Involvement of D1- and D2-like dopamine receptors in the dentate gyrus in the acquisition, expression, and extinction of the morphine-induced conditioned place preference in rats

In the current study, we investigated the role of intra-dentate gyrus (DG) administration of D1 and/or D2 receptor antagonists on the expression, acquisition, and extinction of morphine-CPP. Cannulae were implanted bilaterally into the DG region in male Wistar rats and CPP was induced by the subcutaneous injection of morphine (5 mg/kg) during a 3-day conditioning phase. Three experimental designs were separately employed in the CPP paradigm during the acquisition, expression and extinction phases, and different doses (0.25, 1, or 4 μg/0.5 μl saline) of SCH23390, as a selective D1-like receptor antagonist, and sulpiride (0.25, 1, or 4 μg/0.5 μl DMSO), as a selective D2-like receptor antagonist, were bilaterally microinjected into the DG region. Conditioning scores and locomotor activities were recorded during the test. Results showed that the injection of the antagonists into the DG region dose-dependently attenuated the acquisition and expression of the morphine-induced CPP and sulpiride revealed prominent behavioral results compared to SCH23390 in both mentioned phases. Moreover, the blockade of D1- and D2-like receptors shortened the extinction phase of the morphine-induced CPP but had no effect on the locomotor activity. We found that the dopamine receptors within the DG region are involved in the acquisition and expression of morphine-CPP and have a critical role in the association between a morphine-paired context and the rewarding proprieties of morphine.