Toxicodynamic insights of 2C and NBOMe drugs - Is there abuse potential?

Drug use represents a prevalent and multifaceted societal problem, with profound implications for public health, social welfare, and economic stability. To circumvent strict international drug control regulations, there is a growing trend in the development and market introduction of novel psychoactive substances (NPS), encompassing a wide range of compounds with psychoactive properties. This includes, among other classes of drugs, the phenethylamines. Originally derived from natural sources, these drugs have garnered particular attention due to their psychedelic effects. They comprise a broad spectrum of compounds, including 2,5-dimethoxyphenylethylamine (2C) drugs and their corresponding N-(2,5-dimethoxybenzyl)phenethylamine (NBOMe). Psychedelics are conventionally perceived as having low addiction potential, although recent reports have raised concerns regarding this topic. These substances primarily interact with serotonin receptors, particularly the 5-HT2A subtype, resulting in alterations in sensory perception, mood, and introspective experiences. In addition to their psychedelic properties, 2C and NBOMe drugs have been associated with a multitude of adverse effects, such as cardiovascular complications and neurotoxicity. This manuscript provides a comprehensive review of the psychedelic pathways underlying 2C and NBOMe designer drugs, focusing on their interactions with serotonergic and other neurotransmitter systems, shedding light on their potential for abuse.

5-HT2A receptors: Pharmacology and functional selectivity

Serotonin 5-HT2A receptors were one of the first serotonin receptors to be pharmacologically characterized. In mammals, they are expressed throughout the body in nearly every cell and tissue type, with the highest density in cortical layer V of the brain. They are involved in several aspects of normal physiological processes and behaviors and have been implicated in the etiology of neuropsychiatric diseases such as schizophrenia. Atypical antipsychotics have targeted blockade of 5-HT2A receptors as part of their therapeutic mechanism. More recently, 5-HT2A receptors have come to prominence for their role as the primary target for psychedelic drugs, which activate this receptor subtype to produce their characteristic behavioral effects. 5-HT2A receptor agonists like psilocybin, dimethyltryptamine, and lysergic acid diethylamide have each demonstrated long-lasting therapeutic efficacy in clinical trials for psychiatric disorders such as major depression and substance use disorders. There is a significant effort in both academia and industry to develop new agonists of 5-HT2A receptors with therapeutic efficacy. There are 3 primary scaffolds for agonists: tryptamines, ergolines, and phenylalkylamines, each engaging different subsets of amino acid residues in the receptor binding pocket. Differences can lead to differential responses between ligands for functionally selective outcomes. Here, we provide a historical perspective on 5-HT2A receptors, their key structural features and motifs involved in ligand-receptor interactions, and how these interactions can affect signaling pathways downstream of the receptor. Understanding how ligands interact with the 5-HT2A receptor will fundamentally inform future drug discovery to optimize therapeutics for a variety of disorders. SIGNIFICANCE STATEMENT: Psychedelic drugs have demonstrated long-lasting therapeutic efficacy for several conditions in multiple clinical trials. Their target, serotonin 5-HT2A receptors, are GPCRs with complex pharmacology. Having knowledge of how ligands interact with 5-HT2A receptors in the orthosteric binding pocket at the structural level to induce specific signal transduction pathways will inform on efforts to design and develop functionally selective drugs to potentially treat a variety of diseases.

Common and contrasting effects of 5-HTergic signaling in pyramidal cells and SOM interneurons of the mouse cortex

Serotonin (5-hydroxytryptamine, 5-HT) is a powerful modulator of neuronal activity within the central nervous system and dysfunctions of the serotonergic system have been linked to several neuropsychiatric disorders such as major depressive disorders or schizophrenia. The anterior cingulate cortex (aCC) plays an important role in cognitive capture of stimuli and valence processing and it is densely innervated by serotonergic fibers from the nucleus raphe. In order to understand how pathophysiological 5-HT signalling can lead to neuropsychiatric diseases, it is important to understand the physiological actions of 5-HT on cortical circuits. Therefore, we combined electrophysiological recordings with pharmacology and immunocytochemistry to investigate the effects of 5-HT on Somatostatin-positive interneurons (SOM-INs) and compared these to supragranular pyramidal cells (PCs). This comparison allowed us to identify common and contrasting effects of 5-HT on SOM-INs and PCs of the aCC resulting in a specific modulation of the excitation-to-inhibition balance in PCs but not in SOM-INs.

The immunomodulatory effects of classical psychedelics: A systematic review of preclinical studies

Emerging evidence suggests that classical psychedelics possess immunomodulatory and anti-inflammatory properties; however, these effects are yet to be well-established. This systematic review aims to provide a timely and comprehensive overview of the immunomodulatory effects of classical psychedelics in preclinical studies. A systematic search was conducted on six databases, including CINAHL, EMBASE, MEDLINE, PsychINFO, Scopus, and Web of Science. Eligible studies targeting classical psychedelics for evaluation of their effects on inflammatory markers and immunomodulation have been included for analysis. Data was extracted from 40 out of 2822 eligible articles, and their risk of bias was assessed using the Systematic Review Center for Laboratory Animal Experimentation (SYRCLE) tool and Quality Assessment Tool for In Vitro Studies (QUIN). Studies examined 2,5-dimethoxy-4-iodoamphetamine (DOI; n = 18); psilocybin (4-PO-DMT; n = 9); N,N-dimethyltryptamine (DMT; n = 8); lysergic acid diethylamide (LSD; n = 6); 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT; n = 3); psilocin (4-HO-DMT; n = 3); and mescaline (n = 2). In 36 studies where inflammatory cytokine levels were measured following psychedelic administration, a decrease in at least one inflammatory cytokine was observed in 29 studies. Immune cell activity was assessed in 10 studies and findings were mixed, with an equal number of studies (n = 5 out of 10) reporting either an increase or decrease in immune cell activity. Classical psychedelics were found to alleviate pre-existing inflammation but promote inflammation when administered under normal physiological conditions. This information is anticipated to inform future clinical trials, exploring classical psychedelics' potential to alleviate inflammation in various pathologies.

Flavors of GPCR signaling bias

GPCRs are inherently flexible molecules existing in an equilibrium of multiple conformations. Binding of GPCR agonists shifts this equilibrium. Certain agonists can increase the fraction of active-like conformations that predispose the receptor to coupling to a particular signal transducer or a select group of transducers. Such agonists are called biased, in contrast to balanced agonists that facilitate signaling via all transducers the receptor couples to. These biased agonists preferentially channel the signaling of a GPCR to particular G proteins, GRKs, or arrestins. Preferential activation of particular G protein or arrestin subtypes can be beneficial, as it would reduce unwanted on-target side effects, widening the therapeutic window. However, biasing GPCRs has two important limitations: a) complete bias is impossible due to inherent flexibility of GPCRs; b) receptor-independent functions of signal transducer proteins cannot be directly affected by GPCR ligands or differential receptor barcoding by GRK phosphorylation. This article is part of the Special Issue on "Ligand Bias".

A novel method for quantitative analysis of subjective experience reports: application to psychedelic visual experiences

Introduction

Psychedelic compounds such as LSD, psilocybin, mescaline, and DMT can dramatically alter visual perception. However, the extent to which visual effects of psychedelics consistently vary for different substances is an open question. The visual effects of a given psychedelic compound can range widely both across and within individuals, so datasets with large numbers of participants and descriptions of qualitative effects are required to adequately address this question with the necessary sensitivity.

Methods

Here we present an observational study with narrative self-report texts, leveraging the massive scale of the Erowid experience report dataset. We analyzed reports associated with 103 different psychoactive substances, with a median of 217 reports per substance. Thirty of these substances are standardly characterized as psychedelics, while 73 substances served as comparison substances. To quantitatively analyze these semantic data, we associated each sentence in the self-report dataset with a vector representation using an embedding model from OpenAI, and then we trained a classifier to identify which sentences described visual effects, based on the sentences' embedding vectors.

Results

We observed that the proportion of sentences describing visual effects varies significantly and consistently across substances, even within the group of psychedelics. We then analyzed the distributions of psychedelics' visual effect sentences across different categories of effects (for example, movement, color, or pattern), again finding significant and consistent variation.

Discussion

Overall, our findings indicate reliable variation across psychedelic substances' propensities to affect vision and in their qualitative effects on visual perception.

Serotonergic Psychedelics: A Comparative Review of Efficacy, Safety, Pharmacokinetics, and Binding Profile

Psychedelic compounds, including psilocybin, LSD (lysergic acid diethylamide), DMT (N,N -dimethyltryptamine), and 5-MeO-DMT (5-methoxy-N,N-dimethyltryptamine), all of which are serotonin 2A receptor agonists, are being investigated as potential treatments. This review aims to summarize the current clinical research on these 4 compounds and mescaline to guide future research. Their mechanism(s) of action, pharmacokinetics, pharmacodynamics, efficacy, and safety were reviewed. While evidence for therapeutic indications, with the exception of psilocybin for depression, is still relatively scarce, we noted no differences in psychedelic effects beyond effect duration. Therefore, it remains unclear whether different receptor profiles contribute to the therapeutic potential of these compounds. More research is needed to differentiate these compounds in order to inform which compounds might be best for different therapeutic uses.

The role of brain serotonin signaling in excessive alcohol consumption and withdrawal: A call for more research in females

Alcohol Use Disorder (AUD) is a leading cause of death and disability worldwide, but current treatments are insufficient in fully addressing the symptoms that often lead to relapses in alcohol consumption. The brain's serotonin system has been implicated in AUD for decades and is a major regulator of stress-related behaviors associated with increased alcohol consumption. This review will discuss the current literature on the association between neurobiological adaptations in serotonin systems and AUD in humans as well as the effectiveness of serotonin receptor manipulations on alcohol-related behaviors like consumption and withdrawal. We will further discuss how these findings in humans relate to findings in animal models, including a comparison of systemic pharmacological manipulations modulating alcohol consumption. We next provide a detailed overview of brain region-specific roles for serotonin and serotonin receptor signaling in alcohol-related behaviors in preclinical animal models, highlighting the complexity of forming a cohesive model of serotonin function in AUD and providing possible avenues for more effective therapeutic intervention. Throughout the review, we discuss what is known about sex differences in the sequelae of AUD and the role of serotonin in these sequelae. We stress a critical need for additional studies in women and female animals so that we may build a clearer path to elucidating sex-specific serotonergic mechanisms and develop better treatments.

Ligand bias and inverse agonism on 5-HT2A receptor-mediated modulation of G protein activity in post-mortem human brain

BACKGROUND AND PURPOSE

Whereas biased agonism on the 5-HT2A receptor has been ascribed to hallucinogenic properties of psychedelics, no information about biased inverse agonism on this receptor is available. In schizophrenia, increased 5-HT2A receptor constitutive activity has been suggested, highlighting the therapeutic relevance of inverse agonism. This study characterized the modulation of G protein activity promoted by different drugs, commonly considered as 5-HT2A receptor antagonists, in post-mortem human brain cortex.

EXPERIMENTAL APPROACH

Modulation of [35S]GTPγS binding to different subtypes of Gα proteins exerted by different 5-HT2A receptor drugs was determined by scintillation proximity assays in brain from human, WT and 5-HT2A receptor KO mice.

KEY RESULTS

MDL-11,939 was the only drug having no effect on the basal activity of 5-HT2A receptor. Altanserin and pimavanserin decreased basal activation of Gi1, but not Gq/11 proteins. This effect was blocked by MDL-11,939 and absent in 5-HT2A receptor KO mice. Volinanserin showed 5-HT2A receptor-mediated inverse agonism both on Gi1 and Gq/11 proteins. Ketanserin exhibited 5-HT2A receptor partial agonism exclusively on Gq/11 proteins. On the other hand, eplivanserin and nelotanserin displayed inverse agonism on Gq/11 and/or Gi1 proteins, which was insensitive to MDL-11,939 and was present in KO mice suggesting a role for another receptor.

CONCLUSION AND IMPLICATIONS

The results reveal the existence of constitutively active 5-HT2A receptors in human pre-frontal cortex and demonstrate different pharmacological profiles of various 5-HT2A receptor drugs previously considered antagonists. These findings indicate that altanserin and pimavanserin possess biased inverse agonist profile towards 5-HT2A receptor activation of Gi1 proteins.

Psychedelics: preclinical insights provide directions for future research

Recently, psychedelics have emerged as promising therapeutics for numerous neuropsychiatric disorders. While their potential in the clinic has yet to be fully elucidated, understanding their molecular and biological mechanisms is imperative as these compounds are becoming widely used both in therapeutic and recreational contexts. This review examines the current understanding of basic biology, pharmacology, and structural biology in an attempt to reveal both the knowns and unknowns within the field.

An Overview on the Hallucinogenic Peyote and Its Alkaloid Mescaline: The Importance of Context, Ceremony and Culture

Peyote (Lophophora williamsii) is a cactus that contains various biologically active alkaloids-such as pellotine, anhalonidine, hordenine and mescaline. Here, mescaline induces the psychoactive effects of peyote through the activation of the serotonin 5-HT2A receptor and the subsequent release of calcium (Ca2+) from the endoplasmic reticulum (ER). Moreover, an evaluation of the therapeutic benefits of mescaline is also currently the subject of research. It is important to consider that the outcome of taking a psychedelic drug strongly depends on the mindset of the recipient and the context (set and setting principle), including ceremonies and culture. This overview serves to summarise the current state of the knowledge of the metabolism, mechanism of action and clinical application studies of peyote and mescaline. Furthermore, the benefits of the potential of peyote and mescaline are presented in a new light, setting an example for combining a form of treatment embedded in nature and ritually enriched with our current highly innovative Western medicine.

Psychedelics action and schizophrenia

Psychedelics are compounds acting by serotonin 5-hydroxytryptamine (5-HT)2A receptor activation and induce several behavioral responses. They are of special interest because of their positive effects on neuropsychiatric disorders (depression and posttraumatic stress disorder). However, several findings revealed that some psychedelic actions are similar to symptoms observed in schizophrenia (psychosis, sensorimotor gating impairments, attention, and working memory deficits) which might limit their clinical applications. Psychedelics activate some neurotransmitters, i.e., serotonergic, and glutamatergic, that are also impaired in schizophrenia. Therefore, the neurobiological background of psychedelics and schizophrenia is partially similar. Another important aspect to discuss is the perspective of using psychedelics in schizophrenia therapy. Postmortem studies showed a loss of synapses in schizophrenia, and the positive effects of psychedelics on neuroplasticity (synaptogenesis, neurogenesis, and neuritogenesis) might be essential in the context of schizophrenia therapy. However, because of psychedelics' psychotic action, the recommended doses of psychedelics in schizophrenia treatment are not established, and subpsychedelic dosing or microdosing are considered. Exploratory studies are needed to determine the tolerability of treatment and appropriate dosing regimen. Another therapeutic option is using non-hallucinogenic psychedelic analogs that also induce neuroplastic outcomes but do not have psychotogenic effects. Further preclinical and clinical studies are needed to recognize the potential effectiveness of 5-HT2A agonists in schizophrenia therapy.

Novel multi-target ligands of dopamine and serotonin receptors for the treatment of schizophrenia based on indazole and piperazine scaffolds-synthesis, biological activity, and structural evaluation

Schizophrenia is a chronic mental disorder that is not satisfactorily treated with available antipsychotics. The presented study focuses on the search for new antipsychotics by optimising the compound D2AAK3, a multi-target ligand of G-protein-coupled receptors (GPCRs), in particular D₂, 5-HT_1A, and 5-HT_2A receptors. Such receptor profile may be beneficial for the treatment of schizophrenia. Compounds 1-16 were designed, synthesised, and subjected to further evaluation. Their affinities for the above-mentioned receptors were assessed in radioligand binding assays and efficacy towards them in functional assays. Compounds 1 and 10, selected based on their receptor profile, were subjected to in vivo tests to evaluate their antipsychotic activity, and effect on memory and anxiety processes. Molecular modelling was performed to investigate the interactions of the studied compounds with D₂, 5-HT_1A, and 5-HT_2A receptors on the molecular level. Finally, X-ray study was conducted for compound 1, which revealed its stable conformation in the solid state.

Gα Protein Signaling Bias at Serotonin 1A Receptor

Serotonin 1A receptor (5-HT1AR) is a clinically relevant target because of its involvement in several central and peripheral functions, including sleep, temperature homeostasis, processing of emotions, and response to stress. As a G protein coupled receptor (GPCR) activating numerous Gα i/o/z family members, 5-HT1AR can potentially modulate multiple intracellular signaling pathways in response to different therapeutics. Here, we applied a cell-based bioluminescence resonance energy transfer assay to quantify how ten structurally diverse 5-HT1AR agonists exert biased signaling by differentially stimulating Gα i/o/z family members. Our concentration-response analysis of the activation of each Gα i/o/z protein revealed unique potency and efficacy profiles of selected agonists when compared with the reference 5-hydroxytryptamine, serotonin. Overall, our analysis of signaling bias identified groups of ligands sharing comparable G protein activation selectivity and also drugs with unique selectivity profiles. We observed, for example, a strong bias of F-15599 toward the activation of Gα i3 that was unique among the agonists tested: we found a biased factor of +2.19 when comparing the activation of Gα i3 versus Gα i2 by F-15599, while it was -0.29 for 8-hydroxy-2-(di-n-propylamino) tetralin. Similarly, vortioxetine showed a biased factor of +1.06 for Gα z versus Gα oA, while it was -1.38 for vilazodone. Considering that alternative signaling pathways are regulated downstream of each Gα protein, our data suggest that the unique pharmacological properties of the tested agonists could result in multiple unrelated cellular outcomes. Further investigation is needed to reveal how this type of ligand bias could affect cellular responses and to illuminate the molecular mechanisms underlying therapeutic profile and side effects of each drug. SIGNIFICANCE STATEMENT: Serotonin 1a receptor (5-HT1AR) activates several members of the Gi/o/z protein family. Here, we examined ten structurally diverse and clinically relevant agonists acting on 5-HT1AR and identified distinctive bias patterns among G proteins. Considering the diversity of their intracellular effectors and signaling properties, this data reveal novel mechanisms underlying both therapeutic and undesirable effects.

Serotonin 2C receptors are also important in head-twitch responses in male mice

RATIONALE

Serotonergic psychedelics exert their effects via their high affinity for serotonin (5-HT) receptors, particularly through activating 5-HT2A receptors (5-HT2AR), employing the frontal cortex-dependent head-twitch response (HTR). Although universally believed to be so, studies have not yet fully ascertained whether 5-HT2AR activation is the sole initiator of these psychedelic effects. This is because not all 5-HT2AR agonists exhibit similar pharmacologic properties.

OBJECTIVE

This study aims to identify and discriminate the roles of 5-HT2AR and 5-HT2CR in the HTR induced by Methallylescaline (MAL) and 4-Methyl-2,5,β-trimethoxyphenethylamine (BOD) in male mice. Also, an analysis of their potential neurotoxic properties was evaluated.

METHODS

Male mice treated with MAL and BOD were evaluated in different behavioral paradigms targeting HTR and neurotoxicity effects. Drug affinity, pharmacological blocking, and molecular analysis were also conducted to support the behavioral findings. The HTR induced by DOI has been extensively characterized in male mice, making it a good positive control for this study, specifically for comparing the pharmacological effects of our test compounds.

RESULTS

The activation of 5-HT2CR, alone or in concert with 5-HT2AR, produces a comparable degree of HTRs (at a dose of 1 mg·kg-1), with divergent 5-HT2CR- and 5-HT2AR-Gqα11-mediated signaling and enhanced neurotoxic properties (at a dose of 30 mg·kg-1) coupled with activated pro-inflammatory cytokines. These findings show these compounds' potential psychedelic and neurotoxic effects in male mice.

CONCLUSION

These findings showed that while 5-HT2AR is the main initiator of HTR, the 5-HT2CR also has a distinct property that renders it effective in inducing HTR in male mice.

Serotonin modulates social responses to stressed conspecifics via insular 5-HT2C receptors in rat

Behaviors associated with distress can affect the anxiety-like states in observers and this social transfer of affect shapes social interactions among stressed individuals. We hypothesized that social reactions to stressed individuals engage the serotonergic dorsal raphe nucleus (DRN) which promotes anxiety-like behavior via postsynaptic action of serotonin at serotonin 2C (5-HT2C) receptors in the forebrain. First, we inhibited the DRN by administering an agonist (8-OH-DPAT, 1 μg in 0.5 μL) for the inhibitory 5-HT1A autoreceptors which silences 5-HT neuronal activity. 8-OH-DPAT prevented the approach and avoidance, respectively, of stressed juvenile (PN30) or stressed adult (PN60) conspecifics in the social affective preference (SAP) test in rats. Similarly, systemic administration of a 5-HT2C receptor antagonist (SB242084, 1 mg/kg, i.p.) prevented approach and avoidance of stressed juvenile or adult conspecifics, respectively. Seeking a locus of 5-HT2C action, we considered the posterior insular cortex which is critical for social affective behaviors and rich with 5-HT2C receptors. SB242084 administered directly into the insular cortex (5 μM in 0.5 μL bilaterally) interfered with the typical approach and avoidance behaviors observed in the SAP test. Finally, using fluorescent in situ hybridization, we found that 5-HT2C receptor mRNA (htr2c) is primarily colocalized with mRNA associated with excitatory glutamatergic neurons (vglut1) in the posterior insula. Importantly, the results of these treatments were the same in male and female rats. These data suggest that interactions with stressed others require the serotonergic DRN and that serotonin modulates social affective decision-making via action at insular 5-HT2C receptors.

Structure-Activity Assessment and In-Depth Analysis of Biased Agonism in a Set of Phenylalkylamine 5-HT2A Receptor Agonists

Serotonergic psychedelics are described to have activation of the serotonin 2A receptor (5-HT2A) as their main pharmacological action. Despite their relevance, the molecular mechanisms underlying the psychedelic effects induced by certain 5-HT2A 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 miniGαq to the 5-HT2A, allowing us to assess structure-activity relationships and biased agonism. All test compounds exhibited agonist properties with a relatively large range of both EC50 and Emax values. Interestingly, the lipophilicity of the 2C-X phenethylamines was correlated with their efficacy in both assays but yielded a stronger correlation in the miniGαq- 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-HT2A 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).

Development and Characterization of Novel Selective, Non-Basic Dopamine D2 Receptor Antagonists for the Treatment of Schizophrenia

The dopamine D₂ receptor, which belongs to the family of G protein-coupled receptors (GPCR), is an important and well-validated drug target in the field of medicinal chemistry due to its wide distribution, particularly in the central nervous system, and involvement in the pathomechanism of many disorders thereof. Schizophrenia is one of the most frequent diseases associated with disorders in dopaminergic neurotransmission, and in which the D₂ receptor is the main target for the drugs used. In this work, we aimed at discovering new selective D₂ receptor antagonists with potential antipsychotic activity. Twenty-three compounds were synthesized, based on the scaffold represented by the D2AAK2 compound, which was discovered by our group. This compound is an interesting example of a D₂ receptor ligand because of its non-classical binding to this target. Radioligand binding assays and SAR analysis indicated structural modifications of D2AAK2 that are possible to maintain its activity. These findings were further rationalized using molecular modeling. Three active derivatives were identified as D₂ receptor antagonists in cAMP signaling assays, and the selected most active compound 17 was subjected to X-ray studies to investigate its stable conformation in the solid state. Finally, effects of 17 assessed in animal models confirmed its antipsychotic activity in vivo.

Discovery of novel arylpiperazine-based DA/5-HT modulators as potential antipsychotic agents – Design, synthesis, structural studies and pharmacological profiling

Schizophrenia is a mental disorder with a complex pathomechanism involving many neurotransmitter systems. Among the currently used antipsychotics, classical drugs acting as dopamine D₂ receptor antagonists, and drugs of a newer generation, the so-called atypical antipsychotics, can be distinguished. The latter are characterized by a multi-target profile of action, affecting, apart from the D₂ receptor, also serotonin receptors, in particular 5-HT_2A and 5-HT_1A. Such profile of action is considered superior in terms of both efficacy in treating symptoms and safety. In the search for new potential antipsychotics of such atypical receptor profile, an attempt was made to optimize the arylpiperazine based virtual hit, D2AAK3, which in previous studies displayed an affinity for D₂, 5-HT_1A and 5-HT_2A receptors, and showed antipsychotic activity in vivo. In this work, we present the design of D2AAK3 derivatives (1-17), their synthesis, and structural and pharmacological evaluation. The obtained compounds show affinities for the receptors of interest and their efficacy as antagonists/agonists towards them was confirmed in functional assays. For the selected compound 11, detailed structural studies were carried out using molecular modeling and X-ray methods. Additionally, ADMET parameters and in vivo antipsychotic activity, as well as influence on memory and anxiety processes were evaluated in mice, which indicated good therapeutic potential and safety profile of the studied compound.

Serotonin modulates social responses to stressed conspecifics via insular 5-HT 2C receptors in rat

Social interaction allows for the transfer of affective states among individuals, and the behaviors and expressions associated with pain and fear can evoke anxiety-like states in observers which shape subsequent social interactions. We hypothesized that social reactions to stressed individuals engage the serotonergic dorsal raphe nucleus (DRN) which promotes anxiety-like behavior via postsynaptic action of serotonin at serotonin 2C (5-HT _2C ) receptors in the forebrain. First, we inhibited the DRN by administering an agonist (8-OH-DPAT, 1µg in 0.5µL) for the inhibitory 5-HT _1A autoreceptors which silences 5-HT neuronal activity via G-protein coupled inward rectifying potassium channels. 8-OH-DPAT prevented the approach and avoidance, respectively, of stressed juvenile (PN30) or stressed adult (PN50) conspecifics in the social affective preference (SAP) test in rats. Similarly, systemic administration of a 5-HT _2C receptor antagonist (SB242084, 1mg/kg, i.p.) prevented approach and avoidance of stressed juvenile or adult conspecifics, respectively. Seeking a locus of 5-HT _2C action, we considered the posterior insular cortex which is critical for social affective behaviors and rich with 5-HT _2C receptors. SB242084 administered directly into the insular cortex (5µM bilaterally in 0.5µL ) interfered with the typical approach and avoidance behaviors observed in the SAP test. Finally, using fluorescent in situ hybridization, we found that 5-HT _2C receptor mRNA ( htr2c) is primarily colocalized with mRNA associated with excitatory glutamatergic neurons ( vglut1 ) in the posterior insula. Importantly, the results of these treatments were the same in male and female rats. These data suggest that interactions with stressed others require the serotonergic DRN and that serotonin modulates social affective decision-making via action at insular 5-HT _2C receptors.

Enhanced Bystander BRET (ebBRET) Biosensors as Biophysical Tools to Map the Signaling Profile of Neuropsychiatric Drugs Targeting GPCRs

Bioluminescence resonance energy transfer (BRET) is a non-radiative energy transfer between a bioluminescent donor and a fluorescent acceptor with far-reaching applications in detecting physiologically relevant protein-protein interactions. The recently developed enhanced bystander BRET (ebBRET) biosensors have made it possible to rapidly determine the signaling profile of a series of ligands across a large number of GPCRs and their signaling repertoires, which has tremendous implications in the drug discovery process. Here we describe BRET and the ebBRET biosensors as investigational tools in establishing functional selectivity downstream of GPCRs.

Effects of 2,5-Dimethoxy-4-Methylamphetamine (DOM) and 2-Piperazin-1-yl-Quinoline (Quipazine) on Fentanyl Versus Food Choice in Rhesus Monkeys

There has been increasing interest in the potential therapeutic effects of drugs with agonist properties at serotonin 2A subtype (5-HT2A) receptors (e.g., psychedelics), including treatment of substance use disorders. Studying interactions between 5-HT2A receptor agonists and other drugs is important for understanding potential therapeutic effects as well as adverse interactions. Direct-acting 5-HT2A receptor agonists such as 2,5-dimethoxy-4-methylamphetamine (DOM) and 2-piperazin-1-yl-quinoline (quipazine) enhance some (e.g., antinociceptive) effects of opioids; however, it is unclear whether they alter the abuse-related effects of opioids. This study examined whether DOM and quipazine alter the reinforcing effects of fentanyl in rhesus monkeys (n = 6) responding under a food versus drug choice procedure. Responding on one lever delivered sucrose pellets and responding on the other lever delivered intravenous (i.v.) infusions. In one set of experiments, fentanyl (0.1-3.2 µg/kg/infusion) versus food choice sessions were preceded by noncontingent i.v. pretreatments with DOM (0032-0.32 mg/kg), quipazine (0.32-1.0 mg/kg), naltrexone (0.032 mg/kg), or heroin (0.1 mg/kg). In another set of experiments, fentanyl was available during choice sessions in combination with DOM (0.32-100 µg/kg/infusion) or quipazine (3.2-320 µg/kg/infusion) in varying dose ratios. Naltrexone decreased and heroin increased fentanyl choice, demonstrating sensitivity of responding to pharmacological manipulation. However, whether given as a pretreatment or made available in combination with fentanyl as a mixture, neither DOM nor quipazine significantly altered fentanyl choice. These results suggest that 5-HT2A receptor agonists do not enhance the reinforcing effects of opioids and, thus, will not likely enhance abuse potential. SIGNIFICANCE STATEMENT: Serotonin 2A subtype receptor agonists enhance some (e.g., antinociceptive) effects of opioids, suggesting they could be combined with opioids in some therapeutic contexts such as treating pain. However, it is unclear whether they also enhance adverse effects of opioids, including abuse. Results of this study indicate that serotonin 2A subtype receptor agonists do not reliably enhance opioid self-administration and, thus, are unlikely to enhance the abuse potential of opioids.

Molecular Pathways of the Therapeutic Effects of Ayahuasca, a Botanical Psychedelic and Potential Rapid-Acting Antidepressant

Ayahuasca is a psychoactive brew traditionally used in indigenous and religious rituals and ceremonies in South America for its therapeutic, psychedelic, and entheogenic effects. It is usually prepared by lengthy boiling of the leaves of the bush Psychotria viridis and the mashed stalks of the vine Banisteriopsis caapi in water. The former contains the classical psychedelic N,N-dimethyltryptamine (DMT), which is thought to be the main psychoactive alkaloid present in the brew. The latter serves as a source for β-carbolines, known for their monoamine oxidase-inhibiting (MAOI) properties. Recent preliminary research has provided encouraging results investigating ayahuasca's therapeutic potential, especially regarding its antidepressant effects. On a molecular level, pre-clinical and clinical evidence points to a complex pharmacological profile conveyed by the brew, including modulation of serotoninergic, glutamatergic, dopaminergic, and endocannabinoid systems. Its substances also interact with the vesicular monoamine transporter (VMAT), trace amine-associated receptor 1 (TAAR1), and sigma-1 receptors. Furthermore, ayahuasca's components also seem to modulate levels of inflammatory and neurotrophic factors beneficially. On a biological level, this translates into neuroprotective and neuroplastic effects. Here we review the current knowledge regarding these molecular interactions and how they relate to the possible antidepressant effects ayahuasca seems to produce.

Animal Behavior in Psychedelic Research

Psychedelic-assisted psychotherapy holds great promise in the treatment of mental health disorders. Research into 5-hydroxytryptamine 2A receptor (5-HT2AR) agonist psychedelic compounds has increased dramatically over the past two decades. In humans, these compounds produce drastic effects on consciousness, and their therapeutic potential relates to changes in the processing of emotional, social, and self-referential information. The use of animal behavior to study psychedelics is under debate, and this review provides a critical perspective on the translational value of animal behavior studies in psychedelic research. Acute activation of 5-HT2ARs produces head twitches and unique discriminative cues, disrupts sensorimotor gating, and stimulates motor activity while inhibiting exploration in rodents. The acute treatment with psychedelics shows discrepant results in conventional rodent tests of depression-like behaviors but generally induces anxiolytic-like effects and inhibits repetitive behavior in rodents. Psychedelics impair waiting impulsivity but show discrepant effects in other tests of cognitive function. Tests of social interaction also show conflicting results. Effects on measures of time perception depend on the experimental schedule. Lasting or delayed effects of psychedelics in rodent tests related to different behavioral domains appear to be rather sensitive to changes in experimental protocols. Studying the effects of psychedelics on animal behaviors of relevance to effects on psychiatric symptoms in humans, assessing lasting effects, publishing negative findings, and relating behaviors in rodents and humans to other more translatable readouts, such as neuroplastic changes, will improve the translational value of animal behavioral studies in psychedelic research. SIGNIFICANCE STATEMENT: Psychedelics like LSD and psilocybin have received immense interest as potential new treatments of psychiatric disorders. Psychedelics change high-order consciousness in humans, and there is debate about the use of animal behavior studies to investigate these compounds. This review provides an overview of the behavioral effects of 5-HT2AR agonist psychedelics in laboratory animals and discusses the translatability of the effects in animals to effects in humans. Possible ways to improve the utility of animal behavior in psychedelic research are discussed.

Significance of mammalian N, N-dimethyltryptamine (DMT): A 60-year-old debate

N,N-dimethyltryptamine (DMT) is a potent psychedelic naturally produced by many plants and animals, including humans. Whether or not DMT is significant to mammalian physiology, especially within the central nervous system, is a debate that started in the early 1960s and continues to this day. This review integrates historical and recent literature to clarify this issue, giving special attention to the most controversial subjects of DMT's biosynthesis, its storage in synaptic vesicles and the activation receptors like sigma-1. Less discussed topics, like DMT's metabolic regulation or the biased activation of serotonin receptors, are highlighted. We conclude that most of the arguments dismissing endogenous DMT's relevance are based on obsolete data or misleading assumptions. Data strongly suggest that DMT can be relevant as a neurotransmitter, neuromodulator, hormone and immunomodulator, as well as being important to pregnancy and development. Key experiments are addressed to definitely prove what specific roles DMT plays in mammalian physiology.

Multi-target derivatives of D2AAK1 as potential antipsychotics - the effect of the substitution in the indole moiety

Schizophrenia is a complex disease which is best treated with multi-target drugs, such as atypical antipsychotics. Previously, using structure-based virtual screening we found a virtual hit D2AAK1 with nanomolar affinity to dopamine and serotonin receptors important in schizophrenia pharmacotherapy. As a part of optimization campaign of D2AAK1 we obtained its 17 derivatives also displaying a multi-target profile. Selected compounds were tested against off-targets in schizophrenia, i.e. histamine H 1 receptor and muscarinic M 1 receptor and did not display considerable affinity to these receptors. Two most promising compounds were subjected to behavioral studies. These compounds decreased amphetamine-induced hyperactivity in mice which indicates their antipsychotic potential. The compounds did not interfere with the memory consolidation in mice as determined in the passive avoidance test. The favorable pharmacological profile of the compounds was rationalized using molecular modeling.

Biochemical Mechanisms Underlying Psychedelic-Induced Neuroplasticity

In addition to producing profound subjective effects following acute administration, psychedelic compounds can induce beneficial behavioral changes relevant to the treatment of neuropsychiatric disorders that last long after the compounds have been cleared from the body. One hypothesis with the potential to explain the remarkable enduring effects of psychedelics is related to their abilities to promote structural and functional neuroplasticity in the prefrontal cortex (PFC). A hallmark of many stress-related neuropsychiatric diseases, including depression, post-traumatic stress disorder (PTSD), and addiction, is the atrophy of neurons in the PFC. Psychedelics appear to be particularly effective catalysts for the growth of these key neurons, ultimately leading to restoration of synaptic connectivity in this critical brain region. Furthermore, evidence suggests that the hallucinogenic effects of psychedelics are not directly linked to their ability to promote structural and functional neuroplasticity. If we are to develop improved alternatives to psychedelics for treating neuropsychiatric diseases, we must fully characterize the molecular mechanisms that give rise to psychedelic-induced neuroplasticity. Here, I review our current understanding of the biochemical signaling pathways activated by psychedelics and related neuroplasticity-promoting molecules, with an emphasis on key unanswered questions.

In vitro assays for the functional characterization of (psychedelic) substances at the serotonin receptor 5-HT2A R

Serotonergic psychedelics are substances that induce alterations in mood, perception, and thought, and have the activation of serotonin (5-HT) 2A receptors (5-HT_2A Rs) 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-HT_2A R 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.

Psychedelic-inspired approaches for treating neurodegenerative disorders

Psychedelics are increasingly being recognized for their potential to treat a wide range of brain disorders including depression, post-traumatic stress disorder (PTSD), and substance use disorder. Their broad therapeutic potential might result from an ability to rescue cortical atrophy common to many neuropsychiatric and neurodegenerative diseases by impacting neurotrophic factor gene expression, activating neuronal growth and survival mechanisms, and modulating the immune system. While the therapeutic potential of psychedelics has not yet been extended to neurodegenerative disorders, we provide evidence suggesting that approaches based on psychedelic science might prove useful for treating these diseases. The primary target of psychedelics, the 5-HT_2A receptor, plays key roles in cortical neuron health and is dysregulated in Alzheimer's disease. Moreover, evidence suggests that psychedelics and related compounds could prove useful for treating the behavioral and psychological symptoms of dementia (BPSD). While more research is needed to probe the effects of psychedelics in models of neurodegenerative diseases, the robust effects of these compounds on structural and functional neuroplasticity and inflammation clearly warrant further investigation.

Molecular insights into psychedelic drug action

A confluence of factors has renewed interest in the scientific understanding and translational potential of psychedelic drugs such as lysergic acid diethylamide (LSD), mescaline, and psilocybin: the desire for additional approaches to mental health care, incremental progress in basic and clinical research, and the reconsideration and relaxation of existing drug policies. With the United States Food and Drug Administration's designation of psilocybin as a "Breakthrough Therapy" for treatment-resistant depression, a new path has been forged for the conveyance of psychedelics to the clinic. Essential to the further development of such applications, however, is a clearer understanding of how these drugs exert their effects at the molecular level. Here we review the current knowledge regarding the molecular details of psychedelic drug actions and suggest that these discoveries can facilitate new insights into their hallucinogenic and therapeutic mechanisms.

Low Intrinsic Efficacy Alone Cannot Explain the Improved Side Effect Profiles of New Opioid Agonists

In a recent report in Science Signaling (Gillis, A., et al. Low intrinsic efficacy for G protein activation can explain the improved side effect profiles of new opioid agonists. Sci. Signaling 2020, 13, eaaz3140 10.1126/scisignal.aaz3140), it was suggested that low intrinsic agonism, and not biased agonism, leads to an improvement in the separation of potency in opioid-induced respiratory suppression versus antinociception. Although many of the compounds that were tested have been shown to display G protein signaling bias in prior publications, the authors conclude that because they cannot detect biased agonism in their cellular signaling studies the compounds are therefore not biased agonists. Rather, they conclude that it is low intrinsic efficacy that leads to the therapeutic window improvement. Intrinsic efficacy is the extent to which an agonist can stimulate a G protein-coupled receptor response in a system, while biased agonism takes into consideration not only the intrinsic efficacy but also the potency of an agonist in an assay. Herein, we have reanalyzed the data presented in the published work (10.1126/scisignal.aaz3140) [including the recent Erratum (10.1126/scisignal.abf9803)] to derive intrinsic efficacy and bias factors as ΔΔlog(τ/K_A) and ΔΔlog(Emax/EC₅₀), respectively. On the basis of this reanalysis, the data support the conclusion that biased agonism, favoring G protein signaling, was observed. Moreover, a conservation of rank order intrinsic efficacy was not observed upon comparing responses in each assay, further suggesting that multiple active receptor states were present. These observations agree with prior studies in which oliceridine, PZM21, and SR-17018 were first described as biased agonists with improvement in antinociception over respiratory suppression in mice. Therefore, the data in the Science Signaling paper provide strong corroborating evidence that G protein signaling bias may be a means of improving opioid analgesia while avoiding certain undesirable side effects.

Catalysts for change: the cellular neurobiology of psychedelics

The resurgence of interest in the therapeutic potential of psychedelics for treating psychiatric disorders has rekindled efforts to elucidate their mechanism of action. In this Perspective, we focus on the ability of psychedelics to promote neural plasticity, postulated to be central to their therapeutic activity. We begin with a brief overview of the history and behavioral effects of the classical psychedelics. We then summarize our current understanding of the cellular and subcellular mechanisms underlying these drugs' behavioral effects, their effects on neural plasticity, and the roles of stress and inflammation in the acute and long-term effects of psychedelics. The signaling pathways activated by psychedelics couple to numerous potential mechanisms for producing long-term structural changes in the brain, a complexity that has barely begun to be disentangled. This complexity is mirrored by that of the neural mechanisms underlying psychiatric disorders and the transformations of consciousness, mood, and behavior that psychedelics promote in health and disease. Thus, beyond changes in the brain, psychedelics catalyze changes in our understanding of the neural basis of psychiatric disorders, as well as consciousness and human behavior.

Psychedelic-inspired drug discovery using an engineered biosensor

Ligands can induce G protein-coupled receptors (GPCRs) to adopt a myriad of conformations, many of which play critical roles in determining the activation of specific signaling cascades associated with distinct functional and behavioral consequences. For example, the 5-hydroxytryptamine 2A receptor (5-HT2AR) is the target of classic hallucinogens, atypical antipsychotics, and psychoplastogens. However, currently available methods are inadequate for directly assessing 5-HT2AR conformation both in vitro and in vivo. Here, we developed psychLight, a genetically encoded fluorescent sensor based on the 5-HT2AR structure. PsychLight detects behaviorally relevant serotonin release and correctly predicts the hallucinogenic behavioral effects of structurally similar 5-HT2AR ligands. We further used psychLight to identify a non-hallucinogenic psychedelic analog, which produced rapid-onset and long-lasting antidepressant-like effects after a single administration. The advent of psychLight will enable in vivo detection of serotonin dynamics, early identification of designer drugs of abuse, and the development of 5-HT2AR-dependent non-hallucinogenic therapeutics.

Molecular and Functional Imaging Studies of Psychedelic Drug Action in Animals and Humans

Hallucinogens are a loosely defined group of compounds including LSD, N,N-dimethyltryptamines, mescaline, psilocybin/psilocin, and 2,5-dimethoxy-4-methamphetamine (DOM), which can evoke intense visual and emotional experiences. We are witnessing a renaissance of research interest in hallucinogens, driven by increasing awareness of their psychotherapeutic potential. As such, we now present a narrative review of the literature on hallucinogen binding in vitro and ex vivo, and the various molecular imaging studies with positron emission tomography (PET) or single photon emission computer tomography (SPECT). In general, molecular imaging can depict the uptake and binding distribution of labelled hallucinogenic compounds or their congeners in the brain, as was shown in an early PET study with N1-([11C]-methyl)-2-bromo-LSD ([11C]-MBL); displacement with the non-radioactive competitor ketanserin confirmed that the majority of [11C]-MBL specific binding was to serotonin 5-HT2A receptors. However, interactions at serotonin 5HT1A and other classes of receptors and pleotropic effects on second messenger pathways may contribute to the particular experiential phenomenologies of LSD and other hallucinogenic compounds. Other salient aspects of hallucinogen action include permeability to the blood-brain barrier, the rates of metabolism and elimination, and the formation of active metabolites. Despite the maturation of radiochemistry and molecular imaging in recent years, there has been only a handful of PET or SPECT studies of radiolabeled hallucinogens, most recently using the 5-HT2A/2C agonist N-(2[11CH3O]-methoxybenzyl)-2,5-dimethoxy- 4-bromophenethylamine ([11C]Cimbi-36). In addition to PET studies of target engagement at neuroreceptors and transporters, there is a small number of studies on the effects of hallucinogenic compounds on cerebral perfusion ([15O]-water) or metabolism ([18F]-fluorodeoxyglucose/FDG). There remains considerable scope for basic imaging research on the sites of interaction of hallucinogens and their cerebrometabolic effects; we expect that hybrid imaging with PET in conjunction with functional magnetic resonance imaging (fMRI) should provide especially useful for the next phase of this research.

Hallucinogenic/psychedelic 5HT2A receptor agonists as rapid antidepressant therapeutics: Evidence and mechanisms of action

Major depressive disorder (MDD) is among the most prevalent mental health disorders worldwide, and it is associated with a reduced quality of life and enormous costs to health care systems. Available drug treatments show low-to-moderate response in most patients, with almost a third of patients being non-responders (treatment-resistant). Furthermore, most currently available medications need several weeks to achieve therapeutic effects, and the long-term use of these drugs is often associated with significant unwanted side effects and resultant reductions in treatment compliance. Therefore, more effective, safer, and faster-acting antidepressants with enduring effects are needed. Together with ketamine, psychedelics (or classic or serotoninergic hallucinogens) such as lysergic acid diethylamide (LSD), psilocybin, and ayahuasca are among the few compounds with recent human evidence of fast-acting antidepressant effects. Several studies in the 1950s to 1970s reported antidepressive and anxiolytic effects of these drugs, which are being confirmed by modern trials (LSD, one trial; psilocybin, five trials; ayahuasca, two trials). The effects of these drugs appear to be produced primarily by their agonism at serotonin (5-hydroxytryptamine, 5-HT) receptors, especially the 5-HT_2A receptor. Considering the overall burden of MDD and the necessity of new therapeutic options, the promising (but currently limited) evidence of safety and efficacy of psychedelics has encouraged the scientific community to explore more fully their beneficial effects in MDD.

GRK2 mediates β-arrestin interactions with 5-HT2 receptors for JC polyomavirus endocytosis

JC polyomavirus (JCPyV) infects the majority of the population, establishing a lifelong, asymptomatic infection in the kidney of healthy individuals. People that become severely immunocompromised may experience JCPyV reactivation, which can cause progressive multifocal leukoencephalopathy (PML), a neurodegenerative disease. Due to a lack of therapeutic options, PML results in fatality or significant debilitation among affected individuals. Cellular internalization of JCPyV is mediated by serotonin 5-hydroxytryptamine subfamily 2 receptors (5-HT₂Rs) via clathrin-mediated endocytosis. The JCPyV entry process requires the clathrin-scaffolding proteins β-arrestin, adaptor protein 2 (AP2), and dynamin. Further, a β-arrestin interacting domain, the Ala-Ser-Lys (ASK) motif, within the C-terminus of 5-HT_2AR is important for JCPyV internalization and infection. Interestingly, 5-HT₂R subtypes A, B, and C equally support JCPyV entry and infection, and all subtypes contain an ASK motif, suggesting a conserved mechanism for viral entry. However, the role of the 5-HT₂R ASK motifs and the activation of β-arrestin-associated proteins during internalization has not been fully elucidated. Through mutagenesis, the ASK motifs within 5-HT_2BR and 5-HT_2CR were identified as critical for JCPyV internalization and infectivity. Further, utilizing biochemical pulldown techniques, mutagenesis of the ASK motifs in 5-HT_2BR and 5-HT_2CR resulted in reduced β-arrestin binding. Utilizing small-molecule chemical inhibitors and RNA interference, G-protein receptor kinase 2 (GRK2) was determined to be required for JCPyV internalization and infection by mediating interactions between β-arrestin and the ASK motif of 5-HT₂Rs. These findings demonstrate that GRK2 and β-arrestin interactions with 5-HT₂Rs are critical for JCPyV entry by clathrin-mediated endocytosis and resultant infection.IMPORTANCE As intracellular parasites, viruses require a host cell to replicate and cause disease. Therefore, virus-host interactions contribute to viral pathogenesis. JC polyomavirus (JCPyV) infects most of the population, establishing a lifelong asymptomatic infection within the kidney. Under conditions of severe immunosuppression JCPyV may spread to the central nervous system, causing the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML). Individuals living with HIV or undergoing immunomodulatory therapies are at risk for developing PML. The mechanisms of how JCPyV uses specific receptors on the surface of host cells to initiate internalization and infection is a poorly understood process. We have further identified cellular proteins involved in JCPyV internalization and infection and elucidated their specific interactions that are responsible for activation of receptors. Collectively, these findings illuminate how viruses usurp cellular receptors during infection, contributing to current development efforts for therapeutic options for the treatment or prevention of PML.

Biased agonism at adenosine receptors

Adenosine modulates many aspects of human physiology and pathophysiology through binding to the adenosine family of G protein-coupled receptors, which are comprised of four subtypes, the A₁R, A_2AR, A_2BR and A₃R. Modulation of adenosine receptor function by exogenous agonists, antagonists and allosteric modulators can be beneficial for a number of conditions including cardiovascular disease, Parkinson's disease, and cancer. Unfortunately, many preclinical drug candidates targeting adenosine receptors have failed in clinical trials due to limited efficacy and/or severe on-target undesired effects. To overcome the key barriers typically encountered when transitioning adenosine receptor ligands into the clinic, research efforts have focussed on exploiting the phenomenon of biased agonism. Biased agonism provides the opportunity to develop ligands that favour therapeutic signalling pathways, whilst avoiding signalling associated with on-target undesired effects. Recent studies have begun to define the structure-function relationships that underpin adenosine receptor biased agonism and establish how this phenomenon can be harnessed therapeutically. In this review we describe the recent advancements made towards achieving therapeutically relevant biased agonism at adenosine receptors.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Aging-related modifications to G protein-coupled receptor signaling diversity

Aging is a highly complex molecular process, affecting nearly all tissue systems in humans and is the highest risk factor in developing neurodegenerative disorders such as Alzheimer's and Parkinson's disease, cardiovascular disease and Type 2 diabetes mellitus. The intense complexity of the aging process creates an incentive to develop more specific drugs that attenuate or even reverse some of the features of premature aging. As our current pharmacopeia is dominated by therapeutics that target members of the G protein-coupled receptor (GPCR) superfamily it may be prudent to search for effective anti-aging therapeutics in this fertile domain. Since the first demonstration of GPCR-based β-arrestin signaling, it has become clear that an enhanced appreciation of GPCR signaling diversity may facilitate the creation of therapeutics with selective signaling activities. Such 'biased' ligand signaling profiles can be effectively investigated using both standard molecular biological techniques as well as high-dimensionality data analyses. Through a more nuanced appreciation of the quantitative nature across the multiple dimensions of signaling bias that drugs possess, researchers may be able to further refine the efficacy of GPCR modulators to impact the complex aberrations that constitute the aging process. Identifying novel effector profiles could expand the effective pharmacopeia and assist in the design of precision medicines. This review discusses potential non-G protein effectors, and specifically their potential therapeutic suitability in aging and age-related disorders.

Contribution of serotonin receptor subtypes to hallucinogenic activity of 25I-NBOMe and to its effect on neurotransmission

BACKGROUND

4-Iodo-2,5-dimethoxy-N-(2-methoxybenzyl)phenethylamine (25I-NBOMe) is a potent serotonin (5-HT) receptor agonist with hallucinogenic properties. The aim of our research was to examine the role of the 5-HT2A, 5-HT2C and 5-HT1A serotonin receptor subtypes in 25I-NBOMe hallucinogenic activity and its effect on dopamine (DA), 5-HT and glutamate release in the rat frontal cortex.

METHODS

Hallucinogenic activity was investigated using the wet dog shake (WDS) test. The release of DA, 5-HT and glutamate in the rat frontal cortex was studied using a microdialysis in freely moving rats. Neurotransmitter levels were analyzed by HPLC with electrochemical detection. The selective antagonists of the 5-HT2A, 5-HT2C and 5-HT1A serotonin receptor subtypes: M100907, SB242084 and WAY100635, respectively were applied through a microdialysis probe.

RESULTS

The WDS response to 25I-NBOMe (1 and 3 mg/kg) was significantly reduced by local administration of M100907 and SB242084 (100 nM). The 25I-NBOMe-induced increase in glutamate, DA and 5-HT release was inhibited by M100907 and SB242084. WAY100635 had no effect on 25I-NBOMe-induced WDS and glutamate release, while it decreased DA and 5-HT release from cortical neuronal terminals.

CONCLUSION

The obtained results suggest that 5-HT2A and 5-HT2C receptors play a role in 25I-NBOMe-induced hallucinogenic activity and in glutamate, DA and 5-HT release in the rat frontal cortex as their respective antagonists attenuated the effect of this hallucinogen. The disinhibition of GABA cells by the 5-HT1A receptor antagonist seems to underlie the mechanism of decreased DA and 5-HT release from neuronal terminals in the frontal cortex.

Biased signaling as allosteric probe dependence

Signaling 'bias' is a phenomenon whereby the natural allosteric probe dependence of seven transmembrane receptors allows different receptor conformations (stabilized by different agonists) to activate some signaling pathways (coupled to pleiotropically coupled receptors) more than others at the expense of those other pathways. There are a number of relevant scenarios where such an activity could be therapeutically beneficial therefore there are practical reasons why this property of receptors should be exploited. This paper discusses recent ideas around attempts to harness this potentially useful idea and also the limitations around the current methods available to do so. Specifically, the determination of a quantitative value for the receptor bias of a given agonist that may translate to useful in vivo has been particularly elusive and studies need to be directed to solving this problem.

Molecular, biochemical and behavioural evidence for a novel oxytocin receptor and serotonin 2C receptor heterocomplex

The complexity of oxytocin-mediated functions is strongly associated with its modulatory effects on other neurotransmission systems, including the serotonin (5-hydroxytryptamine, 5-HT) system. Signalling between oxytocin (OT) and 5-HT has been demonstrated during neurodevelopment and in the regulation of specific emotion-based behaviours. It is suggested that crosstalk between neurotransmitters is driven by interaction between their specific receptors, particularly the oxytocin receptor (OTR) and the 5-hydroxytryptamine 2C receptor (5-HTR_2C), but evidence for this and the downstream signalling consequences that follow are lacking. Considering the overlapping central expression profiles and shared involvement of OTR and 5-HTR_2C in certain endocrine functions and behaviours, including eating behaviour, social interaction and locomotor activity, we investigated the existence of functionally active OTR/5-HTR_2C heterocomplexes. Here, we demonstrate evidence for a potential physical interaction between OTR and 5-HTR_2Cin vitro in a cellular expression system using flow cytometry-based FRET (fcFRET). We could recapitulate this finding under endogenous expression levels of both receptors via in silico analysis of single cell transcriptomic data and ex vivo proximity ligation assay (PLA). Next, we show that co-expression of the OTR/5-HTR_2C pair resulted in a significant depletion of OTR-mediated Gαq-signalling and significant changes in receptor trafficking. Of note, attenuation of OTR-mediated downstream signalling was restored following pharmacological blockade of the 5-HTR_2C. Finally, we demonstrated a functional relevance of this novel heterocomplex, in vivo, as 5-HTR_2C antagonism increased OT-mediated hypoactivity in mice. Overall, we provide compelling evidence for the formation of functionally active OTR/5-HTR_2C heterocomplexes, adding another level of complexity to OTR and 5-HTR_2C signalling functionality. This article is part of the special issue on Neuropeptides.

Interclass GPCR heteromerization affects localization and trafficking

Membrane trafficking processes regulate G protein-coupled receptor (GPCR) activity. Although class A GPCRs are capable of activating G proteins in a monomeric form, they can also potentially assemble into functional GPCR heteromers. Here, we showed that the class A serotonin 5-HT2A receptors (5-HT2ARs) affected the localization and trafficking of class C metabotropic glutamate receptor 2 (mGluR2) through a mechanism that required their assembly as heteromers in mammalian cells. In the absence of agonists, 5-HT2AR was primarily localized within intracellular compartments, and coexpression of 5-HT2AR with mGluR2 increased the intracellular distribution of the otherwise plasma membrane-localized mGluR2. Agonists for either 5-HT2AR or mGluR2 differentially affected trafficking through Rab5-positive endosomes in cells expressing each component of the 5-HT2AR-mGluR2 heterocomplex alone, or together. In addition, overnight pharmacological 5-HT2AR blockade with clozapine, but not with M100907, decreased mGluR2 density through a mechanism that involved heteromerization between 5-HT2AR and mGluR2. Using TAT-tagged peptides and chimeric constructs that are unable to form the interclass 5-HT2AR-mGluR2 complex, we demonstrated that heteromerization was necessary for the 5-HT2AR-dependent effects on mGluR2 subcellular distribution. The expression of 5-HT2AR also augmented intracellular localization of mGluR2 in mouse frontal cortex pyramidal neurons. Together, our data suggest that GPCR heteromerization may itself represent a mechanism of receptor trafficking and sorting.

THERAPEUTIC TARGETING OF BRAIN ARACHIDONIC ACID CASCADE IN BIPOLAR DISORDER BY LOW DOSE ASPIRIN AND CELECOXIB

BACKGROUND

Studies in unanesthetized rats suggest that mood stabilizers approved for treating bipolar disorder downregulate brain arachidonic acid (AA) metabolism. AA plays a role in neurotransmission and neuroinflammation, among other processes. Other drugs that reduce brain AA metabolism may add to mood stabilizer action.

METHODS

We reviewed randomized controlled trials (RCTs) and population studies to examine whether celecoxib, a selective cyclooxygenase (COX)-2 inhibitor, and acetylsalicylate (aspirin), a COX-1 and COX-2 inhibitor and acetylator, were useful in bipolar disorder patients on mood stabilizers. COX-1 and COX-2 metabolize AA to bioactive eicosanoids.

RESULTS

Celecoxib significantly enhanced mood stabilizer efficacy in two 6-week RCTs involving 86 manic bipolar inpatients, and in one 8-week RCT on 49 patients with treatment-resistant bipolar depression. With regard to aspirin, a Dutch pharmacoepidemiological study involving 5145 subjects taking lithium reported symptom reduction with added chronic low dose 30-80 mg/day aspirin, while a Danish study on 321,350 subjects taking chronic 75-150 mg/day aspirin found fewer manic episodes than in subjects not on aspirin. Finally, a recent 6-week RCT using low-dose aspirin and/or minocycline showed a specific positive effect of aspirin.

CONCLUSIONS

Efficacy of both celecoxib and aspirin as adjuncts to mood stabilizers in the treatment of bipolar disorder is consistent with the AA hypothesis for mood stabilizer action in that disorder.

Central 5-HT receptors and their function; present and future

Since our review of central 5-HT receptors and their function twenty years ago, no new 5-HT receptor has been discovered and there is little evidence that this situation will change in the near future. Nevertheless, over this time significant progress has been made in our understanding of the properties of these receptors and in the clinical translation of this information, and some of these developments are highlighted herein. Such highlights include extensive mapping of 5-HT receptors in both animal and human brain, culminating in readily-accessible brain atlases of 5-HT receptor distribution, as well as emerging data on how 5-HT receptors are distributed within complex neural circuits. Also, a range of important pharmacological and genetic tools have been developed that allow selective 5-HT receptor manipulation, in cells through to whole organism models. Moreover, unexpected complexity in 5-HT receptor function has been identified including agonist-dependent signalling that goes beyond the pharmacology of canonical 5-HT receptor signalling pathways set down in the 1980s and 1990s. This new knowledge of 5-HT signalling has been extended by the discovery of combined signalling of 5-HT and co-released neurotransmitters, especially glutamate. Another important advance has been the progression of a large number of 5-HT ligands through to experimental medicine studies and clinical trials, and some such agents have already become prescribed therapeutic drugs. Much more needs to be discovered and understood by 5-HT neuropharmacologists, not least how the diverse signalling effects of so many 5-HT receptor types interact with complex neural circuits to generate neurophysiological changes which ultimately lead to altered cognitions and behaviour. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

100 years of modelling ligand-receptor binding and response: A focus on GPCRs

Experimental pharmacologists rely on the application of models to describe biological observations in order to learn about a drug's effective concentration, the strength with which it binds its target and drives a response (at either molecular or system level), and the nature of more complex drug actions (allosterism/functional selectivity). Models in current use build upon decades of basic principles, going back to the beginning of the last century. Yet often, researchers are only partially familiar with these underlying principles, creating the potential for confusion due to failure to recognise the underpinning assumptions of the models that are used. Here, we describe the history of receptor theory as it underpins receptor stimulus-response models in use today, emphasising particularly attributes and models relevant to GPCRs-and point to some current aims of model development.

Essential fatty acids and Barratt impulsivity in gambling disorder

BACKGROUND

Polyunsaturated fatty acids (PUFA) have been long implicated in the etiopathogenesis of mental illnesses, including disorders characterized by high impulsivity. The objective of most of the studies in this field is to determine the effect of omega-3 supplementation on the impulsive symptoms. In contrast, studies analyzing basal PUFA composition in patients with impulsive behaviors are very scarce, results are not yet conclusive, and to date, no publication has specifically evaluated this in gambling disorder. Therefore, the main purpose of this research is to examine the relationship between basal PUFA composition of plasma and erythrocyte membrane and impulsivity in subjects with gambling disorder.

METHODS

It is an observational and cross-sectional study. The sample consisted of fifty-five men with gambling disorder, who voluntarily accepted to participate. Basal composition of PUFA in plasma and erythrocyte membrane was assessed by gas chromatography and mass spectrometry. Trait impulsivity was measured by the Barratt Impulsiveness Scale version 11 (BIS-11).

RESULTS

Arachidonic acid (AA)/eicosapentaenoic acid (EPA) ratio in the erythrocyte membrane was negatively correlated with total scores in BIS-11. It was also observed that impulsive gamblers had a higher proportion of EPA and a lower value of AA/EPA and AA/docosahexaenoic acid (DHA) ratio in erythrocyte membrane than non-impulsive gamblers.

CONCLUSIONS

These results support the hypothesis that alteration of basal PUFA composition exists in disorders characterized by high impulsivity, although the direction of this is still unknown. Unfortunately, the empirical literature on this field is non-existent at the time and we have no direct means to support or refute these outcomes. Further research is needed to determine the relationship between essential fatty acids and disorders characterized by high impulsivity.