Abstract
The present study investigated the influence of aging upon jaw muscle activity evoked by the application of an algesic chemical, mustard oil (MO), to the tooth pulp. Forty-six male Sprague-Dawley rats anesthetized with urethane-chloralose were divided into four groups according to age (7, 10, 20 and 40wks); another group served as the control group (10 wk). MO was applied to the left first maxillary molar pulp and the EMG activities of the left digastric muscle and left and right masseter muscles were recorded. The opiate antagonist naloxone (1.2mg/kg, i. v.) was administered after subsidence of the MO-evoked EMG activity to examine its effect on EMG activity. In the control group, mineral oil application instead of MO induced no significant increase in EMG activity. However, in the experimental groups, MO application increased jaw muscle EMG; two phases of increased activity were often seen, and the increase in activity was reactivated by the administration of naloxone. EMG activities induced by MO application had a tendency to decrease (p<0.05, one-way ANOVA), and the latency of the second phase response tended to prolong with the age (p<0.05, one-way ANOVA), but no significant difference was found in the latency of the first phase MO-evoked activity. The results of the present study suggest that age-related changes in the peripheral and/or central nervous system affect jaw muscle EMG activity reflexively evoked by noxious chemical stimulation of the tooth pulp.
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