NORADRENERGIC AND OPIOID SYSTEMS INTERACT TO ALTER THE DETECTION OF NOXIOUS THERMAL STIMULI AND FACIAL SCRATCHING IN MONKEYS

We examined the ability of the alpha2-adrenoceptor agonist, ST-91, mic roinjected into the medullary dorsal horn (MDH), to diminish the senso ry-discriminative features of noxious heat stimuli in awake behaving m onkeys. Two monkeys performed a noxious thermal detection task and the time to detection of small increases in heat served as a measure of t he perceived intensity of pain. ST-91 microinjected into the MDH (1.0, 3.0, 10.0 and 30.0 mug/0.4 mul) produced dose-dependent increases in detection time to graded temperature increases (0.4-1.0-degrees-C) fro m a noxious 46-degrees-C base line. These dose-dependent effects were attenuated by the systemic administration of the alpha2-adrenoceptor a ntagonist, idazoxan (2.0 mg/kg, i.m.), but not by the alpha1-adrenocep tor antagonist, prazosin (0.5 mg/kg, i.m.) or the opioid-receptor anta gonist, naloxone (0.5 mg/kg, i.m.). The effect of ST-91 on detection l atency of thermal stimuli was not the result of alterations in attenti onal, motivational or motoric aspects of the monkeys' behavior, becaus e detection of visual stimuli and non-noxious temperature coolings (36 .0-34.5-degrees-C) in a similar paradigm were not consistently altered . Microinjection of morphine (3.0 mg) into the MDH also increased dete ction latency of the noxious heat stimuli. Systemic administration of the opioid-receptor antagonist, naloxone (0.5 mg/kg), and the alpha2-a drenoceptor antagonist, idazoxan (2.0 mg/kg, i.m.) attenuated these ef fects of morphine. In a separate experiment, morphine (5.0 mug) microi njected into the MDH induced facial scratching behavior. Idazoxan (2.0 mg/kg) was effective at attenuating this scratching behavior. We have thus shown participation of MDH alpha2-adrenoceptors in the process u nderlying the perception of the intensity of noxious thermal stimulati on in monkeys. Further, opioid and noradrenergic systems interacted in the noxious heat detection paradigm and a paradigm where facial scrat ching behavior was studied.