Chronic exposure to mu-opioid agonists produces constitutive activation of mu-opioid receptors in direct proportion to the efficacy of the agonist used for pretreatment.

Chronic morphine treatment has been shown to produce constitutive activation of mu-opioid receptors, and this transition might contribute to the development of tolerance and dependence. The apparent ability of chronic morphine to increase the spontaneous, agonist-independent activation of mu-opioid receptors may be unique, due to its distinct partial agonist properties of possessing a relatively high intrinsic activity coupled with a poor ability to produce desensitization and down-regulation. Therefore, the present study tested the hypothesis that prolonged exposure to morphine would produce greater constitutive activity of mu-opioid receptors than exposure to the full agonist [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO). GH(3) cells expressing mu-opioid receptors were exposed to chronic morphine, DAMGO, or no opioid under conditions determined to produce maximal desensitization, down-regulation, and cAMP rebound. After chronic treatment, the mu-opioid antagonists naloxone and beta-chlornaltrexamine (beta-CNA) were evaluated in two assays predictive of inverse agonist activity. Both antagonists produced a concentration-dependent inhibition of [(35)S]GTP gamma S binding only in membranes prepared from cells chronically exposed to opioids. This effect was reversed by the neutral mu-opioid antagonist CTAP. Additionally, conditions known to uncouple G protein-coupled receptors from G proteins produced a leftward shift in the competition curve of beta-CNA for [(3)H]DAMGO binding only in membranes prepared from chronically treated cells. In contrast, these conditions produced no shift in the competition curve by the neutral antagonist CTAP in cells exposed to chronic DAMGO. Therefore, prolonged exposure of GH(3)MOR cells to opioids produced constitutive activation of mu-opioid receptors. Surprisingly, chronic treatment with the more efficacious agonist DAMGO produced greater increases in both measures of inverse agonist activity than did morphine. These observations may lend novel insight into the mechanisms of opioid tolerance and dependence.