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Serotonergic psychedelics are substances that induce alterations in mood, perception, and thought, and have the activation of serotonin (5‐HT) 2A receptors (5‐HT2ARs) as a main pharmacological mechanism. Besides their appearance on the (illicit) drug market, e.g. as new psychoactive substances, their potential therapeutic application is increasingly explored. This group of substances demonstrates a broad structural variety, leading to insufficiently described structure‐activity relationships, hence illustrating the need for better functional characterization. This review therefore elaborates on the in vitro molecular techniques that have been used the most abundantly for the characterization of (psychedelic) 5‐HT2AR agonists. More specifically, this review covers assays to monitor the canonical G protein signaling pathway (e.g. measuring G protein recruitment/activation, inositol phosphate accumulation, or Ca2+ mobilization), assays to monitor non‐canonical G protein signaling (such as arachidonic acid release), assays to monitor β‐arrestin recruitment or signaling, and assays to monitor receptor conformational changes. In particular, focus lies on the mechanism behind the techniques, and the specific advantages and challenges that are associated with these. Additionally, several variables are discussed that one should consider when attempting to compare functional outcomes from different studies, both linked to the specific assay mechanism and linked to its specific execution, as these may heavily impact the assay outcome. [ABSTRACT FROM AUTHOR]

Classical psychedelics are a group of hallucinogens which trigger non‐ordinary states of consciousness through activation of the 5‐HT2A receptor (5‐HT2AR) in the brain. However, the exact mechanism of how 5‐HT2AR agonism alters perception remains elusive. When studying receptor signaling, tools which work at the same spatiotemporal resolution as the receptor are exceptionally useful. To create such a tool, we designed a set of photoswitchable ligands based on the classical psychedelic N,N‐dimethyltryptamine (DMT). By incorporation of the DMT‐indole ring into the photoswitchable system, we obtained red‐shifted ligands which can be operated by visible light. Among these azo‐DMTs, compound 2 h ("Photo‐DMT") stands out as its cis isomer exhibits DMT like activity while the trans isomer acts as weak partial agonist. Such a cis‐on "efficacy switch" substantially expands the pharmacological toolbox to investigate the complex mechanisms of 5‐HT2AR signaling. [ABSTRACT FROM AUTHOR]

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The P2Y2 receptor (P2Y2R) is a G protein-coupled receptor that is activated by extracellular ATP and UTP, to a similar extent. This allows it to play roles in the cell's response to the (increased) release of these nucleotides, e.g., in response to stress situations, including mechanical stress and oxygen deprivation. However, despite its involvement in important (patho)physiological processes, the intracellular signaling induced by the P2Y2R remains incompletely described. Therefore, this study implemented a NanoBiT® functional complementation assay to shed more light on the recruitment of β-arrestins (βarr1 and βarr2) upon receptor activation. More specifically, upon determination of the optimal configuration in this assay system, the effect of different (receptor) residues/regions on βarr recruitment to the receptor in response to ATP or UTP was estimated. To this end, the linker was shortened, the C-terminal tail was truncated, and phosphorylatable residues in the third intracellular loop of the receptor were mutated, in either singly or multiply adapted constructs. The results showed that none of the introduced adaptations entirely abolished the recruitment of either βarr, although EC50 values differed and time-luminescence profiles appeared to be qualitatively altered. The results hint at the C-terminal tail modulating the interaction with βarr, while not being indispensable. [ABSTRACT FROM AUTHOR]

The serotonin 7 receptor (5-HT 7 R) regulates various processes in the central nervous system, including mood, learning, and circadian rhythm control, among others. Receptor activation can lead to activation of the Gα s protein and a subsequent increase of intracellular cyclic adenosine monophosphate (cAMP). Receptor interaction with inverse agonists results in a decrease of basal cAMP levels and therefore a downstream effect of reduced neuronal excitability and neurotransmission. Recently, pellotine ( 1a ), a Lophophora alkaloid, was unexpectedly shown to be an inverse agonist of the 5-HT 7 R. Therefore, we evaluated close analogs of compound 1a , both naturally occurring and synthetic analogs, as inverse agonists of the 5-HT 7 R. Functional evaluation in a GloSensor cAMP assay revealed a preference for an 8-hydroxy-6,7-dimethoxy substitution pattern over 6,7,8-trimethoxy analogs or 8-hydroxy-6,7-methylenedioxy analogs. This was supported by molecular dynamics simulations, where the 8-hydroxy substitution allowed more robust interaction with the 5-HT 7 R, which correlated with inverse agonism efficacy. Additionally, N -methylation (as in 1a ) improved the potency of the evaluated analogs. In this series, the most potent inverse agonist was anhalidine ( 1b ) (EC 50 = 219 nM, E max = -95.4%), which lacks the 1-methyl, compared to pellotine ( 1a ), and showed a 2-fold higher functional potency. Altogether, these results provide key insights for the further development of potent low molecular weight inverse agonists of the 5-HT 7 R.

2-Arylethynyl (N)-methanocarba adenosine 5'-methylamides are selective A 3 adenosine receptor (AR) agonists containing a preestablished receptor-preferred pseudoribose conformation. Here, we compare analogues having bulky 2-substitution, either containing or lacking an ethynyl spacer between adenine and a cyclic group. 2-Aryl compounds 9 - 11 , 13 , 14 , 19 , 22 , 23 , 27 , 29 , 31 , and 34 , lacking a spacer, had human (h) A 3 AR K i values of 2-30 nM, and others displayed lower affinity. Mouse (m) A 3 AR affinity varied, with 2-arylethynyl having a higher affinity than 2-aryl analogues ( 7 , 8 > 3c , 3d > 3b ). However, 2-aryl-4'-truncated derivatives had greatly reduced hA 3 AR affinity, even containing affinity-enhancing N 6 -dopamine-derived substituents. Molecular modeling, including molecular dynamics simulation, predicted stable poses in the canonical A 3 AR agonist binding site, but 2-aryl (ECL2 interactions) and 2-arylethynyl (TM2 interactions) substituents have different conformations and environments. In a hA 3 AR miniGα i recruitment assay, 31 (MRS8062) was (slightly) more potent compared to a β-arrestin2 recruitment assay, both in engineered HEK293T cells, and its maximal efficacy ( E max ) was much higher (165%) than reference agonist NECA's. Thus, in the 2-aryl series, A 3 AR affinity and selectivity were variable and generally reduced compared to the 2-arylethynyl series, with a greater dependence on the specific aryl group present. Selected compounds were studied in vivo in an ischemic model of peripheral artery disease (PAD). Rigidified 2-arylethynyl analogues 3a - 3c were protective in this model of skeletal muscle ischemia-reperfusion injury/claudication, as previously shown only for moderately A 3 AR-selective ribosides or (N)-methanocarba derivatives. Thus, we have expanded the A 3 AR agonist SAR for (N)-methanocarba adenosines., Competing Interests: The authors declare no competing financial interest., (© 2024 American Chemical Society.)

The A 3 adenosine receptor (A 3 AR) is implicated in a variety of (patho)physiological conditions. While most research has focused on agonists and antagonists, inverse agonism at A 3 AR has been scarcely studied. Therefore, this study aimed at exploring inverse agonism, using two previously engineered cell lines (hA 3 ARLgBiT-SmBiTβarr2 and hA 3 ARLgBiT-SmBiTminiGα i ), both employing the NanoBiT technology. The previously established inverse agonist PSB-10 showed a decrease in basal signal in the β-arrestin 2 (βarr2) but not the miniGα i recruitment assay, indicative of inverse agonism in the former assay. Control experiments confirmed the specificity and reversibility of this observation. Evaluation of a set of presumed neutral antagonists (MRS7907, MRS7799, XAC, and MRS1220) revealed that all displayed concentration-dependent signal decreases when tested in the A 3 AR-βarr2 recruitment assay, yielding EC 50 and E max values for inverse agonism. Conversely, in the miniGα i recruitment assay, no signal decreases were observed. To assess whether this observation was caused by the inability of the ligands to induce inverse agonism in the G protein pathway, or rather by a limitation inherent to the employed A 3 AR-miniGα i recruitment assay, a GloSensor cAMP assay was performed. The outcome of the latter also suggests inverse agonism by the presumed neutral antagonists in this latter assay. These findings emphasize the importance of prior characterization of ligands in the relevant test system. Moreover, it showed the suitability of the NanoBiT βarr2 recruitment and the GloSensor cAMP assays to capture inverse agonism at the A 3 AR, as opposed to the NanoBiT miniGα i recruitment assay., Competing Interests: The authors declare no competing financial interest., (© 2023 American Chemical Society.)

Serotonergic psychedelics are described to have activation of the serotonin 2A receptor (5-HT 2A ) as their main pharmacological action. Despite their relevance, the molecular mechanisms underlying the psychedelic effects induced by certain 5-HT 2A agonists remain elusive. One of the proposed hypotheses is the occurrence of biased agonism, defined as the preferential activation of certain signaling pathways over others. This study comparatively monitored the efficiency of a diverse panel of 4-position-substituted (and N -benzyl-derived) phenylalkylamines to induce recruitment of β-arrestin2 (βarr2) or miniGα q to the 5-HT 2A , allowing us to assess structure-activity relationships and biased agonism. All test compounds exhibited agonist properties with a relatively large range of both EC 50 and E max values. Interestingly, the lipophilicity of the 2C-X phenethylamines was correlated with their efficacy in both assays but yielded a stronger correlation in the miniGα q - than in the βarr2-assay. Molecular docking suggested that accommodation of the 4-substituent of the 2C-X analogues in a hydrophobic pocket between transmembrane helices 4 and 5 of 5-HT 2A may contribute to this differential effect. Aside from previously used standard conditions (lysergic acid diethylamide (LSD) as a reference agonist and a 2 h activation profile to assess a compound's activity), serotonin was included as a second reference agonist, and the compounds' activities were also assessed using the first 30 min of the activation profile. Under all assessed circumstances, the qualitative structure-activity relationships remained unchanged. Furthermore, the use of two reference agonists allowed for the estimation of both "benchmark bias" (relative to LSD) and "physiology bias" (relative to serotonin).

The serotonin 2A receptor (5-HT 2A R) is the mediator of the psychedelic effects of serotonergic psychedelics, which have shown promising results in clinical studies for several neuropsychiatric indications. The 5-HT 2A R is able to signal through the Gα q and β-arrestin effector proteins, but it is currently not known how the different signaling pathways contribute to the therapeutic effects mediated by serotonergic psychedelics. In the present work, we have evaluated the subtype-selective 5-HT 2A R agonist 25CN-NBOH and a series of close analogues for biased signaling at this receptor. These ligands were designed to evaluate the role of interactions with Ser159 3×36 . The lack of interaction between this hydroxyl moiety and Ser159 3×36 resulted in detrimental effects on potency and efficacy in both βarr2 and miniGα q recruitment assays. Remarkably, Gα q -mediated signaling was considerably more affected. This led to the development of the first efficacious βarr2-biased 5-HT 2A R agonists 4a-b and 6e-f , βarr2 preferring, relative to lysergic acid diethylamide (LSD).

Serotonergic psychedelics are defined as compounds having serotonin 2A receptor (5-HT 2A R) activation as an important pharmacological mechanism. These compounds include the phenylalkylamine class, containing substances with e.g. 2C-X structures (phenethylamines) or their N -methoxybenzyl analogues (NBOMes). Besides their abuse potential, psychedelics are increasingly recognized for having therapeutic benefits. However, many psychedelics remain incompletely characterized, even concerning their structure-activity relationships. Here, five positional isomers of 25H-NBOMe, with two methoxy groups on the different positions of the phenyl ring of the phenethylamine moiety, were subjected to split-nanoluciferase assays assessing the in vitro recruitment of cytosolic proteins to the 5-HT 2A R. Furthermore, molecular docking at the 5-HT 2A R allowed estimation of which residues interact with the specific isomers' methoxy groups. Although the optimal substitution pattern of N -unsubstituted phenylalkylamines has been extensively studied, this is the first comparative evaluation of the functional effects of the positioning of the methoxy groups in the phenethylamine moiety of NBOMes., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)