Psychedelic Drug Actions at Serotonin Receptors and the Path Towards Safer Treatments for Mental Disorders

How can we capture the therapeutic potential that psychedelic drugs have demonstrated against mental disorders while fading their risks in the discovery of new medicines?

A key insight highlighted using fluorescent biosensors is that non-hallucinogenic analogues of these drugs are characterised by relatively modest intrinsic efficacy at the serotonin 5-HT2A receptor.

Psychedelic drugs that activate the serotonin 5-HT2A receptor (5-HT_2AR) have gained massive scientific interest for their therapeutic potential against a range of psychiatric disorders, including depression, anxiety, obsessive-compulsive disorder, and addiction¹. In particular, many researchers are considering approaches for developing non-hallucinogenic analogues to make these treatments more scalable, affordable, and applicable to larger populations due to the extensive medical supervision required in the current model of psychedelic-assisted psychotherapy and the counter-indications against patients with risks of psychosis and self-harm^2-4. However, greater evidence is necessary on the molecular mechanisms by which psychedelics produce their hallucinogenic effects through the 5-HT_2AR, with different studies indicating that either strong intrinsic efficacy⁵ or biased agonism between G protein signalling and β-arrestin recruitment pathways could be responsible^6-9. Notably, this investigation is challenged by various methodological factors such as the sensitivity, time-dependence, and physiological relevance of bioassays that must be addressed to effectively determine and translate drug activity.

In a recent study published by British Journal of Pharmacology, University of Oxford researchers in collaboration with the biotechnology company Compass Pathways implemented Montana Molecular’s fluorescent biosensors to elucidate the mechanism of psychedelic drug actions at the 5-HT_2AR¹⁰. Towards addressing this need, the authors examined various 5-HT_2AR agonists at the Gα_q/11 signalling and β-arrestin recruitment pathways in recombinant human SH-SY5Y neuroblastoma cells (Figure 1). Using Montana Molecular’s Borealis biosensor and the associated signalling kinetics analyses developed by Dr Sam Hoare^11-13 enabled the authors to reliably and accurately measure β-arrestin recruitment to the 5-HT2AR with several practical advantages. The assay operated with an unmodified version of the receptor, important for preserving its native conformational dynamics and functionality¹⁴. The G protein-coupled receptor (GPCR) kinases (GRKs) included in the kit were crucial for potentiating the interaction between the 5-HT_2AR and β-arrestin to ensure that a response was detected even from weak agonists, avoiding a misleading lack of response that is commonly found in other assays of this pathway, due to the fact that it contains no inherent signal amplification^15-18. Being able to monitor β-arrestin recruitment in real-time with the fluorescent biosensor facilitated quantification of the steady state response, avoiding inconsistent comparisons of activity that can arise at early timepoints when the response is unknowingly incomplete^{11,13,15,18-23}. The BacMam vector packaging the 5-HT_2AR, β-arrestin recruitment biosensor, and GPCR kinase allowed these components of the assay to be abundantly expressed with a simple workflow and minimal variability in cells of biological relevance to the human brain. SH-SY5Y cells are known to more appropriately represent neuronal function than commonly used human embryonic kidney 293 (HEK293) or Chinese hamster ovary (CHO) cells and were compatibly employed for this reason despite their typically lower transfectability^24-28. Lastly, the readily titratable nature of this expression provided the capability to adjust the sensitivity of the assay to ensure that all drugs produced a partial agonist maximal response, which is essential for interpreting differences in intrinsic efficacy^16,29.

Figure 1. Borealis sensor for β-arrestin recruitment assists in demonstrating that the non-hallucinogenic 5-HT2A receptor (5-HT_2AR) agonists lisuride and TBG can be distinguished from psychedelic drugs by modest intrinsic efficacy rather than biased signalling in a recent study by Ippolito and colleagues¹⁰. Concentration-response curves of serotonin (5-HT) and a range of 5-HT_2AR agonists at the (A) Gα_q/11 signalling pathway using the inositol monophosphate (IP1) accumulation assay and (B) β-arrestin2 recruitment pathway using Montana Molecular’s Borealis fluorescent biosensor in recombinant human SH-SY5Y neuroblastoma cells. (C) 5-HT_2AR agonist activity relative to serotonin for the IP1 accumulation and β-arrestin2 recruitment pathways, where deviation from the line of unity reflects biased signalling.

This approach revealed that non-hallucinogenic analogues can be distinguished from psychedelic drugs on the basis of modest intrinsic efficacy rather than biased agonism between Gα_q/11 signalling and β-arrestin recruitment. The former compounds produced considerably lower maximal responses in both signalling pathways (Figure 1A,B), whereas no clear pattern emerged for their bias (Figure 1C). This finding adds to the evidence for this mechanism controlling the hallucinogenic effects of 5-HT_2AR agonists⁵, and aligns with similar results that have been demonstrated for drugs targeting other receptors where different grades of efficacy are required to achieve certain physiological outcomes and moderate efficacy can offer a greater safety profile. For instance, moderate intrinsic efficacy agonists of the µ-opioid receptor produce analgesia with limited respiratory depression¹⁷, and those of the GABA_A receptor produce anxiolysis with limited sedation.^30-36 Promisingly, non-hallucinogenic 5-HT_2AR agonists have in some cases been shown to retain the therapeutic benefits of inducing neuroplasticity, producing antidepressant-like effects, and reducing drug-seeking behaviour in preclinical models^6,9,37,38.

The work of Ippolito and colleagues, facilitated by the biosensor technology provided by Montana Molecular, paves the way for developing safer and more accessible treatments for psychiatric disorders based on the therapeutic potential of psychedelic drugs through efforts focusing on moderate intrinsic efficacy 5-HT_2AR agonists.

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