Metabotropic glutamate receptor 5 localized in the limbic forebrain is critical for the development of morphine-induced rewarding effect in mice

The aim of the present study was to clarify the role of the metabotropic glutamate 5 (mGlu5) receptor subtype in the development of rewarding effect induced by a prototypical mu-opioid receptor agonist morphine in the mouse. In the conditioned place preference paradigm, intracerebroventricular (i.c.v.) administration of a selective mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), attenuated the morphine-induced rewarding effects. Using immunoblot analysis, we confirmed that the increased level of protein kinase Cgamma (PKCgamma) isoform was observed in the limbic forebrain of ICR mice conditioned with morphine. Here we found for the first time that the treatment with MPEP significantly inhibited the up-regulation of PKCgamma isoform in the limbic forebrain of mice showing the significant place preference. Furthermore, it should be mentioned that the protein level of mGlu5 was significantly increased in membrane preparations of the limbic forebrain obtained from morphine-conditioned mice compared to those from saline-conditioned mice. As well as the result from the immunoblot analysis, we demonstrated using the receptor binding assay that the number of mGlu5 receptors in the mouse limbic forebrain was significantly increased by morphine conditioning. The present data provide direct evidence that the activation of mGlu5 receptor linked to the increased PKCgamma isoform in the mouse limbic forebrain is implicated in the development of rewarding effect of morphine.