Jaw electromyographic activity induced by the application of algesic chemicals to the rat tooth pulp

The aim of this study was to determine if the application of mustard oil (M O), a small-fiber excitant and inflammatory irritant, or other algesic chem icals (capsaicin, CAP, and bradykinin, BK) to the rat maxillary molar tooth pulp induces electromyographic (EMG) activity of the masseter and digastri c muscles, and also to determine if endogenous opioid mechanisms may be inv olved in any documented EMG changes. Application of MO to the tooth pulp in duced a significant increase in EMG activity of the ipsilateral masseter up to 30 min. The application of mineral oil to the pulp or MO application to the pulp-extirpated tooth did not induce any significant EMG increases. Th e application of CAP or BK to the pulp in contrast had much weaker effects on EMG activity of the jaw muscles. CAP produced a small but prolonged incr ease in masseter EMG activity, and BK induced a short-lasting increase in d igastric EMG activity. The systemic administration of the opiate antagonist naloxone significantly reactivated (i.e. rekindled) the EMG response evoke d by MO application to the pulp. Naloxone did not produce any such signific ant rekindling effect on EMG activity following CAP, BK or mineral oil appl ication to the pulp or following MO application to the pulp-extirpated toot h. The MO, BK and especially CAP groups showed histological evidence of vas odilatation and polymorphonuclear leukocyte infiltration in the pulp tissue and a significant increase in plasma extravasation of Evans Blue dye, wher eas mineral oil did not induce these changes. These findings suggest that p ulp afferent inputs to the central nervous system evoked by BK, CAP and esp ecially MO may induce enhanced jaw muscle activity. In addition, the naloxo ne data suggest that an opioid suppressive mechanism may be induced by the pulpal afferent inputs evoked by MO, and may serve to limit the jaw muscle activity elicited by these inputs. (C) 1999 International Association for t he Study of Pain. Published by Elsevier Science B.V.