title: Opposite conditioned place preference responses to endomorphin-1 and endomorphin-2 in the mouse.
abstract: An unbiased conditioned place preference paradigm was used to evaluate the reward effect of selective endogenous mu-opioid ligands, endomorphin-1 and endomorphin-2, in male CD-1 mice. Pre- and post-conditioning free-movement were measured on day 1 and day 5, respectively. Conditioning sessions were conducted twice daily from day 2 through day 4 consisting of the alternate injection of conditioning drug or vehicle. Intracerebroventricular (i.c.v.) injection of endomorphin-1 (0.3-10 microg) induced place preference in a dose-dependent manner; whereas, endomorphin-2 (1-10 microg) dose-dependently induced place aversion. Both endomorphin-1-induced place preference and endomorphin-2-induced place aversion were blocked by pretreatment i.c.v. with mu-opioid receptor antagonist, beta-funaltrexamine. Selective delta-opioid receptor antagonist, naltrindole, co-administered i.c.v. with endomorphin-1 or endomorphin-2 did not affect reward effect. However, endomorphin-2-induced place aversion, but not endomorphin-1-induced place preference, was blocked by the i.c.v.-administered selective kappa-opioid receptor antagonist, WIN 44,441-3. It is concluded that endomorphin-1 produces conditioned place preference, which is mediated by the stimulation of mu-, but not delta- or kappa-opioid receptors, while endomorphin-2 produces conditioned place aversion, which is mediated by the stimulation of mu- and kappa-, but not delta-opioid receptors.
authors.affiliation: Department of Anesthesiology, Medical College of Wisconsin, Medical Education Building, M4308 8701 Watertown Plank Road, Milwaukee, WI 53226, USA.
authors.forename: Hsiang-en
authors.initials: HE
authors.lastname: Wu
authors.forename: Ryan S
authors.initials: RS
authors.lastname: MacDougall
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Clithero
authors.forename: Randy J
authors.initials: RJ
authors.lastname: Leitermann
authors.forename: Maia
authors.initials: M
authors.lastname: Terashvili
authors.forename: Leon F
authors.initials: LF
authors.lastname: Tseng
mesh_terms: D000818:Animals; D001392:Azocines / Q000494:pharmacology; D003216:Conditioning, Operant / Q000187:drug effects*; D007276:Injections, Intraventricular; D008024:Ligands; D008297:Male; D051379:Mice; D009271:Naltrexone / Q000031:analogs & derivatives* / Q000494:pharmacology; D009842:Oligopeptides / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology*; D017465:Receptors, Opioid, delta / Q000037:antagonists & inhibitors; D017473:Receptors, Opioid, kappa / Q000037:antagonists & inhibitors; D017450:Receptors, Opioid, mu / Q000037:antagonists & inhibitors / Q000378:metabolism
chemical_list: D001392:Azocines; D008024:Ligands; D009842:Oligopeptides; D017465:Receptors, Opioid, delta; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; C105293:endomorphin 1; C105294:endomorphin 2; D009271:Naltrexone; C024524:beta-funaltrexamine; C035969:quadazocine