Possible involvement of histamine in muscular fatigue in temporomandibulardisorders: Animal and human studies

As an approach to clarifying the molecular basis of pain and fatigue in mus cles involved in temporomandibular disorders, we examined the activity of h istidine decarboxylase (HDC), the enzyme which forms histamine, in the mass eter muscles of mice. In the resting muscle, HDC activity was very low. Dir ect electrical stimulation of the muscle markedly elevated HDC activity. HD C activity rose within 3 hrs of the electrical stimulation, peaked at 6 to 8 hrs, and then gradually declined. Intraperitoneal injection of a small am ount of interleukin-l (IL-1) (from 1 to 10 mu g/kg) produced a similar elev ation of HDC activity in the masseter muscle. We also examined the effect o f an antihistamine, chlorphenylamine (CP), on temporomandibular disorders i n humans and compared it with that of an antiinflammatory analgesic, flurbi profen (FB). Two groups received one or the other of the drugs daily for 7 days, and they were asked about their signs and symptoms before and after t he treatment. A positive evaluation of their treatment was made by 74% of t he CP group, but by only 48% of the FB group. Although the effects of CP on the limitation of mouth-opening and on joint noise were negligible, about 50% of the CP group answered positively concerning the drug's effect on spo ntaneous pain or pain induced by chewing or mouth-opening. The positive eva luation for CP (50%) in relieving associated symptoms (headache or shoulder stiffness) was significantly greater than for FB (13%). FB showed effectiv eness similar to but sometimes weaker than that of CP on several symptoms. On the basis of these and previous results and the known actions of histami ne, we propose that the histamine newly formed following the induction of H DC activity, which is itself mediated by IL-1, may be involved in inducing pain and, possibly, stiffness in muscles in temporomandibular disorders.