Biosynthesis and Extracellular Concentrations of N,N-dimethyltryptamine (DMT) in Mammalian Brain

Abstract

N,N-dimethyltryptamine (DMT), a psychedelic compound identified endogenously in mammals, is biosynthesized by aromatic-L-amino acid decarboxylase (AADC) and indolethylamine-N-methyltransferase (INMT). Whether DMT is biosynthesized in the mammalian brain is unknown. We investigated brain expression of INMT transcript in rats and humans, co-expression of INMT and AADC mRNA in rat brain and periphery, and brain concentrations of DMT in rats. INMT transcripts were identified in the cerebral cortex, pineal gland, and choroid plexus of both rats and humans via in situ hybridization. Notably, INMT mRNA was colocalized with AADC transcript in rat brain tissues, in contrast to rat peripheral tissues where there existed little overlapping expression of INMT with AADC transcripts. Additionally, extracellular concentrations of DMT in the cerebral cortex of normal behaving rats, with or without the pineal gland, were similar to those of canonical monoamine neurotransmitters including serotonin. A significant increase of DMT levels in the rat visual cortex was observed following induction of experimental cardiac arrest, a finding independent of an intact pineal gland. These results show for the first time that the rat brain is capable of synthesizing and releasing DMT at concentrations comparable to known monoamine neurotransmitters and raise the possibility that this phenomenon may occur similarly in human brains.

Figure 1

INMT mRNA is expressed in the brain. Images show results of in situ hybridization of INMT mRNA probes on rat (first column) or human (second column) brain FFPE sections. Where applicable, small black and white arrows denote exemplary cells positive and negative for INMT mRNA expression, respectively. Insets are magnifications of these cells. INMT mRNA expression is seen as pink punctate dots in cortical cells within rat visual cortex (A), in cells of the human medial frontal cerebral cortex (B), in pinealocytes within rat (C) and human (D) pineal gland, and in select ependymal cells of rat (E) and human (F) choroid plexus. In this and the additional in situ panels in Figs 2 and 3, nuclear counterstain (blue/gray staining in all images) identifies all cells/nuclei = 50% hematoxylin. All large panel images = 100x oil magnification. Histology sections are all coronal.

Figure 2

INMT mRNA is colocalized with AADC mRNA in the brain. Images show results of in situ hybridization on rat brain FFPE sections using INMT (green) and AADC (pink) mRNA probes. (A) INMT and AADC mRNA are colocalized in cells of the visual cortex (panel A), in cells of the hippocampus (B), in the pinealocytes (C), and in the ependymal cells of the choroid plexus (D).

Figure 3

INMT and AADC mRNA expression is largely non-overlapping in rat peripheral tissues. Two representative regions (Area A and Area B) are shown for each tissue type. Insets show magnifications of areas of the main images wherein cells are positive (black arrows) or negative (white arrows) for either INMT (A,D–G), AADC (C), or both INMT and AADC (B) transcripts. INMT mRNA (green punctate dots) was highly expressed in the adrenal cortex (A), the renal tubules of the kidney medulla (D), the lung (E,F), and the heart (G). INMT transcripts were not uniformly distributed in these tissues, as INMT is found at very low levels in the adrenal medulla (B), renal cortex (C), and select areas of the heart (H). AADC mRNA (pink punctate dots) was expressed in adrenal medulla (B), where it is colocalized with INMT mRNA in select cells (black arrows in B), and in renal cortex (C) where it is not found together with INMT in the same cells. In the lung (E,F) and heart (G,H), AADC mRNA was nearly absent.

Figure 4

DMT is present in the brain at concentrations comparable to known monoamine neurotransmitters. (A) DMT levels in pineal-intact rats (n = 25) rose following cardiac arrest. DMT, sampled 0.5 hr before cardiac arrest shown (retention time = 7.2 minutes) on a representative raw chromatogram (a, blue tracing), ranged between 0.05–1.8 nM (averaged 0.56 nM; b, blue dots) during baseline. DMT levels increased within one hr of cardiac arrest (a, red tracing), averaging 1.35 nM with maximum 5.01 nM (b, red dots). DMT increase was significant (***p = 0.00027; mean of differences = 0.80; SD = 0.93; 95% CI = 0.41 to 1.18; t(24) = −4.26). (B) DMT levels persisted in brains of pinealectomized rats (n = 11). DMT levels sampled 0.5–1 hr following cardiac arrest shown (retention time = 7.5 minutes) on a representative raw chromatogram (a, blue tracing), ranged between 0.25–2.2 nM (averaged 1.02 nM; b, blue dots) during baseline. DMT levels increased within one hr of cardiac arrest (a, red tracing), averaging 1.83 nM with maximum of 5.11 nM (b, red dots). DMT increase was significant (*p = 0.034; mean of differences = 0.81; SD = 1.10; 95% CI = 0.075 to 1.55; t(10) = 2.45). (C) There was no difference in brain concentrations of DMT at baseline (a) between rats without (−pineal; mean = 1.02 nM; SD, blue bar = 0.63) and with ( + pineal; mean = 0.56 nM; SD, blue bar = 0.49) the pineal gland (p = 0.05, Welch’s unpaired t-test). No difference between brain concentrations of DMT following cardiac arrest (b) without (− pineal; mean = 1.83 nM; SD, red bar = 1.38) and with (+pineal; mean = 1.35 nM; SD, red bar = 1.27) the pineal gland was detected (p = 0.34, unpaired t-test with Welch’s correction). Baseline levels of DMT were about half those of 5-HT levels (c; *p = 0.026; mean of differences = 1.08; SD = 1.37; 95% CI = 0.16 to 2.0; t(10) = −2.62) in the visual cortex of pinealectomized rats (mean DMT = 1.02 nM; SD, blue bar = 0.63; mean 5-HT = 2.10 nM; SD, blue bar = 1.67).