CHRONIC OPIOID TREATMENT INDUCES ADENYLYL-CYCLASE-V SUPERACTIVATION -INVOLVEMENT OF G-BETA-GAMMA

It has been known for some time that chronic treatment of neuronal cel ls and tissues with opioids, contrary to their acute effect, leads to an increase in cAMP accumulation. This phenomenon, defined as adenylyl cyclase superactivation, has been implicated in opiate addiction, yet the mechanism by which it is induced remains unclear. Here, we show t hat this phenomenon can be reproduced and studied in COS-7 cells cotra nsfected with adenylyl cyclase type V and mu-opioid receptor cDNAs. Th ese cells display acute opioid inhibition of adenylyl cyclase activity , whereas prolonged exposure to the mu-agonist morphine or [D-Ala(2), N-methyl-Phe(4), Gly-ol(5)]enkephalin leads to a time-dependent supera ctivation of adenylyl cyclase. This superactivated state is reversible , because it is gradually lost following agonist withdrawal, Adenylyl cyclase superactivation can be prevented by pertussis toxin pretreatme nt, indicating the involvement of G(i/o) proteins, or by cotransfectio n with the carboxyl terminus of beta-adrenergic receptor kinase or wit h alpha-transducin (scavengers of G(beta gamma) dimers), indicating a role for the G protein beta gamma dimers in adenylyl cyclase superacti vation. However, contrary to several other G(beta gamma)-dependent sig nal transduction mechanisms (e.g. the extracellular signal-regulated k inase 2/MAP kinase pathway), adenylyl cyclase superactivation is not a ffected by the Has dominant negative mutant N17-Ras.