Binding sites for brain trace amines

Food sources and biomolecular targets of tyramine

Tyramine is a biogenic trace amine that is generated via the decarboxylation of the amino acid tyrosine. At pico- to nanomolar concentrations, it can influence a multitude of physiological mechanisms, exhibiting neuromodulatory properties as well as cardiovascular and immunological effects. In humans, the diet is the primary source of physiologically relevant tyramine concentrations, which are influenced by a large number of intrinsic and extrinsic factors. Among these factors are the availability of tyrosine in food, the presence of tyramine-producing bacteria, the environmental pH, and the salt content of food. The process of fermentation provides a particularly good source of tyramine in human nutrition. Here, the potential impact of dietary tyramine on human health was assessed by compiling quantitative data on the tyramine content in a variety of foods and then conducting a brief review of the literature on the physiological, cellular, and systemic effects of tyramine. Together, the data sets presented here may allow both the assessment of tyramine concentrations in food and the extrapolation of these concentrations to gauge the physiological and systemic effects in the context of human nutrition.

Trace Amines and the Trace Amine-Associated Receptor 1: Pharmacology, Neurochemistry, and Clinical Implications

Biogenic amines are a collection of endogenous molecules that play pivotal roles as neurotransmitters and hormones. In addition to the "classical" biogenic amines resulting from decarboxylation of aromatic acids, including dopamine (DA), norepinephrine, epinephrine, serotonin (5-HT), and histamine, other biogenic amines, present at much lower concentrations in the central nervous system (CNS), and hence referred to as "trace" amines (TAs), are now recognized to play significant neurophysiological and behavioral functions. At the turn of the century, the discovery of the trace amine-associated receptor 1 (TAAR1), a phylogenetically conserved G protein-coupled receptor that is responsive to both TAs, such as β-phenylethylamine, octopamine, and tyramine, and structurally-related amphetamines, unveiled mechanisms of action for TAs other than interference with aminergic pathways, laying the foundations for deciphering the functional significance of TAs and its mammalian CNS receptor, TAAR1. Although, its molecular interactions and downstream targets have not been fully elucidated, TAAR1 activation triggers accumulation of intracellular cAMP, modulates PKA and PKC signaling and interferes with the β-arrestin2-dependent pathway via G protein-independent mechanisms. TAAR1 is uniquely positioned to exert direct control over DA and 5-HT neuronal firing and release, which has profound implications for understanding the pathophysiology of, and therefore designing more efficacious therapeutic interventions for, a range of neuropsychiatric disorders that involve aminergic dysregulation, including Parkinson's disease, schizophrenia, mood disorders, and addiction. Indeed, the recent development of novel pharmacological tools targeting TAAR1 has uncovered the remarkable potential of TAAR1-based medications as new generation pharmacotherapies in neuropsychiatry. This review summarizes recent developments in the study of TAs and TAAR1, their intricate neurochemistry and pharmacology, and their relevance for neurodegenerative and neuropsychiatric disease.

Anatomical and functional evidence for trace amines as unique modulators of locomotor function in the mammalian spinal cord

The trace amines (TAs), tryptamine, tyramine, and β-phenylethylamine, are synthesized from precursor amino acids via aromatic-L-amino acid decarboxylase (AADC). We explored their role in the neuromodulation of neonatal rat spinal cord motor circuits. We first showed that the spinal cord contains the substrates for TA biosynthesis (AADC) and for receptor-mediated actions via trace amine-associated receptors (TAARs) 1 and 4. We next examined the actions of the TAs on motor activity using the in vitro isolated neonatal rat spinal cord. Tyramine and tryptamine most consistently increased motor activity with prominent direct actions on motoneurons. In the presence of N-methyl-D-aspartate, all applied TAs supported expression of a locomotor-like activity (LLA) that was indistinguishable from that ordinarily observed with serotonin, suggesting that the TAs act on common central pattern generating neurons. The TAs also generated distinctive complex rhythms characterized by episodic bouts of LLA. TA actions on locomotor circuits did not require interaction with descending monoaminergic projections since evoked LLA was maintained following block of all Na⁺-dependent monoamine transporters or the vesicular monoamine transporter. Instead, TA (tryptamine and tyramine) actions depended on intracellular uptake via pentamidine-sensitive Na⁺-independent membrane transporters. Requirement for intracellular transport is consistent with the TAs having much slower LLA onset than serotonin and for activation of intracellular TAARs. To test for endogenous actions following biosynthesis, we increased intracellular amino acid levels with cycloheximide. LLA emerged and included distinctive TA-like episodic bouts. In summary, we provided anatomical and functional evidence of the TAs as an intrinsic spinal monoaminergic modulatory system capable of promoting recruitment of locomotor circuits independent of the descending monoamines. These actions support their known sympathomimetic function.

Trace amine-associated receptor 1-Family archetype or iconoclast?

Interest has recently been rekindled in receptors that are activated by low molecular weight, noncatecholic, biogenic amines that are typically found as trace constituents of various vertebrate and invertebrate tissues and fluids. The timing of this resurgent focus on receptors activated by the "trace amines" (TA) beta-phenylethylamine (PEA), tyramine (TYR), octopamine (OCT), synephrine (SYN), and tryptamine (TRYP) is the direct result of 2 publications that appeared in 2001 describing the cloning of a novel G protein-coupled receptor (GPCR) referred to by their discoverers Borowsky et al. as TA1 and Bunzow et al. as TA receptor 1 (TAR1). When heterologously expressed in Xenopus laevis oocytes and various eukaryotic cell lines, recombinant rodent and human TAR dose-dependently couple to the stimulation of adenosine 3',5'-monophosphate (cAMP) production. Structure-activity profiling based on this functional response has revealed that in addition to the TA, other biologically active compounds containing a 2-carbon aliphatic side chain linking an amino group to at least 1 benzene ring are potent and efficacious TA receptor agonists with amphetamine (AMPH), methamphetamine, 3-iodothyronamine, thyronamine, and dopamine (DA) among the most notable. Almost 100 years after the search for TAR began, numerous TA1/TAR1-related sequences, now called TA-associated receptors (TAAR), have been identified in the genome of every species of vertebrate examined to date. Consequently, even though heterologously expressed TAAR1 fits the pharmacological criteria established for a bona fide TAR, a major challenge for those working in the field is to discern the in vivo pharmacology and physiology of each purported member of this extended family of GPCR. Only then will it be possible to establish whether TAAR1 is the family archetype or an iconoclast.

The Trace Amine 1 receptor knockout mouse: an animal model with relevance to schizophrenia

Trace amines have been implicated in a number of neuropsychiatric disorders including depression and schizophrenia. Although long known to modulate neurotransmission indirectly through the release of catecholamines, the identification of the Trace Amine 1 receptor (TA1) offers a mechanism by which trace amines can influence synaptic activity directly. TA1 binds and is activated by trace amines such as beta-phenylethylamine and tyramine. Our pharmacological characterization of mouse TA1 showed that, as in rat and primate, amphetamine is an agonist at this receptor but with surprisingly high potency. Without selective ligands for TA1 that do not also possess catecholamine-releasing properties, however, it has not been possible to study its physiological role in the central nervous system. To that end, a line of mice lacking the TA1 receptor was generated to characterize its contribution to the regulation of behavior. Compared with wild-type littermates, TA1 knockout (KO) mice displayed a deficit in prepulse inhibition. Knockout animals, in which the TA1-agonist influence of amphetamine was absent, showed enhanced sensitivity to the psychomotor-stimulating effect of this drug, which was temporally correlated with significantly larger increases in the release of both dopamine and norepinephrine in the dorsal striatum and associated with a 262% increase in the proportion of striatal high-affinity D2 receptors. TA1 therefore appears to play a modulatory role in catecholaminergic function and represents a potentially novel mechanism for the treatment of neuropsychiatric disorders. Furthermore, the TA1 KO mouse may provide a useful model for the development of treatments for some positive symptoms of schizophrenia.

Trace amine-associated receptors and their ligands

Classical biogenic amines (adrenaline, noradrenaline, dopamine, serotonin and histamine) interact with specific families of G protein-coupled receptors (GPCRs). The term 'trace amines' is used when referring to p-tyramine, beta-phenylethylamine, tryptamine and octopamine, compounds that are present in mammalian tissues at very low (nanomolar) concentrations. The pharmacological effects of trace amines are usually attributed to their interference with the aminergic pathways, but in 2001 a new gene was identified, that codes for a GPCR responding to p-tyramine and beta-phenylethylamine but not to classical biogenic amines. Several closely related genes were subsequently identified and designated as the trace amine-associated receptors (TAARs). Pharmacological investigations in vitro show that many TAAR subtypes may not respond to p-tyramine, beta-phenylethylamine, tryptamine or octopamine, suggesting the existence of additional endogenous ligands. A novel endogenous thyroid hormone derivative, 3-iodothyronamine, has been found to interact with TAAR1 and possibly other TAAR subtypes. In vivo, micromolar concentrations of 3-iodothyronamine determine functional effects which are opposite to those produced on a longer time scale by thyroid hormones, including reduction in body temperature and decrease in cardiac contractility. Expression of all TAAR subtypes except TAAR1 has been reported in mouse olfactory epithelium, and several volatile amines were shown to interact with specific TAAR subtypes. In addition, there is evidence that TAAR1 is targeted by amphetamines and other psychotropic agents, while genetic linkage studies show a significant association between the TAAR gene family locus and susceptibility to schizophrenia or bipolar affective disorder.

A renaissance in trace amines inspired by a novel GPCR family

Trace amines (TAs) are endogenous compounds that are related to biogenic amine neurotransmitters and are present in the mammalian nervous system in trace amounts. Although their pronounced pharmacological effects and tight link to major human disorders such as depression and schizophrenia have been studied for decades, the understanding of their molecular mode of action remained incomplete because of the apparent absence of specialized receptors. However, the recent discovery of a novel family of G-protein-coupled receptors (GPCRs) that includes individual members that are highly specific for TAs indicates a potential role for TAs as vertebrate neurotransmitters or neuromodulators, although the majority of these GPCRs so far have not been demonstrated to be activated by TAs. The unique pharmacology and expression pattern of these receptors make them prime candidates for targets in drug development in the context of several neurological diseases. Current research focuses on dissecting their molecular pharmacology and on the identification of endogenous ligands for the apparently TA-insensitive members of this receptor family.

Trace Amines Depress GABAB Response in Dopaminergic Neurons by Inhibiting G-βγ-Gated Inwardly Rectifying Potassium Channels

Trace amines (TAs) are present in the central nervous system in which they up-regulate catecholamine release and are implicated in the pathogenesis of addiction, attention-deficit/hyper-activity disorder, Parkinson's disease, and schizophrenia. By using intracellular and patch-clamp recordings from dopaminergic cells in the rat midbrain slices, we report a depressant postsynaptic action of two TAs, beta-phenylethylamine (beta-PEA) and tyramine (TYR) on the GABA(B)-mediated slow inhibitory postsynaptic potential and baclofen-activated outward currents. beta-PEA and TYR activated G-proteins, interfering with the coupling between GABA(B) receptors and G-betagamma-gated inwardly rectifying potassium channels. This is the first demonstration that beta-PEA and TYR depress inhibitory synaptic potentials in neurons of the central nervous system, supporting their emerging role as neuromodulators.

The biogenic trace amine tyramine induces a pronounced hydroxyl radical production via a monoamine oxidase dependent mechanism: an in vivo microdialysis study in mouse striatum

Tyramine is a biogenic trace amine that releases monoamines and is a good substrate for monoamine oxidase (MAO)-A/B. Here we investigated whether tyramine affects hydroxyl radical formation in the intact and lesioned dopaminergic system. Male C57bl/6 mice received systemic and local tyramine administrations. Hydroxyl radical formation and dopamine (DA) overflow were determined in the striatum using in vivo microdialysis in combination with the salicylate hydroxylation assay. Systemic injection of tyramine neither enhanced extracellular dopamine nor induced hydroxyl radical formation. In contrast, when tyramine was incorporated into the dialysate fluid, hydroxyl radical formation and extracellular dopamine levels were significantly enhanced. Systemic pretreatment with the MAO-A/B inhibitor tranylcypromine or with the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) significantly diminished the tyramine-induced hydroxyl radical formation by 73.1% and 80.6%, respectively. We conclude that the mechanism of tyramine-induced hydroxyl free radical formation involves MAO metabolism and requires an intact dopaminergic system. Pharmacological intervention on the MAO-mediated formation of hydroxyl free radicals seems to be a promising strategy to prevent oxidative damage in the nigrostriatal dopaminergic system.

Pharmacological studies with endogenous enhancer substances: beta-phenylethylamine, tryptamine, and their synthetic derivatives

The discovery of enhancer regulation in the mesencephalon and the concept that it plays a key role in the operation of innate and acquired drives [Neurochem. Res. 28 (2003) 1187] sets the trace amines (TAs) in their true physiological perspective. The regulation is defined as the existence of enhancer-sensitive neurons in the brain capable of working in a split-second on a high activity level due to endogenous enhancer substances. For the time being, only beta-phenylethylamine (PEA) and tryptamine are the experimentally analyzed examples. (-)-Deprenyl (selegiline), widely used in Parkinson's disease and Alzheimer's disease today, and known as the first selective monoamine oxidase (MAO) type-B inhibitor for decades, was identified as a PEA-derived synthetic mesencephalic enhancer substance. An important and convincing confirmation of the enhancer concept was the recent development of a highly specific and potent tryptamine-derived synthetic mesencephalic enhancer substance, (-)-1-(benzofuran-2-yl)-2-propylaminopentane [(-)-BPAP]. This substance, which is specific and hundreds of times more potent than selegiline, is now the best experimental tool to study the enhancer regulation in the mesencephalon and a promising candidate to significantly surpass the therapeutic efficiency of selegiline in depression, Parkinson's disease, and Alzheimer's disease.

Effects of beta-phenylethylamine on the hypothalamo-pituitary-adrenal axis in the male rat

beta-Phenylethylamine (PEA) is a trace neuroactive amine implicated in the regulation of the hypothalamic-pituitary-adrenal (HPA) response to stress. To test this hypothesis, effects of subchronic levels of PEA (50 mg/kg/day treatment for 10 days) on the corticotroph function were studied. PEA treatment induces: (i) a significant increase of corticotrophin releasing hormone (CRH) immunoreactivity in the median eminence (ME), as measured by semi-quantitative immunofluorescence labeling techniques, (ii) a significant increase in CRH mRNA levels in paraventricular nuclei, as detected by in situ hybridization, and (iii) an increase in plasma adreno-corticotrophin hormone (ACTH) and corticosterone levels in responses to stress. PEA treatment has no effect on the number of binding sites and on the dissociation constant of the glucocorticoid receptors in any structure studied. Results of the dexamethasone suppression test were similar in PEA- and saline-treated rats. Taken together, these results suggest that PEA treatment stimulated the HPA axis activity levels directly via the CRH hypothalamic neurons, without altering the negative feed back control exerted by the glucocorticoids.

Investigations into the biochemical basis of neuromodulation by 2-phenylethylamine: effect on microtubule protein

In order to understand the role of 2-phenylethylamine (PE) on neuronal responses, membrane changes have been studied using ESR probes. We report that the anticipated change in lipid membrane fluidity generally implicated in signal transduction has not been observed when PE is added to synaptosomes. As cytoskeletal architecture of presynaptic terminals appears to be involved in synaptic transmission, we non-specifically labeled synaptosomal membrane proteins with the sulfhydryl spin probe N-(2,2,6,6-tetramethyl-piperidine-1-oxyl-4-yl) maleimide (4-MAL-TEMPO). The addition of 2-phenylethylamine was found to induce conformational changes, in decreasing the ratio of weakly to strongly immobilized spin label (W/S) to 65% of the control. Of the membrane proteins labeled, 70-90% of the 4-MAL-TEMPO is covalently incorporated into cytoskeletal proteins. In isolated synaptosomes, incorporated with spin-labeled tubulin, the addition of PE reduced the W/S ratio to 51.6% of that obtained for polymerized microtubules. In vitro, PE reduced tau R of polymerized microtubules by 37%. We propose that the PE interaction with tubulin changes microtubule dynamics which may lead to its neuromodulatory action. The state of microtubular assembly can modulate the responsiveness of second messengers in the cell to the effect of stimulatory agents. The nature and physiological significance of PE interaction with tubulin is currently under investigation.

2-Phenylethylamine-induced changes in catecholamine receptor density: implications for antidepressant drug action

It is now established that (1) concentrations of 2-phenylethylamine (PEA) are greatly increased in brain following administration of monoamine oxidase inhibitor (MAOI) antidepressants; (2) PEA is a metabolite of the MAOI antidepressant phenelzine; and (3) PEA may be a neuromodulator of catecholamine activity. On the basis of these observations, the effects of long term increases in brain PEA on catecholamine receptors have been assessed. Both PEA and antidepressants induced a reduction in the behavioural response to the beta 2 adrenoceptor agonist salbutamol. Radioligand binding measurements revealed that 28 day administration of PEA in combination with the type B MAOI (-)-deprenyl results in a decrease in the density of beta 1 adrenoceptors but not beta 2 adrenoceptors in rat cerebral cortex and cerebellum. (-)-Deprenyl alone also induced a significant decrease in beta 1-adrenoceptors but when PEA was added to this treatment there was a further decrease in beta 1-adrenoceptor density. Only changes in beta 1 adrenoceptor density were evident following 28 day administration of MAOI antidepressants. PEA also induced a decrease in the density of D1-like dopamine (DA) receptors in the rat striatum. MAOI antidepressants induced a decrease in the density of both D1-like and D2-like DA receptors. These data are discussed in terms of a possible role of PEA-catecholamine interactions in antidepressant drug action.

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.

Effects of age and of chronic antidepressant treatment on [3H]tryptamine and [3H]dihydroalprenolol binding to rat cortical membranes

1. The effects of age and of chronic antidepressant treatment on [3H]tryptamine and [3H]dihydroalprenolol binding site density were measured in brain cortical membranes from male Sprague-Dawley rats. 2. The density but not the affinity of [3H]tryptamine binding sites was increased in 18-month-old rats relative to 3-month-old rats. Neither the density nor the affinity of [3H]dihydroalprenolol binding sites was affected by age. 3. Chronic administration (28 days s.c. via Alzet osmotic minipumps) of tricyclic antidepressant drugs (daily doses: imipramine.HCl, 30 mg kg-1; desipramine.HCl, 10 mg kg-1; clomipramine.HCl, 10 mg kg-1) resulted in decreases in [3H]dihydroalprenolol binding site density but no changes in [3H]tryptamine binding site density; no changes in affinity of either site were observed. 4. Chronic administration (s.c. via Alzet osmotic minipumps) of monoamine oxidase inhibitor antidepressant drugs (daily doses: tranylcypromine.HCl, 0.5 and 1.0 mg kg-1; phenelzine sulfate, 5 and 10 mg kg-1, each for 28 days; clorgyline.HCl, 1.0 mg kg-1; (-)-deprenyl.HCl, 1.0 mg kg-1, each for 14 days) resulted in decreases in [3H]tryptamine binding site density, without any effects on the affinity of this site. In addition, each of these monoamine oxidase inhibitors except (-)-deprenyl resulted in a decrease in [3H]dihydroalprenolol binding site density. No affinity changes were observed. 5. These data indicate that the [3H]tryptamine binding site exhibits physiological changes with aging and is differentially sensitive to the actions of tricyclic antidepressants and monoamine oxidase inhibitor antidepressants, respectively.

Absence of 2-phenylethylamine binding after monoamine oxidase inhibition in rat brain

Earlier work has suggested the existence of saturable and highly specific binding sites for [3H]2-phenylethylamine in rat forebrain membranes. Since monoamine oxidase (MAO) was not inhibited during the assay, the [3H]2-phenylethylamine binding may have been affected by an interaction between 2-phenylethylamine and the enzyme. This is an investigation of [3H]2-phenylethylamine binding to rat forebrain membranes in the presence of two MAO inhibitors, (-)-deprenyl and pargyline. The results show that the high affinity specific binding of [3H]2-phenylethylamine to rat forebrain membranes is inhibited by pretreatment of the membrane with the MAO inhibitors and in vivo injection of the MAO inhibitors in a concentration-dependent manner. In the presence of higher concentrations of MAO inhibitors, the specific binding of [3H]2-phenylethylamine is completely blocked, suggesting that the binding sites reported earlier represent binding to MAO-B.

Down-regulation of tryptamine binding sites following chronic molindone administration. A comparison with responses of dopamine and 5-hydroxytryptamine receptors

The present study assessed changes of tryptamine, dopamine D2, 5-HT1 and 5-HT2 binding sites in rat brain following chronic treatment with low (5 mg/kg/day) and high (40 mg/kg/day) doses of molindone, a clinically effective psychotropic drug. The high-dose molindone treatment produced a decrease in the number of tryptamine binding sites while both high and low doses caused an increase in the number of dopamine D2 binding sites in the striatum. No significant changes were observed in either 5-HT1 or 5-HT2 binding sites in the cerebral cortex. Competition binding experiments showed that molindone was a potent inhibitor at dopamine D2 but less effective at tryptamine, 5-HT1 and 5-HT2 binding sites. The inhibition activity of molindone towards type A monoamine oxidase produced a significant increase in endogenous tryptamine accumulation rate which was much higher than that of dopamine and 5-HT. These findings suggest that the reduction in the number of tryptamine binding sites produced by chronic molindone administration is related to monoamine oxidase inhibition and that the increase in the number of dopamine D2 binding sites is correlated to receptor blocking activity of the drug.