The biochemical analysis of methadone modulation on morphine-induced tolerance and dependence in the rat brain.

We have recently demonstrated that the combination of methadone and morphine enhances the ability of morphine to induce mu-opioid peptide (MOP) receptor endocytosis. As a result, rats receiving both drugs show reduced morphine tolerance and dependence. In the present study, we identify the biochemical basis for the protective effect of the drug combination. In rats treated with morphine alone, the inhibitory effect of DAMGO on forskolin-stimulated adenylyl cyclase activity was significantly reduced in a brain-region-selective manner. Importantly, these reductions were prevented in animals receiving the drug combination. We found that these changes were not due to alterations in MOP receptor density, or MOP receptor-G protein coupling, as no significant change in these parameters was observed. Together these data demonstrate that neither changes in receptor number nor function are required for morphine tolerance and dependence. Rather, brain-region-selective changes in adenylyl cyclase signal transduction are critical, and both these biochemical changes and the behavioral effects are prevented by facilitating endocytosis of the MOP receptor.