Alpha adrenoceptor modulation of the jaw-opening reflex

In the present study, the nature, role and location of the alpha-adrenoceptors, modulating the tooth pulp stimulation (TPS)-evoked jaw-opening reflex (JOR), was investigated. Clonidine, a lipophilic alpha 2-adrenoceptor agonist, rapidly, reversibly and significantly elevated the threshold of the digastric electromyogram (dEMG) evoked by tooth pulp stimulation which was antagonized by yohimbine or piperoxane (two alpha 2-adrenoceptor antagonists) but not by prazosin (an alpha 1-adrenoceptor antagonist). 4-Hydroxyclonidine and ST-91 non lipophilic alpha 2 adrenoceptor agonists, were ineffective on the tooth pulp stimulation-evoked threshold of the digastric electromyogram. Clonidine was ineffective upon digastric muscle stimulation (DMS)-evoked threshold of the digastric electromyogram of the curarized digastricus and mylohyoid (motor) nerve stimulation (MNS)-evoked digastric EMG. It is concluded that modulation by clonidine of the tooth pulp stimulation-evoked jaw-opening reflex is devoid of a peripheral efferent component and that such modulation is predominantly expressed through interactions with centrally located alpha 2 adrenoceptors.