue. The existence of this system could lead to the development of alternative medications to those currently in use. The system might function as a cholinergic anti-inflammatory pathway in synovial tissue. Our observations show that synovial tissue of patients with marked RA or OA, a tissue in which cholinergic innervation is not proven to exist, is supplied with acetylcholine via production in non-neuronal cells within the tissue.