Classical psychedelics alter consciousness by activating the 5-HT2A receptor in the brain, but the precise mechanism is not fully understood. To study this receptor's signaling with high spatiotemporal precision, researchers designed photoswitchable ligands based on the psychedelic N,N-dimethyltryptamine (DMT). By incorporating the DMT-indole ring into a photoswitchable system, they created red-shifted ligands operable by visible light. Among these azo-DMTs, compound 2h (Photo-DMT) stands out: its cis isomer shows DMT-like activity, while the trans isomer acts as a weak partial agonist. This cis-on efficacy switch expands the pharmacological toolbox for investigating the complex signaling of the 5-HT2A receptor.
A photoswitchable ligand for the serotonin 2A receptor (5-HT2AR) was designed to independently study G protein- and β-arrestin2-dependent signaling pathways. The cis-photoisomer binds the receptor with greater affinity than the trans-isomer, at nanomolar concentrations. In functional assays, this ligand showed a preference for recruiting β-arrestin2 over mini-Gαq relative to LSD, offering a tool to investigate β-arrestin2's role in 5-HT2AR signaling and its potential involvement in psychedelic effects.
The efficacy of photoswitchable ligands for the human 5-HT2A receptor depends on the vertical depth of ligand insertion into the orthosteric binding pocket, determined by the position of a methoxy substituent. Molecular dynamics simulations show that a para-methoxy ligand (compound 1) acts as a weak antagonist in its trans form because it forms hydrogen bonds with Asp231 and Thr160, anchoring it deeper and preventing engagement with activation-critical residues. A meta-methoxy ligand (compound 2) lacks these anchoring interactions, adopts a shallower pose, and maintains agonist activity in both forms. The authors propose that ligand insertion depth is a critical determinant of efficacy, offering a framework for designing light-sensitive GPCR ligands with tunable signaling.