Sensitization of the Adenylyl Cyclase System in Cloned κ-Opioid Receptor-Transfected Cells Following Sustained Agonist Treatment: A Chimeric Study Using G Protein α_i2/α_q Subunits

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 κ-opioid receptor and some chimeric G protein α_i2/α_q subunits. In CHO cells co-expressing the κ-opioid receptor and pertussis toxin-insensitive chimeric α_i2/α_q subunits with α_i2 residues Met²⁴⁴–Asn³³¹, despite pretreatment with pertussis toxin, acute treatment with the κ-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 κ-opioid-receptor–selective antagonist norbinaltorphimine (sensitization of the AC system). On the other hand, in CHO cells co-expressing the κ-opioid receptor and pertussis toxin-insensitive chimeric α_i2/α_q subunits without α_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 α_i2 (Met²⁴⁴-Asn³³¹), which was reported to be responsible for the inhibition of AC, and continuous inhibition of AC by α_i2 is necessary for the development of sensitization.