Augmentation of Morphine-Induced Sensitization but Reduction in Morphine Tolerance and Reward in Delta-Opioid Receptor Knockout Mice

Studies in experimental animals have shown that individuals exhibiting enhanced sensitivity to the locomotor-activating and rewarding properties of drugs of abuse are at increased risk for the development of compulsive drug-seeking behavior. The purpose of the present study was to assess the effect of constitutive deletion of delta-opioid receptors (DOPr) on the rewarding properties of morphine as well as on the development of sensitization and tolerance to the locomotor-activating effects of morphine. Locomotor activity testing revealed that mice lacking DOPr exhibit an augmentation of context-dependent sensitization following repeated, alternate injections of morphine (20 mg/kg; s.c.; 5 days). In contrast, the development of tolerance to the locomotor-activating effects of morphine following chronic morphine administration (morphine pellet: 25 mg: 3 days) is reduced relative to WT mice. The conditioned rewarding effects of morphine were reduced significantly in DOPrKO mice as compared to WT controls. Similar findings were obtained in response to pharmacological inactivation of DOPr in WT mice, indicating that observed effects are not due to developmental adaptations that occur as a consequence of constitutive deletion of DOPr. Together, these findings indicate that the endogenous DOPr system is recruited in response to both repeated and chronic morphine administration and that this recruitment serves an essential function in the development of tolerance, behavioral sensitization, and the conditioning of opiate reward. Importantly, they demonstrate that DOPr has a distinct role in the development of each of these drug-induced adaptations. The anti-rewarding and tolerance-reducing properties of DOPr antagonists may offer new opportunities for the treatment and prevention of opioid dependence as well as for the development of effective analgesics with reduced abuse liability.