Mu-opioid receptor activation by methamphetamine metabolite 4-hydroxyamphetamine: a computational insight into a...

Methamphetamine addiction remains a significant global health concern, yet its molecular mechanisms remain incompletely understood. Methamphetamine and its major metabolite, 4-hydroxyamphetamine (4HAMP), can readily cross the blood-brain barrier; however, their interactions with the Mu opioid receptor (MOR) remain unexplored. In this study, molecular docking, molecular dynamics simulations, principal component analysis, free energy landscape mapping, and advanced free energy calculations, including MM/GBSA and adaptive biasing free energy (ABFE) methods were employed, to examine the binding interactions and conformational effects on MOR. The results revealed that both methamphetamine and 4HAMP showed comparable docking and MM/GBSA binding scores, while absolute binding free energy (ABFE) calculations indicated a more favorable value for the 4HAMP:MOR complex. The 4HAMP:MOR complex was stabilized through a salt bridge with ASP149, hydrogen bonding with HSD299, π-π T-shaped interactions with HSD299 and ILE298, and van der Waals contacts with ALA242, MET153, TRP295, and VAL302. These interactions were associated with active-like conformational features of the receptor under the present computational conditions. During 250 ns molecular dynamics simulations, the 4HAMP:MOR complex maintained these key interactions and showed stable conformational behavior with localized free energy minima. Methamphetamine displayed comparable docking and MM/GBSA scores but a less favorable absolute binding free energy under the applied ABFE protocol. Overall, the computational findings suggest that 4HAMP may show compatibility with the MOR orthosteric site, generating a hypothesis of possible opioid-system involvement in methamphetamine addiction that requires experimental validation. These results provide mechanistic insight and warrant further experimental validation to clarify the functional implications of 4HAMP:MOR interactions.