Takayuki Nakagawa, Tohru Ozawa, Takeshi Watanabe, Masabumi Minami and
Department of Molecular Pharmacology, Faculty of Pharmaceutical Sciences, Kyoto University, Kyoto 606 - 8501, Japan
* To whom correspondence should be addressed.
Abstract: Chronic and/or sustained opioid treatment has been shown to result in development of
sensitization of the adenylyl cyclase (AC) system or cAMP overshoot. In this study, we investigated the molecular mechanism responsible for sensitization of the AC system using CHO cells co-expressing cloned È-opioid receptor and some chimeric G protein ¿i2/¿q subunits. In CHO cells co-expressing the È-opioid receptor and pertussis toxin-insensitive chimeric ¿i2/¿q subunits with ¿i2 residues Met244 - Asn331, despite pretreatment with pertussis toxin, acute treatment with the È-opioid-receptor - selective agonist U69,593
suppressed forskolin-stimulated cAMP accumulation, while sustained treatment with U69,593 (4 h) induced cAMP overshoot over the naive level by the È-opioid-receptor - selective antagonist norbinaltorphimine
(sensitization of the AC system). On the other hand, in CHO cells co-expressing the È-opioid receptor and pertussis toxin-insensitive chimeric ¿i2/¿q subunits without ¿i2 residues Met244 - Asn331, pretreatment with pertussis toxin completely blocked these acute and sustained effects of U69,593 on cAMP accumulation. These results suggested that the presence of the specific region of ¿i2 (Met244 - Asn331), which was reported to be responsible for the inhibition of AC, and continuous inhibition of AC by ¿i2 is necessary for the development of sensitization.
Keywords: È-Opioid receptor, Adenylyl cyclase, Sensitization, Chimeric G protein ¿ subunit, CHO cell