RESUMO
Previous research has demonstrated that acute and chronic opiate treatment alters receptor- and postreceptor-mediated adenylyl cyclase activity. This study examined the regulation of G protein- and forskolin-mediated adenylyl cyclase activity in mouse striatum and cortex after short- and long-term opiate exposure. To directly measure adenylyl cyclase enzymatic activity, assays were done in the presence of catalytic site activator forskolin. To measure G protein-mediated adenylyl cyclase activity, assays were performed in the presence of non-hydrolyzable guanosine 5'-triphosphate (GTP) analogue, 5'-guanylyl-imidodiphosphate. Short-term in vitro morphine exposure produced reductions in forskolin-stimulated adenylyl cyclase activity in striatal and cortical tissues. Long-term morphine treatment in mice was performed via morphine- or placebo-pellet implantation for 72 h; this treatment has been shown to produce opiate dependence and withdrawal. In both opiate-dependent and opiate withdrawing mice (1 h post-naloxone induction), there were significant increases in G protein-mediated adenylyl cyclase activity in the striatum (vs. controls). In opiate-dependent mice, there was an decrease in G protein-mediated adenylyl cyclase activity in cortex. In opiate-dependent mice, there were no changes in forskolin-stimulated adenylyl cyclase in the striatum or cortex. Increases in striatal G protein-mediated adenylyl cyclase could represent a compensatory adaptation that opposes the persistent inhibition of adenylyl cyclase by chronic opiate treatment contributing to the expression of opiate dependence and withdrawal.
