The up-regulation of the cyclic AMP system and enhanced phosphorylation of protein substrates after either sudden interruption of chronic opioid treatment or antagonist administration has been proposed to account for the various behavioral responses observed during withdrawal. Using in situ hybridization histochemistry, we show here for the first time that type VIII adenylyl cyclase mRNA is selectively increased, as early as 12 h after morphine treatment in the locus coeruleus and the amygdala, two brain regions suggested to be important in morphine dependence expression. Moreover, the time course of morphine-induced changes in type VIII adenylyl cyclase mRNA in locus coeruleus is related to the incidence of jumping, the most important sign of morphine withdrawal in mice. In addition, the overexpression of type VIII adenylyl cyclase mRNA in thalamic nuclei could add to morphine tolerance. These findings offer a strong support, at the molecular level, for an altered regulation of the cyclic AMP system in opiate tolerance and dependence. The present measure relates only to type VIII adenylyl cyclase mRNA and similar tests with other cyclases are needed to explore fully this relationship.