title: Differential effects of morphine and cocaine on locomotor activity and sensitization in mu-opioid receptor knockout mice.
abstract: The present study was undertaken to investigate the hypothesis that the mu-opioid receptors play a crucial role in locomotor activity and sensitization to cocaine and morphine in wild-type and mu-opioid receptor knockout mice. Our results show that morphine and cocaine increased locomotor activity in wild-type mice, but failed to increase locomotor activity in mu-opioid receptor knockout mice, suggesting a contribution of mu-opioid receptor. Repeated morphine treatment induced sensitization in wild-type mice, but this was not observed in mu-opioid receptor knockout mice. In contrast repeated cocaine treatment produced sensitization in mu-opioid receptor knockout mice, but not in wild-type mice on day 6. However, the sensitization to cocaine was observed in mu-opioid receptor knockout and wild-type mice on day 12. These results suggest that the expression of mu-opioid receptor may contribute to locomotor sensitization induced by morphine, but that mu-opioid receptor does not play an important role in mediating sensitization to cocaine.
authors.affiliation: Department of Pharmacology, College of Pharmacy, Sungkyunkwan University, Cheoncheon-dong, Jangan-ku, Suwon, 440-746, South Korea.
authors.forename: Ji-Hoon
authors.initials: JH
authors.lastname: Yoo
authors.forename: Eun-Mi
authors.initials: EM
authors.lastname: Yang
authors.forename: Seok-Yong
authors.initials: SY
authors.lastname: Lee
authors.forename: Horace H
authors.initials: HH
authors.lastname: Loh
authors.forename: Ing K
authors.initials: IK
authors.lastname: Ho
authors.forename: Choon-Gon
authors.initials: CG
authors.lastname: Jang
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D000818:Animals; D003042:Cocaine / Q000494:pharmacology; D018765:Dopamine Uptake Inhibitors / Q000494:pharmacology*; D051379:Mice; D018345:Mice, Knockout; D009020:Morphine / Q000494:pharmacology; D009043:Motor Activity / Q000187:drug effects* / Q000502:physiology; D009294:Narcotics / Q000494:pharmacology*; D017450:Receptors, Opioid, mu / Q000235:genetics* / Q000502:physiology*
chemical_list: D018765:Dopamine Uptake Inhibitors; D009294:Narcotics; D017450:Receptors, Opioid, mu; D009020:Morphine; D003042:Cocaine