BLOCKADE OF OREXIN RECEPTOR 1 ATTENUATES THE DEVELOPMENT OF MORPHINE TOLERANCE AND PHYSICAL DEPENDENCE IN RATS

Abstract The goals of this study were to evaluate the effects of pretreatment by orexin receptor-1 antagonist on the development of morphine tolerance and physical dependence in rat. Animals were rendered dependent on morphine by subcutaneous (SC) injection of morphine sulfate (10 mg/kg) at set intervals of 12 h for 10 days. Just before the morphine administration, the animals received SB-334867, a selective orexin receptor 1 (OXR1) antagonist. To assess morphine tolerance, the antinociceptive responses of morphine were measured using the warm-water tail immersion test before and after its administration. On day 11, naloxone was injected 2 h after morphine administration and the physical dependence evaluated by quantifying/scoring naloxone-precipitated withdrawal signs for 30 min. The effect of chronic SB-334867 on locomotion was carried out by calculating the number of grid crossings as a measure of locomotor activity. Our findings demonstrated that although morphine-tolerance tended to develop in response to repeated injections of morphine, pre-treatment of OXR1 antagonist prevented this effect, causing a delay in the development of morphine-tolerance. Moreover, co-administration of orexin receptor 1 antagonist with morphine significantly decreased the somatic signs of withdrawal including diarrhea, teeth chattering, jumping, and defecation. Administration of SB-334867 alone or in a chronic co-administration with morphine failed to change locomotor activity. These results suggest that the activation of OXR1 might be involved in the development of morphine tolerance and dependence.