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

The molecular structure, vibrational spectra, solvation effect, non-covalent interactions investigations of psilocin

Psilocin, or 4-HO-DMT (or 3-(2-dimethylaminoethyl)-1H-indol-4-ol), is a psychoactive alkaloid substance from the tryptamine family, isolated from Psilocybe mushrooms. This substance is being studied by various research groups because it has a clear therapeutic effect in certain dosages. In this work, the study of the structure and properties of psilocin was carried using theoretical methods: the effects of polar solvents (acetonitrile, dimethylsulfoxide, water, and tetrahydrofuran) on the structural parameters, spectroscopic properties (Raman, IR, and UV-Vis), frontier molecular orbital (FMO), molecular electrostatic potential (MEP) surface, and nonlinear optical parameters (NLO). Theoretical calculations were performed at the B3LYP/6-311++G(d,p) level by the density functional theory (DFT) method. IEFPCM was used to account for solvent effects. The types and nature of non-covalent interactions (NCI) between psilocin and solvent molecules were determined using Atoms in Molecules (AIM), the reduced density gradient method (RDG), the electron localization function (ELF), and the localization orbital locator (LOL). Experimental and calculated FT-IR, FT-Raman, and UV-Vis spectra were compared and found to be in good agreement.

Psychedelics: From Cave Art to 21st-Century Medicine for Addiction

BACKGROUND

Psychedelic substance use in ritualistic and ceremonial settings dates back as early as 8,500 BCE. Only in recent years, from the mid-20th century, we have seen the re-emergence of psychedelics in a therapeutic setting and more specifically for the treatment of addiction. This article aims to review research over the past 40 years using classic (psilocybin, lysergic acid diethylamide [LSD], dimethyltryptamine [DMT], mescaline) and atypical (ketamine, ibogaine, 5-MeO-DMT, 3,4-methylenedioxymethamphetamine) psychedelics for the treatment of addiction.

SUMMARY

We will start with an overview of the pharmacology and physiological and psychological properties of psychedelic substances from pre-clinical and clinical research. We will then provide an overview of evidence gathered by studies conducted in controlled research environments and naturalistic and ceremonial settings, while we identify the proposed therapeutic mechanisms of each psychedelic substance.

KEY MESSAGES

Classic and atypical psychedelics show promise as therapeutic alternatives for the treatment of addiction, through the improvement of psychological and physiological symptoms of dependence. A more comprehensive understanding of the ancient and present-day knowledge of the therapeutic potential of psychedelics can facilitate hope for psychedelic therapeutics in the treatment of addiction, especially for individuals who have failed other conventional treatment methods.

Human pluripotent stem cells as a translational toolkit in psychedelic research in vitro

Psychedelics, recognized for their impact on perception, are resurging as promising treatments with rapid onset for mood and substance use disorders. Despite increasing evidence from clinical trials, questions persist about the cellular and molecular mechanisms and their precise correlation with treatment outcomes. Murine neurons and immortalized non-neural cell lines harboring overexpressed constructs have shed light on neuroplastic changes mediated by the serotonin 2A receptor (5-HT2AR) as the primary mechanism. However, limitations exist in capturing human- and disease-specific traits. Here, we discuss current accomplishments and prospects for incorporating human pluripotent stem cells (PSCs) to complement these models. PSCs can differentiate into various brain cell types, mirroring endogenous expression patterns and cell identities to recreate disease phenotypes. Brain organoids derived from PSCs resemble cell diversity and patterning, while region-specific organoids simulate circuit-level phenotypes. PSC-based models hold significant promise to illuminate the cellular and molecular substrates of psychedelic-induced phenotypic recovery in neuropsychiatric disorders.

Review of Psilocybin Use for Depression among Cancer Patients after Approval in Oregon

Despite the legalization of psilocybin therapy for depression in terminal illnesses such as advanced cancer through Oregon's Measure 109 in 2020, significant challenges have impeded its implementation. This review synthesizes the empirical data supporting the utilization of psilocybin therapy for addressing cancer-related depression, including an evaluation of its purported benefits and potential adverse effects. It provides a comprehensive examination of therapeutic strategies, dosing regimens, and barriers to ensuring responsible and equitable access. Salient issues explored include the development of ethical protocols, integration within healthcare systems, ensuring statewide availability, resolving legal ambiguities, and defining clinical standards. Oregon's pioneering role serves as a case study, highlighting the necessity of addressing regulatory, logistical, and ethical obstacles to ensure the establishment of rigorous and equitable psilocybin care models.

The novel non-hallucinogenic compound DM506 (3-methyl-1,2,3,4,5,6-hexahydroazepino[4,5-b]indole) induces sedative- and anxiolytic-like activity in mice by a mechanism involving 5-HT2A receptor activation

The anxiolytic and sedative-like effects of 3-methyl-1,2,3,4,5,6-hexahydroazepino[4,5-b]indole (DM506), a non-hallucinogenic compound derived from ibogamine, were studied in mice. The behavioral effects were examined using Elevated O-maze and novelty suppressed feeding (NSFT) tests, open field test, and loss of righting reflex (LORR) test. The results showed that 15 mg/kg DM506 induced acute and long-lasting anxiolytic-like activity in naive and stressed/anxious mice, respectively. Repeated administration of 5 mg/kg DM506 did not cause cumulative anxiolytic activity or any side effects. Higher doses of DM506 (40 mg/kg) induced sedative-like activity, which was inhibited by a selective 5-HT2A receptor antagonist, volinanserin. Electroencephalography results showed that 15 mg/kg DM506 fumarate increased the transition from a highly alert state (fast γ wavelength) to a more synchronized deep-sleeping activity (δ wavelength), which is reflected in the sedative/anxiolytic activity in mice but without the head-twitch response observed in hallucinogens. The functional, radioligand binding, and molecular docking results showed that DM506 binds to the agonist sites of human 5-HT2A (Ki = 24 nM) and 5-HT2B (Ki = 16 nM) receptors and activates them with a potency (EC50) of 9 nM and 3 nM, respectively. DM506 was relatively less potent and behaved as a partial agonist (efficacy <80%) for both receptor subtypes compared to the full agonist DOI (2,5-dimethoxy-4-iodoamphetamine). Our study showed for the first time that the non-hallucinogenic compound DM506 induces anxiolytic- and sedative-like activities in naïve and stressed/anxious mice in a dose-, time-, and volinanserin-sensitive manner, likely through mechanisms involving 5-HT2A receptor activation.

Multimodal Neuroimaging of the Effect of Serotonergic Psychedelics on the Brain

The neurobiological mechanisms underpinning psychiatric disorders such as treatment-resistant major depression, post-traumatic stress disorder, and substance use disorders, remain unknown. Psychedelic compounds, such as psilocybin, lysergic acid diethylamide, and N,N-dimethyltryptamine, have emerged as potential therapies for these disorders because of their hypothesized ability to induce neuroplastic effects and alter functional networks in the brain. Yet, the mechanisms underpinning the neurobiological treatment response remain obscure. Quantitative neuroimaging is uniquely positioned to provide insight into the neurobiological mechanisms of these emerging therapies and quantify the patient treatment response. This review aims to synthesize our current state-of-the-art understanding of the functional changes occurring in the brain following psilocybin, lysergic acid diethylamide, or N,N-dimethyltryptamine administration in human participants with fMRI and PET. We further aim to disseminate our understanding of psychedelic compounds as they relate to neuroimaging with the goal of improved diagnostics and treatment of neuropsychiatric illness.

Unveiling the Psychedelic Journey: An Appraisal of Psilocybin as a Profound Antidepressant Therapy

Depression, a global health concern with significant implications for suicide rates, remains challenging to treat effectively with conventional pharmacological options. The existing pharmaceutical interventions for these illnesses need daily dosing, are accompanied by various adverse effects, and may exhibit limited efficacy in certain cases. However, hope emerges from an unlikely source-Psilocybin, a natural hallucinogen found in certain mushrooms. Recently, this enigmatic compound has garnered attention for its potential therapeutic benefits in addressing various mental health issues, including depression. Psilocybin alters mood, cognition, and perception by acting on a particular subtype of serotonin receptors in the brain. It's feasible that these shifts in consciousness will promote healing development, offering a novel approach to depression management. This comprehensive review explores psilocybin, derived from specific mushrooms, and its implications in the treatment of depression. The study examines new perspectives and therapeutic possibilities surrounding psilocybin, addressing existing gaps in academic literature. It delves into its biosynthesis, unique mechanisms of action, therapeutic applications, and anti-depressive effects. By uncovering the potential of this mind-altering substance, the review aims to advance psychiatric care, offering hope to those globally affected by depression.

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

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

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

RATIONALE

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

A pilot study of cerebral metabolism and serotonin 5-HT2A receptor occupancy in rats treated with the psychedelic tryptamine DMT in conjunction with the MAO inhibitor harmine

Rationale: The psychedelic effects of the traditional Amazonian botanical decoction known as ayahuasca are often attributed to agonism at brain serotonin 5-HT2A receptors by N,N-dimethyltryptamine (DMT). To reduce first pass metabolism of oral DMT, ayahuasca preparations additionally contain reversible monoamine oxidase A (MAO-A) inhibitors, namely β-carboline alkaloids such as harmine. However, there is lacking biochemical evidence to substantiate this pharmacokinetic potentiation of DMT in brain via systemic MAO-A inhibition. Objectives: We measured the pharmacokinetic profile of harmine and/or DMT in rat brain, and tested for pharmacodynamic effects on brain glucose metabolism and DMT occupancy at brain serotonin 5-HT2A receptors. Methods: We first measured brain concentrations of harmine and DMT after treatment with harmine and/or DMT at low sub-cutaneous doses (1 mg/kg each) or harmine plus DMT at moderate doses (3 mg/kg each). In the same groups of rats, we also measured ex vivo the effects of these treatments on the availability of serotonin 5-HT2A receptors in frontal cortex. Finally, we explored effects of DMT and/or harmine (1 mg/kg each) on brain glucose metabolism with [18F]FDG-PET. Results: Results confirmed that co-administration of harmine inhibited the formation of the DMT metabolite indole-3-acetic acid (3-IAA) in brain, while correspondingly increasing the cerebral availability of DMT. However, we were unable to detect any significant occupancy by DMT at 5-HT2A receptors measured ex vivo, despite brain DMT concentrations as high as 11.3 µM. We did not observe significant effects of low dose DMT and/or harmine on cerebral [18F]FDG-PET uptake. Conclusion: These preliminary results call for further experiments to establish the dose-dependent effects of harmine/DMT on serotonin receptor occupancy and cerebral metabolism.

Natural psychedelics in the treatment of depression; a review focusing on neurotransmitters

Natural psychedelic compounds are emerging as potential novel therapeutics in psychiatry. This review will discuss how natural psychedelics exert their neurobiological therapeutic effects, and how different neurotransmission systems mediate the effects of these compounds. Further, current therapeutic strategies for depression, and novel mechanism of action of natural psychedelics in the treatment of depression will be discussed. In this review, our focus will be on N, N-dimethyltryptamine (DMT), reversible type A monoamine oxidase inhibitors, mescaline-containing cacti, psilocybin/psilocin-containing mushrooms, ibogaine, muscimol extracted from Amanita spp. mushrooms and ibotenic acid.

Neuroimaging in psychedelic drug development: past, present, and future

Psychedelic therapy (PT) is an emerging paradigm with great transdiagnostic potential for treating psychiatric disorders, including depression, addiction, post-traumatic stress disorder, and potentially others. 'Classic' serotonergic psychedelics, such as psilocybin and lysergic acid diethylamide (LSD), which have a key locus of action at the 5-HT2A receptor, form the main focus of this movement, but substances including ketamine, 3,4-Methylenedioxymethamphetamine (MDMA) and ibogaine also hold promise. The modern phase of development of these treatment modalities in the early 21st century has occurred concurrently with the wider use of advanced human neuroscientific research methods; principally neuroimaging. This can potentially enable assessment of drug and therapy brain effects with greater precision and quantification than any previous novel development in psychiatric pharmacology. We outline the major trends in existing data and suggest the modern development of PT has benefitted greatly from the use of neuroimaging. Important gaps in existing knowledge are identified, namely: the relationship between acute drug effects and longer-term (clinically-relevant) effects, the precise characterisation of effects at the 5-HT2A receptor and relationships with functional/clinical effects, and the possible impact of these compounds on neuroplasticity. A road-map for future research is laid out, outlining clinical studies which will directly address these three questions, principally using combined Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) methods, plus other adjunct techniques. Multimodal (PET/MRI) studies using modern PET techniques such as the 5-HT2A-selective ligand [11 C]Cimbi-36 (and other ligands sensitive to neuroplasticity changes) alongside MRI measures of brain function would provide a 'molecular-functional-clinical bridge' in understanding. Such results would help to resolve some of these questions and provide a firmer foundation for the ongoing development of PT.

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

The Bright Side of Psychedelics: Latest Advances and Challenges in Neuropharmacology

The need to identify effective therapies for the treatment of psychiatric disorders is a particularly important issue in modern societies. In addition, difficulties in finding new drugs have led pharmacologists to review and re-evaluate some past molecules, including psychedelics. For several years there has been growing interest among psychotherapists in psilocybin or lysergic acid diethylamide for the treatment of obsessive-compulsive disorder, of depression, or of post-traumatic stress disorder, although results are not always clear and definitive. In fact, the mechanisms of action of psychedelics are not yet fully understood and some molecular aspects have yet to be well defined. Thus, this review aims to summarize the ethnobotanical uses of the best-known psychedelic plants and the pharmacological mechanisms of the main active ingredients they contain. Furthermore, an up-to-date overview of structural and computational studies performed to evaluate the affinity and binding modes to biologically relevant receptors of ibogaine, mescaline, N,N-dimethyltryptamine, psilocin, and lysergic acid diethylamide is presented. Finally, the most recent clinical studies evaluating the efficacy of psychedelic molecules in some psychiatric disorders are discussed and compared with drugs already used in therapy.

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Three Naturally-Occurring Psychedelics and Their Significance in the Treatment of Mental Health Disorders

Classical psychedelics represent a family of psychoactive substances with structural similarities to serotonin and affinity for serotonin receptors. A growing number of studies have found that psychedelics can be effective in treating various psychiatric conditions, including post-traumatic stress disorder, major depressive disorder, anxiety, and substance use disorders. Mental health disorders are extremely prevalent in the general population constituting a major problem for the public health. There are a wide variety of interventions for mental health disorders, including pharmacological therapies and psychotherapies, however, treatment resistance still remains a particular challenge in this field, and relapse rates are also quite high. In recent years, psychedelics have become one of the promising new tools for the treatment of mental health disorders. In this review, we will discuss the three classic serotonergic naturally occurring psychedelics, psilocybin, ibogaine, and N, N-dimethyltryptamine, focusing on their pharmacological properties and clinical potential. The purpose of this article is to provide a focused review of the most relevant research into the therapeutic potential of these substances and their possible integration as alternative or adjuvant options to existing pharmacological and psychological therapies.

Monoamine Oxidase Inhibition by Plant-Derived β-Carbolines; Implications for the Psychopharmacology of Tobacco and Ayahuasca

The monoamine oxidases (MAOs) are flavin-containing amine oxidoreductases responsible for metabolism of many biogenic amine molecules in the brain and peripheral tissues. Whereas serotonin is the preferred substrate of MAO-A, phenylethylamine is metabolized by MAO-B, and dopamine and tyramine are nearly ambivalent with respect to the two isozymes. β-Carboline alkaloids such as harmine, harman(e), and norharman(e) are MAO inhibitors present in many plant materials, including foodstuffs, medicinal plants, and intoxicants, notably in tobacco (Nicotiana spp.) and in Banisteriopsis caapi, a vine used in the Amazonian ayahuasca brew. The β-carbolines present in B. caapi may have effects on neurogenesis and intrinsic antidepressant properties, in addition to potentiating the bioavailability of the hallucinogen N,N-dimethyltryptamine (DMT), which is often present in admixture plants of ayahuasca such as Psychotria viridis. Tobacco also contains physiologically relevant concentrations of β-carbolines, which potentially contribute to its psychopharmacology. However, in both cases, the threshold of MAO inhibition sufficient to interact with biogenic amine neurotransmission remains to be established. An important class of antidepressant medications provoke a complete and irreversible inhibition of MAO-A/B, and such complete inhibition is almost unattainable with reversible and competitive inhibitors such as β-carbolines. However, the preclinical and clinical observations with synthetic MAO inhibitors present a background for obtaining a better understanding of the polypharmacologies of tobacco and ayahuasca. Furthermore, MAO inhibitors of diverse structures are present in a wide variety of medicinal plants, but their pharmacological relevance in many instances remains to be established.

Mycotherapy: Potential of Fungal Bioactives for the Treatment of Mental Health Disorders and Morbidities of Chronic Pain

Mushrooms have been used as traditional medicine for millennia, fungi are the main natural source of psychedelic compounds. There is now increasing interest in using fungal active compounds such as psychedelics for alleviating symptoms of mental health disorders including major depressive disorder, anxiety, and addiction. The anxiolytic, antidepressant and anti-addictive effect of these compounds has raised awareness stimulating neuropharmacological investigations. Micro-dosing or acute dosing with psychedelics including Lysergic acid diethylamide (LSD) and psilocybin may offer patients treatment options which are unmet by current therapeutic options. Studies suggest that either dosing regimen produces a rapid and long-lasting effect on the patient post administration with a good safety profile. Psychedelics can also modulate immune systems including pro-inflammatory cytokines suggesting a potential in the treatment of auto-immune and other chronic pain conditions. This literature review aims to explore recent evidence relating to the application of fungal bioactives in treating chronic mental health and chronic pain morbidities.

Administration of N,N-dimethyltryptamine (DMT) in psychedelic therapeutics and research and the study of endogenous DMT

As with all drugs, the route, form, and/or dose of a substance administered or applied can play a defining role in its overall pharmacology and use as a therapeutic. This review will focus on these factors as they relate to the psychedelic N,N-dimethyltryptamine (DMT). It will examine the positive and negative aspects of different formulations and routes of administration of DMT and the observed effects from such administrations in the form of ayahuasca teas; oral "pharmahuasca"; injections by intravenous (IV) and intramuscular (IM) routes; inhalation, insufflation; and other routes; and high-dose, low-dose, and "micro-dose" effects. The review will consider possible oral route of administration alternatives that would not require concomitant use of a monoamine oxidase inhibitor. The review will then address the current research findings for DMT from in vivo and in vitro studies as well as the possibility that these findings may be revealing the role of endogenous DMT in normal brain function.

Serotonergic Psychedelics in Neural Plasticity

Psychedelics, compounds that can induce dramatic changes in conscious experience, have been used by humans for centuries. Recent studies have shown that certain psychedelics can induce neural plasticity by promoting neurite growth and synapse formation. In this review, we focus on the role of classical serotonergic psychedelics in neural plasticity and discuss its implication for their therapeutic potentials.

The Effects of Tryptamine Psychedelics in the Brain: A meta-Analysis of Functional and Review of Molecular Imaging Studies

There is an increasing interest in the neural effects of psychoactive drugs, in particular tryptamine psychedelics, which has been incremented by the proposal that they have potential therapeutic benefits, based on their molecular mimicry of serotonin. It is widely believed that they act mainly through 5HT2A receptors but their effects on neural activation of distinct brain systems are not fully understood. We performed a quantitative meta-analysis of brain imaging studies to investigate the effects of substances within this class (e.g., LSD, Psilocybin, DMT, Ayahuasca) in the brain from a molecular and functional point of view. We investigated the question whether the changes in activation patterns and connectivity map into regions with larger 5HT1A/5HT2A receptor binding, as expected from indolaemine hallucinogens (in spite of the often reported emphasis only on 5HT2AR). We did indeed find that regions with changed connectivity and/or activation patterns match regions with high density of 5HT2A receptors, namely visual BA19, visual fusiform regions in BA37, dorsal anterior and posterior cingulate cortex, medial prefrontal cortex, and regions involved in theory of mind such as the surpramarginal gyrus, and temporal cortex (rich in 5HT1A receptors). However, we also found relevant patterns in other brain regions such as dorsolateral prefrontal cortex. Moreover, many of the above-mentioned regions also have a significant density of both 5HT1A/5HT2A receptors, and available PET studies on the effects of psychedelics on receptor occupancy are still quite scarce, precluding a metanalytic approach. Finally, we found a robust neuromodulatory effect in the right amygdala. In sum, the available evidence points towards strong neuromodulatory effects of tryptamine psychedelics in key brain regions involved in mental imagery, theory of mind and affective regulation, pointing to potential therapeutic applications of this class of substances.