title: (+)-Morphine attenuates the (-)-morphine-produced conditioned place preference and the mu-opioid receptor-mediated dopamine increase in the posterior nucleus accumbens of the rat.
abstract: An unbiased conditioned place preference paradigm and the microdialysis technique was used to evaluate the effect of (+)-morphine pretreatment on the conditioned place preference produced by (-)-morphine and the increased release of the dopamine produced by mu-opioid ligand endomorphin-1, respectively, in the posterior nucleus accumbens shell of the male CD rat. (-)-Morphine (2.5-10 microg) microinjected into the posterior nucleus accumbens shell dose-dependently produced the conditioned place preference. Pretreatment with (+)-morphine (0.1-10 pg) given into the posterior accumbens shell for 45 min dose-dependently attenuated the conditioned place preference produced by (-)-morphine (5 microg) given into the same posterior accumbens shell. However, higher doses of (+)-morphine (0.1 and 1 ng) were less effective in attenuating the (-)-morphine-produced conditioned place preference. Thus, like given systemically, (+)-morphine given into the posterior nucleus accumbens shell also induces a U-shaped dose-response curve for attenuating the (-)-morphine-produced conditioned place preference. Microinjection of mu-opioid agonist endomorphin-1 (1-10 microg) given into the ventral tegmental area dose-dependently increased the release of the extracellular dopamine in the posterior nucleus accumbens shell in the urethane-anesthetized rats. The increased dopamine caused by endomorphin-1 (10 microg) was completed blocked by the (+)-morphine (10 pg) pretreatment given into ventral tegmental area. It is concluded that (+)-morphine attenuates the (-)-morphine-produced conditioned place preference and the mu-opioid receptor-mediated increase of extracellular dopamine in the posterior nucleus accumbens shell of the rat.
authors.affiliation: Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, WI 53226, USA.
authors.forename: Maia
authors.initials: M
authors.lastname: Terashvili
authors.forename: Hsiang-En
authors.initials: HE
authors.lastname: Wu
authors.forename: Emma T
authors.initials: ET
authors.lastname: Schwasinger
authors.forename: Kuei-Chun
authors.initials: KC
authors.lastname: Hung
authors.forename: Jau-Shyong
authors.initials: JS
authors.lastname: Hong
authors.forename: Leon F
authors.initials: LF
authors.lastname: Tseng
mesh_terms: D000701:Analgesics, Opioid / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000494:pharmacology*; D000758:Anesthesia; D000818:Animals; D002851:Chromatography, High Pressure Liquid; D003216:Conditioning, Operant / Q000187:drug effects*; D004298:Dopamine / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004563:Electrochemistry; D008297:Male; D008845:Microinjections; D009020:Morphine / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009714:Nucleus Accumbens / Q000033:anatomy & histology / Q000187:drug effects / Q000378:metabolism*; D009842:Oligopeptides / Q000008:administration & dosage / Q000494:pharmacology; D051381:Rats; D017450:Receptors, Opioid, mu / Q000187:drug effects*; D013237:Stereoisomerism; D017557:Ventral Tegmental Area
chemical_list: D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D009842:Oligopeptides; D017450:Receptors, Opioid, mu; C105293:endomorphin 1; D009270:Naloxone; D009020:Morphine; D004298:Dopamine