Tryptamine may couple dopaminergic and serotonergic transmission in the brain

Maternal Inflammation with Elevated Kynurenine Metabolites Is Related to the Risk of Abnormal Brain Development and Behavioral Changes in Autism Spectrum Disorder

Several studies show that genetic and environmental factors contribute to the onset and progression of neurodevelopmental disorders. Maternal immune activation (MIA) during gestation is considered one of the major environmental factors driving this process. The kynurenine pathway (KP) is a major route of the essential amino acid L-tryptophan (Trp) catabolism in mammalian cells. Activation of the KP following neuro-inflammation can generate various endogenous neuroactive metabolites that may impact brain functions and behaviors. Additionally, neurotoxic metabolites and excitotoxicity cause long-term changes in the trophic support, glutamatergic system, and synaptic function following KP activation. Therefore, investigating the role of KP metabolites during neurodevelopment will likely promote further understanding of additional pathophysiology of neurodevelopmental disorders, including autism spectrum disorder (ASD). In this review, we describe the changes in KP metabolism in the brain during pregnancy and represent how maternal inflammation and genetic factors influence the KP during development. We overview the patients with ASD clinical data and animal models designed to verify the role of perinatal KP elevation in long-lasting biochemical, neuropathological, and behavioral deficits later in life. Our review will help shed light on new therapeutic strategies and interventions targeting the KP for neurodevelopmental disorders.

The endogenous substrates of brain CYP2D

CYP2D6, one of the major cytochrome P450 isoforms present in the human brain, is associated with the incidence and prevalence of central nervous system (CNS) diseases. Human CYP2D6 and rat CYP2D are involved in the metabolism of various neurotransmitters and neurosteroids. Brain CYP2D can be regulated by endogenous steroids, including sex hormones. The alteration of CYP2D-mediated metabolism induced by endogenous steroids may cause changes in sensitivity to environmental and industrial toxins and carcinogens as well as physiological and pathophysiological processes controlled by biologically active compounds. This review summarizes the current knowledge regarding the distribution, endogenous substrates, and regulation of brain CYP2D.

The mysterious trace amines: protean neuromodulators of synaptic transmission in mammalian brain

The trace amines are a structurally related group of amines and their isomers synthesized in mammalian brain and peripheral nervous tissues. They are closely associated metabolically with the dopamine, noradrenaline and serotonin neurotransmitter systems in mammalian brain. Like dopamine, noradrenaline and serotonin the trace amines have been implicated in a vast array of human disorders of affect and cognition. The trace amines are unique as they are present in trace concentrations, exhibit high rates of metabolism and are distributed heterogeneously in mammalian brain. While some are synthesized in their parent amine neurotransmitter systems, there is also evidence to suggest other trace amines may comprise their own independent neurotransmitter systems. A substantial body of evidence suggests that the trace amines may play very significant roles in the coordination of biogenic amine-based synaptic physiology. At high concentrations, they have well-characterized presynaptic "amphetamine-like" effects on catecholamine and indolamine release, reuptake and biosynthesis; at lower concentrations, they possess postsynaptic modulatory effects that potentiate the activity of other neurotransmitters, particularly dopamine and serotonin. The trace amines also possess electrophysiological effects that are in opposition to these neurotransmitters, indicating to some researchers the existence of receptors specific for the trace amines. While binding sites or receptors for a few of the trace amines have been advanced, the absence of cloned receptor protein has impeded significant development of their detailed mechanistic roles in the coordination of catecholamine and indolamine synaptic physiology. The recent discovery and characterization of a family of mammalian G protein-coupled receptors responsive to trace amines such as beta-phenylethylamine, tyramine, and octopamine, including socially ingested psychotropic drugs such as amphetamine, 3,4-methylenedioxymethamphetamine, N,N-dimethyltryptamine, and lysergic acid diethylamide, have revitalized the field of scientific studies investigating trace amine synaptic physiology, and its association with major human disorders of affect and cognition.

Current concepts in neuroendocrine cancer metabolism

Neuroendocrine (NE) cancers occur in multiple anatomic locations and range in prognosis from indolent to aggressive. In addition, adenocarcinomas can express gene products associated with NE cells, referred to as NE differentiation (NED), which correlates with poor prognosis and aggressive disease. Several metabolites and peptides produced by NE cells have been discovered that engage in cellular signaling and have autocrine and paracrine effects on cancer cell proliferation. This review focuses on the current knowledge of small molecule metabolism in NE cancers involving the synthesis of biogenic amine, polyamine, and amino acid neurotransmitters. Systems biology-directed approaches to NE cancer metabolism using gene expression profiling, liquid chromatography/mass spectrometry (LC/MS) and nuclear magnetic resonance (NMR) are also discussed. Furthermore, knowledge of metabolic and signaling pathways in NE cancers has led to the successful implementation of therapeutic regimens in cell culture and animal models of NE carcinogenesis.

Synergic effects of tryptamine and octopamine on ophiuroid luminescence(Echinodermata)

In ophiuroids, bioluminescence is under nervous control. Previous studies have shown that acetylcholine is the main neurotransmitter triggering light emission in Amphipholis squamata and Amphiura filiformis. By contrast, none of the neurotransmitters tested so far induced luminescence in two other ophiuroid species, Ophiopsila aranea and Ophiopsila californica. The aim of this work was thus to investigate the putative involvement of two biogenic amines, tryptamine and octopamine, in light emission of three ophiuroid species. A. filiformis responds to both tryptamine and octopamine, mainly on its arm segments, while O. californica only responds to tryptamine stimulation. By contrast,tryptamine and octopamine do not seem to be involved in O. aranealuminescence control since none of these substances induced light emission in this species. The synergic effects of several other drugs with tryptamine and octopamine were also tested.

Possible parent-of-origin effect of Dopa decarboxylase in susceptibility to bipolar affective disorder

Dopa decarboxylase (DDC) catalyses the synthesis of both dopamine and serotonin as well as trace amines suggested to possess neuromodulating capabilities. We have previously reported evidence suggesting an association between DDC and bipolar affective disorder (BPAD) [Børglum et al., 1999]. To further investigate the possible role of DDC in BPAD, we analyzed a 1- and a 4-bp deletion variant-both of putative functional significance-in two new samples: a case-control sample with 140 cases and 204 controls, and 100 case-parents trios. We also tested for association in subjects with familial disease in both the new and the previously investigated samples. The previously reported association was not replicated in either of the new samples. However, a preponderance of the 1-bp deletion was increased by analysis of the familial cases separately for all case-control samples investigated, indicating a possible association with familial disease (combined analysis, P = 0.02). In the trio sample, a preferential paternal transmission of the 4-bp deletion was observed (P = 0.006). DDC is located next to the imprinted gene GRB10, which is expressed specifically from the paternal allele in fetal brains. Increased transmission of paternal DDC alleles has also been suggested in attention deficit hyperactivity disorder. We suggest that DDC might confer susceptibility to BPAD predominantly when paternally transmitted.

Cellular localization and regional distribution of CYP2D6 mRNA and protein expression in human brain

The cytochrome P4502D6 (CYP2D6) is involved in the biotransformation of many drugs which predominantly act in the central nervous system (CNS), including opioids, various psychotrophic drugs and neurotoxins. Until now, however, only controversial information is available regarding the presence of CYP2D6 in CNS. In this study, the regional and cellular expression of CYP2D6 transcripts and proteins in postmortem brain tissues of three individuals was analysed. A combination of in-situ hybridization coupled with immunohistochemistry on adjacent sections allowed simultaneous detection of CYP2D6 mRNA and protein. However, discrepancies existed in the results such that the mRNA was more widely distributed in the brain areas analysed compared to the protein. Neuronal cells, as well as glial cells, showed labelling for mRNA in brain regions such as the neocortex, caudate nucleus, putamen, globus pallidus, hippocampus, hypothalamus, thalamus, substantia nigra and cerebellum. In contrast, CYP2D6 protein was primarily localized in large principal neurons such as pyramidal cells of the cortex, pyramidal cells of the hippocampus, and Purkinje cells of the cerebellum. In glial cells, CYP2D6 protein was absent. These results provide clear evidence of CYP2D6 expression in certain regions of the CNS and may indicate the role CYP2D6 plays in a number of drug interactions that are of potential clinical importance for neurological diseases.

Tryptamine, serotonin and catecholamines: an immunocytochemical study in the central nervous system

Tryptamine, a serotonin-related indolamine, could be involved in the modulation of catecholaminergic and serotoninergic systems interaction. Despite previous reports on this topic, the morphological relationship among these systems is not well described. We studied the interaction among serotoninergic and catecholaminergic with tryptaminergic systems by double immunostaining at the level of light microscopy. Mesencephalic rat brain sections treated according to the Schiff quenching method were double immunostained using peroxidase and fluorescein labeled antibodies. Primary antibodies to anti-tryptophan hydroxylase (TrpOH), anit-tyrosine hydroxylase (TH) and anti-tryptamine (T) were used to demonstrate serotoninergic, catecholaminergic and tryptaminergic neurons respectively. A morphometric study was performed in order to analyze the different morphological characteristics of each system. The results showed that (i) T+ and TrpOH+ neurons are localized in the same areas but their morphology is significantly different. Moreover morphometric parameters of T+ neurons were significantly different from those TrpOH+ or TH+ neurons; (ii) The number of TrpOH+ neurons was larger than T+ neurons; (iii) T+ neurons were dominant in the lateral dorsal raphe nucleus. TrpOH+ neurons were more numerous in the central area of the dorsal raphe nucleus; (iv) Coexpression of TrpOH and T was demonstrated in the somata of dorsal raphe nucleus neurons; (v) TrpOH+ neurons from raphe nuclei and TH+ neurons from substantia nigra are contacted by T+ fibres. The present morphological evidence supports a functional relationship among these three aminergic systems.

Aromatic L-amino acid decarboxylase: a neglected and misunderstood enzyme

Classically, aromatic L-amino acid decarboxylase (AADC) has been regarded as an unregulated, rather uninteresting enzyme. In this review, we describe advances made during the past 10 years, demonstrating that AADC is regulated both pre- and post-translation. The significance of such regulatory mechanisms is poorly understood at present, but the presence of tissue specific control of expression raises the real possibility of AADC being involved in processes other than neuro-transmitter synthesis. We further discuss clinical and physiological situations in which such regulatory mechanisms may be important, including the intriguing possibility of AADC gene regulation being linked to that of factors thought to have a role in apoptosis and its prevention.

Aromatic L-amino acid decarboxylase: biological characterization and functional role

1. Aromatic L-amino acid decarboxylase is the enzyme responsible for the decarboxylation step in both the catecholamine and the indolamine synthetic pathways. Immunological and molecular biological studies suggest that it is a single enzyme with one catalytic site but with different locations for attachment of the substrates. The enzyme is widely distributed in the brain and in peripheral tissues. 2. Recent investigations have shown that the enzyme is regulated by short term mechanisms that may involve activation of adenyl cyclase or protein kinase C. In addition, a long-term mechanism of activation by altered gene expression has also been suggested.

Neurochemical and some related psychopharmacological aspects of Tourette's syndrome: an update

Neurochemical investigations of Tourette's syndrome (TS) suggest that the symptoms of this disorder may be the result of an imbalance among several neurotransmitter and/or neuromodulator systems. Neurochemicals which have been studied included: catecholamines; acetylcholine; tryptophan and its metabolites; the amino acids γ-aminobutyric acid (GABA), glutamate, phenylalanine and p-tyrosine; trace amines; opioid peptides; cyclic AMP and androgenic hormones. A suitable animal model of TS would do much to advance our understanding of this disorder, and there are some interesting recent developments in this regard.

The trace amines and their acidic metabolites in depression--an overview

1. Investigations of the role of the trace amines (phenylethylamine, tryptamine, m- and p-tyramine) and their acidic metabolites (phenylacetic, indoleacetic, m- and p-hydroxyphenylacetic acids) in depression are reviewed. 2. The evidence for the phenylethylamine hypothesis of depression is mixed. 3. Reduced phenylacetic acid levels in urine, plasma and CSF and changes in those levels during treatment with antidepressants show potential as state markers for depression. 4. Impaired p-tyramine conjugation following a tyramine challenge may be a good trait marker for depression.

Effects of low- and high-dose tranylcypromine on [3H]tryptamine binding sites in the rat hippocampus and striatum

Chronic studies were initiated in rats to determine the effects of high- and low-dose tranylcypromine (TCP) on [3H]tryptamine (3H-T) binding sites. Male Sprague-Dawley rats were administered TCP (0.5 or 2.5 mg/kg/day) or vehicle (distilled water) for 4, 10 or 28 days via Alzet minipumps. After decapitation, the hippocampus and striatum were used to prepare membrane fragments for single point 3H-T binding. Hippocampal 3H-T binding was reduced after 10 and 28 days with the low dose and after 4, 10 and 28 days with the high dose. Striatal 3H-T binding was reduced by both doses at all time intervals. The high dose resulted in a significantly greater reduction in striatal 3H-T binding than did the low-dose after 4, 10, and 28 days. These results suggest that a more rapid reduction of 3H-T binding in the hippocampus and/or a greater reduction of 3H-T binding in the striatum by high-dose TCP than by low-dose TCP may be contributing factors in the reported efficacy of the former in refractory depression.

Effects of tryptamine on plasma glucagon levels in mice

Our previous study indicated that tryptamine induces a dose-related increase in plasma glucagon levels of mice and that this effect is mediated by the peripheral serotonin2 (5-HT2) receptor. The present paper further investigated the involvement of serotonergic and catecholaminergic systems in hyperglucagonemia elicited by tryptamine. An inhibitor of 5-HT synthesis, p-chlorophenylalanine, did not affect tryptamine-induced increases in plasma glucagon levels. Tryptamine-induced hyperglucagonemia was not inhibited by adrenalectomy or by an inhibition of catecholamine synthesis by alpha-methyl-p-tyrosine. These findings indicate that tryptamine-induced hyperglucagonemia is elicited by its direct activation of 5-HT2 receptors and is not mediated by levels of endogenous 5-HT and catecholamines. The results further suggest that the peripheral 5-HT2 receptor has a possible role in the release of glucagon.

Comparison among the effects of nifedipine, nimodipine and nisoldipine on the brain biogenic amines of normal or haloperidol treated rats

1. Previous studies have shown that dihydropyridine calcium antagonists affected the metabolism of the brain biogenic amines more extensively than the non-dihydropyridine ones. The effects of three dihydropyridines (nifedipine, nimodipine or nisoldipine, 0.05-0.10 mmol/kg i.p.) on brain monoamines and metabolites have been evaluated on both normal and haloperidol treated rats. 2. The small changes induced by the drugs on normal rats were markedly increased in the haloperidol treated rats, clearly showing an inhibition of dopaminergic systems. 3. These effects were selective for the striatum and cerebral cortex, with nisoldipine being more effective than nimodipine or nifedipine. 4. The effects of the three drugs on serotonergic systems were similar or more marked on haloperidol treated rats than on normal rats. These effects, showing activation of serotonergic systems, displayed some regional specificity and different potency of the three compounds. Nisoldipine also appeared to be the most effective drug on serotonergic systems. 5. In conclusion nisoldipine, as well as other calcium antagonists, may have a place in the treatment of some mood disorders.

Trace amines and Tourette's syndrome

There has been considerable interest in recent years in possible neurochemical abnormalities in Tourette's Syndrome (TS). In studies combining neuropsychological and neurochemical measurements, we have investigated the possible roles of trace amines in this disorder. Urinary levels of free beta-phenylethylamine (PEA) and plasma levels of its precursor amino acid phenylalanine were decreased in TS patients when compared to values in normal children. These urinary PEA levels in TS patients were inversely related to several scores from the Tourette's Syndrome Global Scale (TSGS). Further investigation of the group of subjects with low urinary PEA indicated that they also had low levels of MHPG, normetanephrine, 5-HT and m- and p-tyramine. Patients with low PEA were also compared on an extensive battery of neuropsychological measures and observed to perform significantly worse than TS patients with normal urinary PEA levels. Biochemical measurements also suggest a possible abnormality in tryptamine turnover in TS since urinary levels of indole-3-acetic acid (IAA; the acid metabolite of tryptamine) are significantly lower in TS patients than in normal controls.

Identification of tryptamine and tryptamine-serotonin neurons in the rat dorsal raphe nuclei using specific antibodies

Previous studies have shown that tryptamine (T) is closely associated with the serotonergic system but have failed to determine whether neurons containing only T exist in the raphe nuclei (RN) and/or if T colocalizes with serotonin (5-HT). The use of rabbit-conjugated T antisera (TAS) and rat-conjugated 5-HT antisera (5-HTAS) in a double-labelling technique has made the study of the colocalization of T and 5-HT in neurons of the rat dorsal RN (DR) possible. Slices taken from the same zone of the DR were treated following four different procedures of double-immunolabelling: DAB coloration for the rabbit TAS and DAB-nickel coloration for the rat 5-HTAS either first or second; or DAB coloration for the 5-HTAS and DAB-nickel coloration for the TAS first or second. Control sections were treated according to a single immunoperoxidase staining in the same zone. The immunolabelled neurons were computed using the Biocom 200 program at the same magnification. The results of this double-immunolabelling show that three different cell types exist in the rat DR: (1) T-only-containing neurons, (2) 5-HT-only-containing neurons displaying either homogeneous DAB or homogeneous DAB-nickel, and (3) neurons where T colocalizes with 5-HT (T-positive/5-HT-positive neurons) displaying heterogeneous DAB-nickel coloration. The results were identical whatever the procedure performed or the order of the DAB-nickel revelation.(ABSTRACT TRUNCATED AT 250 WORDS)

Tryptamine: a metabolite of tryptophan implicated in various neuropsychiatric disorders

Although early interest in the biomedical relevance of tryptamine has waned in recent years, it is clear from the above discussion that the study of tryptamine is worthy of serious consideration as a factor in neuropsychiatric disorders. The study of [3H]-tryptamine binding sites indicates an adaptive responsiveness characteristic of functional receptors. The question raised by Jones (1982d) on whether tryptamine is acting centrally as a neurotransmitter or a neuromodulator still remains mostly unanswered, although the evidence cited within this review strongly suggests a modulatory role for this neuroactive amine (see also Juorio and Paterson, 1990). The synthesis and degradative pathways of tryptamine, as well as the intricate neurochemical and behavioral consequences of altering these pathways, are now more fully understood. It is not yet clear what the role of tryptamine is under normal physiological [homeostatic] conditions, however, its role during pathological conditions such as mental and physical stress, hepatic dysfunction and other disorders of metabolism (i.e. electrolyte imbalance, increased precursor availability, enzyme induction or alterations in enzyme co-factor availability) may be quite subtle, perhaps accounting for various sequelae hitherto considered idiopathic. The evidence for a primary role for tryptamine in the etiology of mental or neurological diseases is still relatively poor, although the observations that endogenous concentrations of tryptamine are particularly susceptible to pharmacological as well as physiological manipulations serve to reinforce the proposition that this indoleamine is not simply a metabolic accident but rather a neuroactive compound in its own right. Finally, one might wonder what proportion of the data attributed to modifications of 5-HT metabolism might, in fact, involve unrecognized changes in the concentrations of other neuroactive metabolites of tryptophan such as tryptamine.

Gene expression of aromatic L-amino acid decarboxylase in cultured rat glial cells

Northern blot hybridization was performed to detect aromatic L-amino acid decarboxylase (AADC) mRNA in primary cultures of astrocytes and C6 glioma cells. The cDNA probe for rat AADC was generated by reverse transcription from rat adrenal gland total RNA and was amplified by the polymerase chain reaction method. AADC mRNA from cultured astrocytes and C6 glioma cells was present as a single band, 2.2 kbp in size, that comigrated with the RNA from rat kidney. Western immunoblot showed a single protein band at 52 kDa for AADC enzyme protein. These findings demonstrate that AADC is expressed in rat glial cells.

Urinary indoleamines in Tourette syndrome patients with obsessive-compulsive characteristics

Tourette syndrome patients with high levels of obsessive-compulsive symptoms were compared with patients without these symptoms on urinary measures of serotonin and its major metabolite, 5-hydroxyindoleacetic acid (5HIAA). Both groups were compared with normal controls, and it was hypothesized that patients with obsessive-compulsive symptoms would have lower levels of serotonin. Both groups of Tourette syndrome patients had lower levels than controls, but there was no difference between them. Obsessive symptoms were related to higher levels of 5HIAA and to a higher turnover of serotonin.

The metabolic fate of infused L-tryptophan in men: possible clinical implications of the accumulation of circulating tryptophan and tryptophan metabolites

L-Tryptophan (Trp) was widely used as a natural tool for the support of serotonin-mediated brain functions and as a challenge probe for the assessment of serotonin-mediated neuroendocrine responses. The metabolic fate of the administered Trp and the kinetics of the accumulation of Trp metabolites in the circulation, however, have never thoroughly been investigated. This study describes the time- and dose-dependent alterations in the plasma levels of various Trp metabolites and large neutral amino acids after the infusion of Trp to healthy young men (1, 3 and 5 g; placebo-controlled, double-blind, cross-over study during day- and night-time). The major Trp metabolites (kynurenine, indole acetic acid and indole lactic acid) in plasma increased dose-dependently but rather slowly after Trp administration to reach their maximal plasma levels (up to 10-fold after the 5 g dose) at about 3 h p.i., and remained at an elevated level (about 5-fold) for up to 8 h. N-acetyl-Trp and 5-hydroxy-Trp rose rapidly and massively after Trp infusions, at the 5 g dose more than 200- and 20-fold, respectively, and declined rapidly to about 5-fold baseline levels within 2 h. Whole blood serotonin levels were almost unaffected by the Trp infusions. A rather slow increase of 5-hydroxyindole acetic acid was seen, reaching maximum values (3-fold at the 5 g dose) at about 2 h after the infusion of Trp. Additionally, a dose-dependent rise of circulating melatonin was observed after L-Trp infusions. The administration of L-Trp caused a depletion of the concentrations of the other large neutral amino acids and a dose dependent decrease of the ratio between plasma tyrosine and the sum of the plasma concentrations of the other large neutral amino acids. Apparently, none of the existing pathways of peripheral Trp metabolism is saturated by its substrate, Trp in men. At least some of the central effects reported after L-Trp administration may be mediated by the Trp-stimulated formation of neuroactive metabolites or by the decreased availability of tyrosine for catecholamine synthesis.

Phenylethylaminergic modulation of catecholaminergic neurotransmission

1. PE is present in the brain in tiny quantities; it is heterogeneously distributed and present in synaptosomes. 2. It is synthesised from phenylalanine by L-AADC and oxidatively deaminated by MAO-B. Its turnover is remarkably fast. 3. Its concentration, particularly in the caudate nucleus, is affected by MAO inhibition (increased), lesion of the Substantia nigra (decreased), amine depletion (increased) and antipsychotic drugs (increased). 4. When iontophoresed (or injected) it amplifies the effects of DA and NA (and their agonists) but is without effect on other neurotransmitters. 5. It is suggested that it acts postsynaptically as a neuromodulator of catecholaminergic neurotransmission and that it is involved in the mechanism of action of Deprenyl; it is also suggested that it, or its principal metabolite PAA, may be involved in the aetiology of schizophrenia, depression and aggression as well as perhaps in other neuropsychiatric conditions.