Male rats require testosterone to develop contralateral digastric muscle activity in response to noxious stimulation of the temporomandibular joint.

The influence of testosterone on the postnatal development of reflex electromyographic (EMG) jaw muscle activity evoked by injection of mustard oil (MO) into the temporomandibular joint region and the later recurrence of this EMG activity after intravenous injection of naloxone, was studied in male rats. MO-evoked EMG activity in the contralateral digastric muscle and naloxone-induced recurrence of this EMG activity were fully developed in intact, 8-week-old rats. Castration at 4 weeks of age inhibited the development of the contralateral MO-evoked EMG activity, but did not influence the naloxone-induced recurrence. Contralateral MO-evoked responses were observed in 8-week-old castrated rats if they received testosterone replacement therapy beginning at 4 weeks of age. These data suggest that testosterone is required for the development of a contralateral nociceptive reflex in the digastric muscle of male rats.