(+)-Morphine attenuates the (−)-morphine-produced conditioned place preference and the µ-opioid receptor-mediated dopamine increase in the posterior nucleus accumbens of the rat

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 µ-opioid ligand endomorphin-1, respectively, in the posterior nucleus accumbens shell of the male CD rat. (−)-Morphine (2.5-10 µg) 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 µg) 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 µ-opioid agonist endomorphin-1 (1-10 µg) 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 µg) 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 µ-opioid receptor-mediated increase of extracellular dopamine in the posterior nucleus accumbens shell of the rat.