Characterization of methadone as a β-arrestin-biased μ-opioid receptor agonist

Background: Methadone is a unique mu-opioid receptor agonist. Although several researchers have insisted that the pharmacological effects of methadone are mediated through the blockade of NMDA receptor, the underlying mechanism by which methadone exerts its distinct pharmacological effects compared to those of other mu-opioid receptor agonists is still controversial. In the present study, we further investigated the pharmacological profile of methadone compared to those of fentanyl and morphine as measured mainly by the discriminative stimulus effect and in vitro assays for NMDA receptor binding, mu-opioid receptor-internalization, and mu-opioid receptor-mediated beta-arrestin recruitment. Results: We found that fentanyl substituted for the discriminative stimulus effects of methadone, whereas a relatively high dose of morphine was required to substitute for the discriminative stimulus effects of methadone in rats. Under these conditions, the non-competitive NMDA receptor antagonist MK-801 did not substitute for the discriminative stimulus effects of methadone. In association with its discriminative stimulus effect, methadone failed to displace the receptor binding of MK801 using mouse brain membrane. Methadone and fentanyl, but not morphine, induced potent mu-opioid receptor internalization accompanied by the strong recruitment of beta-arrestin-2 in mu-opioid receptor-overexpressing cells. Conclusions: These results suggest that methadone may, at least partly, produce its pharmacological effect as a beta-arrestin-biased mu-opioid receptor agonist, similar to fentanyl, and NMDA receptor blockade is not the main contributor to the pharmacological profile of methadone.