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The κ-opioid receptor (KOR) represents a highly desirable therapeutic target for treating not only pain but also addiction and affective disorders1. However, the development of KOR analgesics has been hindered by the associated hallucinogenic side effects2. The initiation of KOR signalling requires the Gi/o-family proteins including the conventional (Gi1, Gi2, Gi3, GoA and GoB) and nonconventional (Gz and Gg) subtypes. How hallucinogens exert their actions through KOR and how KOR determines G-protein subtype selectivity are not well understood. Here we determined the active-state structures of KOR in a complex with multiple G-protein heterotrimers—Gi1, GoA, Gz and Gg—using cryo-electron microscopy. The KOR–G-protein complexes are bound to hallucinogenic salvinorins or highly selective KOR agonists. Comparisons of these structures reveal molecular determinants critical for KOR–G-protein interactions as well as key elements governing Gi/o-family subtype selectivity and KOR ligand selectivity. Furthermore, the four G-protein subtypes display an intrinsically different binding affinity and allosteric activity on agonist binding at KOR. These results provide insights into the actions of opioids and G-protein-coupling specificity at KOR and establish a foundation to examine the therapeutic potential of pathway-selective agonists of KOR.

Initial interest in the value of psychedelic drugs (“psychotomimetics”) in psychiatry began in the early 20th century, with explorations of the possibility that mescaline or peyote could produce psychosis-like effects. Over time, interest was focused on whether the effects of psychedelics could inform as to the underlying basis for psychiatric disorders. As research continued, and especially after the discovery of LSD in 1943, increasing interest in a role for psychedelics as adjuncts to psychotherapy began to evolve and became the major focus of work with psychedelics up to the present day.

Psilocybin (4-phosphoryloxy-N,N-dimethyltryptamine) is an indole-based secondary metabolite produced by numerous species of mushrooms. South American Aztec Indians referred to them as teonanacatl, meaning "god's flesh," and they were used in religious and healing rituals. Spanish missionaries in the 1500s attempted to destroy all records and evidence of the use of these mushrooms. Nevertheless, a 16th century Spanish Franciscan friar and historian mentioned teonanacatl in his extensive writings, intriguing 20th century ethnopharmacologists and leading to a decades-long search for the identity of teonanacatl. Their search ultimately led to a 1957 photo-essay in a popular magazine, describing for the Western world the use of these mushrooms. Specimens were ultimately obtained, and their active principle identified and chemically synthesized. In the past 10-15 years several FDA-approved clinical studies have indicated potential medical value for psilocybin-assisted psychotherapy in treating depression, anxiety, and certain addictions. At present, assuming that the early clinical studies can be validated by larger studies, psilocybin is poised to make a significant impact on treatments available to psychiatric medicine.

Psychedelics are a relatively recent field of research that had not gained much support half a century ago, yet it developed into a much acknowledged, highly relevant field that extends to many people's lives. Psychedelics have demonstrated profound and durable therapeutic potential for the treatment of several psychiatric disorders including depression, anxiety, and substance use disorders, among others. In this special issue, basic science of psychedelics is reviewed with respect to fundamental cellular, molecular, and genetic mechanisms, all the way up to the human systems level with clinical reviews. We hope the articles, authored by leading scientists in their field, will help to understand better the role of the serotonin 5-HT

Serotonin (5-hydroxytryptamine [5-HT]) 5-HT2-family receptors represent essential targets for lysergic acid diethylamide (LSD) and all other psychedelic drugs. Although the primary psychedelic drug effects are mediated by the 5-HT

SummaryLung organogenesis requires precisely timed shifts in the spatial organization and function of parenchymal cells, especially during the later stages of lung development. To investigate the mechanisms governing lung parenchymal dynamics during development, we performed a single cell RNA sequencing (scRNA-seq) time-series yielding 92,238 epithelial, endothelial, and mesenchymal cells across 8 time points from embryonic day 12 (E12) to postnatal day 14 (P14) in mice. We combined new computational analyses with RNAin situhybridization to explore transcriptional velocity, fate likelihood prediction, and spatiotemporal localization of cell populations during the transition between the saccular and alveolar stages. We interrogated this atlas to illustrate the complexity of type 1 pneumocyte function during the saccular and alveolar stages, and we demonstrate an integrated view of the cellular dynamics during lung development.

Hallucinogens like lysergic acid diethylamide (LSD), psilocybin, and substituted N-benzyl phenylalkylamines are widely used recreationally with psilocybin being considered as a therapeutic for many neuropsychiatric disorders including depression, anxiety, and substance abuse. How psychedelics mediate their actions—both therapeutic and hallucinogenic—are not understood, although activation of the 5-HT(2A) serotonin receptor (HTR2A) is key. To gain molecular insights into psychedelic actions, we determined the active-state structure of HTR2A bound to 25-CN-NBOH—a prototypical hallucinogen—in complex with an engineered Gαq heterotrimer by cryoelectron microscopy (cryo-EM). We also obtained the X-ray crystal structures of HTR2A complexed with the arrestin-biased ligand LSD or the inverse agonist methiothepin. Comparisons of these structures reveal determinants responsible for HTR2A-Gαq protein interactions as well as the conformational rearrangements involved in active-state transitions. Given the potential therapeutic actions of hallucinogens, these findings could accelerate the discovery of more selective drugs for the treatment of a variety of neuropsychiatric disorders.

RationaleThe lysergamide lysergic acid diethylamide (LSD) is a prototypical classical hallucinogen with remarkably high potency. LSD remains a popular recreational drug but is also becoming an important research tool for medical and neuroscience studies. Recently, several lysergamides that are close structural analogs of LSD have been sold as recreational drugs, which suggests that further studies are needed to explore the pharmacological properties of these compounds.ObjectiveIn this present investigation, another LSD congener, N-ethyl-N-cyclopropyl lysergamide (ECPLA), which to date has not been marketed as a recreational substance, was evaluated for its pharmacological features relative to those previously reported for LSD. The experiments focused on interactions with the 5-HT2A receptor, which is responsible for mediating the psychedelic effects of LSD and other hallucinogens.MethodsCompetitive binding assays were performed to measure the affinity of ECPLA for 27 monoamine receptors. The ability of ECPLA to activate human 5-HT2 receptor subtypes was assessed using calcium mobilization assays. Head twitch response (HTR) studies were conducted in C57BL/6J mice to determine whether ECPLA activates 5-HT2A receptors in vivo. Two other N-alkyl substituted lysergamides, N-methyl-N-isopropyl lysergamide (MIPLA) and N-methyl-N-propyl lysergamide (LAMPA), were also tested in the HTR paradigm for comparative purposes.ResultsECPLA has high affinity for most serotonin receptors, α2-adrenoceptors, and D2-like dopamine receptors. Additionally, ECPLA was found to be a potent, highly efficacious 5-HT2A agonist for Gq-mediated calcium flux. Treatment with ECPLA induced head twitches in mice with a median effective dose (ED50) of 317.2nmol/kg (IP), which is ~ 40% of the potency observed previously for LSD. LAMPA (ED50 = 358.3nmol/kg) was virtually equipotent with ECPLA in the HTR paradigm whereas MIPLA (ED50 = 421.7nmol/kg) was slightly less potent than ECPLA.ConclusionsThese findings demonstrate that the pharmacological properties of ECPLA, MIPLA, and LAMPA are reminiscent of LSD and other lysergamide hallucinogens.

Hallucinogenic drugs potently alter human behavior and have a millennia-long history of use for medicinal and religious purposes. Interest is rapidly growing in their potential as CNS modulators and therapeutic agents for brain conditions. Antimuscarinic cholinergic drugs, such as atropine and scopolamine, induce characteristic hyperactivity and dream-like hallucinations and form a separate group of hallucinogens known as "deliriants". Although atropine and scopolamine are relatively well-studied drugs in cholinergic physiology, deliriants represent the least-studied class of hallucinogens in terms of their behavioral and neurological phenotypes. As such, novel approaches and new model organisms are needed to investigate the CNS effects of these compounds. Here, we comprehensively evaluate the preclinical effects of deliriant hallucinogens in various animal models, their mechanisms of action, and potential interplay with other signaling pathways. We also parallel experimental and clinical findings on deliriant agents and outline future directions of translational research in this field.

Lysergic acid diethylamide (LSD) is one of the most potent psychoactive agents known, producing dramatic alterations of consciousness after submilligram (≥20 μg) oral doses. Following the accidental discovery of its potent psychoactive effects in 1943, it was supplied by Sandoz Laboratories as an experimental drug that might be useful as an adjunct for psychotherapy, or to give psychiatrists insight into the mental processes in their patients. The finding of serotonin in the mammalian brain in 1953, and its structural resemblance to LSD, quickly led to ideas that serotonin in the brain might be involved in mental disorders, initiating rapid research interest in the neurochemistry of serotonin. LSD proved to be physiologically very safe and nonaddictive, with a very low incidence of adverse events when used in controlled experiments. Widely hailed by psychiatry as a breakthrough in the 1950s and early 1960s, clinical research with LSD ended by about 1970, when it was formally placed into Schedule 1 of the Controlled Substances Act of 1970 following its growing popularity as a recreational drug. Within the past 5 years, clinical research with LSD has begun in Europe, but there has been none in the United States. LSD is proving to be a powerful tool to help understand brain dynamics when combined with modern brain imaging methods. It remains to be seen whether therapeutic value for LSD can be confirmed in controlled clinical trials, but promising results have been obtained in small pilot trials of depression, anxiety, and addictions using psilocybin, a related psychedelic molecule.

The pineal gland has a romantic history, from pharaonic Egypt, where it was equated with the eye of Horus, through various religious traditions, where it was considered the seat of the soul, the third eye, etc. Recent incarnations of these notions have suggested that N,N-dimethyltryptamine is secreted by the pineal gland at birth, during dreaming, and at near death to produce out of body experiences. Scientific evidence, however, is not consistent with these ideas. The adult pineal gland weighs less than 0.2 g, and its principal function is to produce about 30 µg per day of melatonin, a hormone that regulates circadian rhythm through very high affinity interactions with melatonin receptors. It is clear that very minute concentrations of N,N-dimethyltryptamine have been detected in the brain, but they are not sufficient to produce psychoactive effects. Alternative explanations are presented to explain how stress and near death can produce altered states of consciousness without invoking the intermediacy of N,N-dimethyltryptamine.

Lysergic acid diethylamide (LSD) is perhaps one of the best-known psychoactive substances and many structural modifications of this prototypical lysergamide have been investigated. Several lysergamides were recently encountered as 'research chemicals' or new psychoactive substances (NPS). Although lysergic acid morpholide (LSM-775) appeared on the NPS market in 2013, there is disagreement in the literature regarding the potency and psychoactive properties of LSM-775 in humans. The present investigation attempts to address the gap of information that exists regarding the analytical profile and pharmacological effects of LSM-775. A powdered sample of LSM-775 was characterized by X-ray crystallography, nuclear magnetic resonance spectroscopy (NMR), gas chromatography mass spectrometry (GC-MS), high mass accuracy electrospray MS/MS, high performance liquid chromatography (HPLC) diode array detection, HPLC quadrupole MS, and GC solid-state infrared analysis. Screening for receptor affinity and functional efficacy revealed that LSM-775 acts as a nonselective agonist at 5-HT1A and 5-HT2A receptors. Head twitch studies were conducted in C57BL/6J mice to determine whether LSM-775 activates 5-HT2A receptors and produces hallucinogen-like effects in vivo. LSM-775 did not induce the head twitch response unless 5-HT1A receptors were blocked by pretreatment with the antagonist WAY-100,635 (1mg/kg, subcutaneous). These findings suggest that 5-HT1A activation by LSM-775 masks its ability to induce the head twitch response, which is potentially consistent with reports in the literature indicating that LSM-775 is only capable of producing weak LSD-like effects in humans.

The psychoactive properties of lysergic acid diethylamide (LSD) have fascinated scientists across disciplines and the exploration of other analogs and derivatives has been motivated by deepening the understanding of ligand-receptor interactions at the molecular level as well as by the search for new therapeutics. Several LSD congeners have appeared on the new psychoactive substances (NPS) market in the form of blotters or powders. Examples include 1-propionyl-LSD (1P-LSD), AL-LAD and LSZ. The absence of analytical data for novel compounds is a frequent challenge encountered in clinical and toxicological investigations. Two newly emerging lysergamides N6-ethyl-6-norlysergic acid diethylamide (ETH-LAD) and 1P-ETH-LAD were characterized by gas chromatography mass spectrometry (GC-MS), low and high mass accuracy electrospray MS(/MS), GC solid-state infrared analysis, high performance liquid chromatography diode array detection as well as nuclear magnetic resonance spectroscopy. Limited analytical data for ETH-LAD were previously available, whereas information about 1P-ETH-LAD has not previously been encountered in scientific literature. This study extends the characterization of lysergamides distributed on the NPS market, which will help to make analytical data available to clinicians, toxicologists and other stakeholders who are likely to encounter these substances. The analysis of a test incubation of 1P-ETH-LAD with human serum at 37°C by LC single quadrupole MS at various time points (0–6 h, once per hour and one measurement after 24 h) revealed the formation of ETH-LAD, suggesting that 1P-ETH-LAD might serve as a pro-drug. 1P-ETH-LAD was still detectable in serum after 24 h.

Dopamine (DA) is required for reinforcement learning. Hence, disruptions in DA signaling may contribute to the learning deficits associated with psychiatric disorders. The DA D1 receptor (D1R) has been linked to learning and is a target for cognitive/motivational enhancement in patients with schizophrenia. Separating the striatal D1R contribution to learning vs. motivation, however, has been challenging. We suppressed striatal D1R expression in mice using a D1R-targeting short hairpin RNA (shRNA), delivered locally to the striatum via an adeno-associated virus (AAV). We then assessed reward- and punishment-associative learning using a probabilistic learning task and motivation using a progressive-ratio breakpoint procedure. We confirmed suppression of striatal D1Rs immunohistochemically and by testing locomotor activity after the administration of (+)-doxanthrine, a full D1R agonist, in control mice and those treated with the D1RshRNA. D1RshRNA-treated mice exhibited impaired reward-associative learning, while punishment-associative learning was spared. This deficit was unrelated to general learning impairments or amotivation, because the D1shRNA-treated mice exhibited normal Barnes maze learning and normal motivation in the progressive-ratio breakpoint procedure. Suppression of striatal D1Rs selectively impaired reward-associative learning whereas punishment-associative learning, aversion-motivated learning, and appetitive motivation were spared. Because patients with schizophrenia exhibit similar reward-associative learning deficits, D1R-targeted treatments should be investigated to improve reward learning in these patients.

Hallucinogens (psychedelics) are substances that induce profound changes in perception and cognition. The closely related 3,4-methylenedioxymethamphetamine (MDMA) produces euphoria and a feeling of empathy, with minimal sensory distortion. Both of these classes of substances produce their effects by interacting with the serotonergic system. This chapter will review the receptor interactions that contribute to the behavioral effects of serotonergic hallucinogens and MDMA. Importantly, recent human studies confirm that the 5-HT2A receptor plays a primary role in mediating the subjective effects of psilocybin. Evidence from rodent studies, however, suggests that 5-HT1A and 5-HT2C receptors, and dopamine receptors may play a secondary role. By contrast, the effects of MDMA are largely attributable to its potent release of monoamines in serotonergic, dopaminergic, and noradrenergic systems. There is a growing consensus that these drugs are likely to show therapeutic efficacy in the treatment of certain psychiatric disorders such as PTSD, depression, and some addictions.

Scientific interest in serotonergic psychedelics (e.g., psilocybin and LSD; 5-HT2A receptor agonists) has dramatically increased within the last decade. Clinical studies administering psychedelics with psychotherapy have shown preliminary evidence of robust efficacy in treating anxiety and depression, as well as addiction to tobacco and alcohol. Moreover, recent research has suggested that these compounds have potential efficacy against inflammatory diseases through novel mechanisms, with potential advantages over existing antiinflammatory agents. We propose that psychedelics exert therapeutic effects for psychiatric disorders by acutely destabilizing local brain network hubs and global network connectivity via amplification of neuronal avalanches, providing the occasion for brain network "resetting" after the acute effects have resolved. Antiinflammatory effects may hold promise for efficacy in treatment of inflammation-related nonpsychiatric as well as potentially for psychiatric disorders. Serotonergic psychedelics operate through unique mechanisms that show promising effects for a variety of intractable, debilitating, and lethal disorders, and should be rigorously researched.

Lysergic N,N-diethylamide (LSD) is perhaps one of the most intriguing psychoactive substances known and numerous analogs have been explored to a varying extent in previous decades. In 2013, N6-allyl-6-norlysergic acid diethylamide (AL-LAD) and (2’S,4’S)-lysergic acid 2,4-dimethylazetidide (LSZ) have appeared on the ‘research chemicals’ / new psychoactive substances (NPS) market in both powdered and blotter form. This study reports the analytical characterization of powdered AL-LAD and LSZ tartrate samples and their semi-quantitative determination on blotter paper. Included in this study was the use of nuclear magnetic resonance spectroscopy, gas chromatography mass spectrometry (MS), low and high-resolution electrospray MS(/MS), high performance liquid chromatography diode array detection and GC solid-state infrared analysis. One feature shared by serotonergic psychedelics, such as LSD, is the ability to mediate behavioral responses via activation of 5-HT2A receptors. Both AL-LAD and LSZ displayed LSD-like responses in male C57BL/6J mice when employing the head-twitch response (HTR) assay. AL-LAD and LSZ produced nearly identical inverted-U-shaped dose dependent effects, with the maximal responses occurring at 200 µg/kg. Analysis of the dose-responses by nonlinear regression confirmed that LSZ (ED50 = 114.2 nmol/kg) was equipotent to LSD (ED50 = 132.8 nmol/kg) in mice, whereas AL-LAD was slightly less potent (ED50 = 174.9 nmol/kg). The extent to which a comparison in potency can be translated directly to humans requires further investigation. Availability of both chemical and pharmacological data obtained from NPS as they appear on the market provides important data to research communities that are interested in various aspects related substance use and forensic identification.

Lysergic acid diethylamide (LSD) is the prototypical psychedelic drug, but its effects on the human brain have never been studied before with modern neuroimaging. Here, three complementary neuroimaging techniques: arterial spin labeling (ASL), blood oxygen level-dependent (BOLD) measures, and magnetoencephalography (MEG), implemented during resting state conditions, revealed marked changes in brain activity after LSD that correlated strongly with its characteristic psychological effects. Increased visual cortex cerebral blood flow (CBF), decreased visual cortex alpha power, and a greatly expanded primary visual cortex (V1) functional connectivity profile correlated strongly with ratings of visual hallucinations, implying that intrinsic brain activity exerts greater influence on visual processing in the psychedelic state, thereby defining its hallucinatory quality. LSD’s marked effects on the visual cortex did not significantly correlate with the drug’s other characteristic effects on consciousness, however. Rather, decreased connectivity between the parahippocampus and retrosplenial cortex (RSC) correlated strongly with ratings of “ego-dissolution” and “altered meaning,” implying the importance of this particular circuit for the maintenance of “self” or “ego” and its processing of “meaning.” Strong relationships were also found between the different imaging metrics, enabling firmer inferences to be made about their functional significance. This uniquely comprehensive examination of the LSD state represents an important advance in scientific research with psychedelic drugs at a time of growing interest in their scientific and therapeutic value. The present results contribute important new insights into the characteristic hallucinatory and consciousness-altering properties of psychedelics that inform on how they can model certain pathological states and potentially treat others.

Background: In the past few years, the issue of ‘microdosing’ psychedelics has been openly discussed in the public arena where claims have been made about their positive effect on mood state and cognitive processes such as concentration. However, there are very few scientific studies that have specifically addressed this issue, and there is no agreed scientific consensus on what microdosing is. Aim: This critique paper is designed to address questions that need to be answered by future scientific studies and to offer guidelines for these studies. Approach: Owing to its proximity for a possible approval in clinical use and short-lasting pharmacokinetics, our focus is predominantly on psilocybin. Psilocybin is allegedly, next to lysergic acid diethylamide (LSD), one of the two most frequently used psychedelics to microdose. Where relevant and available, data for other psychedelic drugs are also mentioned. Conclusion: It is concluded that while most anecdotal reports focus on the positive experiences with microdosing, future research should also focus on potential risks of (multiple) administrations of a psychedelic in low doses. To that end, (pre)clinical studies including biological (e.g. heart rate, receptor turnover and occupancy) as well as cognitive (e.g. memory, attention) parameters have to be conducted and will shed light on the potential negative consequences microdosing could have.

RationaleSubstances based on the N-(2-methoxybenzyl)phenethylamine template ('NBOMe' derivatives) play an important role in medicinal research but some of these derivatives have also appeared as 'research chemicals' for recreational use which has attracted attention worldwide. A major challenge associated with newly emerging substances includes the lack of analytical data and the ability to correctly identify positional isomers.Methods Six N-benzylphenethylamines based on the 2,5-dimethoxy-4-iodophenethylamine structure ('25I') and twelve substituted N-benzyl-5-methoxytryptamines ('5MT') have been prepared and extensively characterized. Techniques used for characterization were gas chromatography/ion trap mass spectrometry in electron and chemical ionization mode, liquid chromatography/diode array detection (DAD), infrared spectroscopy, electrospray high mass accuracy quadrupole time-of-flight tandem mass spectrometry, and triple quadrupole tandem mass spectrometry.ResultsThe characterization of 18 'NBOMe' compounds provided a comprehensive collection of chromatographic and spectral data. Four groups of three positional isomers, i.e. 25I-NB2OMe, 25I-NB3OMe, 25I-NB4OMe, 25I-NB2B, 25I-NB3B, 25I-NB4B and their 5-methoxytryptamine counterparts, were included and assessed for ability to obtain differentiation. Six meta-substituted N-benzyl derivatives of 5-methoxytryptamine (CF3, F, CH3, Cl, I, SCH3) were also studied.Conclusions The implementation of mass spectral techniques was helpful for the differentiation between isomers, for example, when considering the difference in a number of ion ratios. This was considered beneficial in cases where chromatographic separation was only partially achieved under liquid chromatography (LC) conditions. The use of LC/DAD analysis was also found to be valuable for this particular purpose, which confirmed the integrative value of complementary techniques used in areas related to forensic toxicology.

Although ligand-activated GPCRs induce both G-protein and β-arrestin dependent signaling, gaining precise spatio-temporal control of β-arrestin signaling has proven elusive. Here we describe a platform for specifically activating β-arrestin-dependent signaling in situ. The platform, which we have dubbed GA-PAIR (GPCR/β-Arrestin-Plant protein and Abscisic acid Induced Recruitment), can be controlled by the inert phytochemical S-(+)-abscisic acid (ABA). ABA induces interaction between ABI1 (ABA Insensitive 1) and PYL1 (Pyrabactin Resistance (PYR) 1-Like), two plant proteins with no mammalian counterparts. We fused ABI to the engineered human muscarinic M3 G protein-coupled receptor (hM3Dq) and PYL1 to β-arrestin2. Addition of ABA induced rapid and nearly complete translocation of the PYL-β-arrestin fusion protein and, importantly, induced both ERK and Akt signaling. Photo-uncaging a new photo-caged ABA analogue allowed us to gain relatively precise spatio-temporal control over β-arrestin translocation. Because GA-PAIR facilitates the exclusive activation of endogenous β-arrestin signaling pathways in the absence of a GPCR ligand or G protein, the GA-PAIR system will facilitate deconvoluting GPCR signaling in situ.

A series of N-benzylated-5-methoxytryptamine analogues was prepared and investigated, with special emphasis on substituents in the meta position of the benzyl group. A parallel series of several N-benzylated analogues of 2,5-dimethoxy-4-iodophenethylamine (2C-I) also was included for comparison of the two major templates (i.e., tryptamine and phenethylamine). A broad affinity screen at serotonin receptors showed that most of the compounds had the highest affinity at the 5-HT2 family receptors. Substitution at the para position of the benzyl group resulted in reduced affinity, whereas substitution in either the ortho or the meta position enhanced affinity. In general, introduction of a large lipophilic group improved affinity, whereas functional activity often followed the opposite trend. Tests of the compounds for functional activity utilized intracellular Ca(2+) mobilization. Function was measured at the human 5-HT2A, 5-HT2B, and 5-HT2C receptors, as well as at the rat 5-HT2A and 5-HT2C receptors. There was no general correlation between affinity and function. Several of the tryptamine congeners were very potent functionally (EC50 values from 7.6 to 63 nM), but most were partial agonists. Tests in the mouse head twitch assay revealed that many of the compounds induced the head twitch and that there was a significant correlation between this behavior and functional potency at the rat 5-HT2A receptor.

LSD (lysergic acid diethylamide) was discovered almost 75 years ago, and has been the object of episodic controversy since then. While initially explored as an adjunctive psychiatric treatment, its recreational use by the general public has persisted and on occasion has been associated with adverse outcomes, particularly when the drug is taken under suboptimal conditions. LSD's potential to cause psychological disturbance (bad trips) has been long understood, and has rarely been associated with accidental deaths and suicide. From a physiological perspective, however, LSD is known to be non-toxic and medically safe when taken at standard dosages (50-200μg). The scientific literature, along with recent media reports, have unfortunately implicated "LSD toxicity" in five cases of sudden death. On close examination, however, two of these fatalities were associated with ingestion of massive overdoses, two were evidently in individuals with psychological agitation after taking standard doses of LSD who were then placed in maximal physical restraint positions (hogtied) by police, following which they suffered fatal cardiovascular collapse, and one case of extreme hyperthermia leading to death that was likely caused by a drug substituted for LSD with strong effects on central nervous system temperature regulation (e.g. 25i-NBOMe). Given the renewed interest in the therapeutic potential of LSD and other psychedelic drugs, it is important that an accurate understanding be established of the true causes of such fatalities that had been erroneously attributed to LSD toxicity, including massive overdoses, excessive physical restraints, and psychoactive drugs other than LSD.

Psychedelic drugs, such as lysergic acid diethylamide (LSD), mescaline, and psilocybin, exert profound effects on brain and behavior. After decades of difficulties in studying these compounds, psychedelics are again being tested as potential treatments for intractable biomedical disorders. Preclinical research of psychedelics complements human neuroimaging studies and pilot clinical trials, suggesting these compounds as promising treatments for addiction, depression, anxiety, and other conditions. However, many questions regarding the mechanisms of action, safety, and efficacy of psychedelics remain. Here, we summarize recent preclinical and clinical data in this field, discuss their pharmacological mechanisms of action, and outline critical areas for future studies of psychedelic drugs, with the goal of maximizing the potential benefits of translational psychedelic biomedicine to patients.

This chapter will summarize structure-activity relationships (SAR) that are known for the classic serotonergic hallucinogens (aka psychedelics), focusing on the three chemical types: tryptamines, ergolines, and phenethylamines. In the brain, the serotonin 5-HT

Chronic administration of lysergic acid diethylamide (LSD) every other day to rats results in a variety of abnormal behaviors. These build over the 90 day course of treatment and can persist at full strength for at least several months after cessation of treatment. The behaviors are consistent with those observed in animal models of schizophrenia and include hyperactivity, reduced sucrose-preference, and decreased social interaction. In order to elucidate molecular changes that underlie these aberrant behaviors, we chronically treated rats with LSD and performed RNA-Sequencing on the medial prefrontal cortex (mPFC), an area highly associated with both the actions of LSD and the pathophysiology of schizophrenia and other psychiatric illnesses. We observed widespread changes in the neurogenetic state of treated animals four weeks after cessation of LSD treatment. QPCR was used to validate a subset of gene expression changes observed with RNA-Seq, and confirmed a significant correlation between the two methods. Functional clustering analysis indicates differentially expressed genes are enriched in pathways involving neurotransmission (Drd2, Gabrb1), synaptic plasticity (Nr2a, Krox20), energy metabolism (Atp5d, Ndufa1) and neuropeptide signaling (Npy, Bdnf), among others. Many processes identified as altered by chronic LSD are also implicated in the pathogenesis of schizophrenia, and genes affected by LSD are enriched with putative schizophrenia genes. Our results provide a relatively comprehensive analysis of mPFC transcriptional regulation in response to chronic LSD, and indicate that the long-term effects of LSD may bear relevance to psychiatric illnesses, including schizophrenia.

Many psychoactive drugs are used recreationally, particularly by young people. This use and its perceived dangers have led to many different classes of drugs being banned under national laws and international conventions. Indeed, the possession of cannabis, 3,4-methylenedioxy-N-methylamphetamine (MDMA; also known as ecstasy) and psychedelics is stringently regulated. An important and unfortunate outcome of the controls placed on these and other psychoactive drugs is that they make research into their mechanisms of action and potential therapeutic uses - for example, in depression and post-traumatic stress disorder - difficult and in many cases almost impossible.

This chapter will summarize structure–activity relationships (SAR) that are known for the classic serotonergic hallucinogens (aka psychedelics), focusing on the three chemical types: tryptamines, ergolines, and phenethylamines. In the brain, the serotonin 5-HT2A receptor plays a key role in regulation of cortical function and cognition, and also appears to be the principal target for hallucinogenic/psychedelic drugs such as LSD. It is one of the most extensively studied of the 14 known types of serotonin receptors. Important structural features will be identified for activity and, where possible, those that the psychedelics have in common will be discussed. Because activation of the 5-HT2A receptor is the principal mechanism of action for psychedelics, compounds with 5-HT2A agonist activity generally are quickly discarded by the pharmaceutical industry. Thus, most of the research on psychedelics can be related to activation of 5-HT2A receptors. Therefore, much of the discussion will include not only clinical or anecdotal studies, but also will consider data from animal models as well as a certain amount of molecular pharmacology where it is known.

Recently, an alternative theory concerning the method by which olfactory proteins are activated has garnered attention. This theory proposes that the activation of olfactory G protein-coupled receptors occurs by an inelastic electron tunneling mechanism that is mediated through the presence of an agonist with an appropriate vibrational state to accept the inelastic portion of the tunneling electron’s energy. In a recent series of papers, some suggestive theoretical evidence has been offered that this theory may be applied to nonolfactory G protein-coupled receptors (GPCRs), including those associated with the central nervous system (CNS). [Chee HK, June OS (2013) Genomics Inform 11(4):282–288; Chee HK, et al. (2015) FEBS Lett 589(4):548–552; Oh SJ (2012) Genomics Inform 10(2):128–132]. Herein, we test the viability of this idea, both by receptor affinity and receptor activation measured by calcium flux. This test was performed using a pair of well-characterized agonists for members of the 5-HT2 class of serotonin receptors, 2,5-dimethoxy-4-iodoamphetamine (DOI) and N,N-dimethyllysergamide (DAM-57), and their respective deuterated isotopologues. No evidence was found that selective deuteration affected either the binding affinity or the activation by the selected ligands for the examined members of the 5-HT2 receptor class.

Lysergic N,N-diethylamide (LSD) is perhaps one of the most intriguing psychoactive substances known and numerous analogs have been explored to a varying extent in previous decades. In 2013, N6-allyl-6-norlysergic acid diethylamide (AL-LAD) and (2’S,4’S)-lysergic acid 2,4-dimethylazetidide (LSZ) have appeared on the ‘research chemicals’ / new psychoactive substances (NPS) market in both powdered and blotter form. This study reports the analytical characterization of powdered AL-LAD and LSZ tartrate samples and their semi-quantitative determination on blotter paper. Included in this study was the use of nuclear magnetic resonance spectroscopy, gas chromatography mass spectrometry (MS), low and high-resolution electrospray MS(/MS), high performance liquid chromatography diode array detection and GC solid-state infrared analysis. One feature shared by serotonergic psychedelics, such as LSD, is the ability to mediate behavioral responses via activation of 5-HT2A receptors. Both AL-LAD and LSZ displayed LSD-like responses in male C57BL/6J mice when employing the head-twitch response (HTR) assay. AL-LAD and LSZ produced nearly identical inverted-U-shaped dose-dependent effects, with the maximal responses occurring at 200 µg/kg. Analysis of the dose-responses by nonlinear regression confirmed that LSZ (ED50 = 114.2 nmol/kg) was equipotent to LSD (ED50 = 132.8 nmol/kg) in mice, whereas AL-LAD was slightly less potent (ED50 = 174.9 nmol/kg). The extent to which a comparison in potency can be translated directly to humans requires further investigation. Availability of both chemical and pharmacological data obtained from NPS as they appear on the market provides important data to research communities that are interested in various aspects related substance use and forensic identification.

Psychedelics (serotonergic hallucinogens) are powerful psychoactive substances that alter perception and mood and affect numerous cognitive processes. They are generally considered physiologically safe and do not lead to dependence or addiction. Their origin predates written history, and they were employed by early cultures in many sociocultural and ritual contexts. After the virtually contemporaneous discovery of (5R,8R)-(+)-lysergic acid-N,N-diethylamide (LSD)-25 and the identification of serotonin in the brain, early research focused intensively on the possibility that LSD and other psychedelics had a serotonergic basis for their action. Today there is a consensus that psychedelics are agonists or partial agonists at brain serotonin 5-hydroxytryptamine 2A receptors, with particular importance on those expressed on apical dendrites of neocortical pyramidal cells in layer V. Several useful rodent models have been developed over the years to help unravel the neurochemical correlates of serotonin 5-hydroxytryptamine 2A receptor activation in the brain, and a variety of imaging techniques have been employed to identify key brain areas that are directly affected by psychedelics. Recent and exciting developments in the field have occurred in clinical research, where several double-blind placebo-controlled phase 2 studies of psilocybin-assisted psychotherapy in patients with cancer-related psychosocial distress have demonstrated unprecedented positive relief of anxiety and depression. Two small pilot studies of psilocybin-assisted psychotherapy also have shown positive benefit in treating both alcohol and nicotine addiction. Recently, blood oxygen level–dependent functional magnetic resonance imaging and magnetoencephalography have been employed for in vivo brain imaging in humans after administration of a psychedelic, and results indicate that intravenously administered psilocybin and LSD produce decreases in oscillatory power in areas of the brain’s default mode network.

This work describes the identification of a novel class of octahydrobenzo[f]quinolines as dopamine D(3)-selective full agonists. We developed a facile method that utilizes Suzuki coupling for easy incorporations of various substituted pendant rings into the scaffold. A small focused library of octahydrobenzo[f]quinolines 5 was synthesized, and these compounds demonstrated at least 14-fold D(2)-like selectivity over D(1) in native porcine striatal tissue. Furthermore, n-propyl analog 5f was found to be a high affinity (K(i)=1.1 nM) D(3) dopamine full agonist with 145-fold selectivity over the D(2) receptor and about 840-fold selectivity over the D(1) receptor.

A strategy to replace the ethylamine side chain of 2,5-dimethoxy-4-iodoamphetamine (DOI, 1a), and 2,5-dimethoxy-4-bromoamphetamine (DOB, 1b) with a cyclopropylamine moiety was successful in leading to compounds with high affinity at the 5-HT2 family of receptors; and the more potent stereoisomer of the cyclopropane analogues had the expected (−)-(1R,2S)-configuration. Screening for affinity at various serotonin receptor subtypes, however, revealed that the cyclopropane congeners also had increased affinity at several sites in addition to the 5-HT2A and 5-HT2B receptors. Therefore, at appropriate doses – although (−)-4 and (−)-5 may be useful as tools to probe 5-HT2 receptor function – one would need to be mindful that their selectivity for 5-HT2A receptors is somewhat less than for DOI itself.

Of the 14 known types of serotonin receptors one of the most extensively studied is the 5-HT2A (5-hydroxytryptamine) receptor. In the brain, this receptor plays a key role in regulation of cortical function and cognition, appears to be the principal target for the hallucinogenic/psychedelic drugs such as lysergic acid diethylamide (LSD), and also is a target for the newest atypical antipsychotic agents, which are antagonists or inverse agonists at this site. Among the structure–activity relationships that are known for this receptor type, there are three chemical classes of agonists: tryptamines, ergolines, and phenethylamines. Important structural features are identified for agonist activity and some of these agonists have features in common. In addition to effects at the receptor will be the focus, these drugs are also hallucinogenic (psychedelic) agents, and much of the SAR was developed on the basis of effects in humans, before modern pharmacological techniques were available. A certain amount of our knowledge therefore relies on those human studies. WIREs Membr Transp Signal 2012, 1:559–579. doi: 10.1002/wmts.42 For further resources related to this article, please visit the WIREs website.

To refine further the structure-activity relationships of D(1) dopamine receptor agonists, we investigated the roles of three conserved serine residues [Ser198(5.42), Ser199(5.43), and Ser202(5.46)] in agonist binding and receptor activation. These transmembrane domain 5 (TM5) residues are believed to engage catechol ligands through polar interactions. We stably expressed wild-type or mutant (S198A, S199A, and S202A) D(1) receptors in human embryonic kidney cells. These receptors were expressed at similar levels (approximately 2000 fmol/mg) and bound the radioligand [(3)H]R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390), although S198A and S199A displayed significant losses of affinity compared with that for wild-type receptors. The endogenous agonist, dopamine, had losses of potency at each of the mutant receptors. We tested cyclohexyl-substituted isochroman, carbocyclic, and chroman bicyclic dopamine analogs and found that the mutations affected the chroman to a lesser extent than the other compounds. These results support our hypothesis that the decreased D(1) activity of chroman analogs results from a ligand intramolecular hydrogen bond that impairs the ability of the catechol to engage the receptor. Sensitivities of these rigid catechol agonists to the effects of the serine mutations were dependent on ligand geometry, particularly with respect to the rotameric conformation of the ethylamine side chain and the distance between the amino group and each catechol hydroxyl. Functional experiments in striatal tissue suggest that the ability to engage TM5 serines is largely correlated with agonist efficacy for cAMP stimulation. These results provide a new understanding of the complexities of D(1) ligand recognition and agonist activation and have implications for the design of rigid catechol ligands.

Efforts to develop selective agonists for dopamine D(1)-like receptors led to the discovery of dihydrexidine and doxanthrine, two bioisosteric β-phenyldopamine-type full agonist ligands that display selectivity and potency at D(1)-like receptors. We report herein an improved methodology for the synthesis of substituted chromanoisoquinolines (doxanthrine derivatives) and the evaluation of several new compounds for their ability to bind to D(1)- and D(2)-like receptors. Identical pendant phenyl ring substitutions on the dihydrexidine and doxanthrine templates surprisingly led to different effects on D(1)-like receptor binding, suggesting important differences between the interactions of these ligands with the D(1) receptor. We propose, based on the biological results and molecular modeling studies, that slight conformational differences between the tetralin and chroman-based compounds lead to a shift in the location of the pendant ring substituents within the receptor.

Preclinical evidence indicates that D1 dopamine receptor full agonists have potential as therapeutic agents for a variety of neurological conditions. Dihydrexidine (DHX) was the first high potency selective D1 dopamine receptor full agonist and has been studied as a possible treatment for Parkinson's disease (PD). Recently, we discovered doxanthrine (DOX), an oxygen bioisostere of DHX that has even greater selectivity for the D1 dopamine receptor. Using the unilateral 6-hydroxydopamine-lesioned rat model of PD, DOX and DHX were compared at several doses (0.625, 1.25, 2.5, or 5.0 mg/kg) for their ability to elicit contralateral rotation by either intraperitoneal injection or oral gavage. After intraperitoneal administration, both DOX and DHX showed robust contralateral rotation at doses of 2.5 and 5.0 mg/kg compared to vehicle. In addition, after intraperitoneal administration at doses of 2.5 and 5.0 mg/kg, DHX had a significantly longer duration of action than DOX (p

Ayahuasca is a psychoactive tea prepared from a combination of plants that contain a hallucinogenic tryptamine and monoamine oxidase inhibitors (MAOIs). Behavioral pattern monitor (BPM) experiments demonstrated that the combination of 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) and a behaviorally inactive dose of an MAOA inhibitor such as harmaline or clorgyline induces biphasic effects on locomotor activity in rats, initially reducing locomotion and then increasing activity as time progresses. The present study investigated whether the biphasic locomotor profile induced by the combination of 5-MeO-DMT and an MAOI is a consequence of a reduction in the rate of 5-MeO-DMT metabolism. This hypothesis was tested using a deuterated derivative of 5-MeO-DMT (α,α,β,β-tetradeutero-5-MeO-DMT) that is resistant to metabolism by MAO. Confirming our previous findings, 1.0 mg/kg 5-MeO-DMT (s.c.) had biphasic effects on locomotor activity in rats pretreated with a behaviorally inactive dose of the nonselective MAOI pargyline (10 mg/kg). Administration of 5-MeO-DMT alone, even at doses greater than 1.0 mg/kg, produced only reductions in locomotor activity. Although low doses of α,α,β,β-tetradeutero-5-MeO-DMT (0.3 and 1.0 mg/kg, s.c.) produced only hypoactivity in the BPM, a dose of 3.0 mg/kg induced a biphasic locomotor profile similar to that produced by the combination of 5-MeO-DMT and an MAOI. Receptor binding studies demonstrated that deuterium substitution had little effect on the affinity of 5-MeO-DMT for a wide variety of neurotransmitter binding sites. The finding with α,α,β,β-tetradeutero-5-MeO-DMT indicates that the hyperactivity induced by 5-MeO-DMT after MAO inhibition is a consequence of reduced metabolism of 5-MeO-DMT, leading to prolonged occupation of central serotonin receptors. These results demonstrate that deuterated tryptamines may be useful in behavioral and pharmacological studies to mimic the effects of tryptamine/MAOI combinations.