Genetic Survey of Psilocybe Natural Products

Substrate recognition by the 4-hydroxytryptamine kinase PsiK in psilocybin biosynthesis

Psilocybin, the natural hallucinogen from Psilocybe (magic) mushrooms, is a highly promising drug candidate for the treatment of depression and several other mental health conditions. Biosynthesis of psilocybin from the amino acid l-tryptophan involves four strictly sequential modifications. The third of these, ATP-dependent phosphorylation of the intermediate 4-hydroxytryptamine, is catalysed by PsiK. Here we present a crystallographic analysis and a structure-based mutagenesis study of this kinase, providing insight into its mode of substrate recognition. The results of our work will support future bioengineering efforts aimed at generating variants of psilocybin with enhanced therapeutic properties.

Wood-loving magic mushrooms from Australia are saprotrophic invaders in the Northern Hemisphere

Magic mushrooms are fungi that produce psilocybin, an entheogen with long-term cultural use and a breakthrough compound for treatment of mental health disorders. Fungal populations separated by geography are candidates for allopatric speciation, yet species connectivity typically persists because there is minimal divergence at functional parts of mating compatibility genes. We studied whether connectivity is maintained across populations of a widespread species complex of magic mushrooms that has infiltrated the Northern Hemisphere from a hypothesised centre of origin in Australasia. We analysed 89 genomes of magic mushrooms to examine erosion of species connectivity in disparate populations with support from gene flow, kinship, structure, allelic diversity, and mating compatibility. We used comparative genomics and synteny to test whether the genes that produce psilocybin are under selection in natural populations of magic mushrooms. Despite phenotypic plasticity and intercontinental distribution, sexual compatibility is maintained across geographically isolated populations of magic mushrooms. Psilocybin loci have high allelic diversity and evidence of balancing selection. Australasia is the centre of origin of wood-degrading magic mushrooms and geographically separated populations are fully sexually compatible, despite minimal gene flow since differentiation from a shared ancestor. Movement of woodchips, mulch, or plants has most likely facilitated invasion of these mushrooms in the Northern Hemisphere. Citation: McTaggart AR, Scarlett K, Slot JC, Barlow C, Appleyard C, Gardiner DM, Fechner N, Tilden J, Hass D, Voogelbreinder S, Lording WJ, Lloyd RA, Shuey LS, Drenth A, James TY (2024). Wood-loving magic mushrooms from Australia are saprotrophic invaders in the Northern Hemisphere. Fungal Systematics and Evolution 14: 209-217. doi: 10.3114/fuse.2024.14.14.

Effect of chemically synthesized psilocybin and psychedelic mushroom extract on molecular and metabolic profiles in mouse brain

Psilocybin, a naturally occurring, tryptamine alkaloid prodrug, is currently being investigated for the treatment of a range of psychiatric disorders. Preclinical reports suggest that the biological effects of psilocybin-containing mushroom extract or "full spectrum" (psychedelic) mushroom extract (PME), may differ from those of chemically synthesized psilocybin (PSIL). We compared the effects of PME to those of PSIL on the head twitch response (HTR), neuroplasticity-related synaptic proteins and frontal cortex metabolomic profiles in male C57Bl/6j mice. HTR measurement showed similar effects of PSIL and PME over 20 min. Brain specimens (frontal cortex, hippocampus, amygdala, striatum) were assayed for the synaptic proteins, GAP43, PSD95, synaptophysin and SV2A, using western blots. These proteins may serve as indicators of synaptic plasticity. Three days after treatment, there was minimal increase in synaptic proteins. After 11 days, PSIL and PME significantly increased GAP43 in the frontal cortex (p = 0.019; p = 0.039 respectively) and hippocampus (p = 0.015; p = 0.027) and synaptophysin in the hippocampus (p = 0.041; p = 0.05) and amygdala (p = 0.035; p = 0.004). PSIL increased SV2A in the amygdala (p = 0.036) and PME did so in the hippocampus (p = 0.014). In the striatum, synaptophysin was increased by PME only (p = 0.023). There were no significant effects of PSIL or PME on PSD95 in any brain area when these were analyzed separately. Nested analysis of variance (ANOVA) showed a significant increase in each of the 4 proteins over all brain areas for PME versus vehicle control, while significant PSIL effects were observed only in the hippocampus and amygdala and were limited to PSD95 and SV2A. Metabolomic analyses of the pre-frontal cortex were performed by untargeted polar metabolomics utilizing capillary electrophoresis - Fourier transform mass spectrometry (CE-FTMS) and showed a differential metabolic separation between PME and vehicle groups. The purines guanosine, hypoxanthine and inosine, associated with oxidative stress and energy production pathways, showed a progressive decline from VEH to PSIL to PME. In conclusion, our synaptic protein findings suggest that PME has a more potent and prolonged effect on synaptic plasticity than PSIL. Our metabolomics data support a gradient of effects from inert vehicle via chemical psilocybin to PME further supporting differential effects. Further studies are needed to confirm and extend these findings and to identify the molecules that may be responsible for the enhanced effects of PME as compared to psilocybin alone.

Unravelling the Function of the Sesquiterpene Cyclase STC3 in the Lifecycle of Botrytis cinerea

The genome sequencing of Botrytis cinerea supplies a general overview of the map of genes involved in secondary metabolite synthesis. B. cinerea genomic data reveals that this phytopathogenic fungus has seven sesquiterpene cyclase (Bcstc) genes that encode proteins involved in the farnesyl diphosphate cyclization. Three sesquiterpene cyclases (BcStc1, BcStc5 and BcStc7) are characterized, related to the biosynthesis of botrydial, abscisic acid and (+)-4-epi-eremophilenol, respectively. However, the role of the other four sesquiterpene cyclases (BcStc2, BcStc3, BcStc4 and BcStc6) remains unknown. BcStc3 is a well-conserved protein with homologues in many fungal species, and here, we undertake its functional characterization in the lifecycle of the fungus. A null mutant ΔBcstc3 and an overexpressed-Bcstc3 transformant (OvBcstc3) are generated, and both strains show the deregulation of those other sesquiterpene cyclase-encoding genes (Bcstc1, Bcstc5 and Bcstc7). These results suggest a co-regulation of the expression of the sesquiterpene cyclase gene family in B. cinerea. The phenotypic characterization of both transformants reveals that BcStc3 is involved in oxidative stress tolerance, the production of reactive oxygen species and virulence. The metabolomic analysis allows the isolation of characteristic polyketides and eremophilenols from the secondary metabolism of B. cinerea, although no sesquiterpenes different from those already described are identified.

Genetic regulation of L-tryptophan metabolism in Psilocybe mexicana supports psilocybin biosynthesis

BACKGROUND

Although Basidiomycota produce pharmaceutically and ecologically relevant natural products, knowledge of how they coordinate their primary and secondary metabolism is virtually non-existent. Upon transition from vegetative mycelium to carpophore formation, mushrooms of the genus Psilocybe use L-tryptophan to supply the biosynthesis of the psychedelic tryptamine alkaloid psilocybin with the scaffold, leading to a strongly increased demand for this particular amino acid as this alkaloid may account for up to 2% of the dry mass. Using Psilocybe mexicana as our model and relying on genetic, transcriptomic, and biochemical methods, this study investigated if L-tryptophan biosynthesis and degradation in P. mexicana correlate with natural product formation.

RESULTS

A comparative transcriptomic approach of gene expression in P. mexicana psilocybin non-producing vegetative mycelium versus producing carpophores identified the upregulation of L-tryptophan biosynthesis genes. The shikimate pathway genes trpE1, trpD, and trpB (encoding anthranilate synthase, anthranilate phosphoribosyltransferase, and L-tryptophan synthase, respectively) were upregulated in carpophores. In contrast, genes idoA and iasA, encoding indole-2,3-dioxygenase and indole-3-acetaldehyde synthase, i.e., gateway enzymes for L-tryptophan-consuming pathways, were massively downregulated. Subsequently, IasA was heterologously produced in Escherichia coli and biochemically characterized in vitro. This enzyme represents the first characterized microbial L-tryptophan-preferring acetaldehyde synthase. A comparison of transcriptomic data collected in this study with prior data of Psilocybe cubensis showed species-specific differences in how L-tryptophan metabolism genes are regulated, despite the close taxonomic relationship.

CONCLUSIONS

The upregulated L-tryptophan biosynthesis genes and, oppositely, the concomitant downregulated genes encoding L-tryptophan-consuming enzymes reflect a well-adjusted cellular system to route this amino acid toward psilocybin production. Our study has pilot character beyond the genus Psilocybe and provides, for the first time, insight in the coordination of mushroom primary and secondary metabolism.

Methyl transfer in psilocybin biosynthesis

Psilocybin, the natural hallucinogen produced by Psilocybe ("magic") mushrooms, holds great promise for the treatment of depression and several other mental health conditions. The final step in the psilocybin biosynthetic pathway, dimethylation of the tryptophan-derived intermediate norbaeocystin, is catalysed by PsiM. Here we present atomic resolution (0.9 Å) crystal structures of PsiM trapped at various stages of its reaction cycle, providing detailed insight into the SAM-dependent methylation mechanism. Structural and phylogenetic analyses suggest that PsiM derives from epitranscriptomic N6-methyladenosine writers of the METTL16 family, which is further supported by the observation that bound substrates physicochemically mimic RNA. Inherent limitations of the ancestral monomethyltransferase scaffold hamper the efficiency of psilocybin assembly and leave PsiM incapable of catalysing trimethylation to aeruginascin. The results of our study will support bioengineering efforts aiming to create novel variants of psilocybin with improved therapeutic properties.

Phylogenomics of the psychoactive mushroom genus Psilocybe and evolution of the psilocybin biosynthetic gene cluster

Psychoactive mushrooms in the genus Psilocybe have immense cultural value and have been used for centuries in Mesoamerica. Despite the recent surge of interest in these mushrooms due to the psychotherapeutic potential of their natural alkaloid psilocybin, their phylogeny and taxonomy remain substantially incomplete. Moreover, the recent elucidation of the psilocybin biosynthetic gene cluster is known for only five of ~165 species of Psilocybe, four of which belong to only one of two major clades. We set out to improve the phylogeny of Psilocybe using shotgun sequencing of fungarium specimens, from which we obtained 71 metagenomes including from 23 types, and conducting phylogenomic analysis of 2,983 single-copy gene families to generate a fully supported phylogeny. Molecular clock analysis suggests the stem lineage of Psilocybe arose ~67 mya and diversified ~56 mya. We also show that psilocybin biosynthesis first arose in Psilocybe, with 4 to 5 possible horizontal transfers to other mushrooms between 40 and 9 mya. Moreover, predicted orthologs of the psilocybin biosynthetic genes revealed two distinct gene orders within the biosynthetic gene cluster that corresponds to a deep split within the genus, possibly a signature of two independent acquisitions of the cluster within Psilocybe.

Domestication through clandestine cultivation constrained genetic diversity in magic mushrooms relative to naturalized populations

Fungi that are edible or fermentative were domesticated through selective cultivation of their desired traits. Domestication is often associated with inbreeding or selfing, which may fix traits other than those under selection, and causes an overall decrease in heterozygosity. A hallucinogenic mushroom, Psilocybe cubensis, was domesticated from its niche in livestock dung for production of psilocybin. It has caused accidental poisonings since the 1940s in Australia, which is a population hypothesized to be introduced from an unknown center of origin. We sequenced genomes of 38 isolates from Australia and compared them with 86 genomes of commercially available cultivars to determine (1) whether P. cubensis was introduced to Australia, and (2) how domestication has impacted commercial cultivars. Our analyses of genome-wide SNPs and single-copy orthologs showed that the Australian population is naturalized, having recovered its effective population size after a bottleneck when it was introduced, and it has maintained relatively high genetic diversity based on measures of nucleotide and allelic diversity. In contrast, domesticated cultivars generally have low effective population sizes and hallmarks of selfing and clonal propagation, including low genetic diversity, low heterozygosity, high linkage disequilibrium, and low allelic diversity of mating-compatibility genes. Analyses of kinship show that most cultivars are founded from related populations. Alleles in the psilocybin gene cluster are identical across most cultivars of P. cubensis with low diversity across coding sequence; however, unique allelic diversity in Australia and some cultivars may translate to differences in biosynthesis of psilocybin and its analogs.

A journey with psychedelic mushrooms: From historical relevance to biology, cultivation, medicinal uses, biotechnology, and beyond

Psychedelic mushrooms containing psilocybin and related tryptamines have long been used for ethnomycological purposes, but emerging evidence points to the potential therapeutic value of these mushrooms to address modern neurological, psychiatric health, and related disorders. As a result, psilocybin containing mushrooms represent a re-emerging frontier for mycological, biochemical, neuroscience, and pharmacology research. This work presents crucial information related to traditional use of psychedelic mushrooms, as well as research trends and knowledge gaps related to their diversity and distribution, technologies for quantification of tryptamines and other tryptophan-derived metabolites, as well as biosynthetic mechanisms for their production within mushrooms. In addition, we explore the current state of knowledge for how psilocybin and related tryptamines are metabolized in humans and their pharmacological effects, including beneficial and hazardous human health implications. Finally, we describe opportunities and challenges for investigating the production of psychedelic mushrooms and metabolic engineering approaches to alter secondary metabolite profiles using biotechnology integrated with machine learning. Ultimately, this critical review of all aspects related to psychedelic mushrooms represents a roadmap for future research efforts that will pave the way to new applications and refined protocols.

Cultivation, chemistry, and genome of Psilocybe zapotecorum

Psilocybe zapotecorum is a strongly blue-bruising psilocybin mushroom used by indigenous groups in southeastern Mexico and beyond. While this species has a rich history of ceremonial use, research into its chemistry and genetics have been limited. Herein, we detail mushroom morphology and report on cultivation parameters, chemical profile, and the full genome sequence of P. zapotecorum . First, growth and cloning methods are detailed that are simple, and reproducible. In combination with high resolution microscopic analysis, the strain was barcoded, confirming species-level identification. Full genome sequencing reveals the architecture of the psilocybin gene cluster in P. zapotecorum, and can serve as a reference genome for Psilocybe Clade I. Characterization of the tryptamine profile revealed a psilocybin concentration of 17.9±1.7 mg/g, with a range of 10.6-25.7 mg/g (n=7), and similar tryptamines (psilocin, baeocystin, norbaeocystin, norpsilocin, aeruginascin, 4-HO-tryptamine, and tryptamine) in lesser concentrations for a combined tryptamine concentration of 22.5±3.2 mg/g. These results show P. zapotecorum to be a potent - and variable - Psilocybe mushroom. Chemical profiling, genetic analysis, and cultivation assist in demystifying these mushrooms. As clinical studies with psilocybin gain traction, understanding the diversity of psilocybin mushrooms will assure that psilocybin therapy does not become synonymous with psilocybin mushrooms.

A "Magic Mushroom" Multi-Product Sesquiterpene Synthase

Psilocybe "magic mushrooms" are chemically well understood for their psychotropic tryptamines. However, the diversity of their other specialized metabolites, in particular terpenoids, has largely remained an open question. Yet, knowledge on the natural product background is critical to understand if other compounds modulate the psychotropic pharmacological effects. CubA, the single clade II sesquiterpene synthase of P. cubensis, was heterologously produced in Escherichia coli and characterized in vitro, complemented by in vivo product formation assays in Aspergillus niger as a heterologous host. Extensive GC-MS analyses proved a function as multi-product synthase and, depending on the reaction conditions, cubebol, β-copaene, δ-cadinene, and germacrene D were detected as the major products of CubA. In addition, mature P. cubensis carpophores were analysed chromatographically which led to the detection of β-copaene and δ-cadinene. Enzymes closely related to CubA are encoded in the genomes of various Psilocybe species. Therefore, our results provide insight into the metabolic capacity of the entire genus.

Bifurcate evolution of quinone synthetases in basidiomycetes

BACKGROUND

The terphenylquinones represent an ecologically remarkable class of basidiomycete natural products as they serve as central precursors of pigments and compounds that impact on microbial consortia by modulating bacterial biofilms and motility. This study addressed the phylogenetic origin of the quinone synthetases that assemble the key terphenylquinones polyporic acid and atromentin.

RESULTS

The activity of the Hapalopilus rutilans synthetases HapA1, HapA2 and of Psilocybe cubensis PpaA1 were reconstituted in Aspergilli. Liquid chromatography and mass spectrometry of the culture extracts identified all three enzymes as polyporic acid synthetases. PpaA1 is unique in that it features a C-terminal, yet catalytically inactive dioxygenase domain. Combined with bioinformatics to reconstruct the phylogeny, our results demonstrate that basidiomycete polyporic acid and atromentin synthetases evolved independently, although they share an identical catalytic mechanism and release structurally very closely related products. A targeted amino acid replacement in the substrate binding pocket of the adenylation domains resulted in bifunctional synthetases producing both polyporic acid and atromentin.

CONCLUSIONS

Our results imply that quinone synthetases evolved twice independently in basidiomycetes, depending on the aromatic α-keto acid substrate. Furthermore, key amino acid residues for substrate specificity were identified and changed which led to a relaxed substrate profile. Therefore, our work lays the foundation for future targeted enzyme engineering.

The evolution and ecology of psilocybin in nature

Fungi produce diverse metabolites that can have antimicrobial, antifungal, antifeedant, or psychoactive properties. Among these metabolites are the tryptamine-derived compounds psilocybin, its precursors, and natural derivatives (collectively referred to as psiloids), which have played significant roles in human society and culture. The high allocation of nitrogen to psiloids in mushrooms, along with evidence of convergent evolution and horizontal transfer of psilocybin genes, suggest they provide a selective benefit to some fungi. However, no precise ecological roles of psilocybin have been experimentally determined. The structural and functional similarities of psiloids to serotonin, an essential neurotransmitter in animals, suggest that they may enhance the fitness of fungi through interference with serotonergic processes. However, other ecological mechanisms of psiloids have been proposed. Here, we review the literature pertinent to psilocybin ecology and propose potential adaptive advantages psiloids may confer to fungi.

Genome sequencing progenies of magic mushrooms (Psilocybe subaeruginosa) identifies tetrapolar mating and gene duplications in the psilocybin pathway

Knowledge of breeding systems and genetic diversity is critical to select and combine desired traits that advance new cultivars in agriculture and horticulture. Mushrooms that produce psilocybin, magic mushrooms, may potentially be used in therapeutic and wellness industries, and stand to benefit from genetic improvement. We studied haploid siblings of Psilocybe subaeruginosa to resolve the genetics behind mating compatibility and advance knowledge of breeding. Our results show that mating in P. subaeruginosa is tetrapolar, with compatibility controlled at a homeodomain locus with one copy each of HD1 and HD2, and a pheromone/receptor locus with four homologs of the receptor gene STE3. An additional two pheromone/receptor loci homologous to STE3 do not appear to regulate mating compatibility. Alleles in the psilocybin gene cluster did not vary among the five siblings and were likely homozygous in the parent. Psilocybe subaeruginosa and its relatives have three copies of PsiH genes but their impact on production of psilocybin and its analogues is unknown. Genetic improvement in Psilocybe will require access to genetic diversity from the centre of origin of different species, identification of genes behind traits, and strategies to avoid inbreeding depression.

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.

DNA Authentication and Chemical Analysis of Psilocybe Mushrooms Reveal Widespread Misdeterminations in Fungaria and Inconsistencies in Metabolites

The mushroom genus Psilocybe is best known as the core group of psychoactive mushrooms, yet basic information on their diversity, taxonomy, chemistry, and general biology is still largely lacking. In this study, we reexamined 94 Psilocybe fungarium specimens, representing 18 species, by DNA barcoding, evaluated the stability of psilocybin, psilocin, and their related tryptamine alkaloids in 25 specimens across the most commonly vouchered species (Psilocybe cubensis, Psilocybe cyanescens, and Psilocybe semilanceata), and explored the metabolome of cultivated P. cubensis. Our data show that, apart from a few well-known species, the taxonomic accuracy of specimen determinations is largely unreliable, even at the genus level. A substantial quantity of poor-quality and mislabeled sequence data in public repositories, as well as a paucity of sequences derived from types, further exacerbates the problem. Our data also support taxon- and time-dependent decay of psilocybin and psilocin, with some specimens having no detectable quantities of them. We also show that the P. cubensis metabolome possibly contains thousands of uncharacterized compounds, at least some of which may be bioactive. Taken together, our study undermines commonly held assumptions about the accuracy of names and presence of controlled substances in fungarium specimens identified as Psilocybe spp. and reveals that our understanding of the chemical diversity of these mushrooms is largely incomplete. These results have broader implications for regulatory policies pertaining to the storage and sharing of fungarium specimens as well as the use of psychoactive mushrooms for recreation and therapy. IMPORTANCE The therapeutic use of psilocybin, the active ingredient in "magic mushrooms," is revolutionizing mental health care for a number of conditions, including depression, posttraumatic stress disorder (PTSD), and end-of-life care. This has spotlighted the current state of knowledge of psilocybin, including the organisms that endogenously produce it. However, because of international regulation of psilocybin as a controlled substance (often included on the same list as cocaine and heroin), basic research has lagged far behind. Our study highlights how the poor state of knowledge of even the most fundamental scientific information can impact the use of psilocybin-containing mushrooms for recreational or therapeutic applications and undermines critical assumptions that underpin their regulation by legal authorities. Our study shows that currently available chemical studies are mainly inaccurate, irreproducible, and inconsistent, that there exists a high rate of misidentification in museum collections and public databases rendering even names unreliable, and that the concentration of psilocybin and its tryptamine derivatives in three of the most commonly collected Psilocybe species (P. cubensis, P. cyanescens, and P. semilanceata) is highly variable and unstable in museum specimens spanning multiple decades, and our study generates the first-ever insight into the highly complex and largely uncharacterized metabolomic profile for the most commonly cultivated magic mushroom, P. cubensis.

Psilocybin containing mushrooms: a rapidly developing biotechnology industry in the psychiatry, biomedical and nutraceutical fields

Humans have collected and used hallucinogenic mushrooms for ethnic medicinal, recreational, and religious purposes since before recorded history. Currently, the use of these mushrooms is illegal in most countries, but where their use is legal they are applied as self medication. Psilocybin and psilocin, two psychoactive alkaloids, are naturally synthesized by hallucinogenic mushrooms. The chemical structure of these compounds are similar to the neurotransmitter serotonin. Activation of this system by psilocybin and psilocin may produce temporary changes in the brain that induce hallucinations and feelings of euphoria. Adjustment of the serotonin system in this way can moderate symptoms of related mental disorders. This review summarizes relevant and current information regarding the discovery of hallucinogenic mushrooms and their contained psychoactive compounds, the events that lead to their criminalization and decriminilization, and the state of knowledge of psilocybin, psilocin, and derivatives. Last, research on the psychoactive properties of these mushrooms is placed in perspective to possible applications for human dysfunctions.

Structure-Activity Relationships for Psilocybin, Baeocystin, Aeruginascin, and Related Analogues to Produce Pharmacological Effects in Mice

4-Phosphoryloxy-N,N-dimethyltryptamine (psilocybin) is a naturally occurring tertiary amine found in many mushroom species. Psilocybin is a prodrug for 4-hydroxy-N,N-dimethyltryptamine (psilocin), which induces psychedelic effects via agonist activity at the serotonin (5-HT) 2A receptor (5-HT2A). Several other 4-position ring-substituted tryptamines are present in psilocybin-containing mushrooms, including the secondary amine 4-phosphoryloxy-N-methyltryptamine (baeocystin) and the quaternary ammonium 4-phosphoryloxy-N,N,N-trimethyltryptamine (aeruginascin), but these compounds are not well studied. Here, we investigated the structure-activity relationships for psilocybin, baeocystin, and aeruginascin, as compared to their 4-acetoxy and 4-hydroxy analogues, using in vitro and in vivo methods. Broad receptor screening using radioligand binding assays in transfected cells revealed that secondary and tertiary tryptamines with either 4-acetoxy or 4-hydroxy substitutions display nanomolar affinity for most human 5-HT receptor subtypes tested, including the 5-HT2A and the serotonin 1A receptor (5-HT1A). The same compounds displayed affinity for 5-HT2A and 5-HT1A in mouse brain tissue in vitro and exhibited agonist efficacy in assays examining 5-HT2A-mediated calcium mobilization and β-arrestin 2 recruitment. In mouse experiments, only the tertiary amines psilocin, psilocybin, and 4-acetoxy-N,N-dimethyltryptamine (psilacetin) induced head twitch responses (ED50 0.11-0.29 mg/kg) indicative of psychedelic-like activity. Head twitches were blocked by 5-HT2A antagonist pretreatment, supporting 5-HT2A involvement. Both secondary and tertiary amines decreased body temperature and locomotor activity at higher doses, the effects of which were blocked by 5-HT1A antagonist pretreatment. Across all assays, the pharmacological effects of 4-acetoxy and 4-hydroxy compounds were similar, and these compounds were more potent than their 4-phosphoryloxy counterparts. Importantly, psilacetin appears to be a prodrug for psilocin that displays substantial serotonin receptor activities of its own.