Environmental Enrichment Ameliorates Psychological Dependence Symptoms and Voluntary Morphine Consumption in Morphine Withdrawn Rats Under Methadone Maintenance Treatment

INTRODUCTION: Previous studies have shown that physical and psychological dependence and the vulnerability to relapse are still present during MMT. Thus, this study examined whether Enriched Environment (EE) would attenuate anxiety, depressive, and obsessive-compulsive-like behaviors, as well as voluntary morphine consumption following Methadone Maintenance Treatment (MMT) in morphine withdrawn rats. METHODS: The rats were injected bi-daily doses (10 mg/kg, 12-h interval) of morphine for 14 days. Then, the rats were reared in a Standard Environment (SE) or EE for 30 more days during morphine withdrawal, simultaneous with receiving MMT. The rats were tested for anxiety (the Elevated Plus Maze [EPM]) and depression (Sucrose Preference Test [SPT]), Obsessive-Compulsive Disorder (OCD) as grooming behavior, and voluntary morphine consumption using a Two-Bottle Choice (TBC) paradigm. RESULTS: The findings revealed that EE experience in morphine withdrawn rats under MMT significantly increased the EPM open-arm time and higher sucrose preference than SE rats. Also, we found that the EE decreased the self-grooming behavior and morphine preference ratio in morphine withdrawn rats receiving MMT compared to the SE group. CONCLUSION: We conclude that exposure to EE decreased methadone-induced anxiety, depressive and OCD-like behaviors, and voluntary morphine consumption in morphine withdrawn rats under MMT. Thus, the EE seems to be one of the strategies for reducing MMT-induced behavioral dysfunction and the risk of relapse induced by morphine withdrawal.