Effects of central administration of opioids on facial scratching in monkeys.

Epidural and intrathecal administration of opioids to humans can produce facial pruritus and scratching that is naloxone reversible. It has been proposed that opioids may act at the level of the medulla to produce facial pruritus and associated scratching behavior. We investigated the effects of mu, delta and kappa opioid-receptor agonists microinjected unilaterally into the medullary dorsal horn (MDH) on facial scratching in cynomolgus monkeys. The selective mu opioid-receptor agonist, DAMGO (3.1-25.0 ng) produced large dose-dependent, naloxone-reversible increases in facial scratches. The selective delta opioid-receptor agonist, DPDPE (1.0-5.0 micrograms) and the selective kappa opioid-receptor agonist, U-50,488H (0.1-5.0 micrograms) did not produce significant increases in facial scratching behavior. We conclude that the MDH is a site where DAMGO, a mu opioid-receptor agonist, can act to produce facial scratching in monkeys, and that the MDH is likely the site where centrally administered opioids act to produce facial pruritus in humans.