The effects of medetomidine, an alpha-2-adrenoceptor agonist, and cocaine on the tooth pulp-evoked jaw-opening reflex in cat.

In the pentobarbitone-anesthetized cat, the threshold of the tooth pulp-elicited jaw-opening reflex was elevated in a dose-dependent way (30-100 micrograms/kg, IP) following the administration of medetomidine, an alpha-2-adrenoceptor agonist. This elevation was significantly reduced by atipamezole (1 mg/kg, IP), an alpha-2-adrenoceptor antagonist. The inhibitory interaction between two successive dental stimuli applied to the same tooth (in-field inhibition) was suppressed by a lower dose of medetomidine (30 micrograms/kg) than the threshold elevation to single electrical pulses (55 micrograms/kg). Only the highest dose of medetomidine used (100 micrograms/kg) significantly influenced the temporally facilitated (in-field facilitation) response. In comparison, cocaine, a nonspecific monoaminergic agent, did not produce a significant threshold elevation of the tooth pulp-elicited jaw-opening reflex (1-25 mg/kg, IP). It is concluded that medetomidine, through an action on alpha-2-adrenoceptors, can suppress a predominantly nociceptive trigeminal reflex in anesthetized cats. The threshold evoked by single electric pulses, in-field inhibition, and in-field facilitation display differential sensitivities to medetomidine effects.