Regulation of adenosine- and G protein-mediated signalling in opiate dependence

Regulation of several receptor- and G protein-mediated adenylate cyclase (AC) pathways, including adenosine receptor signalling, is hypothesized to have a role in mediating opiate dependence. We examined the effects of short-term in vitro and chronic in vivo morphine exposure on adenosine receptor- and G protein-mediated AC activity in brain tissues. Implantation of morphine or placebo pellets in CD-1 mice (72 hr treatment) produced a state of naloxone-induced opiate withdrawal (jumping, wet dog shakes, tremors, diarrhea) and produced tolerance to the acute stimulant effects of IP morphine. In brain tissues of morphine- or placebo-pellet implanted mice, AC activity was measured via a competition binding assay for [3H]cAMP. In vitro morphine exposure in cortex and striatum from untreated mice significantly inhibited Porskolin-stimulated AC activity. Chronic in vivo morphine treatment significantly enhanced striatal Gpp[NH]p-stimulated (GTP analogue) AC dose-response curves and increased Emax values by 31% from 830 (± 10) to 1085 (39) pmol cAMP/mg/min. Additionally, EC50 values were decreased by 46% from 1.95 μM to 0.95 μM in the morphine treated membranes. These results suggest chronic morphine produces adaptive upregulation and sensitization of G protein-mediated AC activity in striatum. In cortex, morphine significantly altered Gpp[NH]p dose-response curves and decreased Emax values by 16%. Adenosine A1 agonist N6-cyclopentyladenosine inhibited AC activity and this inhibition was significantly enhanced in cortical membranes from morphine treated mice. Chronic morphine treatment significantly reduced adenosine A2a agonist (CGS 21680) stimulation of AC in striatum in comparison to vehicle treatment. Emax values from these curves were decreased by 13% from 556 (± 14) to 486 (11) pmol cAMP/mg/min by morphine treatment. These data support the hypothesis that enhanced A1 adenosine receptor and decreased A2a adenosine receptor function develops in opiate dependence and this may be an adaptive response that opposes opiate-mediated upregulation of AC.