Distribution of inducible nitric oxide synthase, interleukin-1β, and interleukin-1 receptor in the temporomandibular joint of normal rats

Nitric oxide (NO) is generated from L-arginine by NO synthase (NOS) and has multiple functions under both physiological and pathological conditions. One isoform of NOS, inducible NOS (iNOS) is expressed in the cells such as macrophages after induction with various cytokines or mechanical stress. In this study, we investigated the distribution of iNOS, interleukin-1β (IL-1β) and its receptor, the IL-1 receptor (IL-1R), in the synovial membrane of the temporomandibular joint (TMJ) of normal rats using immunolight and immunoelectron microscopy. By light microscopy, an immunopositive reaction for iNOS and IL-1β was found in the superficial cells of the synovial membrane of both the anterior and posterior portions of the articular disc. Immunoelectron microscopy revealed that iNOS-immunoreactive products were deposited in the cytoplasm and vesicles, and on the plasma membrane of type-A (macrophage-like) and B (fibroblast-like) cells of the superficial layer. IL-1R-positive products were found both on the plasma membrane and in the vesicles of type-A cells of the synovial lining, and were observed in macrophages in the sublining layer. These results reveal that iNOS and IL-1β localize to the synovial membrane of the rat TMJ under physiological conditions. Therefore, it is likely that autocrine/paracrine effects of IL-1β induce NO generation by iNOS via the IL-1R in type-A cells. It is considered that cytokine-induced NO may play an important role in the physiological maintenance, e.g. self-protection, by synovial lining cells of the synovial membrane in the TMJ.