Structural insights into tryptamine psychedelics: The role of hydroxyl indole ring site in 5-HT2A receptor activation and psychedelic-like activity

Recent advancements in the study of mushroom-derived tryptamines, particularly psilocybin and its metabolite psilocin, highlight their unique psychedelic properties and potential therapeutic applications, especially for mental health conditions like depression. This study examines how the position of the hydroxyl group on the indole ring affects the 5-HT_2A receptor activity and psychedelic-like effects of psilocin analogs. Chemically synthesized psilocin (1) and its analogs bufotenine (2), 6-OH-DMT (3), and 7-OH-DMT (4) were assessed for 5-HT_2A receptor agonistic activity using the Gα_q-Gγ dissociation bioluminescence resonance energy transfer (BRET) assay and for psychedelic-like effects through the head-twitch response assay. Results show that compounds with hydroxyl group at the 4th and 5th positions exhibit significantly higher 5-HT_2A agonistic and psychedelic-like activities than those with hydroxyl group at the 6th and 7th positions. Funnel metadynamics simulations revealed that psilocin (1) and bufotenine (2) have lower binding free energies, correlating with experimental data. Analysis of the simulation trajectories reveals that the formation of a hydrogen bond with residue L229 is crucial for guiding psilocin (1) and bufotenine (2) into the 5-HT_2AR binding site. In contrast, analogs 3 and 4, which lack this interaction, fail to be directed into the orthosteric site. Furthermore, psilocin (1) and bufotenine (2) establish a stable salt bridge and hydrogen bond with residue D155. These interactions are more stable compared to those formed by ligands 3 and 4, contributing to the latter's poor 5-HT_2AR activities. These findings underscore the critical role of the hydroxyl group position on the indole ring in modulating 5-HT_2A receptor activity and the corresponding psychedelic-like effects, offering valuable insights for the development of targeted therapeutics.