Orexin mediates morphine place preference, but not morphine-induced hyperactivity or sensitization.

Orexin (or hypocretin) has been implicated in mediating drug addiction and reward. Here, we investigated orexin's contribution to morphine-induced behavioral sensitization and place preference. Orexin-/- (OKO) mice and littermate wild-type (WT) controls (n=56) and C57BL/6J mice (n=67) were tested for chronic morphine-induced locomotor sensitization or for conditioned place preference (CPP) for a morphine- or a cocaine-paired environment. C57BL/6J mice received the orexin receptor 1 (Ox1r) antagonist, SB-334867, prior to test sessions. OKO mice did not significantly differ from WT controls in locomotor activity following acute- or chronic-morphine treatments. Similarly, mice treated with the Ox1r antagonist did not differ from vehicle controls in locomotor activity following acute- or chronic-morphine treatments. In contrast, while OKO mice did not differ from WT controls in preference for a morphine-paired environment, the Ox1r antagonist significantly attenuated place preference for a morphine-, but not a cocaine-paired, environment. These data suggest that orexin action is not required for locomotor responses to acute and chronic morphine, but Ox1r signaling can influence morphine-seeking in WT animals.