Psilocybin analog 4-OH-DiPT enhances fear extinction and GABAergic inhibition of principal neurons in the basolateral amygdala

Psychedelics such as psilocybin show great promise for the treatment of depression and PTSD, but their long duration of action poses practical limitations for patient access. 4-OH-DiPT is a fast-acting and shorter-lasting derivative of psilocybin. Here we characterized the pharmacological profile of 4-OH-DiPT and examined its impact on fear extinction learning as well as a potential mechanism of action. First, we profiled 4-OH-DiPT at all 12 human 5-HT GPCRs. 4-OH-DiPT showed strongest agonist activity at all three 5-HT2A/2B/2C receptors with near full agonist activity at 5-HT2A. Notably, 4-OH-DiPT had comparable activity at mouse and human 5-HT2A/2B/2C receptors. In a fear extinction paradigm, 4-OH-DiPT significantly reduced freezing responses to conditioned cues in a dose-dependent manner with a greater potency in female mice than male mice. Female mice that received 4-OH-DiPT before extinction training had reduced avoidance behaviors several days later in the light dark box, elevated plus maze and novelty-suppressed feeding test compared to controls, while male mice did not show significant differences. 4-OH-DiPT produced robust increases in spontaneous inhibitory postsynaptic currents (sIPSCs) in basolateral amygdala (BLA) principal neurons and action potential firing in BLA interneurons in a 5-HT2A-dependent manner. RNAscope demonstrates that Htr2a mRNA is expressed predominantly in BLA GABA interneurons, Htr2c mRNA is expressed in both GABA interneurons and principal neurons, while Htr2b mRNA is absent in the BLA. Our findings suggest that 4-OH-DiPT activates BLA interneurons via the 5-HT2A receptor to enhance GABAergic inhibition of BLA principal neurons, which provides a potential mechanism for suppressing learned fear.

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.

Beyond the 5-HT2A Receptor: Classic and Nonclassic Targets in Psychedelic Drug Action

Serotonergic psychedelics, such as psilocybin and LSD, have garnered significant attention in recent years for their potential therapeutic effects and unique mechanisms of action. These compounds exert their primary effects through activating serotonin 5-HT2A receptors, found predominantly in cortical regions. By interacting with these receptors, serotonergic psychedelics induce alterations in perception, cognition, and emotions, leading to the characteristic psychedelic experience. One of the most crucial aspects of serotonergic psychedelics is their ability to promote neuroplasticity, the formation of new neural connections, and rewire neuronal networks. This neuroplasticity is believed to underlie their therapeutic potential for various mental health conditions, including depression, anxiety, and substance use disorders. In this mini-review, we will discuss how the 5-HT2A receptor activation is just one facet of the complex mechanisms of action of serotonergic psychedelics. They also interact with other serotonin receptor subtypes, such as 5-HT1A and 5-HT2C receptors, and with neurotrophin receptors (e.g., tropomyosin receptor kinase B). These interactions contribute to the complexity of their effects on perception, mood, and cognition. Moreover, as psychedelic research advances, there is an increasing interest in developing nonhallucinogenic derivatives of these drugs to create safer and more targeted medications for psychiatric disorders by removing the hallucinogenic properties while retaining the potential therapeutic benefits. These nonhallucinogenic derivatives would offer patients therapeutic advantages without the intense psychedelic experience, potentially reducing the risks of adverse reactions. Finally, we discuss the potential of psychedelics as substrates for post-translational modification of proteins as part of their mechanism of action.

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.

A non-hallucinogenic LSD analog with therapeutic potential for mood disorders

Hallucinations limit widespread therapeutic use of psychedelics as rapidly acting antidepressants. Here we profiled the non-hallucinogenic lysergic acid diethylamide (LSD) analog 2-bromo-LSD (2-Br-LSD) at more than 33 aminergic G protein-coupled receptors (GPCRs). 2-Br-LSD shows partial agonism at several aminergic GPCRs, including 5-HT2A, and does not induce the head-twitch response (HTR) in mice, supporting its classification as a non-hallucinogenic 5-HT2A partial agonist. Unlike LSD, 2-Br-LSD lacks 5-HT2B agonism, an effect linked to cardiac valvulopathy. Additionally, 2-Br-LSD produces weak 5-HT2A β-arrestin recruitment and internalization in vitro and does not induce tolerance in vivo after repeated administration. 2-Br-LSD induces dendritogenesis and spinogenesis in cultured rat cortical neurons and increases active coping behavior in mice, an effect blocked by the 5-HT2A-selective antagonist volinanserin (M100907). 2-Br-LSD also reverses the behavioral effects of chronic stress. Overall, 2-Br-LSD has an improved pharmacological profile compared with LSD and may have profound therapeutic value for mood disorders and other indications.

Pharmacological Mechanism of the Non-hallucinogenic 5-HT2A Agonist Ariadne and Analogs

Ariadne is a non-hallucinogenic analog in the phenylalkylamine chemical class of psychedelics that is closely related to an established synthetic hallucinogen, 2,5-dimethoxy-4-methyl-amphetamine (DOM), differing only by one methylene group in the α-position to the amine. Ariadne has been tested in humans including clinical trials at Bristol-Myers Company that indicate a lack of hallucinogenic effects and remarkable therapeutic effects, such as rapid remission of psychotic symptoms in schizophrenics, relaxation in catatonics, complete remission of symptoms in Parkinson's disease (PD), and improved cognition in geriatric subjects. Despite these provocative clinical results, the compound has been abandoned as a drug candidate and its molecular pharmacology remained unknown. Here, we report a detailed examination of the in vitro and in vivo pharmacology of Ariadne and its analogs, and propose a molecular hypothesis for the lack of hallucinogenic effects and the therapeutic potential of this compound class. We also provide a summary of previous clinical and preclinical results to contextualize the molecular signaling data. Our results show that Ariadne is a serotonin 5-HT₂ receptor agonist, exhibits modest selectivity over 5-HT₁ receptors, has no relevant activity at 5-HT_4,5,7 and other aminergic receptors, and no substantial affinity at plasma membrane monoamine transporters. Compared to DOM, Ariadne shows lower signaling potency and efficacy in multiple signaling pathways examined (G_q, G₁₁, and β-arrestin2) coupled to 5-HT_2A receptors. We confirmed the shift in signaling for an α-propyl analog and provide a molecular docking rationale for the progressive decrease in signaling potency with the growing length of the α-substituent. Ariadne versus DOM exhibits no apparent change in the relative preference between G_q/11 activation and β-arrestin2 recruitment; instead, there is a small but consistent drop in efficacy in these signaling channels. Ariadne acts as a 5-HT_2A agonist in vivo in mice and shows markedly attenuated head twitch response (HTR) in comparison to its hallucinogenic analogs, consistent with previous studies in rabbits, cats, and dogs. Hence, we propose the lower 5-HT_2A receptor signaling efficacy of this compound class as an explanatory model for the lack of hallucinogenic effects of Ariadne in humans and the dramatically attenuated hallucinosis-like effects in animals (5-HT_2A signaling efficacy hypothesis). In terms of reverse translation of the noted clinical therapeutic effects, we used an auxilin knockout model of Parkinson's disease where Ariadne rescued severe motor deficits in this mouse line, on par with the effects of l-DOPA, a notable finding considering Ariadne's lack of activity at dopamine receptors and transporters. Ariadne emerges as a prototype of a new drug class, non-hallucinogenic 5-HT_2A agonists, with considerable therapeutic potential across psychiatric and neurological indications.