5-HT(2A) and 5-HT(2C) receptors exert opposing effects on locomotor activity in mice

Biological sex modulates the efficacy of 2,5-dimethoxy-4-iodoamphetamine (DOI) to mitigate fentanyl demand

BACKGROUND

Overdose deaths remain high for opioid use disorder, emphasizing the need to pursue innovative therapeutics. Classic psychedelic drugs that engage many monoamine receptors mitigate opioid use. Here, we tested the hypothesis that the preferential serotonin 5-HT2AR agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI) could reduce the demand for fentanyl in a preclinical model of fentanyl self-administration.

METHODS

Male and female Sprague-Dawley rats (n = 25-29) were implanted with indwelling jugular catheters and allowed to self-administer fentanyl (3.2μg/kg/infusion). Rats progressed to a novel low price twice within-session threshold procedure where rats sampled the lowest price twice before decreasing the dose of fentanyl by a ¼ log every 10minutes across 11 doses. Once stable, rats were pretreated with saline or DOI (0.01, 0.03, 1mg/kg). Fentanyl consumption was analyzed using an exponentiated demand function to extract the dependent variables, Q0 and α.

RESULTS

Male and female rats acquired fentanyl self-administration in the lowest price twice within-session threshold procedure. DOI dose-dependently altered fentanyl intake such that 5-HT2AR activation decreased Q0 in female rats but increased Q0 in male rats. For demand elasticity, DOI increased α in male rats but did not alter α in female rats. DOI did not alter inactive lever presses or latency.

CONCLUSION

DOI reduces consumption at minimally constrained costs but did not affect the reinforcement value of fentanyl in female rats. Alternatively, DOI significantly reduced the reinforcement value of fentanyl in male rats. Biological sex alters the therapeutic efficacy of DOI and 5-HT2AR activation sex-dependently alters opioid reinforcement.

Ultra-low doses of methamphetamine suppress 5-hydroxytryptophan-induced head-twitch response in mice during aging

The head-twitch response (HTR) in mice is considered a behavioral assay for activation of 5-HT 2A receptors in rodents. It can be evoked by direct-acting 5-HT 2A receptor agonists such as (±)-2,5-dimethoxy-4-iodoamphetamine, 5-hydroxytryptamine precursors [e.g. 5-hydroxytryptophan (5-HTP)], and selective 5-hydroxytryptamine releasers (e.g. d -fenfluramine). The nonselective monoamine releaser methamphetamine by itself does not produce the HTR but can suppress both (±)-2,5-dimethoxy-4-iodoamphetamine- and d -fenfluramine-evoked HTRs across ages via concomitant activation of the inhibitory serotonergic 5-HT 1A or adrenergic α 2 receptors. Currently, we investigated: (1) the ontogenic development of 5-HTP-induced HTR in 20-, 30-, and 60-day-old mice; (2) whether pretreatment with ultra-low doses of methamphetamine (0.1, 0.25, and 0.5 mg/kg, intraperitoneally) can suppress the frequency of 5-HTP-induced HTR at different ages; and (3) whether the inhibitory serotonergic 5-HT 1A or adrenergic α 2 receptors may account for the potential inhibitory effect of methamphetamine on 5-HTP-induced HTR. In the presence of a peripheral decarboxylase inhibitor (carbidopa), 5-HTP produced maximal frequency of HTRs in 20-day-old mice which rapidly subsided during aging. Methamphetamine dose-dependently suppressed 5-HTP-evoked HTR in 20- and 30-day-old mice. The selective 5-HT 1A -receptor antagonist WAY 100635 reversed the inhibitory effect of methamphetamine on 5-HTP-induced HTR in 30-day-old mice, whereas the selective adrenergic α 2 -receptor antagonist RS 79948 failed to reverse methamphetamine's inhibition at any tested age. These findings suggest an ontogenic rationale for methamphetamine's inhibitory 5-HT 1A receptor component of action in its suppressive effect on 5-HTP-induced HTR during development which is not maximally active at a very early age.

Psychedelics: A review of their effects on recalled aversive memories and fear/anxiety expression in rodents

Threatening events and stressful experiences can lead to maladaptive memories and related behaviors. Existing treatments often fail to address these issues linked to anxiety/stress-related disorders effectively. This review identifies dose ranges associated with specific actions across various psychedelics. We examined psilocybin/psilocin, lysergic acid diethylamide (LSD), N,N-dimethyltryptamine (DMT), mescaline, 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), serotonin 2 A/2 C agonists (e.g., DOI) and 3,4-methylenedioxymethamphetamine (MDMA) on aversive memory extinction and reconsolidation, learned fear, anxiety, and locomotion in rodents. Nearly 400 studies published since 1957 were reviewed. Psychedelics often show biphasic effects on locomotion at doses that enhance extinction learning/retention, impair memory reconsolidation, or reduce learned fear and anxiety. Emerging evidence suggests a dissociation between their prospective benefits and locomotor effects. Under-explored aspects include sex differences, susceptibility to interference as memories age and generalize, repeated treatments, and immediate vs. delayed changes. Validating findings in traumatic-like memory and maladaptive fear/anxiety models is essential. Understanding how psychedelics modulate threat responses and post-retrieval memory processes in rodents may inform drug development and human studies, improving therapeutic approaches for related psychiatric conditions.

The serotonergic psychedelic DOI impairs deviance detection in the auditory cortex

Psychedelics are known to induce profound perceptual distortions, yet the neural mechanisms underlying these effects, particularly within the auditory system, remain poorly understood. In this study, we investigated the effects of the psychedelic compound 2,5-Dimethoxy-4-iodoamphetamine (DOI), a serotonin 2A receptor agonist, on the activity of neurons in the auditory cortex of awake mice. We examined whether DOI administration alters sound-frequency tuning, variability in neural responses, and deviance detection (a neural process reflecting the balance between top-down and bottom-up processing). Our results show that while DOI does not alter the frequency selectivity of auditory cortical neurons in a consistent manner, it increases trial-by-trial variability in responses and consistently diminishes the neural distinction between expected (standard) and unexpected (oddball) stimuli. This reduction in deviance detection was primarily driven by a decrease in the response to oddball sounds, suggesting that DOI dampens the auditory cortex's sensitivity to unexpected events. These findings provide insights into how psychedelics disrupt sensory processing and shed light on the neural mechanisms underlying the altered perception of auditory stimuli observed in the psychedelic state.

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.

1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI): From an Obscure to Pivotal Member of the DOX Family of Serotonergic Psychedelic Agents - A Review

1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI, or DOX where X = -I) was first synthesized in 1973 in a structure-activity study to explore the effect of various aryl substituents on the then newly identified, and subsequently controlled, hallucinogenic agent 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM, or DOX where X = -CH3). Over time, DOI was found to be a serotonin (5-HT) receptor agonist using various peripheral 5-HT receptor tissue assays and later, following the identification of multiple families of central 5-HT receptors, an agonist at 5-HT2 serotonin receptors in rat and, then, human brain. Today, classical hallucinogens, currently referred to as serotonergic psychedelic agents, are receiving considerable attention for their potential therapeutic application in various neuropsychiatric disorders including treatment-resistant depression. Here, we review, for the first time, the historical and current developments that led to DOI becoming a unique, perhaps a landmark, agent in 5-HT2 receptor research.

Evaluating bexicaserin for the treatment of developmental epileptic encephalopathies

INTRODUCTION

Developmental epileptic encephalopathies (DEEs) pose significant challenges due to their refractory nature and limited treatment options. Despite advancements in genetic understanding, effective therapies targeting underlying pathophysiology are lacking. Serotoninergic dysfunction has been implicated in epilepsy, sparking interest in serotonin as a therapeutic target.

AREA COVERED

This article explores the potential of bexicaserin, a selective 5-HT2C receptor agonist, as an adjunctive antiseizure medication in DEEs. Bexicaserin is thought to modulate GABAergic neurotransmission, suppressing central hyperexcitability. Preclinical studies demonstrate its efficacy across various seizure models. Clinical trials, including the Pacific Study, reveal promising results in reducing motor seizures. However, challenges such as adverse effects and treatment discontinuation underscore the need for further investigation.

EXPERT OPINION

The efficacy of 5-HT2C serotoninergic agonists, validated in preclinical and clinical studies, highlights serotonin's role in DEEs. Bexicaserin offers new therapeutic possibilities, potentially synergizing with existing antiseizure medications. Polypharmacotherapy, targeting distinct pathways, may enhance therapeutic outcomes. Monitoring pharmacological interactions and addressing central nervous system comorbidities are crucial for optimizing treatment strategies. Further research is needed to elucidate bexicaserin's mechanisms and potential antiepileptogenic effects.

Molecular and Medical Aspects of Psychedelics

Psychedelics belong to the oldest psychoactive drugs. They arouse recent interest due to their therapeutic applications in the treatment of major depressive disorder, substance use disorder, end-of-life anxiety,= and anxiety symptoms, and obsessive-compulsive disorder. In this review, the current state of preclinical research on the mechanism of action, neurotoxicity, and behavioral impact of psychedelics is summarized. The effect of selective 5-HT2A receptor agonists, 25I- and 25B-NBOMe, after acute and repeated administration is characterized and compared with the effects of a less selective drug, psilocybin. The data show a significant effect of NBOMes on glutamatergic, dopaminergic, serotonergic, and cholinergic neurotransmission in the frontal cortex, striatum, and nucleus accumbens. The increases in extracellular levels of neurotransmitters were not dose-dependent, which most likely resulted from the stimulation of the 5-HT2A receptor and subsequent activation of the 5-HT2C receptors. This effect was also observed in the wet dog shake test and locomotor activity. Chronic administration of NBOMes elicited rapid development of tolerance, genotoxicity, and activation of microglia. Acute treatment with psilocybin affected monoaminergic and aminoacidic neurotransmitters in the frontal cortex, nucleus accumbens, and hippocampus but not in the amygdala. Psilocybin exhibited anxiolytic properties resulting from intensification of GABAergic neurotransmission. The data indicate that NBOMes as selective 5-HT2A agonists exert a significant effect on neurotransmission and behavior of rats while also inducing oxidative DNA damage. In contrast to NBOMes, the effects induced by psilocybin suggest a broader therapeutic index of this drug.

Effects of (-)-MBP, a novel 5-HT2C agonist and 5-HT2A/2B antagonist/inverse agonist on brain activity: A phMRI study on awake mice

A novel serotonin ligand (-)-MBP was developed for the treatment of schizophrenia that has 5-HT2A/2B antagonist activity together with 5-HT2C agonist activity. The multi-functional activity of this novel drug candidate was characterized using pharmacological magnetic resonance imaging. It was hypothesized (-)-MBP would affect activity in brain areas associated with sensory perception. Adult male mice were given one of three doses of (-)-MBP (3.0, 10, 18 mg/kg) or vehicle while fully awake during the MRI scanning session and imaged for 15 min post I.P. injection. BOLD functional imaging was used to follow changes in global brain activity. Data for each treatment were registered to a 3D MRI mouse brain atlas providing site-specific information on 132 different brain areas. There was a dose-dependent decrease in positive BOLD signal in numerous brain regions, especially thalamus, cerebrum, and limbic cortex. The 3.0 mg/kg dose had the greatest effect on positive BOLD while the 18 mg/kg dose was less effective. Conversely, the 18 mg/kg dose showed the greatest negative BOLD response while the 3.0 mg/kg showed the least. The prominent activation of the thalamus and cerebrum included the neural circuitry associated with Papez circuit of emotional experience. When compared to vehicle, the 3.0 mg dose affected all sensory modalities, for example, olfactory, somatosensory, motor, and auditory except for the visual cortex. These findings show that (-)-MBP, a ligand with both 5-HT2A/2B antagonist and 5-HT2C agonist activities, interacts with thalamocortical circuitry and impacts areas involved in sensory perception.

A suite of engineered mice for interrogating psychedelic drug actions

Psychedelic drugs like lysergic acid diethylamide (LSD) and psilocybin have emerged as potentially transformative therapeutics for many neuropsychiatric diseases, including depression, anxiety, post-traumatic stress disorder, migraine, and cluster headaches. LSD and psilocybin exert their psychedelic effects via activation of the 5-hydroxytryptamine 2A receptor (HTR2A). Here we provide a suite of engineered mice useful for clarifying the role of HTR2A and HTR2A-expressing neurons in psychedelic drug actions. We first generated Htr2a-EGFP-CT-IRES-CreERT2 mice (CT:C-terminus) to independently identify both HTR2A-EGFP-CT receptors and HTR2A-containing cells thereby providing a detailed anatomical map of HTR2A and identifying cell types that express HTR2A. We also generated a humanized Htr2a mouse line and an additional constitutive Htr2A-Cre mouse line. Psychedelics induced a variety of known behavioral changes in our mice validating their utility for behavioral studies. Finally, electrophysiology studies revealed that extracellular 5-HT elicited a HTR2A-mediated robust increase in firing of genetically-identified pyramidal neurons--consistent with a plasma membrane localization and mode of action. These mouse lines represent invaluable tools for elucidating the molecular, cellular, pharmacological, physiological, behavioral, and other actions of psychedelic drugs in vivo.

Mechanisms and molecular targets surrounding the potential therapeutic effects of psychedelics

Psychedelics, also known as classical hallucinogens, have been investigated for decades due to their potential therapeutic effects in the treatment of neuropsychiatric and substance use disorders. The results from clinical trials have shown promise for the use of psychedelics to alleviate symptoms of depression and anxiety, as well as to promote substantial decreases in the use of nicotine and alcohol. While these studies provide compelling evidence for the powerful subjective experience and prolonged therapeutic adaptations, the underlying molecular reasons for these robust and clinically meaningful improvements are still poorly understood. Preclinical studies assessing the targets and circuitry of the post-acute effects of classical psychedelics are ongoing. Current literature is split between a serotonin 5-HT2A receptor (5-HT2AR)-dependent or -independent signaling pathway, as researchers are attempting to harness the mechanisms behind the sustained post-acute therapeutically relevant effects. A combination of molecular, behavioral, and genetic techniques in neuropharmacology has begun to show promise for elucidating these mechanisms. As the field progresses, increasing evidence points towards the importance of the subjective experience induced by psychedelic-assisted therapy, but without further cross validation between clinical and preclinical research, the why behind the experience and its translational validity may be lost.

Discovery of Novel Oleamide Analogues as Brain-Penetrant Positive Allosteric Serotonin 5-HT2C Receptor and Dual 5-HT2C/5-HT2A Receptor Modulators

The serotonin 5-HT2A receptor (5-HT2AR) and 5-HT2CR localize to the brain and share overlapping signal transduction facets that contribute to their roles in cognition, mood, learning, and memory. Achieving selective targeting of these receptors is challenged by the similarity in their 5-HT orthosteric binding pockets. A fragment-based discovery approach was employed to design and synthesize novel oleamide analogues as selective 5-HT2CR or dual 5-HT2CR/5-HT2AR positive allosteric modulators (PAMs). Compound 13 (JPC0323) exhibited on-target properties, acceptable plasma exposure and brain penetration, as well as negligible displacement to orthosteric sites of ∼50 GPCRs and transporters. Furthermore, compound 13 suppressed novelty-induced locomotor activity in a 5-HT2CR-dependent manner, suggesting 5-HT2CR PAM, but not 5-HT2AR, activity at the level of the whole organism at the employed doses of 13. We discovered new selective 5-HT2CR PAMs and first-in-class 5-HT2CR/5-HT2AR dual PAMs that broaden the pharmacological toolbox to explore the biology of these vital receptors.

The current state of ayahuasca research in animal models: A systematic review

RATIONALE

The psychedelic brew ayahuasca is increasingly being investigated for its therapeutic potential. Animal models are essential to investigate the pharmacological effects of ayahuasca since they can control important factors influencing it, such as the set and setting.

OBJECTIVE

Review and summarise data available on ayahuasca research using animal models.

METHODS

We systematically searched five databases (PubMed, Web of Science, EMBASE, LILACS and PsycInfo) for peer-reviewed studies in English, Portuguese or Spanish published up to July 2022. The search strategy included ayahuasca- and animal model-related terms adapted from the SYRCLE search syntax.

RESULTS

We identified 32 studies investigating ayahuasca effects on toxicological, behavioural and (neuro)biological parameters in rodents, primates and zebrafish. Toxicological results show that ayahuasca is safe at ceremonial-based doses but toxic at high doses. Behavioural results indicate an antidepressant effect and a potential to reduce the reward effects of ethanol and amphetamines, while the anxiety-related outcomes are yet inconclusive; also, ayahuasca can influence locomotor activity, highlighting the importance of controlling the analysis for locomotion when using tasks depending on it. Neurobiological results show that ayahuasca affects brain structures involved in memory, emotion and learning and that other neuropathways, besides the serotonergic action, are important in modulating its effects.

CONCLUSIONS

Studies using animal models indicate that ayahuasca is toxicologically safe in ceremonial-comparable doses and indicates a therapeutic potential for depression and substance use disorder while not supporting an anxiolytic effect. Essential gaps in the ayahuasca field can still be sufficed using animal models.

Rearing behaviour in the mouse behavioural pattern monitor distinguishes the effects of psychedelics from those of lisuride and TBG

Psychedelics alter consciousness and may have potential for drug development. As psychedelics are likely therapeutically active, it is important to study their effects and mechanisms using preclinical models. Here, we examined the effects of phenylalkylamine and indoleamine psychedelics on locomotor activity and exploratory behaviour using the mouse Behavioural Pattern Monitor (BPM). DOM, mescaline, and psilocin reduced locomotor activity at high doses and influenced rearings, an exploratory behaviour, in a characteristic inverted U-shaped dose-response function. Pretreatment with the selective 5-HT_2A antagonist M100907 reversed the drug-induced alterations in locomotor activity, rearings, and jumps after systemic administration of DOM at low doses. However, holepoking at the full range of doses tested was not blocked by M100907. Administration of the hallucinogenic 5-HT_2A agonist 25CN-NBOH induced striking similarities in response to that to psychedelics; these alterations were significantly diminished by M100907, whereas the putatively non-hallucinogenic 5-HT_2A agonist TBG did not affect locomotor activity, rearings, or jumps at the most effective doses. The nonhallucinogenic 5-HT_2A agonist lisuride failed to increase rearing. The results of these experiments provide strong evidence that DOM-elicited increases in rearing are due to mediation by the 5-HT_2A receptor. Finally, discriminant analysis was able to distinguish all four psychedelics from lisuride and TBG based on behavioural performance alone. Thus, increased rearing in mice could provide additional evidence of behavioural differences between hallucinogenic and nonhallucinogenic 5-HT_2A agonists.

Psychedelic-Induced Serotonin 2A Receptor Downregulation Does Not Predict Swim Stress Coping in Mice

Serotoninergic psychedelics such as psilocybin have been reported to elicit a long-lasting reduction in depressive symptoms. Although the main target for serotoninergic psychedelics, serotonin type 2A receptor (5-HT_2A), has been established, the possible mechanism of the antidepressant action of psychedelics remains unknown. Using the mouse forced swim test model, we examined whether the administration of the synthetic serotoninergic psychedelic 2,5-dimethoxy-4-iodoamphetamine (DOI) would modulate 5-HT_2A receptor levels in the medial prefrontal cortex (mPFC) and revert stress-induced changes in behavior. Mice subjected to swim stress developed a passive stress-coping strategy when tested in the forced swim test 6 days later. This change in behavior was not associated with the hypothesized increase in 5-HT_2A receptor-dependent head twitch behaviors or consistent changes in 5-HT_2A receptor levels in the mPFC. When DOI was administered 1 day before the forced swim test, a low dose (0.2 mg/kg i.p.) unexpectedly increased immobility while a high dose (2 mg/kg i.p.) had no significant effect on immobility. Nevertheless, DOI evoked a dose-dependent decrease in 5-HT_2A levels in the mPFC of mice previously exposed to swim stress. Our findings do not support the hypothesis that the downregulation of 5-HT_2A receptors in the mPFC contributes to the antidepressant-like properties of serotoninergic psychedelics.

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-HT_2AR) 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-HT_2ARs 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-HT_2AR 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.

Influence of Gestational Chlorpyrifos Exposure on ASD-like Behaviors in an fmr1-KO Rat Model

Based on previous reports, exposure to pesticides could be linked to the prevalence increase of autism spectrum disorders (ASD). Gestational exposure to chlorpyrifos (CPF) has been associated with ASD diagnosis in humans and ASD-like behaviors in rodents. However, ASD severity degree results from the complex relationship between genetic background and environmental factors. Thus, animals with a genetic vulnerability and prenatally exposed to CPF could have a more severe ASD-like phenotype. Fragile X syndrome is one of the most common monogenic causes of ASD, characterized by a mutation in the X chromosome which alters the expression of the fragile X mental retardation protein (FMRP). Based on this, some fmr1 knockout (KO) rodent models have been developed to study the physiological and genetic basis of ASD. Both fmr1-KO and wild-type male rats (F2 generation) were used in the present study. F1 pregnant females were randomly exposed to 1 mg/kg/mL/day of CPF (s.c.) from GD12.5-15.5 or vehicle. Different behavioral, developmental, and molecular variables were analyzed in F2 males. KO rats were heavier, emitted altered USVs, were socially inefficient, reacted more to a novel stimulus, were hyperactive when exploring a new context, but hypoactive when exploring anxiety-inducing environments, and had an upregulated hippocampal expression of the grin2c gene. When exposed to low doses of CPF during gestation, these KO rats showed decreased climbing capacity, dysfunctional social interaction, and increased hippocampal expression for kcc1 and 5ht2c genes. Gestational CPF exposure increased the ASD-like phenotype in those animals with a genetic vulnerability, although its effect was less generalized than expected. It is the first time that this additive effect of CPF exposure and the fmr1-KO genetic vulnerability model is explored concerning social traits or any other behavior.

SAGE-718: A First-in-Class N-Methyl-d-Aspartate Receptor Positive Allosteric Modulator for the Potential Treatment of Cognitive Impairment

N-Methyl-d-aspartate receptor (NMDAR) positive allosteric modulators (PAMs) have received increased interest as a powerful mechanism of action to provide relief as therapies for CNS disorders. Sage Therapeutics has previously published the discovery of endogenous neuroactive steroid 24(S)-hydroxycholesterol as an NMDAR PAM. In this article, we detail the discovery of development candidate SAGE-718 (5), a potent and high intrinsic activity NMDAR PAM with an optimized pharmacokinetic profile for oral dosing. Compound 5 has completed phase 1 single ascending dose and multiple ascending dose clinical trials and is currently undergoing phase 2 clinical trials for treatment of cognitive impairment in Huntington's disease.

Serotonin Receptors as Therapeutic Targets for Autism Spectrum Disorder Treatment

Autism spectrum disorder (ASD) is a group of neurodevelopmental disorders characterized by repetitive and stereotyped behaviors as well as difficulties with social interaction and communication. According to reports for prevalence rates of ASD, approximately 1~2% of children worldwide have been diagnosed with ASD. Although there are a couple of FDA (Food and Drug Administration)—approved drugs for ASD treatment such as aripiprazole and risperidone, they are efficient for alleviating aggression, hyperactivity, and self-injury but not the core symptoms. Serotonin (5-hydroxytryptamine, 5-HT) as a neurotransmitter plays a crucial role in the early neurodevelopmental stage. In particular, 5-HT has been known to regulate a variety of neurobiological processes including neurite outgrowth, dendritic spine morphology, shaping neuronal circuits, synaptic transmission, and synaptic plasticity. Given the roles of serotonergic systems, the 5-HT receptors (5-HTRs) become emerging as potential therapeutic targets in the ASD. In this review, we will focus on the recent development of small molecule modulators of 5-HTRs as therapeutic targets for the ASD treatment.

Effects of a psychedelic 5-HT2A receptor agonist on anxiety-related behavior and fear processing in mice

Psychedelic-assisted psychotherapy gained considerable interest as a novel treatment strategy for fear-related mental disorders but the underlying mechanism remains poorly understood. The serotonin 2A (5-HT2A) receptor is a key target underlying the effects of psychedelics on emotional arousal but its role in fear processing remains controversial. Using the psychedelic 5-HT2A/5-HT2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) and 5-HT2A receptor knockout (KO) mice we investigated the effect of 5-HT2A receptor activation on emotional processing. We show that DOI administration did not impair performance in a spontaneous alternation task but reduced anxiety-like avoidance behavior in the elevated plus maze and elevated zero maze tasks. Moreover, we found that DOI did not block memory recall but diminished fear expression in a passive avoidance task. Likewise, DOI administration reduced fear expression in an auditory fear conditioning paradigm, while it did not affect retention of fear extinction when administered prior to extinction learning. The effect of DOI on fear expression was abolished in 5-HT2A receptor KO mice. Administration of DOI induced a significant increase of c-Fos expression in specific amygdalar nuclei. Moreover, local infusion of the 5-HT2A receptor antagonist M100907 into the amygdala reversed the effect of systemic administration of DOI on fear expression while local administration of DOI into the amygdala was sufficient to suppress fear expression. Our data demonstrate that activation of 5-HT2A receptors in the amygdala suppresses fear expression but provide no evidence for an effect on retention of fear extinction.

"Selective" serotonin 5-HT2A receptor antagonists

Blockade of the serotonin 5-HT2A G protein-coupled receptor (5-HT2AR) is a fundamental pharmacological characteristic of numerous antipsychotic medications, which are FDA-approved to treat schizophrenia, bipolar disorder, and as adjunctive therapies in major depressive disorder. Meanwhile, activation of the 5-HT2AR by serotonergic psychedelics may be useful in treating neuropsychiatric indications, including major depressive and substance use disorders. Serotonergic psychedelics and other 5-HT2AR agonists, however, often bind other receptors, and standard 5-HT2AR antagonists lack sufficient selectivity to make well-founded mechanistic conclusions about the 5-HT2AR-dependent effects of these compounds and the general neurobiological function of 5-HT2ARs. This review discusses the limitations and strengths of currently available "selective" 5-HT2AR antagonists, the molecular determinants of antagonist selectivity at 5-HT2ARs, and the utility of molecular pharmacology and computational methods in guiding the discovery of novel unambiguously selective 5-HT2AR antagonists.

Gs signaling pathway distinguishes hallucinogenic and nonhallucinogenic 5-HT2AR agonists induced head twitch response in mice

5- HT2A receptor is a member of the family A G-protein-coupled receptor. It is involved in many psychiatric disorders, such as depression, addiction and Parkinson's disease. 5-HT2AR targeted drugs play an important role in regulating cognition, memory, emotion and other physiological function by coupling G proteins, and their most notable function is stimulating the serotonergic hallucination. However, not all 5-HT2AR agonists exhibit hallucinogenic activity, such as lisuride. Molecular mechanisms of these different effects are not well illustrated. This study suggested that 5-HT2AR coupled both Gs and Gq protein under hallucinogenic agonists DOM and 25CN-NBOH stimulation, but nonhallucinogenic agonist lisuride and TBG only activates Gq signaling. Moreover, in head twitch response (HTR) model, we found that cAMP analogs 8-Bromo-cAMP and PDE4 inhibitor Rolipram could increase HTR, while Gs protein inhibitor Melittin could reduce HTR. Collectively, these results revealed that Gs signaling is a key signaling pathway that may distinguish hallucinogenic agonists and nonhallucinogenic agonists.

(2-Aminopropyl)benzo[β]thiophenes (APBTs) are novel monoamine transporter ligands that lack stimulant effects but display psychedelic-like activity in mice

Derivatives of (2-aminopropyl)indole (API) and (2-aminopropyl)benzofuran (APB) are new psychoactive substances which produce stimulant effects in vivo. (2-Aminopropyl)benzo[β]thiophene (APBT) is a novel sulfur-based analog of API and APB that has not been pharmacologically characterized. In the current study, we assessed the pharmacological effects of six APBT positional isomers in vitro, and three of these isomers (3-APBT, 5-APBT, and 6-APBT) were subjected to further investigations in vivo. Uptake inhibition and efflux assays in human transporter-transfected HEK293 cells and in rat brain synaptosomes revealed that APBTs inhibit monoamine reuptake and induce transporter-mediated substrate release. Despite being nonselective transporter releasers like MDMA, the APBT compounds failed to produce locomotor stimulation in C57BL/6J mice. Interestingly, 3-APBT, 5-APBT, and 6-APBT were full agonists at 5-HT2 receptor subtypes as determined by calcium mobilization assays and induced the head-twitch response in C57BL/6J mice, suggesting psychedelic-like activity. Compared to their APB counterparts, ABPT compounds demonstrated that replacing the oxygen atom with sulfur results in enhanced releasing potency at the serotonin transporter and more potent and efficacious activity at 5-HT2 receptors, which fundamentally changed the in vitro and in vivo profile of APBT isomers in the present studies. Overall, our data suggest that APBT isomers may exhibit psychedelic and/or entactogenic effects in humans, with minimal psychomotor stimulation. Whether this unique pharmacological profile of APBT isomers translates into potential therapeutic potential, for instance as candidates for drug-assisted psychotherapy, warrants further investigation.

Sex-specific role for serotonin 5-HT2A receptor in modulation of opioid-induced antinociception and reward in mice

Opioids are among the most effective analgesics and the mainstay of pain management. However, concerns about safety and abuse liability have challenged their widespread use by the medical community. Opioid-sparing therapies include drugs that in combination with opioids have the ability to enhance analgesia while decreasing opioid requirement as well as their side effects. Sex differences in antinociceptive responses to opioids have received increasing attention in recent years. However, the molecular mechanisms underlying sex differences related to opioid-sparing adjuncts remain largely unexplored. Using warm water tail-withdrawal as a mouse model of acute thermal nociception, our data suggest that adjunctive administration of the serotonin 5-HT_2A receptor (5-HT_2AR) antagonist volinanserin dose-dependently enhanced potency of the opioid analgesic oxycodone in male, but not female, mice. This antinociceptive-like response induced by oxycodone was also augmented in 5-HT_2AR knockout (5-HT_2AR^-/-) male, but not female mice; an effect that was reversed by Cre-loxP-mediated selective expression of 5-HT_2AR in dorsal root ganglion (DRG) neurons of 5-HT_2AR^-/- littermates. Pharmacological inhibition with volinanserin or genetic deletion in 5-HT_2AR^-/- animals potentiated the ability of oxycodone to reduce DRG excitability in male mice. Adjunctive volinanserin did not affect oxycodone-induced conditioned place preference (CPP), whereas it reduced oxycodone-induced locomotor sensitization in male and female mice. Together, these results suggest that adjunctive volinanserin augments opioid-induced antinociception, but not abuse-related behavior, through a sex-specific signaling crosstalk mechanism that requires 5-HT_2AR expression in mouse DRG neurons. Ultimately, our results may pave the way for the clinical evaluation of volinanserin as a potential sex-specific opioid adjuvant.

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

Chronic presence of blood circulating anti-NMDAR1 autoantibodies impairs cognitive function in mice

High titers of anti-NMDAR1 autoantibodies in brain cause anti-NMDAR1 encephalitis that displays psychiatric symptoms of schizophrenia and/or other psychiatric disorders in addition to neurological symptoms. Low titers of anti-NMDAR1 autoantibodies are reported in the blood of a subset of the general human population and psychiatric patients. Since ~0.1–0.2% of blood circulating antibodies cross the blood-brain barriers and antibodies can persist for months and years in human blood, it is important to investigate whether chronic presence of these blood circulating anti-NMDAR1 autoantibodies may impair human cognitive functions and contribute to the development of psychiatric symptoms. Here, we generated mice carrying low titers of anti-NMDAR1 autoantibodies in blood against a single antigenic epitope of mouse NMDAR1. Mice carrying the anti-NMDAR1 autoantibodies are healthy and display no differences in locomotion, sensorimotor gating, and contextual memory compared to controls. Chronic presence of the blood circulating anti-NMDAR1 autoantibodies, however, is sufficient to impair T-maze spontaneous alternation in the integrity of blood-brain barriers across all 3 independent mouse cohorts, indicating a robust cognitive deficit in spatial working memory and/or novelty detection. Our studies implicate that chronic presence of low titers of blood circulating anti-NMDAR1 autoantibodies may impair cognitive functions in both the general healthy human population and psychiatric patients.

Evaluation of lorcaserin as an anticonvulsant in juvenile Fmr1 knockout mice

Recent preclinical and clinical studies suggest that lorcaserin, a preferential serotonin 2C receptor (5-HT_2CR) agonist that was approved for the treatment of obesity, possesses antiepileptic properties. Here, we tested whether lorcaserin (1, 3, 5.6, 10 mg/kg) is prophylactic against audiogenic seizures (AGSs) in juvenile Fmr1 knockout mice, a mouse model of fragile X syndrome (FXS). MPEP (30 mg/kg), a non-competitive mGluR5 receptor antagonist, was used as a positive control. As lorcaserin likely engages 5-HT_2ARs at therapeutic doses, we pretreated one group of mice with the selective 5-HT_2AR antagonist/inverse agonist, M100907 (0.03 mg/kg), alone or before administering lorcaserin (5.6 mg/kg), to discern putative contributions of 5-HT_2ARs to AGSs. We also assessed lorcaserin's in vitro pharmacology at human (h) and mouse (m) 5-HT_2CRs and 5-HT_2ARs and its in vivo interactions at m5-HT_2CRs and m5-HT_2ARs. MPEP significantly decreased AGS prevalence (P = 0.011) and lethality (P = 0.038). Lorcaserin, 3 mg/kg, attenuated AGS prevalence and lethality by 14 % and 32 %, respectively, however, results were not statistically significant (P = 0.5 and P = 0.06); other doses and M100907 alone or with lorcaserin also did not significantly affect AGSs. Lorcaserin exhibited full efficacy agonist activity at h5-HT_2CRs and m5-HT_2CRs, and near full efficacy agonist activity at h5-HT_2ARs and m5-HT_2ARs; selectivity for activation of 5-HT_2CRs over 5-HT_2ARs was greater for human (38-fold) compared to mouse (13-fold) receptors. Lorcaserin displayed relatively low affinities at antagonist-labeled 5-HT_2CRs and 5-HT_2ARs, regardless of species. Lorcaserin (3 and 5.6 mg/kg) increased the 5-HT_2AR-dependent head-twitch response (HTR) elicited by (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) in mice (P = 0.03 and P = 0.02). At 3 mg/kg, lorcaserin alone did not elicit an HTR. If mice were treated with the selective 5-HT_2CR antagonist SB 242084 (0.5 or 1 mg/kg) plus lorcaserin (3 mg/kg), a significantly increased HTR was observed, relative to vehicle (P = 0.01 and P = 0.03), however, the HTR was much lower than what was elicited by DOI or DOI plus lorcaserin. Lorcaserin, 3 mg/kg, significantly reduced locomotor activity on its own, an effect reversed by SB 242084, and lorcaserin also dose-dependently reduced locomotor activity when administered prior to DOI (Ps<0.002). These data suggest that lorcaserin may engage 5-HT_2CRs as well as 5-HT_2ARs in mice at doses as low as 3 mg/kg. The similar activity at m5-HT_2CRs and m5-HT_2ARs suggests careful dosing of lorcaserin is necessary to selectively engage 5-HT_2CRs in vivo. In conclusion, lorcaserin was ineffective at preventing AGSs in Fmr1 knockout mice. Lorcaserin may not be a suitable pharmacotherapy for seizures in FXS.

Pimavanserin, a 5HT2A receptor inverse agonist, rapidly suppresses Aβ production and related pathology in a mouse model of Alzheimer's disease

Amyloid-β (Aβ) peptide aggregation into soluble oligomers and insoluble plaques is a precipitating event in the pathogenesis of Alzheimer's disease (AD). Given that synaptic activity can regulate Aβ generation, we postulated that 5HT_2A -Rs may regulate Aβ as well. We treated APP/PS1 transgenic mice with the selective 5HT_2A inverse agonists M100907 or Pimavanserin systemically and measured brain interstitial fluid (ISF) Aβ levels in real-time using in vivo microdialysis. Both compounds reduced ISF Aβ levels by almost 50% within hours, but had no effect on Aβ levels in 5HT_2A -R knock-out mice. The Aβ-lowering effects of Pimavanserin were blocked by extracellular-regulated kinase (ERK) and NMDA receptor inhibitors. Chronic administration of Pimavanserin by subcutaneous osmotic pump to aged APP/PS1 mice significantly reduced CSF Aβ levels and Aβ pathology and improved cognitive function in these mice. Pimavanserin is FDA-approved to treat Parkinson's disease psychosis, and also has been shown to reduce psychosis in a variety of other dementia subtypes including Alzheimer's disease. These data demonstrate that Pimavanserin may have disease-modifying benefits in addition to its efficacy against neuropsychiatric symptoms of Alzheimer's disease. Read the Editorial Highlight for this article on page 560.

Investigating the role of 5-HT2A and 5-HT2C receptor activation in the effects of psilocybin, DOI, and citalopram on marble burying in mice

Psychedelic drugs acting as 5-hydroxyptryptamine 2A receptor (5-HT_2AR) agonists have shown promise as viable treatments of psychiatric disorders, including obsessive-compulsive disorder. The marble burying test is a test of compulsive-like behavior in mice, and psychedelics acting as 5-HT_2AR agonists can reduce digging in this test. We assessed the 5-HT₂R contribution to the mechanisms of two 5-HT_2A agonists on digging behavior in female NMRI mice, using citalopram as a reference compound. While the 5-HT_2AR antagonist M100907 blocked the effect of DOI and the 5-HT_2CR antagonist SB242084 blocked the effect of citalopram, neither antagonist blocked the effect of psilocybin. This study confirms 5-HT_2AR agonism as a mechanism for reduced compulsive-like digging in the MB test and suggests that 5-HT_2A and 5-HT_2CRs can work in parallel on this type of behavior. Our results with psilocybin suggest that a 5-HT₂R-independent mechanism also contributes to the effect of psilocybin on repetitive digging behavior.

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.

The Psychopharmacology of Obsessive-Compulsive Disorder: A Preclinical Roadmap

This review evaluates current knowledge about obsessive-compulsive disorder (OCD), with the goal of providing a roadmap for future directions in research on the psychopharmacology of the disorder. It first addresses issues in the description and diagnosis of OCD, including the structure, measurement, and appropriate description of the disorder and issues of differential diagnosis. Current pharmacotherapies for OCD are then reviewed, including monotherapy with serotonin reuptake inhibitors and augmentation with antipsychotic medication and with psychologic treatment. Neuromodulatory therapies for OCD are also described, including psychosurgery, deep brain stimulation, and noninvasive brain stimulation. Psychotherapies for OCD are then reviewed, focusing on behavior therapy, including exposure and response prevention and cognitive therapy, and the efficacy of these interventions is discussed, touching on issues such as the timing of sessions, the adjunctive role of pharmacotherapy, and the underlying mechanisms. Next, current research on the neurobiology of OCD is examined, including work probing the role of various neurotransmitters and other endogenous processes and etiology as clues to the neurobiological fault that may underlie OCD. A new perspective on preclinical research is advanced, using the Research Domain Criteria to propose an adaptationist viewpoint that regards OCD as the dysfunction of a normal motivational system. A systems-design approach introduces the security motivation system (SMS) theory of OCD as a framework for research. Finally, a new perspective on psychopharmacological research for OCD is advanced, exploring three approaches: boosting infrastructure facilities of the brain, facilitating psychotherapeutic relearning, and targeting specific pathways of the SMS network to fix deficient SMS shut-down processes. SIGNIFICANCE STATEMENT: A significant proportion of patients with obsessive-compulsive disorder (OCD) do not achieve remission with current treatments, indicating the need for innovations in psychopharmacology for the disorder. OCD may be conceptualized as the dysfunction of a normal, special motivation system that evolved to manage the prospect of potential danger. This perspective, together with a wide-ranging review of the literature, suggests novel directions for psychopharmacological research, including boosting support systems of the brain, facilitating relearning that occurs in psychotherapy, and targeting specific pathways in the brain that provide deficient stopping processes in OCD.

Acute DOB and PMA Administration Impairs Motor and Sensorimotor Responses in Mice and Causes Hallucinogenic Effects in Adult Zebrafish

The drastic increase in hallucinogenic compounds in illicit drug markets of new psychoactive substances (NPS) is a worldwide threat. Among these, 2, 5-dimetoxy-4-bromo-amphetamine (DOB) and paramethoxyamphetamine (PMA; marketed as “ecstasy”) are frequently purchased on the dark web and consumed for recreational purposes during rave/dance parties. In fact, these two substances seem to induce the same effects as MDMA, which could be due to their structural similarities. According to users, DOB and PMA share the same euphoric effects: increasing of the mental state, increasing sociability and empathy. Users also experienced loss of memory, temporal distortion, and paranoia following the repetition of the same thought. The aim of this study was to investigate the effect of the acute systemic administration of DOB and PMA (0.01–30 mg/kg; i.p.) on motor, sensorimotor (visual, acoustic, and tactile), and startle/PPI responses in CD-1 male mice. Moreover, the pro-psychedelic effect of DOB (0.075–2 mg/kg) and PMA (0.0005–0.5 mg/kg) was investigated by using zebrafish as a model. DOB and PMA administration affected spontaneous locomotion and impaired behaviors and startle/PPI responses in mice. In addition, the two compounds promoted hallucinatory states in zebrafish by reducing the hallucinatory score and swimming activity in hallucinogen-like states.

The selective 5-HT2A receptor agonist 25CN-NBOH: Structure-activity relationship, in vivo pharmacology, and in vitro and ex vivo binding characteristics of [3H]25CN-NBOH

The remarkable effects exhibited by classical psychedelics in recent clinical trials have spawned considerable interest in 5-HT_2A receptor (5-HT_2AR) activation as a treatment strategy for several psychiatric/cognitive disorders. In this study we have continued our development of 25CN-NBOH, one of the most 5-HT_2AR-selective agonists reported to date, as a pharmacological tool for exploration of 5-HT_2AR expression and functions. The importance of the 2' and 3' positions in 25CN-NBOH as structural hotspots for its 5-HT_2AR activity was investigated by synthesis and pharmacological characterization of six novel analogs at 5-HT_2AR and 5-HT_2CR in binding and functional assays. While the 5-HT_2AR activity of 25CN-NBOH was retained in 3'-methyl, 2',3'-chroman, 2',3'-dihydrofuran and 2',3'-furan analogs, the 3'-methoxy and 3'-ethyl analogs displayed substantially lower binding affinities and agonist potencies than 25CN-NBOH. Interestingly, the 2',3'-substitution pattern was also a key determinant of agonist efficacy, as all six analogs exhibited low-efficacy partial agonism or de facto antagonism at the 5-HT_2AR in the functional assays. Systemic administration of 25CN-NBOH and its close structural analog 25CN-NBMD induced robust head-twitch response in mice, a well-established behavioural effect of 5-HT_2AR activation in vivo, and 25CN-NBOH mediated robust reductions in the activity of mice in an anxiety-related marble burying assay, which supports the proposed beneficial effects of 5-HT_2AR activation on disorders characterized by cognitive rigidity. Finally, tritiated 25CN-NBOH exhibited high 5-HT_2AR binding affinity (K_D ~1 nM) and selectivity against 5-HT_2BR and 5-HT_2CR in equilibrium and kinetic binding studies of the recombinant receptors, and in concordance [³H]25CN-NBOH displayed substantial specific, ketanserin-sensitive binding to cortex and small levels of binding to choroid plexus in rat brain slices in autoradiography studies. In conclusion, this work delineates the subtle molecular determinants of the 5-HT_2AR activity in 25CN-NBOH, substantiates the potential in this compound and its analogs as tools for in vivo studies of the 5-HT_2AR, and introduces a novel selective agonist radioligand as another potentially valuable tool for future explorations of this receptor.

Lorcaserin Administration has Pro-Ejaculatory Effects in Rats via 5-HT2C Receptors Activation: A Putative Pharmacologic Strategy to Delayed Ejaculation?

BACKGROUND

Lorcaserin is an anti-obesity drug whose weight loss effect results from 5-hydroxytryptamin (5-HT)_2C receptors activation. The 5-HT_2C receptor was shown to participate in the physiological control of ejaculation, but no data addressing a putative effect of lorcaserin on ejaculation exist.

AIM

To investigate the effects of lorcaserin in different in vitro and in vivo experimental models of ejaculation in rats.

METHODS

Contractile responses to lorcaserin in rat seminal emission organs in vitro (prostatic and epididymal vas deferens, cauda epididymis, and seminal vesicles), analysis of male rat copulatory behavior, and electromyographic recording of bulbospongiosus muscle in anesthetized animals were studied.

MAIN OUTCOME MEASURES

The main outcome measures included in vitro contraction of seminal emission organs and evaluation of the male rat copulatory behavior. The male rat sexual behavior in terms of copulation latency, ejaculation latency, mount and intromission frequency, and ejaculation frequency of sexually experienced adult male rats with a receptive female were also recorded.

RESULTS

Lorcaserin (1.0 nM to 1.0 mM) had no significant effects on the in vitro contractility of seminal emission organs smooth muscle (cauda epididymis, vas deferens, and seminal vesicles). On the other hand, lorcaserin administration (0.3-1.0 mg/kg, intravenous) induced ejaculation in anesthetized rats, which was prevented by the 5-HT_2C-selective antagonist SB 242084 (0.1 and 0.3 mg/kg, intravenous). Single-dose treatment of non-anesthetized male rats with lorcaserin (1.0, 4.0, or 10 mg/kg, per os) induced non-copulating ejaculations in sexually naïve rats. Lorcaserin also had pro-ejaculation effects by decreasing the ejaculation threshold of copulating rats by half. The pro-ejaculatory effects of lorcaserin were reversible as the ejaculation threshold of treated rats recovered after a 1-week washout period.

CLINICAL IMPLICATIONS

Due to its reported clinical safety, repurposing lorcaserin for the treatment of delayed ejaculation may be suggested.

STRENGTHS & LIMITATIONS

The pro-ejaculatory effect of lorcaserin administration and the role of 5-HT_2C were demonstrated in different experimental models of ejaculation in rats. The lack of studies in putative experimental models of delayed ejaculation is a limitation of this study.

CONCLUSION

Our results demonstrate that the clinically approved 5-HT_2C agonist lorcaserin is a strong facilitator of ejaculation in rats. de Almeida Kiguti LR, Pacheco TL, Antunes E, et al. Lorcaserin Administration has Pro-Ejaculatory Effects in Rats via 5-HT2C Receptors Activation: A Putative Pharmacologic Strategy to Delayed Ejaculation? J Sex Med 2020;17:1060-1071.

The 5-hydroxytryptamine 2A receptor agonists DOI and 25CN-NBOH decrease marble burying and reverse 8-OH-DPAT-induced deficit in spontaneous alternation

5-Hydroxytryptamine 2A receptor (5-HT_2AR) agonist psychedelics are increasingly recognized as potentially useful treatments of psychiatric disorders, such as obsessive-compulsive disorder, depression, anxiety, and drug dependence. There is limited understanding of the way they exert their therapeutic action, but inhibition of rigid behavior and cognition has been suggested as a key factor. To examine the role of 5-HT_2ARs in modulating repetitive behavior, we tested two 5-HT_2AR agonists, DOI, and the selective 25CN-NBOH, in two mouse tests of compulsive-like behavior. Using adult C57BL/6JOlaHsd male mice, we examined the effects of the two compounds on digging behavior in the marble burying test and on 8-OH-DPAT-disrupted spontaneous alternation behavior in the Y-maze. Both compounds dose-dependently decreased digging behavior in the marble burying test, indicating anti-compulsivity effects, which were not related to non-specific locomotor inhibition. Both 5-HT_2AR agonists also reversed 8-OH-DPAT-reduced alternation ratio in the spontaneous alternation behavior test, although the effects were less pronounced than in the marble burying test. This suggests that the 5-HT_2AR promotes exploratory behavior, but that the deficit produced by 8-OH-DPAT is too excessive to be fully reversed by 5-HT_2AR agonists. This study shows that agonism of 5-HT_2AR reduces repetitive behavioral patterns, supporting the theory that this is a potential new treatment approach to disorders of cognitive or behavioral inflexibility. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Fully automated head-twitch detection system for the study of 5-HT2A receptor pharmacology in vivo

Head-twitch behavior (HTR) is the behavioral signature of psychedelic drugs upon stimulation of the serotonin 5-HT_2A receptor (5-HT_2AR) in rodents. Following the previous report of a semi-automated detection of HTR based on the dynamics of mouse's head movement, here we present a system for the identification of individual HTR events in a fully automated fashion. The validity of this fully automated HTR detection system was tested with the psychedelic drug DOI in 5-HT_2AR-KO mice, and via evaluation of potential sources of false-positive and false-negative HTR events. The increased throughput in data processing achieved via automation afforded the possibility of conducting otherwise time consuming HTR time-course studies. To further assess the versatility of our system, we also explored the pharmacological interactions between 5-HT_2AR and the metabotropic glutamate receptor 2 (mGluR2). Our data demonstrate the potentiation effect of the mGluR2/3 antagonist LY341495 on DOI-induced HTR, as well as the HTR-blocking effect of the mGluR2/3 agonist and antipsychotic drug in development LY404039. This fully automated system can contribute to speed up our understanding of 5-HT_2AR's pharmacology and its characteristic behavioral outputs in rodents.

Contrasting patterns of ERK activation in the tail of the striatum in response to aversive and rewarding signals

The caudal part of the striatum, also named the tail of the striatum (TS), defines a fourth striatal domain. Determining whether rewarding, aversive and salient stimuli regulate the activity of striatal spiny projections neurons (SPNs) of the TS is therefore of paramount importance to understand its functions, which remain largely elusive. Taking advantage of genetically encoded biosensors (A-kinase activity reporter 3) to record protein kinase A signals and by analyzing the distribution of dopamine D1R- and D2R-SPNs in the TS, we characterized three subterritories: a D2R/A2aR-lacking, a D1R/D2R-intermingled and a D1R/D2R-SPNs-enriched area (corresponding to the amygdalostriatal transition). In addition, we provide evidence that the distribution of D1R- and D2R-SPNs in the TS is evolutionarily conserved (mouse, rat, gerbil). The in vivo analysis of extracellular signal-regulated kinase (ERK) phosphorylation in these TS subterritories in response to distinct appetitive, aversive and pharmacological stimuli revealed that SPNs of the TS are not recruited by stimuli triggering innate aversive responses, fasting, satiety, or palatable signals whereas a reduction in ERK phosphorylation occurred following learned avoidance. In contrast, D1R-SPNs of the intermingled and D2R/A2aR-lacking areas were strongly activated by both D1R agonists and psychostimulant drugs (d-amphetamine, cocaine, 3,4-methyl enedioxy methamphetamine, or methylphenidate), but not by hallucinogens. Finally, a similar pattern of ERK activation was observed by blocking selectively dopamine reuptake. Together, our results reveal that the caudal TS might participate in the processing of specific reward signals and discrete aversive stimuli. Cover Image for this issue: doi: 10.1111/jnc.14526. Open Science: This manuscript was awarded with the Open Materials Badge For more information see: https://cos.io/our-services/open-science-badges/.

Intra-Nasally Administered Oligopeptide Lunasin Acts as a Possible Anti-Psychotic Agent in Mice Models

Background and Objectives: Previously we have shown that synthetic lunasin, a 43 amino acid residue-containing peptide, after its central (intracisternal) administration in mice demonstrated antagonism against dopaminergic drug behavioural effects, indicating a putative antipsychotic/anti-schizophrenic profile of lunasin. The aims of the present studies were: to test whether lunasin would show an influence on the dopaminergic system after intranasal administration, and to examine the effect(s) of lunasin on serotonin and glutamatergic systems, which could play an essential role in antipsychotic action. Materials and Methods: Lunasin was administered intra-nasally at doses 0.1 and 1 nmol/mouse in ICR mice (n = 7–8) and tested in an open field on hyperlocomotion caused by amphetamine; serotonin 5-HT 2A/2C receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)- 2-aminopropane (DOI); and glutamate NMDA receptor antagonist phencyclidine. Following behavioural testing, the contents of neurotransmitters and their metabolites in brain hemispheres (n = 6–8) were assessed by ultra-high-performance liquid chromatography-time of flight mas-spectrometry (UHPLC-TOF-MS) method. Also, lunasin binding to serotonin receptors was assessed. Results: Lunasin intra-nasally fully normalized hyper-locomotion and brain monoamine levels in amphetamine- and DOI-treated mice brains. Phencyclidine behavioural effects were not influenced. In vitro receptor binding data demonstrated a low affinity of lunasin (at µM concentrations) compared with DOI (nM concentrations) for the 5-HT_2A and 5-HT_2C receptors. Conclusions: These results demonstrated, for the first time, that the intranasal administration of oligopeptide lunasin normalized mice behaviour and brain monoamine levels in experimental psychosis mice models. Its neuro-regulatory effects indicated a usefulness of this peptide molecule for the design of novel psychotropic agents.

The Neuronal Activation of Deep Cerebellar Nuclei Is Essential for Environmental Enrichment-Induced Post-Stroke Motor Recovery

The level of cerebellar activity in stroke patients has been shown to correlate with the extent of functional recovery. We reasoned that the cerebellum may be an important player in post-stroke rehabilitation. Because the neurons in the deep cerebellar nuclei (DCN) represent virtually all of the output from the cerebellum, in this study, using environmental enrichment (EE) to promote rehabilitation, we investigated the influence of the optogenetic neuronal modulation of DCN on EE-induced rehabilitation. We found that neuronal inhibition of the DCN almost completely blocked motor recovery in EE treated mice, but the stroke mice with neuronal activation of the DCN achieved a similar recovery level as those in the EE treated group. No difference was observed in anxiety-like behavior. Moreover, Htr2a in the DCN, the gene encoding 5-HT2A receptor, was shown to be a hub gene in the protein-protein interaction network identified using RNA-seq. This indicated that 5-HT2A receptor-mediated signaling may be responsible for DCN-dependent functional improvement in EE. We further verified this using the 5-HT2A receptor antagonist, MDL100907, to inhibit the function of 5-HT2A receptor in the DCN. This treatment resulted in impaired recovery in EE treated mice, who performed at a level as poor as the stroke-only group. Thus, this work contributes to an understanding of the importance of the DCN activation in EE-induced post-stroke rehabilitation. Attempts to clarify the mechanism of 5-HT2A receptor-mediated signaling in the DCN may also lead to the creation of a pharmacological mimetic of the benefits of EE-induced rehabilitation.

Hallucinogen-Like Action of the Novel Designer Drug 25I-NBOMe and Its Effect on Cortical Neurotransmitters in Rats

NBOMes are N-benzylmethoxy derivatives of the 2C family hallucinogens. 4-Iodo-2,5-dimethoxy-N-(2-methoxybenzyl)phenethylamine (25I-NBOMe) is one of the commonly used illicit drugs. It exhibits high binding affinity for 5-HT_2A/C and 5-HT_1A serotonin receptors. Activation of 5-HT_2A receptor induces head-twitch response (HTR) in rodents, a behavioral marker of hallucinogen effect in humans. There is not much data on neurochemical properties of NBOMes. Therefore, we aimed to investigate the effect of 25I-NBOMe on extracellular level of dopamine (DA), serotonin (5-HT), and glutamate (GLU) in the rat frontal cortex, tissue contents of monoamines, and hallucinogenic activity in rats. The extracellular levels of DA, 5-HT, and GLU were studied using microdialysis in freely moving animals. The tissue contents of DA, 5-HT and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) were determined in the rat frontal cortex. We also tested a drug-elicited HTR. 25I-NBOMe at doses 1, 3, and 10 mg/kg (sc) increased extracellular DA, 5-HT, and GLU levels, enhanced tissue content of 5-HT and 5-HIAA, but did not affect tissue level of DA and its metabolites. The compound exhibited an inverted U-shaped dose-response curve with respect to the effect on extracellular DA and 5-HT levels, but a U-shaped dose-response curve was observed for its effect on GLU release and HTR. The data from our study suggest that hallucinogenic activity of 25I-NBOMe seems to be related with the increase in extracellular GLU level-mediated via cortical 5-HT_2A receptors. The influence of 25I-NBOMe on 5-HT_2C and 5-HT_1A receptors may modulate its effect on neurotransmitters and HTR.

Dorsal raphe serotonin neurons inhibit operant responding for reward via inputs to the ventral tegmental area but not the nucleus accumbens: evidence from studies combining optogenetic stimulation and serotonin reuptake inhibition

The monoamine neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) exerts an inhibitory influence over motivation, but the circuits mediating this are unknown. Here, we used an optogenetic approach to isolate the contribution of dorsal raphe nucleus (DRN) 5-HT neurons and 5-HT innervation of the mesolimbic dopamine (DA) system to motivated behavior in mice. We found that optogenetic stimulation of DRN 5-HT neurons enhanced downstream 5-HT release, but this was not sufficient to inhibit operant responding for saccharin, a measure of motivated behavior. However, combining optogenetic stimulation of DRN 5-HT neurons with a low dose of the selective serotonin reuptake inhibitor (SSRI) citalopram synergistically reduced operant responding. We then examined whether these effects could be recapitulated if optogenetic stimulation specifically targeted 5-HT terminals in the ventral tegmental area (VTA) or nucleus accumbens (NAc) of the mesolimbic DA system. Optogenetic stimulation of 5-HT input to the VTA combined with citalopram treatment produced a synergistic decrease in responding for saccharin, resembling the changes produced by targeting 5-HT neurons in the DRN. However, this effect was not observed when optogenetic stimulation targeted 5-HT terminals in the NAc. Taken together, these results suggest that DRN 5-HT neurons exert an inhibitory influence over operant responding for reward through a direct interaction with the mesolimbic DA system at the level of the VTA. These studies support an oppositional interaction between 5-HT and DA systems in controlling motivation and goal-directed behavior, and have important implications for the development and refinement of treatment strategies for psychiatric disorders such as depression and addiction.

Chronic treatment with a metabotropic mGlu2/3 receptor agonist diminishes behavioral response to a phenethylamine hallucinogen

BACKGROUND

There is evidence that mGlu2/3 receptors regulate 5-HT_2A signaling, interactions that have been theorized to play a role in the antipsychotic-like effects of mGlu2/3 agonists as well as the hallucinogenic effects of 5-HT_2A agonists. One approach to unraveling this interaction is through the chronic administration of agonists at the two receptors, which should influence the functional properties of the targeted receptor due to receptor downregulation or desensitization and thereby alter crosstalk between the two receptors. In this study, we investigated whether chronic treatment with the mGlu2/3 agonist LY379268 would alter the behavioral response to a phenethylamine hallucinogen, 25CN-NBOH, which acts as a selective 5-HT_2A agonist.

METHODS

We first conducted a dose response of 25CN-NBOH (0.1, 0.3, 1, 3, or 10 mg/kg) to confirm the effects on head-twitch response (HTR) and then blockade studies with either the M100907 (0.1 mg/kg) or SB242084 (0.1, 0.3, or 1 mg/kg) to determine the contribution of 5-HT2A and 5-HT2C to 25CN-NBOH-induced HTR, respectively. To determine whether an mGlu2/3 agonist could block 25CN-NBOH-induced HTR, mice were pretreated with vehicle or LY379268 (0.1, 1, or 10 mg/kg) prior to 25CN-NBOH, and HTR was assessed. The effects of chronic LY379268 on 5-HT2A agonist-induced HTR were evaluated by treating mice with either vehicle or LY379268 (10 mg/kg) for 21 days and measuring 25CN-NBOH-induced HTR 48 h after the final LY379268 treatment. The following day (72 h after the final LY379268 treatment), the ability of acute LY379268 to block PCP-induced locomotor activity was assessed.

RESULTS

25CN-NBOH dose-dependently increased the HTR, a 5-HT_2A-mediated behavior, in mice. The selective 5-HT_2A antagonist M100907 completely blocked the HTR induced by 25CN-NBOH, whereas the selective 5-HT_2C antagonist SB242084 had no effect on the HTR. Administration of LY379268 (10 mg/kg SC) attenuated the HTR induced by 1 mg/kg 25CN-NBOH by ~ 50%. Chronic treatment (21 days) with LY379268 also attenuated the HTR response to 25CN-NBOH when tested 48 h after the last dose of LY379268. In locomotor tests, acute LY379268 significantly attenuated PCP-induced locomotor activity in the chronic vehicle treatment group; by contrast, there was only a trend for an overall interaction in the chronic LY379268 group, with LY379268 blocking the locomotor-stimulating effects of PCP only during the last 20 min.

CONCLUSIONS

These data are consistent with a functional interaction between mGlu2/3 and 5-HT_2A receptors, although the specific mechanism for the interaction is not known. These data support the hypothesis that mGlu2/3 receptors play a prominent role in modulating the behavioral response to 5-HT_2A receptor activation.

Role of mGlu2 in the 5-HT2A receptor-dependent antipsychotic activity of clozapine in mice

BACKGROUND

Serotonin 5-HT2A and metabotropic glutamate 2 (mGlu2) are neurotransmitter G protein-coupled receptors (GPCRs) involved in the signaling mechanisms underlying psychosis and schizophrenia treatment. Previous findings in mGlu2 knockout (KO) mice suggested that mGlu2 is necessary for head-twitch behavior, a rodent phenotype characteristic of hallucinogenic 5-HT2A receptor agonists. However, the role of mGlu2 in the behavioral effects induced by antipsychotic drugs remains poorly understood. Here, we tested antipsychotic-like behavioral phenotypes induced by the atypical antipsychotic clozapine in mGlu2-KO mice and wild-type control littermates.

METHODS

Locomotor activity was tested in mGlu2-KO mice and control littermates injected (i.p.) with clozapine (1.5 mg/kg) or vehicle followed by MK801 (0.5 mg/kg), PCP (7.5 mg/kg), amphetamine (6 mg/kg), scopolamine (2 mg/kg), or vehicle. Using a virally (HSV) mediated transgene expression approach, the role of frontal cortex mGlu2 in the modulation of MK801-induced locomotor activity by clozapine treatment was also evaluated.

RESULTS

The effect of clozapine on hyperlocomotor activity induced by the dissociative drugs MK801 and phencyclidine (PCP) was decreased in mGlu2-KO mice as compared to controls. Clozapine treatment, however, reduced hyperlocomotor activity induced by the stimulant drug amphetamine and the deliriant drug scopolamine in both wild-type and mGlu2-KO mice. Virally mediated over-expression of mGlu2 in the frontal cortex of mGlu2-KO mice rescued the ability of clozapine to reduce MK801-induced hyperlocomotion.

CONCLUSION

These findings further support the existence of a functionally relevant crosstalk between 5-HT2A and mGlu2 receptors in different preclinical models of antipsychotic activity.

Dark Classics in Chemical Neuroscience: Mescaline

Archeological studies in the United States, Mexico, and Peru suggest that mescaline, as a cactus constituent, has been used for more than 6000 years. Although it is a widespread cactus alkaloid, it is present in high concentrations in few species, notably the North American peyote ( Lophophora williamsii) and the South American wachuma ( Trichocereus pachanoi, T. peruvianus, and T. bridgesii). Spanish 16th century chroniclers considered these cacti "diabolic", leading to their prohibition, but their use persisted to our days and has been spreading for the last 150 years. In the late 1800s, peyote attracted scientific attention; mescaline was isolated, and its role in the psychedelic effects of peyote tops or "mescal buttons" was demonstrated. Its structure was established by synthesis in 1929, and alternative routes were developed, providing larger amounts for pharmacological and biosynthetic research. Although its effects are attributed mainly to its action as a 5-HT_2A serotonin receptor agonist, mescaline binds in a similar concentration range to 5-HT_1A and α_2A receptors. It is largely excreted unchanged in human urine, and its metabolic products are apparently unrelated to its psychedelic properties. Its low potency is probably responsible for its relative neglect by recreational substance users, as the successful search for structure-activity relationships in the hallucinogen field focused largely on finding more potent analogues. Renewed interest in the possible therapeutic applications of psychedelic drugs may hopefully lead to novel insights regarding the commonalities and differences between the actions of individual classic hallucinogens.

Biological Effects and Biodistribution of Bufotenine on Mice

Bufotenine is an alkaloid derived from serotonin, structurally similar to LSD and psilocin. This molecule is able to inhibit the rabies virus infection in in vitro and in vivo models, increasing the survival rate of infected animals. Being a very promising molecule for an incurable disease and because of the fact that there is no consensus regarding its neurological effects, this study aimed to evaluate chronic treatment of bufotenine on behavior, pathophysiology, and pharmacokinetics of mice. Animals were daily treated for 21 consecutive days with 0.63, 1.05, and 2.1 mg/animal/day bufotenine and evaluated by open field test and physiological parameters during all the experiment. After this period, organs were collected for histopathological and biodistribution analysis. Animals treated with bufotenine had mild behavioral alterations compared to the control group, being dose-response relationship. On the other hand, animals showed normal physiological functions and no histological alterations in the organs. With high doses, an inflammatory reaction was observed in the site of injection, but with no cellular damage. The alkaloid could be found in the heart and kidney with all doses and in the lungs and brain with higher doses. These results show that the effective dose, 0.63 mg/day, is safe to be administered in mice, since it did not cause significant effects on the animals' physiology and on the CNS. Higher doses were well tolerated, causing only mild behavioral effects. Thus, bufotenine might be a drug prototype for rabies treatment, an incurable disease.

Design, Synthesis, and Characterization of 4-Undecylpiperidine-2-carboxamides as Positive Allosteric Modulators of the Serotonin (5-HT) 5-HT2C Receptor

An impaired signaling capacity of the serotonin (5-HT) 5-HT_2C receptor (5-HT_2CR) has been implicated in the neurobehavioral processes that promote relapse vulnerability in cocaine use disorder (CUD). Restoration of the diminished 5-HT_2CR signaling through positive allosteric modulation presents a novel therapeutic approach. Several new molecules with the 4-alkylpiperidine-2-carboxamide scaffold were designed, synthesized, and pharmacologically evaluated, leading to the discovery of selective 5-HT_2CR positive allosteric modulators (PAMs). Compound 16 (CYD-1-79) potentiated 5-HT-evoked intracellular calcium release in cells stably expressing the human 5-HT_2CR but not the 5-HT_2AR cells. A topographically distinct allosteric site was identified based on the newly solved 5-HT_2CR structure. Compound 16 modulated 5-HT_2CR-mediated spontaneous ambulation, partially substituted for the training dose of the 5-HT_2CR agonist WAY163909, synergized with a low dose of WAY163909 to substitute fully for the stimulus effects of WAY163909, and attenuated relapse vulnerability as assessed in a rodent self-administration model, indicating its therapeutic promise for CUD.