Involvement of orexin-1 and orexin-2 receptors within the dentate gyrus of the hippocampus in the acquisition, expression and extinction of lateral hypothalamic-induced conditioned place preference in the rats

Orexinergic projections originating from the lateral hypothalamus (LH) have an important role in the acquisition of the LH-stimulation conditioned place preference (CPP). Among the brain areas associated with reward processing, LH orexinergic neurons send projections to the dentate gyrus (DG) region of the hippocampus, and it has been shown that orexin receptors are expressed in the DG. In this study, we investigated the role of intra-DG orexin-1 (OX1) and orexin-2 (OX2) receptors on acquisition, expression and extinction of CPP induced by stimulation of the LH. Rats were unilaterally implanted by two separate cannulae into the LH and DG. The CPP paradigm was done; conditioning scores and locomotor activities were recorded by Ethovision software. The results showed that intra-DG administration of SB334867, a selective OX1r antagonist, and TCS OX2 29, a selective OX2r antagonist, (0.5, 5, 12.5 and 50nM/0.5µl DMSO) before carbachol microinjection (250nM; effective dose) during the 3-days conditioning phase, dose-dependently inhibited the development of LH stimulation-induced CPP in the rats. However, this reduction in OX1r antagonist treated groups was more than that in OX2r antagonist treated animals. In addition, these antagonists decreased the expression of LH-induced CPP. Moreover, OX1r but not OX2r antagonist could shorten the extinction duration of place preference. We conclude that the orexinergic projections from the LH to DG are involved in the development, expression and extinction of CPP induced by LH stimulation.