A general role for adaptations in G-proteins and the cyclic AMP system in mediating the chronic actions of morphine and cocaine on neuronal function

Previous studies have shown that chronic morphine increases levels of the G-protein subunits G_iα and G_oα, adenylate cyclase, cyclic AMP-dependent protein kinase, and certain phosphoproteins in the rat locus coeruleus, but not in several other brain regions studied, and that chronic morphine decreases levels of G_iα and increases levels of adenylate cyclase in dorsal root ganglion/spinal cord (DRG-SC) co-cultures. These findings led us to survey the effects of chronic morphine on the G-protein/cyclic AMP system in a large number of brain regions to determine how widespread such regulation might be. We found that while most regions showed no regulation in response to chronic morphine, nucleus accumbens (NAc) and amygdala did show increases in adenylate cyclase and cyclic AMP-dependent protein kinase activity, and thalamus showed an increase in cyclic AMP-dependent protein kinase activity only. An increase in cyclic AMP-dependent protein kinase activity was also observed in DRG-SC co-cultures. Morphine regulation of G-proteins was variable, with decreased levels of G_iα seen in the NAc, increased levels of G_iα and G_oα amygdala, and no change in thalamus or the other brain regions studied. Interestingly, chronic treatment of rats with cocaine, but not with several non-abused drugs, produced similar changes compared to morphine in G-proteins, adenylate cyclase, and cyclic AMP-dependent protein kinase in the NAc, but not in the other brain regions studied. These results indicate that regulation of the G-protein/cyclic AMP system represents a mechanism by which a number of opiate-sensitive neurons adapt to chronic morphine and thereby develop aspects of opiate tolerance and/or dependence. The findings that chronic morphine and cocaine produce similar adaptations in the NAc, a brain region important for the reinforcing actions of many types of abused substances, suggest further that common mechanisms may underlie psychological aspects of drug addiction mediated by this brain region.