The effects of morphine and nalorphine on the jaw-opening reflex in rabbits

In rabbits lightly anesthetized with pentobarbital, the effects of injecting morphine and its antagonist, nalorphine, directly into the cerebral circulation via a carotid artery cannulation, on the jaw-opening reflex elicited by intradental stimulation, were studied. Using averaged electromyogram signals from the digastric muscle as the index, a single dose of morphine (3 mg/kg) was found to produce only a transient depression of the jaw-opening reflex. After a second, cumulative injection of morphine (3 mg/kg), the jaw-opening reflex was sufficiently inhibited for at least 30 min. The morphine-induced analgesia could be reversed by nalorphine, indicating that such action was truly pharmacological. The likelihood of morphine acting on some central sites and promoting the release of neurotransmitters which produce inhibition of transmission of nociceptive information from the dental pulp is discussed in the light of recent physiological and pharmacological linkage between morphine-triggered and central stimulation-induced analgesia.