Opiate-induced Adenylyl Cyclase Superactivation Is Isozyme-specific

Abstract

While acute activation of inhibitory G i/o -coupled receptors leads to inhibition of adenylyl cyclase, chronic activation of such receptors leads to an increase in cAMP accumulation. This phenomenon, observed in many cell types, has been referred to as adenylyl cyclase superactivation. At this stage, the mechanism leading to adenylyl cyclase superactivation and the nature of the isozyme(s) responsible for this phenomenon are largely unknown. Here we show that transfection of adenylyl cyclase isozymes into COS-7 cells results in an isozymespecific increase in AC activity upon stimulation (e.g. with forskolin, ionomycin, or stimulatory receptor ligands). However, independently of the method used to activate specific adenylyl cyclase isozymes, acute activation of the -opioid receptor inhibited the activity of adenylyl cyclases I, V, VI, and VIII, while types II, IV, and VII were stimulated and type III was not affected. Chronic -opioid receptor activation followed by removal of the agonist was previously shown, in transfected COS-7 cells, to induce superactivation of adenylyl cyclase type V. Here we show that it also leads to superactivation of adenylyl cyclase types I, VI, and VIII, but not of type II, III, IV, or VII, demonstrating that the superactivation is isozyme-specific. Not only were isozymes II, IV, and VII not superactivated, but a reduction in the activities of these isozymes was actually observed upon chronic opiate exposure. These results suggest that the phenomena of tolerance and withdrawal involve specific adenylyl cyclase isozymes. The synthesis of cAMP by adenylyl cyclase (AC)1 is modulated by hormones and neurotransmitters acting via receptors that activate GTP-binding proteins (G proteins). To date, mRNAs encoding nine distinct isozymes of AC have been identified (1-11). Sequence and functional similarities allow the categorization of these ACs into six classes: (a) AC type I (AC-I) is stimulated by Ca 2ϩ /calmodulin, possibly independently of G ␣s stimulation, and is inhibited by G ␤␥ subunits; (b) AC-VIII is stimulated by Ca 2ϩ /calmodulin; (c) AC-V and AC-VI are inhibited by low levels of Ca 2ϩ but are unaffected by G ␤␥ subunits; (d) AC-II, AC-IV, and AC-VII comprise a subfamily, where AC-II and AC-IV are highly activated by G ␤␥ subunits in the presence of activated G ␣s , while AC-II and AC-VII are stimulated by activation of protein kinase C; (e) AC-III is stimulated by a high concentration of Ca 2ϩ /calmodulin in the presence of G ␣s but is unaffected by G ␤␥ subunits; (f) AC-IX, which has only recently been cloned, has thus far only been found to be affected by G ␣s . The activities of all AC isozymes seem to be stimulated by G ␣s , but to different extents (5, 9, 10, 12-15).Stimulation of seven-transmembrane domain inhibitory receptors (e.g. -, ␦-, and -opioid receptors and m 2 -and m 4 -muscarinic receptors) activates G i/o proteins, as a result of which these G proteins dissociate into G ␣ and G ␤␥ dimers (16 -18). The G ␣i subunit interacts with AC, leading to its acute inhibition ...

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Identity of adenylyl cyclase isoform determines the G protein mediating chronic opioid‐induced adenylyl cyclase supersensitivity

“…Due to the relative resistance of endogenous AC type IX to stimulation by forskolin (Avidor-Reiss et al 1997), addition of the diterpene (1 lmol/L) only moderately elevated intracellular cAMP levels ( 3-fold increase). Although [ 3 H]diprenorphine binding experiments verified strong l-OR expression (not shown), neither acute nor chronic morphine treatment had any effect on intracellular cAMP production (Fig.…”

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