title: Microinjection of morphine into the rat medullary dorsal horn produces a dose-dependent increase in facial scratching.
abstract: It has been proposed that opioids act at the level of the medulla to produce facial pruritus. Supporting this hypothesis, microinjection of mu-opioid receptor agonists into the medullary dorsal horn (MDH; trigeminal subnucleus caudalis) of monkeys produces facial scratching behavior. The present study sought to establish a rodent model of opioid-induced facial pruritus. To this end, morphine (0.1, 0.3 or 1.0 micrograms/0.2 microliter) or saline (0.2 microliter) was unilaterally microinjected into the MDH of male Sprague-Dawley rats. Behavior for the 20 min preceding and the 80 min after this microinjection was videotaped. Morphine produced dose-dependent increases in facial scratching behavior ipsilateral to the microinjections with the peak effect at 30-40 min after microinjection. Facial scratching continued for the entire 80 min post-microinjection test period. Morphine also produced a lesser degree of facial scratching contralateral to the microinjections. Increases in facial scratching ipsilateral to the microinjection of 0.3 microgram morphine into the MDH were attenuated by 0.4 mg/kg s.c. naloxone. These findings support the hypothesis that the MDH is a critical site of action of opioid agonists in producing facial pruritus.
authors.affiliation: Department of Anesthesia and Critical Care, University of Chicago, IL 60637, USA.
authors.forename: D A
authors.initials: DA
authors.lastname: Thomas
authors.forename: D L
authors.initials: DL
authors.lastname: Hammond
mesh_terms: D000818:Animals; D004195:Disease Models, Animal; D004305:Dose-Response Relationship, Drug; D005145:Face; D007839:Functional Laterality / Q000502:physiology; D008297:Male; D008526:Medulla Oblongata / Q000502:physiology*; D008845:Microinjections; D009020:Morphine / Q000008:administration & dosage / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D011537:Pruritus / Q000139:chemically induced* / Q000523:psychology; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D009292:Narcotic Antagonists; D009270:Naloxone; D009020:Morphine