Coexistence of enkephalin- and tyrosine hydroxylase-like immunoreactivities in nerve fibers of the temporomandibular joint capsule of the guinea pig

Abstract

Background:The innervation of joints has been suggested to play an important role for their normal function as well as for the pathogenesis of chronic arthritic conditions. It is therefore important to elucidate the functional anatomy of this innervation.Methods: The expression of methionine enkephalin-like immunoreactivity (ENK-LI) and tyrosine hydroxylase (TH)-LI as well as their possible colocalization were examined in the temporomandibular joint of the guinea pig.Results: Nerve fibers with ENK-LI were demonstrated in the synovium of the guinea pig temporomandibular joint capsule with the use of indirect immunofluorescence. The ENK ؉ fibers were found close to the surface of the synovial membrane, deeper in the synovium, and at the walls of blood vessels. Coexistence of ENK-and TH-LI was observed frequently in the synovial nerve fibers. After removal of the superior cervical ganglion (SCG), the ENKcontaining nerve fibers were reduced substantially in number on the operated side or were completely absent if the cervical sympathetic trunk was also excised.Conclusions: The findings indicate that the majority of fibers with ENK-LI derive from the SCG. ENK may act as a neuromodulator on the catecholaminergic nerves and may also have an analgesic effect in the joint.