A possible role for G proteins in contributing to the chronic actions of opiates was investigated in the rat locus coeruleus (LC). The LC is a relatively homogeneous brain region that appears to play an important role in mediating acute and chronic opiate action in animals, as well as in humans. It was found that chronic, but not acute, treatment of rats with morphine, under conditions known to induce states of opiate tolerance and dependence, produced an increase in the level of pertussis toxin-mediated ADP-ribosylation of G proteins in the LC. The morphine-induced increase in ADP-ribosylation occurred in both Gi and Go, and was observed over a 30-fold range of NAD concentrations used. Concomitant treatment of rats with the opiate receptor antagonist naltrexone blocked the ability of morphine to produce this effect. In contrast, chronic morphine had no effect on pertussis toxin-mediated ADP-ribosylation of Gi and Go in the other brain regions studied, including the neostriatum, frontal cortex, and dorsal raphe. Chronic morphine also had no effect on cholera toxin-mediated ADP-ribosylation of Gs in the LC and these other brain regions. Preliminary immunoblot analysis revealed that increased ADP-ribosylation levels of the alpha subunit of Go in the LC were associated with equivalent increases in the immunoreactivity of this protein in this brain region. It is possible that the observed regulation of G-proteins by morphine in the LC represents part of the changes that underlie opiate addiction in these neurons.