mu-Opioid receptor mRNA expression and immunohistochemical localization in the rat temporomandibular joint

This study was undertaken to examine the presence and distribution of the mu-opioid receptor (MOR) in the non-inflamed rat temporomandibular joint (TMJ) using non-radiographic in situ hybridization at the mRNA level and immunohistochemistry at the protein level. MOR mRNA and MOR-like immunoreactivity (MOR-LI) were found around the small blood vessels in the anterior part of the synovial membrane. The number of MOR mRNA signals in the anterior synovial membrane was significantly higher than that in the posterior part. Morphologically, MOR mRNA and MOR-LI were localized in amorphous materials considered to be nervous tissue, as well as some cell types considered to be macrophages, mast cells and endothelial cells. The present study showed the distribution of MOR in the rat TMJ synovial membrane and suggests that the opiate system plays an important role in endogenous analgesia in the TMJ.