25CN-NBOH: A Selective Agonist for in vitro and in vivo Investigations of the Serotonin 2A Receptor

The serotonin(5-HT)2A receptor is involved in the hypersensitivity of bladder afferent neurons in cyclophosphamide-induced cystitis

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a chronic bladder inflammation characterized by the main symptoms of urinary frequency, urgency, and pelvic pain. The hypersensitivity of bladder afferent neurons is considered a significant pathophysiologic mechanism in IC/PBS. Serotonin (5-HT, 5-hydroxytryptamine) receptors are known to be involved in the regulation of the micturition reflex and hyperalgesia, but the effect of 5-HT receptors on cystitis remains unknown. In this study, a rat model of interstitial cystitis induced by intraperitoneal injection of cyclophosphamide (CYP) was used to investigate the role of 5-HT receptors on cystitis. The histology and urodynamics exhibited chronic cystitis and overactive bladder in CYP-treated rats. Notably, among 5-HT1A, 5-HT2A and 5-HT7 receptors, the expression of 5-HT2A receptor was significantly increased in bladder afferent neurons in CYP-treated rats. Intrathecal administration of the 5-HT2A receptor antagonist M100907 could alleviate bladder overactivity and hyperalgesia in CYP-induced cystitis rats. Neuronal calcium imaging of bladder afferent neurons revealed increased calcium influx induced by the 5-HT2A receptor agonist or capsaicin in cystitis rats, which could be inhibited by M100907. Moreover, RNA sequencing indicated that differentially expressed genes were enriched in inflammation-related pathways and cellular calcium homeostasis. These findings suggest that the 5-HT2A receptor is involved in the hypersensitivity of bladder afferent neurons in CYP-induced cystitis, and M100907 could alleviate bladder overactivity and hyperalgesia in CYP-induced cystitis by inhibiting neuronal hypersensitivity in the afferent pathways. The 5-HT2A receptor may be a potential therapeutic target for the treatment of IC/BPS.

5-hydroxytryptamine 2C/1A receptors modulate the biphasic dose response of the head twitch response and locomotor activity induced by DOM in mice

RATIONALE

The phenylalkylamine hallucinogen (-)-2,5-dimethoxy-4-methylamphetamine (DOM) exhibits an inverted U-shaped dose-response curve for both head twitch response (HTR) and locomotor activity in mice. Accumulated studies suggest that HTR and locomotor hyperactivity induced by DOM are mainly caused by the activation of serotonin 5-hydroxytryptamine 2 A receptor (5-HT2A receptor). However, the mechanisms underlying the biphasic dose response of HTR and locomotor activity induced by DOM, particularly at high doses, remain unclear.

OBJECTIVES

The primary objective of this study is to investigate the modulation of 5-HT2A/2C/1A receptors in HTR and locomotor activity, while also exploring the potential receptor mechanisms underlying the biphasic dose response of DOM.

METHODS

In this study, we employed pharmacological methods to identify the specific 5-HT receptor subtypes responsible for mediating the biphasic dose-response effects of DOM on HTR and locomotor activity in C57BL/6J mice.

RESULTS

The 5-HT2A receptor selective antagonist (R)-[2,3-di(methoxy)phenyl]-[1-[2-(4-fluorophenyl)ethyl]piperidin-4-yl]methanol (M100907) (500 µg/kg, i.p.) fully blocked the HTR at every dose of DOM (0.615-10 mg/kg, i.p.) in C57BL/6J mice. M100907 (50 µg/kg, i.p.) decreased the locomotor hyperactivity induced by a low dose of DOM (0.625, 1.25 mg/kg, i.p.), but had no effect on the locomotor hypoactivity induced by a high dose of DOM (10 mg/kg) in C57BL/6J mice. The 5-HT2C antagonist 6-chloro-5-methyl-1-[(2-[2-methylpyrid-3yloxy]pyrid-5yl)carbamoyl]indoline (SB242084) (0.3, 1 mg/kg, i.p.) reduced the HTR induced by a dose of 2.5 mg/kg DOM, but did not affect the response to other doses. SB242084 (1 mg/kg, i.p.) significantly increased the locomotor activity induced by DOM (0.615-10 mg/kg, i.p.) in mice. The 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]N-(2-pyridinyl) cyclohexane carboxamide maleate (WAY100635) (1 mg/kg, i.p.) increased both HTR and locomotor activity induced by DOM in mice. The 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (1 mg/kg, i.p.) significantly reduced both the HTR and locomotor activity induced by DOM in mice. Additionally, pretreatment with the Gαi/o inhibitor PTX (0.25 µg/mouse, i.c.v.) enhanced the HTR induced by DOM and attenuated the effect of DOM on locomotor activity in mice.

CONCLUSIONS

Receptor subtypes 5-HT2C and 5-HT1A are implicated in the inverted U-shaped dose-response curves of HTR and locomotor activity induced by DOM in mice. The biphasic dose-response function of HTR and locomotor activity induced by DOM has different mechanisms in mice.

Photoswitchable TCB-2 for control of the 5-HT2A receptor and analysis of biased agonism

Therapies that target the serotonin 2A receptor (5-HT2AR) are promising. However, probes are needed to better understand the role of 5-HT2AR. Here, we design and synthesize a photoswitch and photoswitchable 5-HT2AR ligand based on highly potent agonist TCB-2 and arylazopyrazole, which also boasts photoswitchable G protein vs. β-arrestin pathway bias.

Discovery and Structure-Activity Relationships of 2,5-Dimethoxyphenylpiperidines as Selective Serotonin 5-HT2A Receptor Agonists

Classical psychedelics such as psilocybin, lysergic acid diethylamide (LSD), and N,N-dimethyltryptamine (DMT) are showing promising results in clinical trials for a range of psychiatric indications, including depression, anxiety, and substance abuse disorder. These compounds are characterized by broad pharmacological activity profiles, and while the acute mind-altering effects can be ascribed to their shared agonist activity at the serotonin 2A receptor (5-HT2AR), their apparent persistent therapeutic effects are yet to be decidedly linked to activity at this receptor. We report herein the discovery of 2,5-dimethoxyphenylpiperidines as a novel class of selective 5-HT2AR agonists and detail the structure-activity investigations leading to the identification of LPH-5 [analogue (S)-11] as a selective 5-HT2AR agonist with desirable drug-like properties.

Impact of Serotonergic 5HT1A and 5HT2A Receptor Activation on the Respiratory Response to Hypercapnia in a Rat Model of Parkinson's Disease

In Parkinson's disease (PD), along with typical motor dysfunction, abnormal breathing is present; the cause of which is not well understood. The study aimed to analyze the effects of stimulation of the serotonergic system with 5-HT1A and 5-HT2A agonists in a model of PD induced by injection of 6-hydroxydopamine (6-OHDA). To model PD, bilateral injection of 6-OHDA into both striata was performed in male Wistar rats. Respiratory disturbances in response to 7% hypercapnia (CO2 in O2) in the plethysmographic chamber before and after stimulation of the serotonergic system and the incidence of apnea were studied in awake rats 5 weeks after 6-OHDA or vehicle injection. Administration of 6-OHDA reduced the concentration of serotonin (5-HT), dopamine (DA) and norepinephrine (NA) in the striatum and the level of 5-HT in the brainstem of treated rats, which have been associated with decreased basal ventilation, impaired respiratory response to 7% CO2 and increased incidence of apnea compared to Sham-operated rats. Intraperitoneal (i.p.) injection of the 5-HT1AR agonist 8-OH-DPAT and 5-HT2AR agonist NBOH-2C-CN increased breathing during normocapnia and hypercapnia in both groups of rats. However, it restored reactivity to hypercapnia in 6-OHDA group to the level present in Sham rats. Another 5-HT2AR agonist TCB-2 was only effective in increasing normocapnic ventilation in 6-OHDA rats. Both the serotonergic agonists 8-OH-DPAT and NBOH-2C-CN had stronger stimulatory effects on respiration in PD rats, compensating for deficits in basal ventilation and hypercapnic respiration. We conclude that serotonergic stimulation may have a positive effect on respiratory impairments that occur in PD.

Serotonin 2A Receptor (5-HT2AR) Agonists: Psychedelics and Non-Hallucinogenic Analogues as Emerging Antidepressants

Psychedelics make up a group of psychoactive compounds that induce hallucinogenic effects by activating the serotonin 2A receptor (5-HT2AR). Clinical trials have demonstrated the traditional psychedelic substances like psilocybin as a class of rapid-acting and long-lasting antidepressants. However, there is a pressing need for rationally designed 5-HT2AR agonists that possess optimal pharmacological profiles in order to fully reveal the therapeutic potential of these agonists and identify safer drug candidates devoid of hallucinogenic effects. This Perspective provides an overview of the structure-activity relationships of existing 5-HT2AR agonists based on their chemical classifications and discusses recent advancements in understanding their molecular pharmacology at a structural level. The encouraging clinical outcomes of psychedelics in depression treatment have sparked drug discovery endeavors aimed at developing novel 5-HT2AR agonists with improved subtype selectivity and signaling bias properties, which could serve as safer and potentially nonhallucinogenic antidepressants. These efforts can be significantly expedited through the utilization of structure-based methods and functional selectivity-directed screening.

A role for the serotonin 2A receptor in the expansion and functioning of human transmodal cortex

Integrating independent but converging lines of research on brain function and neurodevelopment across scales, this article proposes that serotonin 2A receptor (5-HT2AR) signalling is an evolutionary and developmental driver and potent modulator of the macroscale functional organization of the human cerebral cortex. A wealth of evidence indicates that the anatomical and functional organization of the cortex follows a unimodal-to-transmodal gradient. Situated at the apex of this processing hierarchy-where it plays a central role in the integrative processes underpinning complex, human-defining cognition-the transmodal cortex has disproportionately expanded across human development and evolution. Notably, the adult human transmodal cortex is especially rich in 5-HT2AR expression and recent evidence suggests that, during early brain development, 5-HT2AR signalling on neural progenitor cells stimulates their proliferation-a critical process for evolutionarily-relevant cortical expansion. Drawing on multimodal neuroimaging and cross-species investigations, we argue that, by contributing to the expansion of the human cortex and being prevalent at the apex of its hierarchy in the adult brain, 5-HT2AR signalling plays a major role in both human cortical expansion and functioning. Owing to its unique excitatory and downstream cellular effects, neuronal 5-HT2AR agonism promotes neuroplasticity, learning and cognitive and psychological flexibility in a context-(hyper)sensitive manner with therapeutic potential. Overall, we delineate a dual role of 5-HT2ARs in enabling both the expansion and modulation of the human transmodal cortex.

Identification of 5-HT2A receptor signaling pathways associated with psychedelic potential

Serotonergic psychedelics possess considerable therapeutic potential. Although 5-HT2A receptor activation mediates psychedelic effects, prototypical psychedelics activate both 5-HT2A-Gq/11 and β-arrestin2 transducers, making their respective roles unclear. To elucidate this, we develop a series of 5-HT2A-selective ligands with varying Gq efficacies, including β-arrestin-biased ligands. We show that 5-HT2A-Gq but not 5-HT2A-β-arrestin2 recruitment efficacy predicts psychedelic potential, assessed using head-twitch response (HTR) magnitude in male mice. We further show that disrupting Gq-PLC signaling attenuates the HTR and a threshold level of Gq activation is required to induce psychedelic-like effects, consistent with the fact that certain 5-HT2A partial agonists (e.g., lisuride) are non-psychedelic. Understanding the role of 5-HT2A Gq-efficacy in psychedelic-like psychopharmacology permits rational development of non-psychedelic 5-HT2A agonists. We also demonstrate that β-arrestin-biased 5-HT2A receptor agonists block psychedelic effects and induce receptor downregulation and tachyphylaxis. Overall, 5-HT2A receptor Gq-signaling can be fine-tuned to generate ligands distinct from classical psychedelics.

Identification of 5-HT2A Receptor Signaling Pathways Responsible for Psychedelic Potential

Serotonergic psychedelics possess considerable therapeutic potential. Although 5-HT2A receptor activation mediates psychedelic effects, prototypical psychedelics activate both 5-HT2A-Gq/11 and β-arrestin2 signaling, making their respective roles unclear. To elucidate this, we developed a series of 5-HT2A-selective ligands with varying Gq efficacies, including β-arrestin-biased ligands. We show that 5-HT2A-Gq but not 5-HT2A-β-arrestin2 efficacy predicts psychedelic potential, assessed using head-twitch response (HTR) magnitude in male mice. We further show that disrupting Gq-PLC signaling attenuates the HTR and a threshold level of Gq activation is required to induce psychedelic-like effects, consistent with the fact that certain 5-HT2A partial agonists (e.g., lisuride) are non-psychedelic. Understanding the role of 5-HT2A-Gq efficacy in psychedelic-like psychopharmacology permits rational development of non-psychedelic 5-HT2A agonists. We also demonstrate that β-arrestin-biased 5-HT2A receptor agonists induce receptor downregulation and tachyphylaxis, and have an anti-psychotic-like behavioral profile. Overall, 5-HT2A receptor signaling can be fine-tuned to generate ligands with properties distinct from classical psychedelics.

Introducing Conformational Restraints on 25CN-NBOH: A Selective 5-HT2A Receptor Agonist

The N-benzylphenethylamines (NBOMes) are a class of ligands from which compounds with impressive selectivity for the serotonin 2A receptor (5-HT2AR) over the closely related serotonin 2C receptor (5-HT2CR) have emerged. These include 4-(2-((2-hydroxybenzyl)amino)ethyl)-2,5-dimethoxybenzonitrile (25CN-NBOH, 1) and 2-(2,5-dimethoxy-4-bromobenzyl)-6-(2-methoxyphenyl)piperidine (DMPMBB, 2). The present work entails the synthesis and characterization of ligands wherein the structures of these two molecules have been fused. The desired compounds were accessed by a six-step synthetic procedure followed by the chiral resolution of the resulting racemic mixtures, giving one active ((S,S)-3) and three essentially inactive stereoisomers. In silico experiments support that one of the four possible stereoisomers would be active. Further in silico investigations showed that 1, 2, and (S,S)-3 share a common binding mode, further supporting the shared stereochemistry between the active enantiomer ((S,S)-3) and 2.

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.

Discovery of β-Arrestin-Biased 25CN-NBOH-Derived 5-HT2A Receptor Agonists

The serotonin 2A receptor (5-HT2AR) is the mediator of the psychedelic effects of serotonergic psychedelics, which have shown promising results in clinical studies for several neuropsychiatric indications. The 5-HT2AR is able to signal through the Gαq 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-HT2AR 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 Ser1593×36. The lack of interaction between this hydroxyl moiety and Ser1593×36 resulted in detrimental effects on potency and efficacy in both βarr2 and miniGαq recruitment assays. Remarkably, Gαq-mediated signaling was considerably more affected. This led to the development of the first efficacious βarr2-biased 5-HT2AR agonists 4a-b and 6e-f, βarr2 preferring, relative to lysergic acid diethylamide (LSD).