Significance of mammalian N, N-dimethyltryptamine (DMT): A 60-year-old debate

This review (2022) explores the role of endogenously (within the animal) produced DMT in mammalian physiology by exploring 60 years of research. The biosynthesis of DMT, its receptor activity, and regulation are discussed while key experiments are used to prove what role DMT plays in the body such as a neurotransmitter and/or a hormone.

Abstract

“N,N-dimethyltryptamine (DMT) is a potent psychedelic naturally produced by many plants and animals, including humans. Whether or not DMT is significant to mammalian physiology, especially within the central nervous system, is a debate that started in the early 1960s and continues to this day. This review integrates historical and recent literature to clarify this issue, giving special attention to the most controversial subjects of DMT’s biosynthesis, its storage in synaptic vesicles and the activation receptors like sigma-1. Less discussed topics, like DMT’s metabolic regulation or the biased activation of serotonin receptors, are highlighted. We conclude that most of the arguments dismissing endogenous DMT’s relevance are based on obsolete data or misleading assumptions. Data strongly suggest that DMT can be relevant as a neurotransmitter, neuromodulator, hormone and immunomodulator, as well as being important to pregnancy and development. Key experiments are addressed to definitely prove what specific roles DMT plays in mammalian physiology.”

Authors: Javier H. Jiménez & Jose C. Bouso

Summary

Introduction

DMT is a small tertiary indolealkylamine naturally produced by many plants and animals, including humans. It is structurally and metabolically related to tryptamine, serotonin, melatonin, and other endogenous DMTs.

DMT’s structure is found in several drugs, including psychedelics and non-psychedelic compounds such as -carbolines and triptans.

DMT and 5-MeO-DMT are potent classical psychedelic drugs that activate serotonin receptor 2A (5-HT2A) and induce profound changes in cognition, mood and perception. These changes correlate with major alterations in brain activity.

Inspired by the transmethylation hypothesis of schizophrenia, it was initially proposed that endogenous DMTs could underlie mental illness. However, it was also found in normal individuals.

DMTs are debated as being a neurotransmitter, a neuromodulator and a neurohormone, but it is unlikely that they can play any natural role at the concentrations it has been detected.

This work explores historical and recent literature regarding endogenous DMT, with a special interest in those topics that have been more controversial. It offers new insights into the role of DMT inside our bodies.

DMT is a potent psychedelic naturally produced by many plants and animals, including humans. This review discusses the relevance of DMT to mammalian physiology, including its biosynthesis, storage in synaptic vesicles, and activation of receptors like sigma-1.

How much DMT is there in the CNS?

Endogenous DMT has been detected in human cerebrospinal fluid and rat brains in vivo, but the proper measurement of DMT levels has been hindered by technical difficulties and the use of whole brain homogenates which risk diluting higher DMT concentrations that may occur locally in discrete areas.

Dean et al. (2019) measured DMT in the brains of 25 adult rats and found that it ranges from 0.05 to 1.8 nM, which is comparable to 5-HT, dopamine and norepinephrine.

DMT levels vary with time of day and age, and rise with stress. Additionally, DMT levels are doubled in the rat cortex following cardiac arrest, and human CSF and brain tissue levels are comparable to those of 5-HT, DA and NE.

Can DMT be synthesized and degraded in the CNS?

Human serum contains 12.98 g/mL of L-tryptophan, which is enough to produce relevant amounts of DMT. DMT is synthesized by aromatic L-amino acid decarboxylase (AADC), N-methylated twice by indole N-methyltransferase (INMT) and deaminated by MAO, yielding IAA.

Tryptophan’s decarboxylation

AADC is a well-known enzyme that catalyzes 5-HTP and L-DOPA decarboxylation during 5-HT/melatonin and DA/NE biosynthesis, respectively. It can also act directly on amino acids to generate trace amines, such as TA, phenylethylamine, tyramine and octopamine.

Blocking AADC activity impairs TA levels in vivo, but Tryptophan’s affinity for AADC is very low in vitro. D-neurons are found in the rat spinal cord and primate cortex, and express AADC, but neither other enzymes nor cofactors needed in 5-HT/melatonin or DA/NE synthesis.

AADC is an enzyme that plays a key role in the synthesis of neurotransmitters such as DA and 5-HT. It also generates trace amines from amino acids, like TA from Tryptophan.

INMT was first isolated from rabbit lung and described as an alkaline enzyme catalyzing the N-methylation of TA and 5-HT, yielding NMT and N-methylserotonin, respectively, and their subsequent N,N-dimethylation towards DMT and bufotenin, respectively.

Thompson and Weinshilboum (1998) confirmed that TA’s Km at recombinant rabbit INMT was 270 versus 1380 m for 5-HT. In contrast, INMT S-methylates small sulphur-containing amines in acidic pH.

After INMT’s discovery, several studies detected its activity in several tissues of vertebrate species, including human and rat brains. INMT-mediated DMT biosynthesis was proven in vivo by Mandel et al. (1978).

Thompson and colleagues sequenced rabbit and human INMT genes and found that INMT was not expressed in brain tissue, although it was well-detected in spinal cords. TA had very low affinity for recombinant, human INMT.

INMT has been detected at the protein and mRNA levels in the rhesus macaque’s retina, pineal gland and spinal cord, as well as in the rat hippocampus and peripheral tissues. Furthermore, INMT is inducible and is expressed in the mouse striatum after chronic heroin and methamphetamine administration.

Thompson et al. (1999) were misled by the technical limitations they faced, and the recombinant human wild-type INMT has a Km of 850 m and NMT has an 86 m Km.

Enzyme kinetics also depend on the maximum velocity of a reaction. When INMT acts on TA, it yields NMT, but little to no DMT, whereas when NMT is used as a substrate, INMT readily produces DMT.

Initial assays identified a small molecular weight INMT inhibitor that could be removed by dialysis within rat, rabbit and human tissues. DMT is itself an INMT inhibitor with a small weight, and a missense mutation implying the loss of this hydrogen bond is highly frequent in humans.

Endogenous DMT can activate many receptors, but its IC50 is much higher than the EC50. Therefore, DMT-dependent INMT inhibition is a self-regulatory mechanism.

Many studies have detected INMT in several species and tissues, including the human brain. INMT can yield DMT, and its kinetics have been proven in vivo.

DMT’s degradation

MAO is anchored to the outer surface of neuronal mitochondria and mediates the oxidative deamination of monoamines. It is highly efficient and contributes to the rise of DMT levels in the brain in response to ischaemia.

MTHBC and THBC are -carbolines that are formed in the rat brain in vitro and in vivo through DMT cyclization. MTHBC is a selective and potent MAOI with an IC50 of 3 M. DMT is also a potent, but short-acting, selective inhibitor of MAO in the rat brain.

All the metabolites of DMT mentioned above are shown in Figure 3. MAO inhibition promotes a metabolic shift from the highly efficient MAO metabolism towards the less efficient DMT-NO and NMT formation.

Can DMT enter presynaptic neurons and be stored inside synaptic vesicles?

DMT enters presynaptic terminals through the serotonin transporter (SERT) or the NE transporter (NET), and promotes 5-HT release through reverse SERT transport. It has been proposed that DMT may act as a methylenedioxymethamphetamine (MDMA)-like 5-HT releaser in vivo.

Neurotransmitters enter neurons through vesicular storage mechanisms, which are regulated by ATP/H+ and vMAT2. Cozzi et al. (2009) predicted that DMT is a substrate for SERT and vMAT2, but the method was inconsistent.

DMT has been detected within the vesicle-associated subfractions of lysed synaptosomes, and it can be mobilized from the cytoplasm-associated subfraction to the vesicle-associated subfraction in the presence of Mg2+ and ATP. This strongly suggests a vMAT2-mediated transport.

DMT can enter neurons through SERT and NET, and it is predicted to enter synaptic vesicles through vMAT2.

What receptors can DMT activate?

DMT is a promiscuous compound that binds to several 5-HTRs, DA receptors, adrenergic receptors, and imidazoline receptors. It is an agonist at 5-HTRs 1A, 2A, 2B, and 2C, as well as TAAR 1 and sigma 1 receptor.

DMT produces and retains 5-HT2A in the cerebral cortex, and this activation is believed to be at the heart of the complex subjective effects elicited by psychedelics.

Psychedelics release glutamate and induce an excitatory response that depends on synaptic Glu spillover and NMDA receptor activation.

DMT is a partial agonist for the 5-HT2A signalling pathway, activating PLC, IP3, Ca2+ release and PKC. This pathway is subject to biased agonism, which is an essential aspect of psychedelics’ pharmacodynamics.

5-HT2A couples to phospholipase A2, which promotes arachidonic acid cleavage and subsequently activating the mitogen-activated protein kinase (MAPK) cascade. DMTs have a preference for activating PLA2 over PLC cascade, whereas non-dimethylated TAs have no preference for any specific pathway.

The biased activation of 5-HT2A leads to psychedelic-specific transcriptome fingerprints in HEK293 cells and the mouse somatosensory cortex, and induces the formation of new, fully functional synapses in layer V pyramidal neurons of the rat prefrontal cortex.

DMT-biased activation of 5-HT2A has an especially relevant feature: 5-HT2A does not become desensitized to DMT over time, and PKC’s activity is not effectively recruited by DMT.

DMT activates 5-HT2A at nM concentrations, and 5-HT2A is subject to biased agonism. DMT preferably triggers PLA2/AA/MAPK over the PLC pathway, and 5-HT2A does not desensitize to DMT.

Other 5-HTRs

The serotonergic system is a complex system of neurotransmitters, and DMT binds to several 5-HTRs. 5-HT1A and 5-HT2C counteract 5-HT2A excitatory effects, and blocking 5-HT1A exacerbates DMT’s subjective effects.

DMT is a potent 5-HT2C activator, which is located at the choroid plexus. It can modulate several neuronal processes, like electrophysiology and plasticity, as well as acting as a medium for volume transmission.

Endogenous DMT activates 5-HT1D and 5-HT7 receptors, which are also present in regions where DMT has been detected and/or can be produced. 5-HT1D and 5-HT7 receptors are also related to certain types of pain, and 5-HT7 activates the MAPK cascade, mTOR and other intracellular effectors related to development and neuroplasticity.

DMT can activate several 5-HTRs, including 5-HT1A, 5-HT2C, and 5-HT7, which are involved in processes like migraine pain, development, and neuroplasticity.

DMT binds to sigma-2 (1) and -1 receptors and is located at moderate levels in several CNS structures, as well as in peripheral organs. It is also enriched in the C-terminals of some spinal cord motoneurons.

The 1 protein is a chaperone that regulates several proteins in the endoplasmic reticulum, including GPCRs, ion channels and protein kinases, thereby controlling cellular excitability, plasticity, apoptosis and inflammatory processes.

DMT treatment downregulates apoptosis protease-activating factor-1 (APAF-1) and inflammatory cytokines and upregulates brain-derived neurotrophic factor (BDNF) and the anti-inflammatory IL-10 in a rat model of focal brain ischaemic injury and in vitro. DMT also induces cellular differentiation in the hippocampus of adult mice via 1.

Endogenous DMT’s binding affinity for 1 has been argued against, but storage inside synaptic vesicles allows DMT to reach local concentrations that are more than enough to activate 1.

DMT is a natural agonist of a protein called 1. It causes the adult genesis of fully differentiated neurons, oligodendrocytes and astrocytes in mice hippocampi, which is accompanied by improvements in memory tasks.

TAARs

TAARs are GPCRs that are preferentially activated by trace amines over neurotransmitters. DMT can effectively activate rodent TAAR1 but is inactive at human TAAR1.

DMT could be a ligand at TAAR6, a gene that preferentially binds tertiary amines. The functions of TAAR6 are unknown, but some polymorphic variants have been related to psychiatric disorders.

How does DMT affect electrophysiology?

Single-neuron recordings have demonstrated that DMT modulates neuronal firing. DMT exerts mainly inhibitory, although sometimes excitatory effects, and facilitates firing of the cat’s septal nuclei and the CA1 hippocampal region.

DMT has inhibitory effects on the optic nerve’s spontaneous activity and phasic response to light stimulus in cats in vivo. The lower dose, 1 mg/kg intravenous, already saturated the effect.

DMT-derived synaptogenesis increases the amplitude and frequency of spontaneous excitatory postsynaptic currents in layer V pyramidal neurons from the rat’s prefrontal cortex.

DMT and ayahuasca induce pronounced and complex effects on cortical activity broadly. DMT enhances bottom-up, feed-forward, sensory-associated information transfer over top-down feedback projections associated with the interpretation of sensory input, based on previous experience and expectations.

DMT can exert both excitatory and inhibitory effects on single neurons, promote plasticity, and alter cortical EEG oscillations, which correlate with complex visions.

Discussion: What is the relevance of endogenous DMT?

DMT has been considered a trace amine, but recent studies have shown that it can be found at levels that are about half of 5-HT’s. It is also completely inactive at human TAAR1 at 10 mM.

Endogenous DMT has been detected in the brain robustly, and it can be cleared out from a synapse through monoamine transporters. It activates several receptors in a specific way, triggering specific postsynaptic responses. To prove that DMT is a neurotransmitter, it must be proven that it is released following presynaptic depolarization in a calcium-dependent manner. Reserpine can be used to block the transporter, which should impair DMT’s release after depolarization.

INMT produces DMT from injected NMT in vivo, but its relevance to endogenous DMT levels has not been tested. Therefore, rats must be treated with INMT inhibitors and DMT levels must be measured.

DMT is active through 5-HT2A and other 5-HTRs, but it is less potent but has more intrinsic activity than 5-HT-triggering PLA2 in HEK293 systems, and unlike 5-HT, it does not desensitize the receptor over time.

Endogenous DMT and 5-HT do not necessarily have to compete at the same synapse. DMT could be synthesized in neurons that do not contain 5-HT, like D-neurons, which appear at P12, peak at P30 and then fall to mature levels.

D-neurons in the central canal of the spinal cord sense and secrete molecules into the CSF. These neurons may also express INMT, which may explain the relationship between DMT and CSF secretion.

Endogenous DMT plays an active role through 5-HT2A in vivo, and inhibiting INMT should decrease cortical activation after a local perturbation by magnetic transcranial stimulation, as well as subjective and behavioural effects such as dizziness and tiredness.

Since Strassman’s (2001) book, the pineal origin of endogenous DMT has been a hot topic in countercultural pseudoscience. However, several authors have linked endogenous DMT’s rise in stressful situations to near-death experiences, and the pineal gland might divert Tryptophan towards melatonin production, rather than producing DMT.

The pineal gland is known to depend on wake – sleep cycles, and DMT’s effects on delta and theta waves are comparable to EEG changes during rapid eye movement (REM) sleep. Furthermore, eyes-closed DMT is associated with striking changes in cortical dynamics, which are remarkably similar to those observed during actual eyes-open visual stimulation.

The adrenal medulla is the only tissue that co-expresses AADC and INMT transcripts, and DMT can act as a hormone. It also binds to stress-related receptors and increases cortisol levels, although the latter can be attributed to the stress that high-dose DMT experiences can trigger.

Hypoxia induces hyperventilation, which leads to hypocapnia (low blood CO2) and subsequent alkalosis, especially in the lungs. The super-alkaline isoform of INMT is more active than the alkaline isoform, which can combine and add up to MAO impairment causing DMT accumulation.

To test the hypothesis that DMT plays a role in pregnancy, experiments must measure blood levels of DMT and its metabolites in animals after inoculating hypoxic stress, either by cardiac arrest or asphyxia. Additionally, animals must be treated with INMT inhibitors.

Endogenous DMT is not relevant as a trace amine-like neuromodulator, but rather as a putative neurotransmitter, acting through serotonergic receptors. DMT has its own biological meaning, involving neuroplasticity, tissue protection, circadian regulation, an unusual way of exciting postsynaptic neurons, broad cortical dynamics and perhaps CSF secretion and sleep cycles.

Conclusion

After 60 years, we are still unsure about endogenous DMT’s role in mammalian physiology. However, we can improve our knowledge about psychedelic drugs by understanding the natural mechanisms with which they interact.

Endogenous DMT offers new perspectives in many fields, including consciousness and dreams. Funders should support efforts to dispel doubts about DMT’s roles in mammalian physiology.