We examined the effects of antinociceptive agents microinjected into the medullary dorsal horn (MDH) on the ability of monkeys to detect small temperature increases in the noxious heat range. The monkeys detected temperature changes of 0.4, 0.6 and 1.0 degree C (T2) superimposed on an initial temperature shift to 46 degree C (TI). ST-91, an alpha-2 adrenergic agonist (1.0, 3.0, 10.0 and 30.0 micrograms) and morphine sulfate (5.0 micrograms), an opioid-receptor agonist, were microinjected into the MDH and produced increases in the detection latencies to T2 noxious heat stimuli. There was no drug effect on the detection of innocuous coolings or visual trials indicating that the effects on the detection of noxious heat were independent of motivational, motoric or attentional factors. Systemic idazoxan (an alpha-2 receptor specific antagonist), but not prazocin (an alpha-1 receptor specific antagonist), naloxone (an opioid antagonist) or saline, significantly attenuated the effects of ST-91 on all noxious T2s. These data demonstrate a pharmacologically specific effect of an alpha-2 agonist on the perceived intensity of noxious heat stimuli at the MDH. Both systemic naloxone and idazoxan were effective at attenuating disruption of noxious heating detection by MDH-administered morphine sulfate. This may demonstrate an interaction of opioid and adrenergic pain control systems in the MDH. The effect of MDH-administered opioid agonists on facial scratching behavior was also examined. The opioid agonist morphine and the mu-specific opioid-receptor agonist DAMGO produced large numbers of facial scratches. The delta-specific opioid agonist DPDPE did not produce facial scratches. These findings demonstrate that facial itch, the often observed side effect of spinal opioid administration in humans, may be the result of opioids acting at mu receptors in the MDH.
