Deciphering the role of Gi2 in opioid-induced adenylyl cyclase supersensitization.

Prolonged opioid treatment of HEK 293 cells expressing opioid receptors are known to induce adenylyl cyclase supersensitization, a process that requires pertussis toxin (PTX)-sensitive G(i/o) proteins. Here, the role of Gi2 in adenylyl cyclase supersensitization was investigated. A PTX-insensitive G alpha(i2)/z chimera was stably co-expressed with mu-, kappa- or delta-opioid receptors in HEK 293 cells. Functional coupling of G alpha(i2)/z to the opioid receptors was demonstrated by opioid-induced inhibition of adenylyl cyclase and stimulation of ERK1/2 phosphorylation in PTX-treated cells. Chronic opioid treatment of each cell line led to adenylyl cyclase supersensitization but this response was blocked by PTX. Our results demonstrated that although PTX-sensitive G proteins are obligatory for opioid-induced adenylyl cyclase supersensitization, Gi2 alone was insufficient to mediate this response.