Postnatal development of central nociceptive mechanisms modulating jaw muscle activity in the rat

Postnatal changes in the electromyographic (EMG) activity of jaw muscles ev oked by mustard oil (MO) application into the rat temporomandibular joint r egion and the recurrence of increased jaw muscle activities after intraveno us injection of naloxone were compared among 4,6 and 8-week-old rats. In al l the groups, MO application increased EMG activity on the ipsilateral side , however, 4-week old rats showed only a small increase in EMG activity on the contralateral side. The EMG activity on the contralateral side increase d in an age-dependent manner. The recurrence of increased jaw muscle activi ty was not induced in 4-week old rats. These results suggest that a neural circuit for generating contralateral responses and mechanisms for central e xcitation are not established until after 4 weeks postnatally. (C) 2000 Els evier Science Ireland Ltd. All rights reserved.