Microinjection of the mGluR5 antagonist MTEP into the nucleus accumbens attenuates the acquisition but not expression of morphine-induced conditioned place preference in rats

• MTEP injection into NAc did not alter locomotion and did not induce place preference.
• Intra-NAc administration of MTEP dose-dependently reduced the acquisition of morphine CPP.
• Blockade of mGluR5 receptors in NAc did not alter expression of morphine-induced CPP.

Previous studies suggest that metabotropic glutamate receptor type 5 (mGluR5) plays an important role in modulation of the rewarding properties of morphine. Little is known about the role of mGluR5 in the nucleus accumbens (NAc), as one of the important regions of the reward circuitry. In the present study, we investigated the effects of intra-accumbal injection of mGluR5 antagonist, 3-[(2-methyl-4-thiazolyl) ethynyl] pyridine, MTEP, on the acquisition and expression of morphine induced Conditioned Place Preference (CPP) in the rats. Eighty four adult male Wistar rats (220–260 g) were bilaterally implanted with cannulae into the NAc. Subjects were tested in a CPP paradigm. Different doses of MTEP (0.3, 1 and 3 μg/0.5 μl per side) were delivered bilaterally into NAc during 3 conditioning days (Acquisition) or post-conditioning day (Expression).Results showed that bilateral intra accumbal administration of MTEP (1 and 3 μg) significantly attenuated the acquisition but not expression of morphine-induced CPP in a dose-dependent manner.Our findings indicated that blockade of mGluR5 reduces rewarding properties of morphine. Further studies are needed to know the involved mechanisms.