The Involvement of Intra-Hippocampal Dopamine Receptors in the Conditioned Place Preference Induced By Orexin Administration into the Rat Ventral Tegmental Area

The activity of dopamine (DA)-containing neurons in the ventral tegmental area (VTA) is a key mechanism in mesolimbic reward processing that has modulatory effects on different diencephalic structures like hippocampus (HIP), and receives inhibitory feedback and excitatory feed forward control. In addition, within the hippocampus, DA receptors are mostly located in the dorsal part (CA1) and dopaminergic innervations are predominant in this sub-region. The current study aimed to examine the effect of intra-hippocampal CA1 administration of SCH23390 and Sulpiride as D1- and D2-like receptor antagonists on the acquisition of orexin-induced conditioned place preference (CPP), respectively. Cannulas were unilaterally implanted into the VTA and HIP of adult male albino Wistar rats weighing 200-250 g. For induction of CPP, orexin A (10 ng/0.3 µL saline) was daily microinjected into the VTA during a three-day conditioning phase. Thereafter, various doses of SCH23390 and Sulpiride (0.25, 1 and 4 µg) were unilaterally injected into the CA1 during this 3-day conditioning phase after intra-VTA administration. The conditioning score was then calculated. Results revealed that intra-CA1 administration of D1- and D2-like receptor antagonists during the 3-day conditioning phase attenuated the acquisition of place preference by orexin A in a dose-dependent manner. It seems the effect of D2-like receptor antagonist within the CA1 region of hippocampus on this phenomenon was found to be more considerable than that of D1-like receptor antagonist. It is concluded that orexin-induced CPP may be mediated, at least in part, by stimulation of DA receptors in the CA1.