Interest in classical or serotonergic psychedelics has resurged, given their potential to induce rapid, sustained therapeutic effects. Psychedelics are limited by their hallucinogenic effects and can cause confusion and anxiety. Although a recent preclinical study suggested the possibility of disentangling psychedelics from their therapeutic properties, it is unclear which signaling pathways and receptors mediate psychedelic effects.Serotonergic psychedelics are derivatives of different chemical scaffolds, including lysergamides, tryptamines, and phenethylamines, which all activate the serotonin G protein-coupled receptor (GPCR) 5-HT2AR. GPCR ligands stabilize some conformations of the receptor, favoring coupling to downstram transducer proteins. Thus, ligand-dependent bias has implications for clinical pharmacology and drug development.
Current psychedelics activate 5-HT2AR and transduce the inner signal with both β-arrestin2 and Gq proteins; however, the contribution of these pathways in mediating psychedelic effects is uncertain. Although there are non-psychedelic agonists of 5-HT2AR, there is no explanation for the lack of psychedelic action. In a new study, researchers developed 5-HT2AR-biased agonists to unravel a molecular and mechanistic explanation for biased 5-HT2AR agonism. First, they showed that 5-HT2AR strongly coupled to arrestin2, Gq/11, G protein subtypes and β-arrestin 1. Next, classical psychedelics were tested for their effects on Gq dissociation and β-arrestin 2 recruitment. Psychedelics exhibited dynamic, time-dependent activity profiles, which, in some cases, exceeded the activity of the endogenous agonist, serotonin, at longer time points.
The effects of all tested psychedelics on β-arrestin and Gq activity were comparable at equivalent time points, mirroring the endogenous agonist. Moreover, all psychedelics did not have a preference for β-arrestin 2 or Gq, indicating that they were not biased for either transducer.Next, the team aimed to develop biased agonists to interrogate 5-Ht”AR-coupled signaling pathways with psychedelic potential. They focused on the phenethylamine scaffold and selected 25N as the core. The researchers confirmed it was a potent, high-affinity 5-HT2 agonist with weak selectivity for serotonin receptors 2A, 2B and 2C. N-benzylation of 25N generated 25N-NB, which substantially decreased 5-HT2AR efficacy, while retaining potent 5-HT2CR activity. Further, the team synthesized multiple analogs to modify the electrostatic properties of the N-benzyl ring.
They observed that increasing electron density around the C5′ position of the ring increased 5-HT2AR affinity and agonist potency. This led to the discovery of various high-affinity 5-HT2AR agonists. Next, they evaluated the effects of substitutions at the N-benzyl 2-position on 5-HT2AR selectivity. Bulkier 2-iodo or 2-bromo substituent on the ring decreased 5-HT2BR and 5-HT2CR activities but retained potent 5-HT2AR activity, increasing 5-HT2AR selectivity. 25N-NBI was the most selective 5-HT2AR agonist from this series, and several assays confirmed its high selectivity. Besides, 25N-NBI induced a head-twitch response (HTR) in mice, confirming its psychedelic potential. Nevertheless, 25N-NBI lacked preference for Gq or β-arrestin 2 activity. Thus, the team focused on 25N-NB-2-OH-3-Me, another 25N analogy with the highest affinity for 5-HT2AR.
This compound showed a selective decrease in Gq-efficacy but not β-arrestin 2 efficacy, suggesting that steric effects around the 3-position on the ring could influence 5-HT2AR-biased agonism. Additionally, the team explored whether larger N-biphenyl (25N-NBPh) or N-naphthyl (25N-N1-Nap) ring could affect Gq signaling. Interestingly, both 25N-NBPh and 25N-N1-Nap substantially reduced Gq-efficacy but preserved β-arrestin 2 efficacy. These analogs had substantially weaker Gq and β-arrestin 2 activities forreceptor types 2B and 2C. Besides, binding assays indicated these compounds had weak affinities for other 5-HT receptors and off-targets. Further analyses showed that a threshold level of Gq activation was required for psychedelic-like effects and that 5-HT2AR-Gq signaling was necessary for HTR and psychedelic potential.
This is why other serotoninergic drugs like lisuride, 2-Bromo-LSD and 6-MeO-DMT are likely non-psychedelic because of weak 5-HT2AR Gq-directed efficacy. Lastly, the team demonstrated that these two lat compounds induced potent β-arrestin 2-dependent receptor internalization. By contrast, pimavanserin, a known 5-HT2AR antagonist / inverse agonist, failed to induce receptor internalization. These informations have implications for understanding the neurobiological basis of psychedelic effects and reveal strategies for designing non-psychedelic 5-HT2AR agonists that can potentially be used as therapeutics.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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