ANXIOLYTIC AND ANTIDEPRESSANT EFFECTS OF INHIBITORS OF ENDOCANNABINOID DEGRADING ENZYME IN CHRONIC RESTRAINT- STRESSED MICE

Abstract Objective Cannabinoid receptors are the molecular targets of D9-tetrahydrocannabinol, a major ingredient in cannabis sativa. Endocannabinoid such as arachidonate-based lipids 2-arachidonoylglycerol (2-AG) and N-acyl arachidonoyl ethanolamine (anandamide) are the primary endogenous ligands of cannabinoid CB receptors. Endocannabinoid systems play important roles in physiological functions in the central nervous system, such as pain perception, appetite, psychomotor behavior, emotion, reward system and cognitive function. Rimonabant, a cannabinoid CB1 receptor antagonist, has been known to induce psychiatric symptoms such as anxiety and depression. On the other hand, chronic stress is associated with the development of anxiety and depression. Acute psychological stress has been shown to increase anandamide levels in healthy volunteers, while patients with posttraumatic stress disorder have exhibited decreased levels of anandamide and 2-AG in plasma. Based on these backgrounds, we investigated the effects of inhibitors of the cannabinoid degrading enzyme on abnormalities of emotional behavior induced by chronic restraint stress in mice. Method For exposing chronic restraint stress, mice were immobilized for 3 hours a day for 10 consecutive days in a 50mL syringe equipped with numerous small breathing holes to ensure airflow. The control group was isolated with free moving in the homecage without food and water for the same duration as the stress exposing schedule. JZL184 (40 mg/kg, i.p.), a selective inhibitor of monoacylglycerol lipase which hydrolyze 2-AG, URB597 (10 mg/kg, i.p.), a selective inhibitor of fatty acid amide hydrolase which hydrolyze anandamide, fluvoxamine (30 mg/kg, p.o.), a selective serotonin reuptake inhibitor to evaluate the predictive validity of an animal model for depression, or vehicle were administered one hour before the restraint stress. After exposing the chronic restraint stress, we carried out behavioral evaluation including the openfield test, elevated plus-maze test and forced swim test. Result In mice subjected to chronic restraint stress, time spent in the open arm of the elevated plus- maze, as well as time spent in the central area of the openfield apparatus decreased, indicating anxiogenic behavior. Furthermore, in the forced swim test, immobility time increased in mice exposed to chronic restraint stress, implying depressive-like behavior. These behavioral abnormalities in chronic restraint-stressed mice were ameliorated by fluvoxamine administered prior to restraint stress. These results verified the face and predictive validity of animal models of depression. Furthermore, the prior administration of either JZL184 or URB597 also ameliorated both anxiety- and depression-like behaviors in chronic restraint-stressed mice. Conclusion These results suggested that mice exposed to chronic restraint stress have validity as an animal model for depression. Since both anxiety- and depression-like behavior induced by chronic restraint stress were ameliorated by inhibitors of the endocannabinoid degrading enzyme, endocannabinoids in the brain may play a crucial role in regulating anxiety and depression. Furthermore, the inhibitor of the endocannabinoid degrading enzyme could potentially be a candidate for development of novel anxiolytics and/or antidepressants.