Microbiome Metabolome Brain Vagus Nerve Circuit in Disease and Recovery

By Elena L. Paley

Microbiome Metabolome Brain Vagus Nerve Circuit in Disease and Recovery focuses on the emerging hypothesis of a dysfunctional microbiome metabolome vagus nerve brain circuit in Alzheimer’s disease and associated diseases and medical conditions, including dementia, aging, COVID-19, autoimmune conditions, and inflammatory skin condition rosacea, which may increase the risk of other conditions. This book also discusses the vagus nerve-related conditions, including Arnold’s reflex, laryngopharyngeal reflux, duodenogastric reflux, gastroesophageal reflux, and related pulmonary diseases. The subjects covered in the book also address an important question of which one is more important for human health and intellectual abilities: the human genome or the human microbiome? The conceptual model of food and gut microbial tryptamine vagus nerve circuit is also presented in this book.

• Addresses the emerging hypothesis of a dysfunctional microbiome vagus nerve brain circuit in Alzheimer’s disease and associated diseases and medical conditions
• Covers dementia, aging, COVID-19, autoimmune conditions, and inflammatory skin condition rosacea
• Presents the conceptual model of food and gut microbial tryptamine vagus nerve circuit
• Covers human health and intellectual abilities in the context of both the human genome and the human microbiome

• Language: English
• Publisher: Academic Press
• Release date: Jan 25, 2023
• ISBN: 9780443151989

Elena L. Paley

Dr. Paley is Cofounder of the nonprofit Stop Alzheimers Corp and Founder of Expert Biomed, Inc. She holds a PhD degree in biology with specialization in molecular biology from the Engelhardt Institute of Molecular Biology of the Russian Academy of Sciences in the Laboratory of Lev L. Kisselev. Dr. Paley’s research focuses mainly on protein biosynthesis in biology and diseases and is conducted in collaboration with Harvard University, Brandeis University, the University of Miami, Tel Aviv University, the Institut des Vaisseaux et du Sang (Paris, France), and the University of Texas at San Antonio. She is Adjunct Professor at Nova Southeastern University, FL, United States, and is inventor in patents issued and pending.

Book preview

Chapter 1: Introduction: Microbial metabolite interference of protein biosynthesis in neurodegenerative, neurodevelopmental, and other disorders; microbial metabolites hijacking vagus nerve

Abstract

My current studies focus on protein biosynthesis interference induced by microbial metabolites in Alzheimer’s disease and associated medical conditions.

Protein synthesis represents a major metabolic activity of the cell. Multifaceted deregulation of gene expression and protein synthesis was observed with age.

The data reported on microbial and human tryptophan metabolite tryptamine in virus infections including the most recent of 2021 are discussed in this book.

I present here the conceptual model of food and gut microbial tryptamine—vagus nerve circuit.

In this book, I put together a 1758 piece (citations) puzzle to understand a mechanism of neurodegenerative and other associated diseases to be able to test, prevent, and treat the diseases (Life Puzzle).

Keywords

Protein biosynthesis; Alzheimer’s disease; Associated medical conditions; Tryptophan metabolites; Gut microbial tryptamine; Vagus nerve circuit; Life puzzle collection of citations; Conceptual model; N,N-dimethyl-tryptamine (DMT); Redox reaction

My current studies focus on protein biosynthesis interference induced by microbial metabolites in Alzheimer’s disease and associated medical conditions [1,2].

Protein synthesis represents a major metabolic activity of the cell. Multifaceted deregulation of gene expression and protein synthesis was observed with age [3].

The gut microbiota contributes to host physiology through the production of a myriad of metabolites. These metabolites exert their effects within the host as signaling molecules and substrates for metabolic reactions. Although the study of host-microbiota interactions remains challenging due to the high degree of crosstalk both within and between kingdoms, metabolite-focused research has identified multiple actionable microbial targets that are relevant for host health [4].

Human host protein biosynthesis is one of the main microbial targets. We demonstrated that analog of essential amino acid tryptophan—the microbial metabolite an inhibitor of protein biosynthesis biogenic amine tryptamine shows multiple dose-dependent effects in vivo in cell and animal models and in vitro. In our studies, tryptamine induces neurodifferentiation [5,6], neurodegeneration [5–8], cytotoxicity, mitosis, and death of cancer cells [5,6,9,10]. The data by other authors supported our findings of these effects. For instance, it was recently demonstrated that tryptamine administration attenuated clinical signs of paralysis in mice [11]. Thus, tryptamine-induced neurodifferentiation is supported by this study.

In the earlier study, tryptamine was produced in the midintestinal diverticula in rats [12]. Removal of the intestinal pouch results in a reduction of tryptamine metabolite indoleacetic acid to normal levels within 24 h, and oral administration of neomycin promptly reduces the excretion of this compound to normal levels [13]. Colonic diverticular disease may be associated with increased risk for dementia [14]. There were 1057 dementia cases in the diverticular disease cohort during the follow-up period of 315,171 person-years; the overall incidence rate of dementia differed from that of the control group (3.35 vs 2.43 per 1000 person-years, P < .001) [14].

We discovered the gut microbial alterations in Alzheimer’s disease and associated conditions by targeting tryptophan-tryptamine pathways [15–17]. Recently, induction of gut microbial tryptamine by SARS-CoV-2 in nonhuman primate model of COVID-19 disease is consistent with tryptamine-induced model of neurodegeneration is demonstrated [18,19].

Another candidate to be implicated in protein biosynthesis interference induced by microbial metabolite in disease is an amino acid analog β-N-methylamino-l-alanine (BMAA). Particularly, chronic exposure to BMAA may be a risk factor for progressive neurodegenerative disease [20]. The growing scientific literature suggests the following: (1) BMAA exposure causes Guamanian amyotrophic lateral sclerosis/parkinsonism dementia complex (ALS/PDC) among the indigenous Chamorro people of Guam; (2) Guamanian ALS/PDC shares clinical and neuropathological features with Alzheimer’s disease, Parkinson’s disease, and ALS; (3) one possible mechanism for protein misfolds is misincorporation of BMAA into proteins as a substitute for L-serine; and (4) chronic exposure to BMAA through diet or environmental exposures to cyanobacterial blooms can cause neurodegenerative disease [20]. Of note, BMAA is a nonprotein amino acid, an amino acid which is not preferentially used to make human proteins. Over 900 nonprotein amino acids have been reported [20].

A microbial metabolite, histamine—an analog of amino acid histidine, inhibitor of mammalian protein biosynthesis and a diet component is implicated in cytotoxicity and allergies [1,21–23]. Histamine, which has been associated with food poisoning in concentrations of 185 mg/100 g in Swiss cheese and 180–500 mg/100 g in fish, was found in concentrations above 500 mg/100 g in Swiss cheese [24]. Histamine inhibits histidyl-tRNA synthetase from (ATP-PPi exchange reaction catalyzed by histidyl-tRNA synthetase) Salmonella typhimurium by ∼80% [25]. Specificity (Ki) of histidyl-tRNA synthetase in the binding of histidine analog histamine dihydrochloride was determined to be 4 × 10−4 M in tRNA aminoacylation reaction [25]. In addition to inhibiting protein synthesis, l-histidinol acts as an intracellular histamine antagonist [26]. Histidine decarboxylase (HDC) catalyzes production of histamine from histidine. Structural study reveals that Ser-354 determines substrate specificity on human HDC [27]. The Ser-354 to Gly mutation at the active site enlarged the size of the hHDC substrate-binding pocket and resulted in a decreased affinity for histidine (14.5-fold higher Km), but an acquired ability to bind and act on L-DOPA as a substrate (116-fold increased enzymatic activity) [27]. Human myositis autoantibody inhibits protein biosynthesis enzyme histidyl-tRNA synthetase in a model for autoimmunity [28].

A diet component glutamate produced by human microbiome is a protein biosynthesis inhibitor (discussed in this book in more details).

Tryptamine inhibits a protein biosynthesis enzyme tryptophany-tRNA synthetase (TrpRS), which is also a human autoantigen [29] formed due to alternative splicing [30,31]. A master autoantigen-ome links alternative splicing, female predilection, and COVID-19 to autoimmune diseases while TrpRS is an autoantigen detected in COVID-19 [32]. The human adenovirus serotype 5 (Ad5) E4orf6 and E1B55K proteins work in concert (E4orf6/E1B55K E3 ubiquitin ligase complex) to degrade TrpRS and to regulate selective export of late viral mRNAs during productive infection [33]. Degradation frequently involves one of a variety of E3 ubiquitin ligase complexes in which a substrate recognition component introduces the target protein for ubiquitination and subsequent degradation by proteasomes [33]. TrpRS modified by degradation and phosphorylation [34] is a serum tumor biomarker revealed in cancer patients using a blood test [1,35].

Sipilä and colleagues (2021) reported in The Lancet [36] the large, multicohort, observational study, the analysis was based on a primary cohort consisting of pooled individual-level data from three prospective cohort studies in Finland (the Finnish Public Sector study, the Health and Social Support study, and the Still Working study) and an independent replication cohort from the UK Biobank. The authors compared the risk of Alzheimer’s disease and other dementias across a wide range of hospital-treated bacterial and viral infections in two large cohorts with long follow-up periods [36]. In this study, severe infections requiring hospital treatment are associated with long-term increased risk of dementia, including vascular dementia and Alzheimer’s disease. This association is not limited to central nervous system (CNS) infections, suggesting that systemic effects are sufficient to affect the brain. The authors suggested that the absence of infection specificity combined with evidence of dose-response relationships between infectious disease burden and dementia risk support the hypothesis that increased dementia risk is driven by general inflammation rather than specific microbes [36].

In this book, I put together our recent and older data with published reports, present concepts, and a rationale for our emerging hypothesis of a dysfunctional microbiome—vagus nerve—brain circuit in Alzheimer’s disease and associated diseases and medical conditions. The vagus nerve-related anatomical defects including deformation of lower esophageal sphincter linked to acid reflux (heartburn) and diverticula in the intestine linked to diverticular disease are risk factors for neurodegenerative diseases. Human vagus nerve has different anatomical characteristics such as cervical vagus nerve branching. In sudden infant death syndrome, more small and fewer large myelinated vagal fibers were found than in controls. This book discusses the vagus nerve-related conditions including Arnold’s reflex, laryngopharyngeal reflux, duodenogastric reflux, gastroesophageal reflux, and related pulmonary diseases.

The Chapters 22 and 23 discuss rosacea, a common medical condition (5.46% of the world adult population [37]) associated with severe neurodegenerative and other diseases. Rosacea is a common skin disease that affects 16 million Americans [38]. In the preliminary study of high-resolution digital photographs of 2933 women aged 10–70 from the general population, Dr. Alexa Boer Kimball, director of the clinical unit for research trials in skin at Harvard Medical School, found a prevalence rate for rosacea of 16% in Caucasian women. The incidence rate is comparable to the 14% prevalence of rosacea in women and 6% in men in a frequently cited study of 809 office workers in Sweden, published in 1989 [39].

Both serotonin and tryptamine affect vasculature that is impaired in rosacea. Interactions between the microbiome and pathogenesis of rosacea discussed recently (2020, 2021) [40–42]. Although all of the studies showed significant alterations in the composition of the skin, blood, or gut microbiome in rosacea, the results were highly inconsistent, or even, in some cases, contradictory. No one among the micro-organisms such as Helicobacter pylori, Demodex folliculorum, Staphylococcus epidermidis, and Chlamydia pneumonia has been identified as having a real causative role in the disease [40].

The subjects covered in this book also include a question: which one is more important for human health and intellectual abilities human genome or human microbiome?

The data reported on microbial and human tryptophan metabolite tryptamine in virus infections including the most recent of 2021 are included here in Table 1 and discussed in this book.

Table 1

The microbially produced tryptamine is discussed here and in the previous publications as anticancer, apoptotic, necrotic, pro-cancer, mitogenic, neurogenerative, neurodifferentiative, antimicrobial, and antiviral metabolite.

Hence, antibiotics for cancer treatment can be considered as a double-edged sword [52]. Tryptamine and some other biogenic amines are virulence factors. Typically, virulence factors associated with bacteria confer the ability to evade host defenses, adhere to cell surfaces, produce toxins and enzymes that contribute to pathogenic potential [53].

The convulsant effects of tryptamine in cats and rats were described by Laidlaw (1912) and Tedeschi, Tedeschi, and Fellows (1959) and a head shake or twitch has been described in mice (Come, Pickering, and Warner, 1963) [54]. The effects of tryptamine were demonstrated on flexor and extensor lower limb reflexes in spinal and decerebrate cats (Marley and Vane, 1967) [54]. The effects of tryptamine on the flexor reflex were enhanced by pretreating cats with a hydrazide or a hydrazine monoamine oxidase inhibitor [54]. The flexor reflex is initiated by cutaneous receptors, involving an entire limb. This is exemplified by pulling the hand back from a hot object, via flexing of the arm. Spinal flexor reflex pathways are slightly inhibited from descending influences of the brainstem (Moini and Piran, 2020) [55]. The facilitations of the flexor and crossed extensor reflexes were induced by tryptamines [54].

The relationship of the gut microbiota and its metabolites with autism spectrum disorder (ASD)-like behaviors was studied recently in China by Xiao and colleagues (2021) [56]. Autism spectrum disorder (ASD) is a category of neurodevelopmental disorders that are characterized by social and communication impairments and restricted or repetitive behaviors (Hyman, Levy, Myers, 2020) [57]. ASD is a common neurodevelopmental disorder with reported prevalence in the United States of 1 in 59 children (∼1.7%) [57]. Xiao and colleagues (2021) [56] collected fecal samples from typically developing (TD) and autism spectrum disorder (ASD) children, transplanted them into germ-free (GF) mice, and found that the fecal microbiome of ASD children can lead to ASD-like behaviors, different microbial community structures, and altered tryptophan and serotonin metabolism in GF mice. In addition, the researchers demonstrated that tryptophan and serotonin metabolism was also distinct in ASD and TD children. Together, these findings confirmed that the microbiome from children with ASD may lead to ASD-like behavior of GF mice through metabolites, especially tryptophan and serotonin metabolism [56]. Changed metabolites in the tryptophan and serotonin metabolic pathways between TD (TD control) (n = 60) and ASD children (n = 120) were presented by the authors. Differential metabolites were screened according to a fold change of >1.25 or <0.8 and a P value of <.05 (by a t test). *, P < .05; **, P < .01. The authors found that 5-hydroxy-N-formylkynurenine, tryptamine, 5-hydroxytryptophan, and serotonin were more abundant in children with ASD than in TD children and that 6-hydroxymelatonin and 5-hydroxyindole-3-acetic acid were decreased in the children with ASD relative to the TD children [56]. In this study, fecal metabolites in both groups of children were analyzed using untargeted metabolomics [56]. Differential metabolites in the tryptophan and serotonin metabolic pathways at different tissue sites were revealed between the TD-FMT (mice transplanted with the fecal microflora of TD control donors) and ASD-FMT (mice transplanted with the fecal microflora of donors with ASD) groups (n = 8). Particularly, the level of indole-3-acetic, a main metabolite of tryptamine was higher in cecum of ASD-FMT compared to TD-FMT [56]. Vagus nerve stimulation (VNS) paired with behavioral therapy may represent a potential new approach to enhance rehabilitation that could significantly improve the outcomes of individuals with autism and other neurodevelopmental disorders (Engineer and colleagues of The University of Texas at Dallas, 2017) [58].

Tryptamine (185 or 208 mg/kg of fish product) is associated with food poisoning (Hwang and colleagues, 1995) [59]. Sailfish fillets were involved in a 1994 food poisoning outbreak in Western Taiwan. Samples were collected from the victims’ samples and from retail and wholesale suppliers [59]. The incident caused illness of 12 victims from five families. Symptoms appeared soon after eating sailfish fillets and included rashes, urticaria, nausea, vomiting, diarrhea, flushing, tingling, and itching of the skin. The victims recovered within 8 h.

Pseudomonas species can grow in media containing different biogenic amines including tryptamine as carbon and energy sources, a reason why these bacteria are excellent models for studying such catabolic pathways [60]. Putative and incomplete bacterial pathway proposed to explain tryptamine degradation (review by Luengo and Olivera, 2020) [60]. Nevertheless, tryptamine production was detected in 6 out of 7 tested Pseudomonas species that were isolated from Italian cheese (Martuscelli and colleagues, 2005) [61]. Although many microbes are able to degrade different biogenic amines, at least to the best of our knowledge, no pathway able to fully catabolize amines derived from l-tryptophan [tryptamine, serotonin, or their derivatives N-acetylmethoxytryptamine (melatonin), N,N-dimethyltryptamine and psilocybin] has yet been described in bacteria [60]. Endophytic bacterium Pseudomonas fluorescens RG11 colonization increased the endogenous levels of 5-hydroxytryptophan, N-acetylserotonin, and melatonin, but reduced those of tryptamine and serotonin, in the roots of the Red Globe grape cultivar under salt stress conditions. Quantitative real-time PCR revealed that RG11 reduced the transcription of grapevine tryptophan decarboxylase and serotonin N-acetyltransferase genes when compared to the un-inoculated control. These results correlated with decreased reactive oxygen species bursts and cell damage, which were alleviated by RG11 colonization under salt stress conditions (Ma and colleagues, 2016) [62]. Tryptamine was not detected during the in vitro incubation of P. fluorescens RG11. Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT) catalyzes the rate-limiting step in the biosynthesis of the circadian hormone melatonin from serotonin. Both tryptamine (Km = 0.168 ± 0.014 mM) and serotonin (Km = 0.241 ± 0.027 mM) are substrates of AANAT enzyme [63].

Tryptamine was detected in Balkan-style fresh sausages stored at 2°C under anaerobic modified atmosphere storage even at 0 time storage (Carballo and colleagues, 2019) [64]. Fresh sausages must be refrigerator stored and, in contrast to cheese, must be cooked before consumption.

Therefore, the microorganisms producing biogenic amines can be acquired by human with uncooked food. Endophytic bacterium may contain inhibitor/s of tryptophan decarboxylase and arylalkylamine N-acetyltransferase genes transcription.

Three new tryptamine derivatives diaporols T-V were isolated by adding tryptamine into the culture of Diaporthe sp., a fungus obtained from the leaves of plant Rhizophora stylosa (Mangrove). The structures of these compounds were elucidated by NMR spectroscopy and high resolution mass spectroscopic data. Among them, compound 1 showed moderate cytotoxic activity against SW480 cancer cell with IC50 9.84 μM (Chen and colleagues, 2021) [65].

Human indole(ethyl)amine-N-methyltransferase (hINMT) catalyzed methylation of tryptamine, is enhanced (>2-fold) under reducing dithiothreitol (DTT) conditions (Torres and colleagues, 2019) [66]. For N-methyltryptamine (NMT) and N,N-dimethyltryptamine (DMT) synthesis, 5 μg of −80°C frozen hINMT protein was combined with 35 μL 20 mM NaH2PO4 pH 7.9 and tryptamine. Production of NMT/DMT was compared at (−/+) 15 mM DTT (similar data to 15 mM DTT was obtained with tryptamine and 2, 5, or 10 mM DTT). DTT was chosen because of its relative pH neutrality, lack of bulk, and possession of two reducing equivalents per molecule [66].

The steps taken for the discovery of the hallucinogenic effects of N,N-dimethyl-tryptamine (DMT) was described by Dr. Stephen Szára [67].

Reducing agent is a substance that reduces a chemical compound usually by donating electrons. In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is an element or compound in a redox chemical reaction that loses or donates an electron to an electron recipient (called the oxidizing agent, oxidant, oxidizer, or electron acceptor). In other words, a reducer is any substance that reduces another substance.

Thus, redox chemical reaction can result in enhanced production of hallucinogen DMT from microbial tryptamine via hINMT-catalyzed methylation of tryptamine.

Finally yet importantly, I present here the conceptual model of food and gut microbial tryptamine—vagus nerve circuit.

In this book, I put together a 1758 piece (citations) puzzle to understand a mechanism of neurodegenerative and other associated diseases to be able to test, prevent, and treat the diseases.

References

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Chapter 2: COVID-19: Scientific progress

Abstract

This chapter includes sections and subsections that discuss different aspects of COVID-19 including possible sources of SARS-CoV-2 virus transmission, metabolic changes upon this virus infection and pathology: (1) Tryptophan catabolism and tryptamine in viral infections; (1.1) Tryptamine and COVID-19; (1.2) Tryptamine cytotoxicity in human, mammalian animal, protozoan, yeast, bacterial, insect and algae cells; (1.3) Tryptamine-interacting proteins; (2) Hospital-acquired infections; (3) COVID-19 and bacterial infections; (4) COVID-19 in Florida; (5) Possible sources of SARS-CoV-2 virus transmitting/dispersing: leakage of untreated wastewater, medical procedure fecal microbiota transplantation, infected during insulin administration, animals; (5.1) Leakage of untreated wastewater; (5.2) The medical procedure fecal microbiota transplantation; (5.3) Infected during insulin administration; (5.4) Animals harboring coronavirus; (6) COVID-19 and necrotizing medical conditions; (6.1) Gut microbial tryptamine induced by SARS-CoV-2 elicits necrosis; (6.2) Necrotizing pancreatitis.

Keywords

Tryptophan catabolism; Tryptamine and COVID-19; SARS-CoV-2 virus transmission; Hospital-acquired infections and COVID-19; COVID-19 and necrotizing medical conditions

Escandón and colleagues (2021) urge a nuanced understanding of the science and caution against black-or-white messaging, all-or-nothing guidance, and one-size-fits-all approaches. There is a need for meaningful public health communication and science-informed policies that recognize shades of gray, uncertainties, local context, and social determinants of health [1]. The topics of this review [1] are (1) health and lives vs economy and livelihoods, (2) indefinite lockdown vs unlimited reopening, (3) symptomatic vs asymptomatic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, (4) droplet vs aerosol transmission of SARS-CoV-2, (5) masks for all vs no masking, and (6) SARS-CoV-2 reinfection vs no reinfection (Escandón and colleagues, 2021). A false dichotomy is a logical fallacy that involves presenting two opposing facts, views, or options as though they were the only possibilities. The false dichotomy fallacy is often committed when someone thinks one of the two options is obviously true while the other is obviously false. In reality, many more facts, views, and options exist in between, which can be represented as a gradient of gray shades between the extremes of black and white [1].

1: Introduction: Tryptophan catabolism and tryptamine in viral infections

1.1: Tryptamine and COVID-19

The Lancet Editorial publication (2021) emphasizes that the emergence of COVID-19 has indelibly marked science and medicine. Advances in epidemiology, clinical care, prevention, treatment, and the speed of vaccine development have been unprecedented, driven by global collaboration and data sharing [2].

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is considered as a causative factor of coronavirus disease 2019 (COVID-19) [3]. However, SARS-CoV-2 induces only mild symptoms of COVID-19 in nonhuman primates (the macaque) [4]. Specifically, macaques were exposed to a total dose of 2 × 10⁷ PFUs (plaque forming unit) of SARS-CoV-2 via the combination of intranasal and intratracheal routes. This procedure allowed infection in the upper and lower respiratory tracts [4]. The virus strain used in this study was isolated by the National Reference Center for Respiratory Viruses (NRC-VIR, Institute Pasteur, Paris, France) from a nasopharyngeal swab from one of the first French cases [4].

The subjects covered by this chapter partially published elsewhere [5–7]. Here is revisited, revised and updated information. The COVID-19-related subjects also discussed in some new chapters that can be found in the Content of this Book. Along with other subjects, this chapter discusses the recently demonstrated (Sokol et al., 2021) induction of the tryptophan catabolite tryptamine by SARS-CoV-2 in gut microbiome [4]. However, Mehraj and Routy (2015) [8] reported earlier in the review article that pathogens experiencing tryptophan deprivation by indoleamine-2,3-dioxygenase (IDO)-catalyzed degradation include certain bacteria, parasites, and less likely viruses. Nevertheless, chronic viral infections highjack the host immune response to create a state of disease tolerance via kynurenine catabolites of the tryptophan catabolic pathway. This review [8] covers the data involving chronic viral infections such as human immunodeficiency virus (HIV), hepatitis B virus (HBV), hepatitis C virus (HCV), herpes, and cytomegalovirus (CMV) and their cellular interplay with tryptophan catabolites [8]. IDO is an interferon-γ (IFN-γ)-induced tryptophan-degrading enzyme, producing kynurenine that participates in the mechanism of tumor immune tolerance. Thus, IDO inhibition has been considered a strategy for anticancer therapy. Tryptamine and its metabolite N,N-dimethyltryptamine (DMT) have inhibitory effects on recombinant human IDO (rhIDO) activity. Serotonin and melatonin had no effect but tryptamine and DMT modulated the activity of rhIDO as classical noncompetitive inhibitors, with Ki values of 156 and 506 μM, respectively. This inhibitory effect was also observed on constitutively expressed or IFN-γ-induced IDO in the A172 human glioma cell line. Tryptamine and DMT increased the cytotoxic activity of peripheral blood mononuclear cells (PBMCs) in coculture assays [9].

Thus, tryptamine and DMT can affect the production of kynurenine and its metabolites. Moreover, a minor pathway that converts tryptamine into kynuramine via IDO is also described [10]. The earlier literature mainly focused on cardiovascular effects of kynuramine, 5-hydroxykynuramine and their N(1),N(1)-dimethylated analogs, including indirect effects via release of catecholamines or acetylcholine and interference with serotonin receptors [11].

Tryptamine is altered upon Cricket paralysis virus infection as shown by metabolomic analysis [12].

Using a cell-based SARS-CoV-2 infection assay, Piscotta and colleagues (2021) [13] screened culture broth extracts from a collection of phylogenetically diverse human-associated bacteria for the production of small molecules with antiviral activity. Bioassay-guided fractionation uncovered three bacterial metabolites capable of inhibiting SARS-CoV-2 infection. This included the nucleoside analog N6-(Δ²-isopentenyl)adenosine, the 5-hydroxytryptamine receptor agonist tryptamine, and the pyrazine 2,5-bis(3-indolylmethyl)pyrazine [13]. Anti-SARS-CoV-2 activity of naturally occurring tryptamine, and structural relatives (50% inhibitory concentration IC50 μM) are detected: tryptamine 33; tryptophan >250; indole-3-propionic acid >250; N-acetyltryptamine >250; phenylethylamine 132; tyramine >250 [13]. Infection of Huh-7.5 cells (well-differentiated hepatocyte derived cellular carcinoma cell line) by SARS-CoV-2, HCoV-229E (human coronavirus), hPIV-3 (parainfluenza), and YFV 17D (yellow fever) viruses was explored for tryptamine inhibition (IC50, μM) to be: 33, 50, 158, 218, respectively. Distribution of anti-SARS-CoV-2 metabolites among culture broth extracts of human-associated bacteria was demonstrated for representative culture (tryptamine produced by 9 bacteria from 5 Phylum). Culture broth extracts from 50 phylogenetically diverse and commonly seen commensal bacteria were screened in a cell-based assay for the ability to inhibit infection of human cells by SARS-CoV-2 in this study [13].

Tanimoto and colleagues of Hiroshima University, Japan reported in Scientific Reports of Nature (2021) [14] the inhibition of SARS-CoV-2 infection in vitro by suppressing its receptor, angiotensin-converting enzyme 2 (ACE2), via aryl-hydrocarbon receptor signal. The authors focused on the internalization mechanism of SARS-CoV-2 via ACE2. Because RNA-seq analysis suggested that suppressive effects on ACE2 might be inversely correlated with induction of the genes regulated by aryl hydrocarbon receptor (AHR), the AHR agonists 6-formylindolo(3,2-b)carbazole (FICZ) and omeprazole (OMP) were tested to assess whether those treatments affected ACE2 expression. Both FICZ and OMP clearly suppressed ACE2 expression in a dose-dependent manner along with inducing CYP1A1. Knock-down experiments indicated a reduction of ACE2 by FICZ treatment in an AHR-dependent manner. Finally, treatments of AHR agonists inhibited SARS-CoV-2 infection into Vero E6 cells as determined with immunoblotting analyses detecting SARS-CoV-2-specifc nucleocapsid protein. The researchers demonstrate that treatment with AHR agonists, including FICZ, and OMP, decreases expression of ACE2 via AHR activation, resulting in suppression of SARS-CoV-2 infection in mammalian cells [14]. Several other tryptophan metabolites, indole 3-carbinol, indoleacetic acid (10 and 100 μM), tryptamine (1, 10, and 100 μM), and L-kynurenine, and proton pump inhibitors, rabeprazole sodium, lansoprazole, and tenatoprazole, were also tested as to whether they regulate ACE2 gene expression; it was demonstrated that most of them decreased gene expression of ACE2 with a variety of actions and increased gene expression of CYP1A1 (100 μM tryptamine increased expression of CYP1A1) in HepG2 cells (evaluated by quantitative RT-PCR). Similar differences were also observed among cell lines tested in this study. The authors suggested that optimization of each compound will be necessary in the next steps (in vivo experiments and clinical studies), although most of the tryptophan metabolites and proton pump inhibitors seem to be safely applicable to clinical therapeutic research. Furthermore, inhibitory effects of those compounds on infections with SARS-CoV-2 variants would be interesting to test, since many mutations in the virus have been reported so far. On the other hand, one limitation of this strategy might be that these drugs do not target the SARS-CoV-2 virus itself but just modify cellular susceptibility to it. Combination therapies of AHR agonists with antivirus drugs, such as favipiravir or remdesivir, are therefore possible strategies for clinical application [14].

The nutrition contents of breastmilk directly participate in neonatal immune response. The alterations of the components of breastmilk under the context of viral infection not only reflect the physiological changes in mothers but also affect neonatal immunity and metabolism via breastfeeding. The authors from China answered the important questions whether breastmilk production is affected by COVID-19 and whether breastfeeding is still a safe or recommended operation for COVID-19 puerperant women (Yin Zhao and colleagues, 2020) [15]. Proteomic and metabolic/lipidomic profiling of colostrum samples from COVID-19 puerperant women and healthy volunteers is presented with overview of colostrum samples collected from COVID-19 puerperant women (n = 4) and healthy volunteers (n = 2) [15]. Days between birth and breastmilk sample collection were within 3 days for all patients. The pathway analysis revealed the alterations of aminoacyl-tRNA biosynthesis and aromatic amino acid (AAA) metabolism as the notable metabolic signatures. The data showed (Log2 fold change) that tryptophan level (−2.14-fold) and tryptophan catabolism were significantly decreased in breastmilk of COVID-19 patients. For instance, the microbial metabolites, such as indole (−1.97-fold), indoleacetaldehyde (−1.95-fold), indole-3-acetic acid (−1.858-fold), and tryptamine (−1.89-fold), which can be derived from tryptophan, were significantly decreased in breastmilk of COVID-19 patients [15]. Phenethylamine (phenylethylamine, PEA) was also decreased (−0.635) in COVID-19 [15]. The authors suggested that the alterations of breastmilk components were probably a reflection of the mother’s whole-body physiological responses to COVID-19, or caused by SARS-CoV-2-mediated impact on breastmilk production and/or secretion by mammary glands. Besides, COVID-19 probably affects the bacteria in the body of puerperant women, thereby resulting in the alterations of bacterial metabolites that can be secreted to breastmilk [15].

Guo and Tao (2018) selected milk biomarkers of pregnancy recognition in dairy cows to provide a new insight and method for early pregnancy diagnosis. Ten healthy Holstein high-yielding dairy cows with similar body condition and parity were selected. All cows were oestrus synchronized, and their milk samples were collected at day 0 and day 17 of artificial insemination. Metabolic profiles of the 2 groups of milk samples were determined with LC-Q/TOF-MS metabolomics Differentiated metabolites for the 2 groups of milk samples were selected according to VIP values ( > 1) and P values ( < .05). The results showed that (1) milk metabolic profiles at day 0 were obviously different from those at day 17 after artificial insemination and (2) ten metabolites (mevalonolactone, tryptamine, l-tyrosine, dihydroneopterin phosphate, acetylcysteine, N-acetyl-l-tyrosine, l-proline, 1BnTIQ, guanosine, and heptadecanoyl carnitine) were first confirmed as differentiated metabolites between the 2 groups of milk samples (Guo and Tao, 2018) [16].

Thus, tryptamine was detected in human breastmilk an in milk of dairy cows. Furthermore, changes in tryptamine levels were detected in pregnancy (dairy cows) and in puerperant (the period immediately following childbirth) women with COVID-19 (patients).

Delirium is an acute change in attention and cognition occurring in ∼65% of severe SARS-CoV-2 cases. It is also common following surgery and an indicator of brain vulnerability and risk for the development of dementia. Cuperlovic-Culf and colleagues (2021) [17] analyzed the underlying role of metabolism in delirium susceptibility in the postoperative setting using metabolomic profiling of cerebrospinal fluid (CSF) and blood taken from the same patients prior to planned orthopedic surgery. Paired CSF and blood metabolomics analysis were undertaken for 54 age (above 65) and gender-matched patients where 28 of the patients experienced postoperative delirium (based on Confusion Assessment Method test) and 26 did not show any delirium symptoms (control). Elective hip or knee arthroplasty to a single surgical center were eligible for inclusion. Exclusion criteria included a preexisting diagnosis of dementia or other neurodegenerative condition. The incidence of postoperative delirium was 14%. Cohort had a mean age of 74.4 years and 57% were female. Biogenic amine phenethylamine (PEA), the catabolite of aromatic amino acid phenylalanine showed opposite concentration difference in blood and CSF in delirium-prone and delirium-free patient groups [17]. Particularly, PEA is higher in CSF than in blood in control, while PEA is higher in blood than in CSF in delirium. For PEA—Control, adjusted p-value 1.8E-6; for PEA—Delirium, adjusted p-value 1.78 E-10. PEA is a natural monoamine alkaloid that acts as a central nervous system stimulant. In CSF, PEA is overall slightly reduced in the delirium group. Similarly in blood PEA concentration is slightly lower in delirium [17]. Both biogenic amines tryptamine and PEA are the gut microbial metabolites derived from aromatic amino acids. Both tryptamine and PEA cross the blood-brain barrier.

Therefore, (1) tryptophan catabolite tryptamine induced by SARS-CoV-2 in the gut microbiome of nonhuman primates [4]; (2) bioassay-guided fractionation uncovered three bacterial metabolites including tryptamine capable of inhibiting SARS-CoV-2 infection [13]; and (3) tryptamine was significantly decreased in breastmilk of COVID-19 patients following childbirth compared to healthy volunteers in untargeted metabolomics [15]. Furthermore, pregnancy affects tryptamine content in milk of dairy cows [16].

1.2: Tryptamine cytotoxicity in human, mammalian animal, protozoan, yeast, bacterial, insect, and algae cells

In dose-response testing against SARS-CoV-2 infection of Huh-7.5 cells, tryptamine had an IC50 of 33 μM, with some loss in cell viability observed at the highest concentration tested (250 μM) [13]. Some loss of viability of human cultured cells at 250 μM tryptamine is not consistent with the earlier reported data on tryptamine cytotoxicity in human cultured cells with LD50 100 μM and some cell loss at 10 μM (Herrera and colleagues, 2006) [18]. More data on tryptamine-induced cell toxicity are reported by Paley [19]. Tryptamine, a competitive inhibitor of tryptophanyl-tRNA synthetase (TrpRS) with Km of purified bovine TrpRS for tryptophan 0.9 × 10−7 M and Ki for tryptamine 6.0 × 10−7 M in ATP-[³²P]pyrophosphate exchange [20,21]. Tryptamine exerted a cytotoxic effect on human cervical cancer HeLa cells as well as on the simian virus (polyomavirus) SV40-transformed 631 hamster cells, although human cells demonstrated less sensitivity to the tryptamine inhibition than the bovine kidney MDBK and hamster cells. Incubation of HeLa cells with 55 μg/mL tryptamine led to loss of ∼25% of cells. A 50% cytotoxic concentration (IC50) for human HeLa cells was about 110 μg/mL, proved to be almost 4-fold higher than the previously estimated value for bovine kidney cells (30 μg/mL = 150 μM) [22] and ∼2-fold for Chinese hamster 631 cells [21]. IC50 reached 220 μg/mL within 19 months of cultivation of HeLa cells with gradually increasing concentrations of tryptamine. The same concentration led to 96% death of the original cells. Tryptamine at 330 μg/mL exerted 70 and 99.4% cytotoxic effects on resistant HeLa A and control cells, respectively. It was found that the resistant HeLa A cells grew at twice the lower rate in the logarithmic phase than the original counterpart [21]. The different sensitivities of cultured cells to tryptamine can depend on the activity of monoamine oxidase (MAO) in cells and in the serum used for cultivation and thus the tryptamine-sensitivity is cellular MAO-dependent and endogenous MAO inhibitor (MAOI) dependent. Importantly, the TrpRS inhibition-mediated cytotoxicity depends on tryptophan content in the medium for cell cultivation and the tryptophan concentration in the serum added to the medium for cultivation. The glutamine added to the culture medium can also affect tryptamine activity because glutamine inhibits the accumulation and hydroxylation of tryptophan. Glutamine inhibits tryptophan hydroxylase activity [23]. Tryptamine cytotoxicity depends on the cell density and time of incubation [22]. The LD50 (the inhibitor concentration at which one half of the cells are killed) for tryptamine was found to be about 30 μg/mL (150 μM tryptamine) at an inoculation density of 1.5 × l0⁶ cells/per 80-mm petri dishes for 9 days. However, all the cells died at the tryptamine concentration 25 μg/mL and at an inoculation density of 1.6 × l0⁵ cells/plate. The LD50 for tryptophanol was 150 μg/mL at an inoculation density of 2 × l0⁵ cells per dish, but all the cells died at this inhibitor concentration during the second passage [22]. Tryptamine is an effective inhibitor of human malignant cell growth similarly to other well-characterized cytotoxic drugs, the main distinction being the establishment of low resistance to tryptamine requiring almost 2 years of stepwise selection procedures. The same tendency has been observed in the spontaneously transformed bovine kidney MDBK, SV40-transformed Djungarian hamster DM15 cells, SV40-transformed Chinese hamster 631 cells, human osteosarcoma cell line (HOS), acute-phase chronic myeloid leukemia cell line K562, and acute lymphoblastic leukemia cell line Molt-4, which have been cultivated over extended periods of time in the presence of tryptamine by Paley [21]. Tryptamine accumulates at high concentrations in fish sauces (up to 2280 mg/kg = 14.25 mM), certain fish and fish products (up to 362 mg/kg), dairy products such as cheese (up to 312 mg/kg), and certain fermented meat products such as fermented sausages (up to 194 mg/kg)) [European Food Safety Authority (EFSA), 2011] [24]. The IC50 values for β-phenylethylamine and tryptamine were determined at different arbitrary time points (8, 12, 18, and 24 h) and became progressively smaller with increasing incubation time with intestinal epithelium cells HT29 (human colorectal adenocarcinoma). The IC50 values obtained at 24 h exposure showed tryptamine to be more cytotoxic than β-phenylethylamine (IC50 for β-phenylethylamine = 4.09 ± 1.65 mM, and IC50 for tryptamine = 0.67 ± 0.09 mM); its IC50 was around 6 times smaller than that of β-phenylethylamine (Del Rio et al., 2020) [24]. In the study by Jin and colleagues (2014), tryptamine at 50 μM induced loss (∼10%) of CaCo-2 cells (human intestinal epithelial cells) following 24 h of incubation with tryptamine [25]. The 1 mM tryptamine induced 75% cell inhibition (5 × 10³ cells per well were plated in 96-well plates and allowed to attach for 16 h). The medium was changed to DMEM containing 2.5% FBS and a tested compound for 24 h [25]. In the cecal material of 6-week-old C57BL/6 mice tryptamine and its main metabolite indole-3-acetate were abundant, with concentrations ranging from 10–20 to 10–40 μM [25]. Tryptamine (100 μM) and dimethyltryptamine (DMT, 100 μM) increased the cytotoxic activity of peripheral blood mononuclear cells (PBMCs) in coculture assays with the human glioma cell line A172 cells in RPMI-1640 medium [9]. Tryptamine (100 μM) reduced the number of A172 cells too [9]. PBMCs was isolated from three different healthy donors. Peripheral blood mononuclear cells were used for the preparation of co-culture experiments to assess whether tryptamine and DMT influenced the tumor-reactive activity of immune cells. PBMCs comprehend a mixture of different cell types, specially lymphocytes and monocytes.

Significant toxicity for microphages was observed at tryptamine concentrations greater than 500 μM (no details provided) [26].

Bacteria Streptomyces staurosporeus (AM-2282) was found to produce tryptamine [27]. The IC50 values (μg/mL) for tryptamine were examined with human breast cancer (BC1); human lung cancer (Lu1); human colon cancer (Col2); human oral epidermoid (KB); vinblastine-resistant KB (KB-V1) to be 14.2; >20; >20; >20; and 13.3 μg/mL = 83 μM (Yang et al., 1999) [27].

In the study by Arakaki and colleagues, the 83.9% of human T-acute lymphoblastic leukemia Jurkat cells survived after 72 h of incubation in the presence of 100 μM tryptamine (as a percentage of the number of control cells) [28].

Luqman and colleagues (2018) [29] demonstrated (Fig. S3) ∼10% cell loss of human MonoMac6 (acute monocytic leukemia derived suspension cell line) at 7.8 μg/mL tryptamine and about 40% loss at 62.5 μg/mL tryptamine. The 10% loss of human monolayer line HEK293 adherent cells (immortalized human embryonic kidney cells transformed with adenovirus 5 (Ad5)) was demonstrated after 24-h incubation with 125 μg/mL tryptamine. Before the cytotoxicity assay, HEK293 cells were seeded in a 96-well microtiter flat-bottom plate with 10⁵ cells/well and incubated overnight at 37°C in 5% CO2. MonoMac6 cells (10⁵ per well) were seeded and incubated under the same conditions as HEK293 cells for 1 h before treatment. The host cells were treated with various concentrations of biogenic amines. The cytotoxicity assay was performed using the Cell Proliferation Kit I (MTT; Roche, Germany). At 570 and 690 nm (reference), the formed formazan was determined [29]. The medium for cell cultivation in the cytotoxicity testing is not indicated in this article. However, in the same report another cell line HT-29, a human colorectal adenocarcinoma cell line with epithelial morphology was used. HT-29 cells (5 × 10⁵ cells/well) were seeded in a 24-well plate in DMEM medium with 10% fetal bovine serum (FBS) and antibiotic mix and incubated at 37°C in 5% CO2 for 48 h before the addition of bacteria for adherence and internalization assay with the HT-29 cell line [29].

In my view, despite a high cell density for seeding and a short overnight incubation with biogenic amines, tryptamine showed cytotoxicity in both tested human cell lines—adherent HEK293 and suspension MonoMac6. Metabolic cooperation is a form of cell communication in which the mutant phenotype of enzyme deficient cells, as determined by incorporation of labeled substrates, is corrected in culture by contact with normal cells. Cell communication is a more generalized phenomenon among cells in contact than previously appreciated [30]. Cell density-dependent recovery of the tryptamine-sensitive cells we observed due to metabolic cooperation in cell culture [22]. This explains why cells MonoMac6 in suspension culture were more sensitive to tryptamine than adherent cells HEK293.

The example of suggested metabolic cooperation in cultured Madin Darby Bovine Kidney (MDBK) cells is demonstrated in Fig. 1.

Fig. 1

Fig. 1 Cell density-dependent cytotoxicity of tryptamine due to metabolic cooperation in kidney cell culture. (A and B) Effect of tryptamine on MDBK cell viability. (A) 1.6 × l0 ⁶ cells per dish were inoculated into a medium containing a specified tryptamine concentration. Control dishes contained no inhibitors (A and B). (B) Different cell quantities were plated in a medium with tryptamine (25 μg/mL). The number of cells in the control dishes is 100%; cell viability as a percent of the same day control. The 80-mm petri dishes (Falcon), grown in a CO 2 , incubator at 37°C for 9 days in RPMI 1640 medium. (C) Electron microscopy original magnification ×7500. Two neighboring cells with visible contacts in adherent monolayer MDBK kidney culture. Arrows show distinct intensities of immunostaining with antibodies to tryptophanyl-tRNA synthetase (TrpRS) in two adjacent cells in the monolayer culture. A cell with a higher TrpRS level can be more resistant to tryptamine inhibition than a cell with a lower TrpRS level (C). (C) TrpRS, a secreted protein can be secreted from a high level TrpRS cell and then internalized by a cell with a lower TrpRS level via the cell contacts (red arrowhead) . This can cause metabolic cooperation observed in the cell culture (B). Data from E.L. Paley, V.N. Baranov, N.M. Alexandrova, L.L. Kisselev, Tryptophanyl-tRNA synthetase in cell lines resistant to tryptophan analogs, Exp. Cell Res. 195 (1991) 66–78.

We demonstrated the neuronal loss in human neuroblastic cells SH-SY5Y and in hippocampus of the brain of mice [31]. The SH-SY5Y cells subline was treated with 20–100 μg/mL (20 μg/mL = 102 μM) of tryptamine for 6–60 days. Neuronal loss was detected in Balb-c mice treated with tryptamine injections (each injection of 200 μg of tryptamine in 0.2 mL of noncomplete adjuvant for 2.5 week was administered for every second day). Until the end of the experiments, the tryptamine mice were in good health and visibly gained more weight than the control mice.

Blood glucose level, weight progression, and positron emission tomography (PET) studies of glucose utilization after tryptamine administration of 200–400 μg/mouse, body weight 23–26 (200 μg/mouse ∼8 μg/g = 8 mg/kg = 50 μM) were conducted in the Anna-Liisa Brownell PET laboratory of Massachusetts General Hospital, Harvard University (Paley and colleagues, 2007) [31] (Table 1).

Table 1

Indication of tryptamine cytotoxicity in insulin-producing β-cells of pancreatic islets.

Blood glucose level, weight progression, and PET studies of glucose utilization after tryptamine administration 200–400 μg/mouse, body weight 23–26 g (Paley et al., 2007).

The control mice injected with PBS and tryptamine-treated mice were examined. Standardized uptake values (SUVs). PET imaging with (18)F-FDG. Of note, we demonstrated (2004) cytotoxicity and differential sensitivity between two pancreatic cell lines to 2-deoxy-d-glucose, a biologically active molecule commonly used for PET [32].

Tryptamine induced the weight increase in mice (Table 1). Glucose utilization by brain was reduced in cerebellum cingulate, hippocampus, olfactory area, and striatum (Table 1). Of note, olfactory dysfunction is frequent in COVID-19 [33]. In vitro autoradiographic techniques were used to examine the distribution of [3H]tryptamine-binding sites in rat brain. The gross distribution and pharmacological characteristics of binding to brain sections resembled those seen in homogenate studies. Binding sites were found throughout the brain, with a preponderance of sites in the forebrain and limbic structures; highest levels were seen in the choroid plexus and the interpeduncular nucleus. Other regions exhibiting high levels of [3H]tryptamine binding include the cortex (especially lamina I), caudate putamen, hippocampus, anterior olfactory nucleus, olfactory tubercle, nucleus accumbens, amygdala, superior colliculus (superficial gray layer), locus ceruleus, the nucleus of the solitary tract, and the pineal body (Perry, 1986) [34]. Importantly, [3H]tryptamine-binding sites are not identical to MAO location in the rat brain (Perry and colleagues, 1988) [35]. Tryptamine-induced hypoglycemia [36] and hyperinsulinemia [37] correlated with our findings of tryptamine-induced decrease of blood glucose level in mice [31]. The effects of tryptamine on blood glucose levels were studied by Yamada and colleagues (1988) [36]. Tryptamine induced significant hypoglycemia in mice. The hypoglycemia elicited by tryptamine was strongly antagonized by methysergide, an antagonist of both 5-HT1 and 5-HT2 receptors. A 5-HT2 receptor antagonist, ketanserin, partially inhibited the tryptamine-induced hypoglycemia. The effects of tryptamine on serum insulin levels were also investigated by Yamada and colleagues (1990) [37]. Tryptamine induced an apparent increase in serum insulin levels in mice. The elevation in insulin elicited by tryptamine was potently antagonized by the 5-HT1 and 5-HT2 receptor antagonist, methysergide, but partially reduced by the 5-HT2 receptor antagonist, ketanserin. However, the 5-HT3 receptor antagonist, ICS 205-930, was without effect [37]. Hyperinsulinemia is an early indicator of metabolic dysfunction. Hyperinsulinemia is strongly associated with type 2 diabetes. Hyperinsulinemia characterizes a prediabetic state [38,39] (Fig. 2).

Fig. 2

Fig. 2 Early in the long process, which leads to type 2 diabetes, the great majority of type 2 patients have hyperinsulinemia. The representative depiction of the natural history of type 2 diabetes—the fasting glucose and serum insulin [40].

Dopkins and colleagues (2020) [41] tested the ability of tryptamine to ameliorate symptoms of experimental autoimmune encephalomyelitis (EAE), a murine model of multiple sclerosis. Authors found that tryptamine administration attenuated clinical signs of paralysis in EAE mice, decreased the number of infiltrating CD4+ T cells in the CNS, Th17 cells, and RORγ T cells while increasing FoxP3 + Tregs (Tregs is regulatory T cells). Model mice were immunized on day 0 with subcutaneous injections containing 150 μg of myelin oligodendrocyte glycoprotein subunit 35-55 (MOG35-55) and 600 mg heat-killed Mycobacterium tuberculosis (H37Ra) suspended within an emulsion of PBS and Freund’s complete adjuvant On day 0 and 2, mice received a single intraperitoneal injection containing 200 and 400 ng of pertussis toxin, respectively. Beginning on day 1, mice received a 50 μL intraperitoneal injection containing either a vehicle (sterile corn oil (CO) with 2% DMSO v/v) or a treatment suspension (12.5 mg/kg = 12 μg/g = 60 μM tryptamine in sterile CO with 2% DMSO v/v) every 48 h. Authors analyzed mice daily to observe paralysis symptoms and body weight until any individual mice displayed a severity of symptoms that reflected a moribund state and required euthanasia. At this point, observed at day 13, all mice were euthanized for uniform sample collection. Tryptamine-treated mice demonstrated a significant reduction in paralysis symptoms beginning from day 8 that persisted for the remaining duration of the study. The sum of paralysis scores per mouse further demonstrates a significant reduction in observed paralysis symptoms experienced between tryptamine vs vehicle treatment groups. The average weight of mice was also measured throughout the time course of chronic progressive EAE and the data expressed as percent of starting body weight, clearly showed that while the vehicle-treated group started losing weight, especially on days 11–13, the tryptamine-treated mice showed a significant retention of weight when compared to the vehicle controls. These results together demonstrated that tryptamine treatment of EAE mice ameliorates the clinical symptoms of paralysis and weight loss associated with EAE.

These data are consistent with our data on weight progression following tryptamine treatment of mice (Table 1) and our earlier reported data (Paley and colleagues, 2007) [31].

Donor cells were derived from wild type (WT) and Lck-Cre AHRflox/flox mice and treated ex vivo in the presence of vehicle (DMSO) or 100 μM tryptamine prior to induction of disease [41]. The percentage of CD4+ and PCNA+ cells among all lymphocytes collected from immunized mice stimulated in vitro with MOG35-55 in the presence or absence of tryptamine demonstrated a reduction in proliferative activity. Tryptamine treatment in vitro reduced the percentage of proliferating CD4+ T cells as demonstrated by PCNA+ cells. Also, tryptamine treatment in vitro significantly reduced the percentage of CD4+ RORγT as shown in a representative flow cytometric analysis and the percentage of such cells from multiple samples [41].

Thus, tryptamine at 100 μM is cytotoxic for proliferating CD4+ T cells that play important role in immune system.

Authors studied the effects of tryptamine treatment in vivo on the composition of the cecal microbiota in EAE mice [41]. The data demonstrated that the cecal microbiota of treated mice was distinctly clustered from the vehicle group. Dehalobacterium, Bacteroides, and Peptostreptococcaceae were significantly altered in tryptamine-treated groups when compared to the vehicle controls.

Using mass spectrometry, the data revealed a significant increase in n-butyric acid following tryptamine treatment when compared to vehicle controls. These results suggested that tryptamine treatment alters the microbiota in the gut and promotes the induction of n-butyric acid [41].

Jessica Wickline and colleagues (2019) [42] reported the in vitro effects of tryptamine, harmine, and harmaline on Leishmania tarentolae and the possible implications for Leishmaniasis. Leishmaniasis is a disease caused by Leishmania parasitic protozoans affecting people in both the Eastern and Western Hemispheres [42]. The exact number of leishmaniasis cases is not known but is reported to be between 1.5 and 2.0 million new cases per year and over 150 million people affected worldwide. There were no apparent changes in cell motility or shape in treated cells relative to control cells in these trials. However, cells of day 3, 4, and 5 cultures exposed to either harmine, harmaline, or tryptamine exhibit clumping differences relative to control cells. Relative to control cells, exposure to harmine, harmaline, or tryptamine resulted in about a 33%, 25% or 60% increase, respectively, in number of cell clumps on day 5 (data not shown). This is of interest as the cell clumping is reflective of cell stress. Also, the apparent clump size (reflective of the number of cells per clump) and clump shape were not identical to the control cells [42]. Effect of the compounds on L. tarentolae cell viability was examined. In the presence of harmine (from 12.50 to 250.0 μM final concentration), the cell viability was negatively affected at and above 62.50 μM on days 4, 5, and 6 of cultivation. The cultures exposed to 62.50 μM did appear to recover on day 7, whereas above that concentration, cell viability remained very low. The same trend is shown with tryptamine (12.5; 25; 62.50; 125; 187.5; 250 μM). The cells exposed to 12.50 μM (50% inhibition after 1 day incubation and 157% growth stimulation on the day 7; cell viability as a percent of the same day control) did exhibit an apparent recovery effect not observed for any other concentration of tryptamine. For cells exposed to the various concentrations of harmaline, the cell viability appears less inhibited by addition of this compound relative to the other two testes compounds [42]. Effect of tryptamine compounds on L. tarentolae secreted acid phosphatase activity (SAP) was examined. Tryptamine has inhibitory effects on