Abstract
Adaptations to long-term morphine treatment resulting in tolerance are protective by counteracting the consequences of sustained opioid receptor activation. Consequently, the manifestation of specific adenylyl cyclase (AC)-related neurochemical sequelae of long-term morphine treatment should depend on the consequences of short-term μ-opioid receptor (MOR) activation. We tested this by comparing complementary chemical sequelae of long-term morphine treatment among cells in which short-term MOR activation inhibited instead of stimulated AC activity. Short-term activation of MOR in Chinese hamster ovary (CHO) cells stably transfected with MOR (MOR-CHO) inhibits AC activity. Long-term morphine treatment of these cells increased AC and Gβ phosphorylation, membrane protein kinase Cγ (PKCγ) translocation, and MOR Gs association. All converge, shifting the consequences of short-term MOR activation from Gαi/Gαo inhibitory to AC stimulatory signaling. In contrast, overexpression of the Gβγ-stimulated AC isoform AC2 (which converted MOR-coupled inhibition to stimulation of AC) eliminated or reversed these adaptations to long-term morphine treatment; it negated the increase in Gβ phosphorylation and PKCγ translocation while reversing the increase in AC phosphorylation and MOR Gs association. These adaptations greatly attenuated MOR-coupled stimulation of AC activity. Altered overexpression of AC protein per se was not a confounding factor because MOR-CHO overexpressing AC1, which is inhibited by short-term MOR activation, manifested adaptations to long-term morphine treatment qualitatively identical with those of MOR-CHO. These results reveal that adaptations elicited by long-term morphine treatment depend on the effects of short-term MOR activation. This dynamic and pliable nature of tolerance mechanisms could represent a new paradigm for pharmacotherapeutics.
Footnotes
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ABBREVIATIONS: AC, adenylyl cyclase; MOR, μ-opioid receptor; MOR-CHO, Chinese hamster ovary cells stably expressing μ-opioid receptor; AC1-MOR-CHO, Chinese hamster ovary cells stably expressing μ-opioid receptor overexpressing AC1; AC2-MOR-CHO, Chinese hamster ovary cells stably expressing μ-opioid receptor overexpressing AC2; IP, immunoprecipitate; LMMP, longitudinal muscle myenteric plexus; PKCγ, protein kinase Cγ; GPCR, G protein-coupled receptor; DTT, dithiothreitol; GTPγS, guanosine 5′-3-O-(thio)triphosphate; ANOVA, analysis of variance.
- Received September 26, 2007.
- Accepted November 27, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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