CENTRAL NEUROTRANSMITTER SYSTEMS IN THE MORPHINE SUPPRESSION OF JAW-OPENING REFLEX IN RABBITS - DOPAMINERGIC SYSTEM

The possible participation of the central dopaminergic system in the suppression by morphine of the jaw-opening reflex was investigated in rabbits lightly anesthetized with pentobarbital sodium (30 mg/kg, i.v.). The averaged amplitude of EMG signals from the digastric muscle, evoked by intrapulpal stimulation, was used as the pain index. Drugs were administered either directly into the cerebral circulation via a cannulated carotid artery or intraperitoneally. Pretreatment of the animals with haloperidol (0.5 mg/kg) or pimozide (0.5 mg/kg) essentially eliminated the inhibitory action of an optimal dose of morphine (6 mg/kg) on the jaw-opening reflex. Such antagonistic action of pimozide (and to a lesser extent, of haloperidol) on the opiate effect could be reversed by physostigmine (0.1 mg/kg). Apomorphine (0.5 mg/kg) pretreatment sufficiently potentiated the effect of a suboptimal dose of morphine (3 mg/kg), resulting in significant suppression of the evoked digastric activities. We concluded that the dopaminergic system is involved synergistically in morphine suppression of the jaw-opening reflex; the present study also reinforced a previous proposal that shifting of the balance between central neurotransmitter systems may be a key to the precipitation of the analgesic process induced by the opiate. © 1979.