Differential effects of morphine and cocaine on locomotor activity and sensitization in μ-opioid receptor knockout mice

• Published in: Neuroscience letters 2003-06, Vol.344 (1), p.37-40
• Main Authors: Yoo, Ji-Hoon, Yang, Eun-Mi, Lee, Seok-Yong, Loh, Horace H., Ho, Ing K., Jang, Choon-Gon
• Format: Article
• Language: English
• Subjects: Adaptation, Physiological - physiology; Animals; Behavioral sensitization; Biological and medical sciences; Cocaine; Cocaine - pharmacology; Dopamine Uptake Inhibitors - pharmacology; Drug addictions; Knockout mice; Locomotor activity; Medical sciences; Mice; Mice, Knockout; Morphine; Morphine - pharmacology; Motor Activity - drug effects; Motor Activity - physiology; Narcotics - pharmacology; Receptors, Opioid, mu - genetics; Receptors, Opioid, mu - physiology; Toxicology; μ-Opioid receptors
• ISSN: 0304-3940; 1872-7972
• DOI: 10.1016/S0304-3940(03)00410-5

The present study was undertaken to investigate the hypothesis that the μ-opioid receptors play a crucial role in locomotor activity and sensitization to cocaine and morphine in wild-type and μ-opioid receptor knockout mice. Our results show that morphine and cocaine increased locomotor activity in wild-type mice, but failed to increase locomotor activity in μ-opioid receptor knockout mice, suggesting a contribution of μ-opioid receptor. Repeated morphine treatment induced sensitization in wild-type mice, but this was not observed in μ-opioid receptor knockout mice. In contrast repeated cocaine treatment produced sensitization in μ-opioid receptor knockout mice, but not in wild-type mice on day 6. However, the sensitization to cocaine was observed in μ-opioid receptor knockout and wild-type mice on day 12. These results suggest that the expression of μ-opioid receptor may contribute to locomotor sensitization induced by morphine, but that μ-opioid receptor does not play an important role in mediating sensitization to cocaine.