There exist both noradrenergic and opioid based systems of pain control. We examined the effects of activating these systems by injecting antinociceptive agents into the medullary dorsal horn (MDH) and examining the ability of monkeys to detect small increases in noxious thermal stimuli. The effects of these agents on facial scratching behavior in separate experiments were also examined. In the detection paradigm, the monkeys were required to detect temperature changes of 0.4, 0.6 and 1.0 degrees C (T2) superimposed on a 46.0 degrees C (T1) stimuli. Consistent with previous research, 10 micrograms of the noradrenergic agonist, ST-91, and also 3 micrograms of the opioid agonist, morphine, each increased the time to detection of the heating stimuli. Both morphine and ST-91 also produced small increases in the monkeys' time to detection of innocuous cooling stimuli. Neither morphine nor ST-91 interfered with the detection of visual stimuli, indicating the effects of these agents are not the result of changes in motoric, attentional, or motivational aspects of the monkeys' behavior. In separate experiments, morphine administered into the MDH, but not ST-91, was found to produce a great amount of facial scratching behavior. In both the detection and the scratching paradigms, the noradrenergic receptor antagonist, idazoxan (1.0 mg/kg: I.M.) attenuated the effects of both ST-91 and morphine. In the detection paradigm, the opioid receptor antagonist, naloxone (1.0 mg/kg: I.M.) was only effective at attenuating the effects of morphine, not ST-91. These findings demonstrate that the opioid and noradrenergic systems are involved in both pain and scratch behavior. The present data has increased our understanding of how pain control systems interact, which may lead to precise co-activation of opioid and noradrenergic pain control systems in clinical settings. The findings related to scratching indicate that noradrenergic antagonists may be effective antipruritic agents.
