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Trace amine-associated receptor 1 (TAAR1) is a trace amine-associated receptor (TAAR) protein that in humans is encoded by the TAAR1gene.[5]
TAAR1 is a primarily intracellularamine-activated Gs-coupled and Gq-coupledG protein-coupled receptor (GPCR) that is primarily expressed in several peripheral organs and cells (e.g., the stomach, small intestine, duodenum, and white blood cells), astrocytes, and in the intracellular milieu within the presynaptic plasma membrane (i.e., axon terminal) of monoamineneurons in the central nervous system (CNS).[6][7][8][9]
TAAR1 is one of six functional human TAARs, which are so named for their ability to bind endogenous amines that occur in tissues at trace concentrations.[10][11] TAAR1 plays a significant role in regulating neurotransmission in dopamine, norepinephrine, and serotonin neurons in the CNS;[12][13][7][10] it also affects immune system and neuroimmune system function through different mechanisms.[14][15][16][17]
Endogenous ligands of the TAAR1 include trace amines, monoamine neurotransmitters, and certain thyronamines.[18][6] The trace amines β-phenethylamine, tyramine, tryptamine, and octopamine, the monoamine neurotransmitters dopamine and serotonin, and the thyronamine 3-iodothyronamine (3-IT) are all agonists of the TAAR1 in different species.[18][6][19] Other endogenous agonists are also known.[18] A variety of exogenouscompounds and drugs are TAAR1 agonists as well, including various phenethylamines, amphetamines, tryptamines, and ergolines, among others.[18] There are marked species differences in the interactions of ligands with the TAAR1, resulting in greatly differing affinities, potencies, and efficacies of TAAR1 ligands between species.[18] Many compounds that are TAAR1 agonists in rodents are much less potent or inactive at the TAAR1 in humans.[18]
A number of selective TAAR1 ligands have been developed, for instance the TAAR1 full agonistRO5256390, the TAAR1 partial agonistRO5263397, and the TAAR1 antagonistsEPPTB and RTI-7470-44.[18][20] Selective TAAR1 agonists are used in scientific research, and a few TAAR1 agonists, such as ulotaront and ralmitaront, are being developed as novel pharmaceutical drugs, for instance to treat schizophrenia and substance use disorder.[20][21][22]
The TAAR1 was discovered in 2001 by two independent groups, Borowski et al. and Bunzow et al.[19][23]. To find the genetic variants responsible for TAAR1 synthesis, they used mixtures of oligonucleotides with sequences related to G protein-coupled receptors (GPCRs) of serotonin and dopamine to discover novel DNA sequences in rat genomic DNA and cDNA, which they then amplified and cloned. The resulting sequence was not found in any database and coded for TAAR1.[19][23] Further characterization of the functional role of TAAR1 and other receptors from this family was performed by other researchers including Raul Gainetdinov and his colleagues.[18]
TAAR1 shares structural similarities with the class A rhodopsin GPCR subfamily.[23] It has 7 transmembrane domains with short N and C terminal extensions.[24] TAAR1 is 62–96% identical with TAARs2-15, which suggests that the TAAR subfamily has recently evolved; while at the same time, the low degree of similarity between TAAR1 orthologues suggests that they are rapidly evolving.[19] TAAR1 shares a predictive peptidemotif with all other TAARs. This motif overlaps with transmembrane domain VII, and its identity is NSXXNPXX[Y,H]XXX[Y,F]XWF. TAAR1 and its homologues have ligand pocket vectors that utilize sets of 35 amino acids known to be involved directly in receptor-ligand interaction.[11]
All human TAAR genes are located on a single chromosome spanning 109 kb of human chromosome 6q23.1, 192 kb of mouse chromosome 10A4, and 216 kb of rat chromosome 1p12. Each TAAR is derived from a single exon, except for TAAR2, which is coded by two exons.[11] The human TAAR1 gene is thought to be an intronless gene.[25]
To date, TAAR1 has been identified and cloned in five different mammalgenomes: human, mouse, rat, monkey, and chimpanzee. In rats, mRNA for TAAR1 is found at low to moderate levels in peripheral tissues like the stomach, kidney, intestines[26] and lungs, and at low levels in the brain.[19]Rhesus monkey Taar1 and human TAAR1 share high sequence similarity, and TAAR1 mRNA is highly expressed in the same important monoaminergic regions of both species. These regions include the dorsal and ventral caudate nucleus, putamen, substantia nigra, nucleus accumbens, ventral tegmental area, locus coeruleus, amygdala, and raphe nucleus.[6][27] hTAAR1 has also been identified in human astrocytes.[6][14]
Outside of the human central nervous system, hTAAR1 also occurs as an intracellular receptor and is primarily expressed in the stomach, intestines,[26]duodenum,[26]pancreatic β-cells, and white blood cells.[9][26] In the duodenum, TAAR1 activation increases glucagon-likepeptide-1 (GLP-1) and peptide YY (PYY) release;[9] in the stomach, hTAAR1 activation has been observed to increase somatostatin (growth hormone-inhibiting hormone) secretion from delta cells.[9]
hTAAR1 is the only human trace amine-associated receptor subtype that is not expressed within the human olfactory epithelium.[28]
TAAR1 is a primarily intracellular receptor expressed within the presynaptic terminal of monoamine neurons in humans and other animals.[7][10][29] In model cell systems, hTAAR1 has extremely poor membrane expression.[29] A method to induce hTAAR1 membrane expression has been used to study its pharmacology via a bioluminescence resonance energy transfer cAMP assay.[29]
Because TAAR1 is an intracellular receptor in monoamine neurons, exogenous TAAR1 ligands must enter the presynaptic neuron through a membrane transport protein[note 1] or be able to diffuse across the presynaptic membrane in order to reach the receptor and produce reuptake inhibition and neurotransmitter efflux.[10] Consequently, the efficacy of a particular TAAR1 ligand in producing these effects in different monoamine neurons is a function of both its binding affinity at TAAR1 and its capacity to move across the presynaptic membrane at each type of neuron.[10] The variability between a TAAR1 ligand's substrate affinity at the various monoamine transporters accounts for much of the difference in its capacity to produce neurotransmitter release and reuptake inhibition in different types of monoamine neurons.[10] E.g., a TAAR1 ligand which can easily pass through the norepinephrine transporter, but not the serotonin transporter, will produce – all else equal – markedly greater TAAR1-induced effects in norepinephrine neurons as compared to serotonin neurons.
A 2016 study found that most TAAR1 was expressed in intracellular membranes near the nucleus and that 2.3% of TAAR1 was expressed at the cell surface.[30] In addition, TAAR1 signaling via the protein kinase A (PKA) pathway was predominantly associated with cell-surface TAAR1.[30]
TAAR1 ligands have also been found to enter neurons by transporters other than the monoamine transporters.[31][32][33]
TAAR1 forms GPCR oligomers with monoamine autoreceptors in neurons in vivo.[32][34] These and other reported TAAR1 hetero-oligomers include:
[note 2] in the TAAR1- D2sh example shows that TAAR1 can be located at cell membranes, and in the case of enterochromaffin cells in the gut epithelium, TAAR1 can be activated by high doses of dietary 'trace' amines, proximal to vesicles packed with catecholamines, impacting the vagal nerve system and CNS. This raises questions about where T1AM might find TAAR1 and cause similar unexpected nerve firing. TAAR1 gene knockout in mice leads to shifts in microbial community richness and stability.[36]
The known endogenousagonists of the TAAR1 include trace amines like β-phenethylamine (PEA), monoamine neurotransmitters like dopamine, and thyronamines like 3-iodothyronamine (T1AM).[12]
Trace amines are endogenous amines which act as agonists at TAAR1 and are present in extracellular concentrations of 0.1–10 nM in the brain, constituting less than 1% of total biogenic amines in the mammalian nervous system.[37] Some of the human trace amines include tryptamine, phenethylamine (PEA), N-methylphenethylamine, p-tyramine, m-tyramine, N-methyltyramine, p-octopamine, m-octopamine, and synephrine. These share structural similarities with the three common monoamine neurotransmitters: serotonin, dopamine, and norepinephrine. Each ligand has a different potency, measured as increased cyclic AMP (cAMP) concentration after the binding event. The rank order of potency for the primary endogenous ligands at the human TAAR1 is: tyramine > β-phenethylamine > dopamine = octopamine.[6][19]Tryptamine and histamine also interact with the human TAAR1 with lower potency, whereas serotonin and norepinephrine have been found to be inactive.[18][19][38][23]
Thyronamines are molecular derivatives of thyroid hormone involved in endocrine system function. 3-Iodothyronamine (T1AM) is one of the most potent TAAR1 agonists yet discovered. It also interacts with a number of other targets.[13][39][40] Unlike the monoamine neurotransmitters and trace amines, T1A is not a monoamine transporter (MAT) substrate, although it does still weakly interact with the MATs.[39][41] Activation of TAAR1 by T1AM results in the production of large amounts of cAMP. This effect is coupled with decreased body temperature and cardiac output.
Other endogenous TAAR1 agonists include cyclohexylamine, isoamylamine, and trimethylamine, among others.[18][22][42]
Although amphetamine, methamphetamine, and MDMA are potent TAAR1 agonists in rodents, they are less potent at human TAAR1.[18][68][38][69] For example, based on TAAR1 activation EC50 values, amphetamine and methamphetamine show nanomolar potency at rodent TAAR1, but micromolar potency at human TAAR1.[18][68][38][44][69] MDMA shows very low potency and efficacy as a human TAAR1 agonist and has been described as inactive.[18][68][38][70] The magnitude of TAAR1-mediated effects of amphetamines in humans at low doses is uncertain.[71][43][69][72] Given the micromolar EC50 values reported for human TAAR1, TAAR1 agonism may be more relevant at high recreational doses.[43][69] TAAR1 activation can inhibit monoaminergic neuronal firing and is expected to reduce action potential-dependent (vesicular) monoamine release.[73][71][74][55]
While some amphetamines are human TAAR1 agonists, many others are not.[38] As examples, most cathinones (β-ketoamphetamines), such as methcathinone, mephedrone, and flephedrone, as well as other amphetamines, including ephedrine, 4-methylamphetamine (4-MA), para-chloroamphetamine (PCA),[55]para-methoxyamphetamine (PMA), 4-methylthioamphetamine (4-MTA), MDEA, MBDB, 5-APDB, and 5-MAPDB, are inactive as human TAAR1 agonists.[71][38] Many other drugs acting as monoamine releasing agents (MRAs) are also inactive as human TAAR1 agonists, for instance piperazines like benzylpiperazine (BZP), meta-chlorophenylpiperazine (mCPP), and 3-trifluoromethylphenylpiperazine (TFMPP), as well as the alkylaminemethylhexanamine (DMAA).[18][38][75] The negligible TAAR1 agonism with most cathinones might serve to enhance their effects and misuse potential as MRAs compared to their amphetamine counterparts.[71][74]
Monoaminergic activity enhancers (MAEs), such as selegiline, benzofuranylpropylaminopentane (BPAP), and phenylpropylaminopentane (PPAP), have been claimed to act as TAAR1 agonists to mediate their MAE effects, but TAAR1 agonism for BPAP and PPAP has yet to be assessed or confirmed.[76][77] Selegiline is only a weak agonist of the mouse TAAR1, with dramatically lower potency than amphetamine or methamphetamine, and does not seem to have been assessed at the human TAAR1.[46][77]
A few other less well-known TAAR1 antagonists have also been discovered and characterized.[83][79][84][82]
RO5073012 is an antagonist-esque weak partial agonist of the rodent and human TAAR1 (EmaxTooltip maximal efficacy = 24–35%).[18][63][85][86] Similarly, MDA and MDMA are weak to very weak partial agonists or antagonists of the human TAAR1 (Emax = 11% and 26%, respectively), albeit with very low potency.[18][38][44]
It has been claimed that rasagiline may act as a TAAR1 antagonist, but TAAR1 interactions have yet to be assessed or confirmed for this agent.[77]
Before the discovery of TAAR1, trace amines were believed to serve very limited functions. They were thought to induce noradrenaline release from sympathetic nerve endings and compete for catecholamine or serotonin binding sites on cognate receptors, transporters, and storage sites.[37] Today, they are believed to play a much more dynamic role by regulating monoaminergic systems in the brain.[94]
One of the downstream effects of active TAAR1 is to increase cAMP in the presynaptic cell via Gαs G-protein activation of adenylyl cyclase.[94][19][11] This alone can have a multitude of cellular consequences. A main function of the cAMP may be to up-regulate the expression of trace amines in the cell cytoplasm.[95] These amines would then activate intracellular TAAR1. Monoamine autoreceptors (e.g., D2 short, presynaptic α2, and presynaptic 5-HT1A) have the opposite effect of TAAR1, and together these receptors provide a regulatory system for monoamines.[10] Notably, amphetamine and trace amines possess high binding affinities for TAAR1, but not for monoamine autoreceptors.[10][7] The effect of TAAR1 agonists on monoamine transporters in the brain appears to be site-specific.[10] Imaging studies indicate that monoamine reuptake inhibition by amphetamine and trace amines is dependent upon the presence of TAAR1 co-localization in the associated monoamine neurons.[10] As of 2010, co-localization of TAAR1 and the dopamine transporter (DAT) has been visualized in rhesus monkeys, but co-localization of TAAR1 with the norepinephrine transporter (NET) and the serotonin transporter (SERT) has only been evidenced by messenger RNA (mRNA) expression.[10]
In neurons with co-localized TAAR1, TAAR1 agonists can regulate monoamine concentrations in the synaptic cleft through effects on neuronal excitability and monoamine transporter function.[10] Through direct activation of G protein-coupled inwardly-rectifying potassium channels (GIRKs), TAAR1 can reduce the firing rate of dopamine neurons, in turn preventing a hyper-dopaminergic state.[96][90][91] Amphetamine and trace amines can enter the presynaptic neuron either through DAT or by diffusing across the neuronal membrane directly.[10] As a consequence of DAT uptake, amphetamine and trace amines produce competitive reuptake inhibition at the transporter.[97][10] Upon entering the presynaptic neuron, these compounds activate can activate TAAR1 which, through protein kinase A (PKA), protein kinase C (PKC), and Ras Homolog family member A (RhoA) signaling, causes DAT phosphorylation.[98][99] Activation of any these intracellular signaling pathways can result in DAT internalization (non-competitive reuptake inhibition), but PKC-mediated phosphorylation alone induces reverse transporter function (dopamine efflux).[10][100][101]
Electrophysiological studies have linked TAAR1 activation to reduced excitability of midbrain dopamine neurons, including increased spontaneous firing in TAAR1 gene knockout animals and TAAR1-dependent suppression of neuronal firing in wild-type tissue.[99] A review noted that reduced VTA dopamine neuron firing would be expected to decrease extracellular dopamine, whereas TAAR1-mediated regulation of DAT function, including transporter internalization and reverse transport, would be expected to increase extracellular dopamine.[72] To address comparatively limited work on whether TAAR1 agonists affect synaptic dopamine via DAT, a 2025 study on TAAR1-mediated regulation of transporter function compared amphetamine with three other TAAR1 agonists (RO5166017, RO5256390, and ulotaront) and reported that these TAAR1 agonists produce distinct effects on DAT function.[102] In HEK 293T cells stably expressing human TAAR1 (hTAAR1) and human DAT (hDAT), RO5166017 and RO5256390 were shown to bind to hDAT and competively inhibited hDAT-mediated dopamine reuptake, whereas ulotaront did not show detectable hDAT binding.[102] In the same cell system, RO5256390 and ulotaront reduced dopamine reuptake, while RO5166017 increased reuptake, and these reuptake changes were hTAAR1-dependent.[102] RO5166017 increased hDAT expression at the plasma membrane and potentiated amphetamine-induced reverse transport in a hTAAR1-dependent manner, whereas RO5256390 and ulotaront did not alter amphetamine-induced hDAT efflux.[102] The authors concluded that although these hTAAR1 agonists are relatively consistent in their effects on reducing VTA dopamine neuron firing, they are less consistent in effects on hDAT uptake and membrane expression, and only amphetamine induced hDAT-mediated dopamine efflux.[102]
Expression of TAAR1 on lymphocytes is associated with activation of lymphocyte immuno-characteristics.[16] In the immune system, TAAR1 transmits signals through active PKA and PKC phosphorylation cascades.[16] In a 2012 study, Panas et al. observed that methamphetamine had these effects, suggesting that, in addition to brain monoamine regulation, amphetamine-related compounds may have an effect on the immune system.[16] A recent paper showed that, along with TAAR1, TAAR2 is required for full activity of trace amines in PMN cells.[17]
Phytohaemagglutinin upregulates hTAAR1mRNA in circulating leukocytes;[6] in these cells, TAAR1 activation mediates leukocyte chemotaxis toward TAAR1 agonists.[6] TAAR1 agonists (specifically, trace amines) have also been shown to induce interleukin 4 secretion in T-cells and immunoglobulin E (IgE) secretion in B cells.[6]
Astrocyte-localized TAAR1 regulates EAAT2 levels and function in these cells;[14] this has been implicated in methamphetamine-induced pathologies of the neuroimmune system.[14]
Low phenethylamine (PEA) concentration in the brain is associated with major depressive disorder,[19][37][103] and high concentrations are associated with schizophrenia.[103][104] Low PEA levels and under-activation of TAAR1 also appears to be associated with ADHD.[103][104][105] It is hypothesized that insufficient PEA levels result in TAAR1 inactivation and overzealous monoamine uptake by transporters, possibly resulting in depression.[19][37] Some antidepressants function by inhibiting monoamine oxidase (MAO), which increases the concentration of trace amines, which is speculated to increase TAAR1 activation in presynaptic cells.[19][11] Decreased PEA metabolism has been linked to schizophrenia, a logical finding considering excess PEA would result in over-activation of TAAR1 and prevention of monoamine transporter function. Mutations in region q23.1 of human chromosome 6 – the same chromosome that codes for TAAR1 – have been linked to schizophrenia.[11]
Medical reviews from February 2015 and 2016 noted that TAAR1-selective ligands have significant therapeutic potential for treating psychostimulant addictions (e.g., cocaine, amphetamine, methamphetamine, etc.).[7][8] Despite wide distribution outside of the CNS and PNS, TAAR1 does not affect hematological functions and the regulation of thyroid hormones across different stages of ageing. Such data represent that future TAAR1-based therapies should exert little hematological effect and thus will likely have a good safety profile.[106]
A large candidate gene association study published in September 2011 found significant differences in TAAR1 allele frequencies between a cohort of fibromyalgia patients and a chronic pain-free control group, suggesting this gene may play an important role in the pathophysiology of the condition; this possibly presents a target for therapeutic intervention.[107]
In preclinical research on rats, TAAR1 activation in pancreatic cells promotes insulin, peptide YY, and GLP-1 secretion;[108] therefore, TAAR1 is potentially a biological target for the treatment of obesity and diabetes.[108]
Lack of TAAR1 significantly increase mice aggressive behavior [109], but does not affect sexual motivation and routine lipid and metabolic blood biochemical parameters, suggesting that future TAAR1-based therapies should have a favorable safety profile.[110] Moreover, agonist RO5263397 reduces social aggression in brain serotonin-deficient tryptophan hydroxylase 2 knockout rats. This establishes the biological plausibility of TAAR1 agonism as a treatment for aggressive behavior.[111]
Tissue DistributionCNS (region specific) & several peripheral tissues:Stomach > amygdala, kidney, lung, small intestine > cerebellum, dorsal root ganglion, hippocampus, hypothalamus, liver, medulla oblongata, pancreas, pituitary gland, pontine reticular formation, prostate, skeletal muscle, spleen. ...Leukocytes ...Pancreatic islet β cells ... Primary Tonsillar B Cells ... Circulating leukocytes of healthy subjects (upregulation occurs upon addition of phytohaemagglutinin).Species: Human ...In the brain (mouse, rhesus monkey) the TA1 receptor localises to neurones within the momaminergic pathways and there is emerging evidence for a modulatory role for TA1 on function of these systems. Co-expression of TA1 with the dopamine transporter (either within the same neurone or in adjacent neurones) implies direct/indirect modulation of CNS dopaminergic function. In cells expressing both human TA1 and a monoamine transporter (DAT, SERT or NET) signalling via TA1 is enhanced [26,48,50–51]. ...Functional Assays ...Mobilization of internal calcium in RD-HGA16 cells transfected with unmodified human TA1Response measured: Increase in cytopasmic calcium ...Measurement of cAMP levels in human cultured astrocytes.Response measured: cAMP accumulation ...Activation of leukocytesSpecies: HumanTissue: PMN, T and B cellsResponse measured: Chemotactic migration towards TA1 ligands (β-Phenylethylamine, tyramine and 3-iodothyronamine), trace amine induced IL-4 secretion (T-cells) and trace amine induced regulation of T cell marker RNA expression, trace amine induced IgE secretion in B cells.
TAAR1 is a high-affinity receptor for METH/AMPH and DA ... This original observation of TAAR1 and DA D2R interaction has subsequently been confirmed and expanded upon with observations that both receptors can heterodimerize with each other under certain conditions ... Additional DA D2R/TAAR1 interactions with functional consequences are revealed by the results of experiments demonstrating that in addition to the cAMP/PKA pathway (Panas et al., 2012) stimulation of TAAR1-mediated signaling is linked to activation of the Ca++/PKC/NFAT pathway (Panas et al.,2012) and the DA D2R-coupled, G protein-independent AKT/GSK3 signaling pathway (Espinoza et al., 2015; Harmeier et al., 2015), such that concurrent TAAR1 and DA DR2R activation could result in diminished signaling in one pathway (e.g. cAMP/PKA) but retention of signaling through another (e.g., Ca++/PKC/NFA)
TAAR1 is largely located in the intracellular compartments both in neurons (Miller, 2011), in glial cells (Cisneros and Ghorpade, 2014) and in peripheral tissues (Grandy, 2007) ... Existing data provided robust preclinical evidence supporting the development of TAAR1 agonists as potential treatment for psychostimulant abuse and addiction. ... Given that TAAR1 is primarily located in the intracellular compartments and existing TAAR1 agonists are proposed to get access to the receptors by translocation to the cell interior (Miller, 2011), future drug design and development efforts may need to take strategies of drug delivery into consideration (Rajendran et al., 2010).
T1AM has been reported to be a multitarget ligand, since it can also interact with other TAAR subtypes (particularly TAAR5), α2A- and β-adrenergic receptors, TRM8 calcium channels, and membrane amine transporters like dopamine transporter (DAT), norepinephrine transporter (NET), and vesicular monoamine transporter (VMAT) (reviewed by Hoefig et al., 2016). The affinity for these additional targets is substantially lower than the affinity for rat TAAR1, but the difficulties in assessing T1AM concentration at receptor level do not enable to reach a clear conclusion about their physiological relevance. As discussed below, the presence of non-TAAR targets is an important pitfall in the interpretation of experimental results obtained with the administration of natural TAAR1 ligands.
TAAR1 overexpression significantly decreased EAAT-2 levels and glutamate clearance ... METH treatment activated TAAR1 leading to intracellular cAMP in human astrocytes and modulated glutamate clearance abilities. Furthermore, molecular alterations in astrocyte TAAR1 levels correspond to changes in astrocyte EAAT-2 levels and function.
Background: The human trace amine-associated receptor 1 (hTAAR1) plays an important role in the regulation of monoamine neurotransmission and is a potential target for development of novel antipsychotic and antidepressant drugs. However, many aspects of the subcellular distribution and trafficking of hTAARI remain unknown. In this study, we investigated the membrane orientation, cellular localization and the signal transduction of hTAAR1 protein. Methods: A green fluorescent protein (GFP) tag was added to hTAAR1 for analysis of the orientation of hTAAR1 proteins in cells. An acceptor peptide (AP) tag for biotinylation was added to hTAARI for quantification of cell surface hTAARI. The function of hTAAR1 in protein kinase A (PKA) activation was analyzed using the hTAAR1 agonist amphetamine and a CRE-luciferase reporter system. Results: An anti-GFP antibody recognized the hTAAR1 fusion proteins only when GFP was at the N-terminus, but not the C-terminus in transfected HEK293. Only 2.3% (±0.7%) of N-terminal tagged hTAAR1 was detected at the cell surface. Most of the hTAAR1 proteins were localized in intracellular membranes near the nucleus. Amphetamine activated PKA similarly in the cells transfected with hTAAR1 only or with hTAAR1 and the dopamine transporter. Conclusions: hTAAR1 at the cell surface is oriented with the N-terminus in the extracellular space and the C-terminus in the cytosol. A small proportion of hTAAR1 proteins are at the cell surface, but most hTAAR1 proteins are localized within an intracellular membrane compartment. However, hTAAR1 signaling through PKA pathways occurs predominantly via the hTAAR1 at the cell surface.
A more recent study suggested that trace amines action is dependent on intracellular uptake by pentamidine-sensitive Na(+)-independent membrane transporters but not monoamine transporters (Gozal et al., 2014). Gozal EA, O'Neill BE, Sawchuk MA, Zhu H, Halder M, Chou CC, & Hochman S (2014). Anatomical and functional evidence for trace amines as unique modulators of locomotor function in the mammalian spinal cord. Front Neural Circuits, 8, 134. doi: 10.3389/fncir.2014.00134
TAAR1 peripheral and immune localization/functions: It is important to note that in addition to the brain, TAAR1 is also expressed in the spinal cord (Gozal et al., 2014) and periphery (Revel et al., 2012c). It has been shown that TAAR1 is expressed and regulates immune function in rhesus monkey leukocytes (Babusyte et al., 2013; Nelson et al., 2007; Panas et al., 2012). In granulocytes, TAAR1 is necessary for chemotaxic migration of cells towards TAAR1 agonists. In addition, TAAR1 signaling in B and T cells can trigger immunoglobulin and cytokine release, respectively (Babusyte et al., 2013). TAAR1 is also expressed in the islets of Langerhans, stomach and intestines based on LacZ staining patterns carried out on TAAR1-KO LacZ mice (Revel et al., 2012c). Interestingly, the administration of selective TAAR1 partial agonist RO5263397 reverses the side effect of weight gain observed with the antipsychotic olanzapine, indicating that peripheral TAAR1 signalling can regulate metabolic homeostasis (Revel et al., 2012b). ...Monoamine transporters and SLC22A carrier subfamily transport TAAR1 ligand: Studies using the rhesus monkey TAAR1 have shown that this receptor interacts with the monoamine transporters DAT, SERT, and NET in HEK cells (Miller et al., 2005; Xie and Miller, 2007; Xie et al., 2007). It has been hypothesized that TAAR1 interaction with these transporters might provide a mechanism by which TAAR1 ligands can enter the cytoplasm and bind to TAAR1 in intracellular compartments. A recent study has shown that in rat neonatal motor neurons, trace-amine specific signalling requires the presence and function of the transmembrane solute carrier SLC22A but not that of monoamine transporters (DAT, SERT, and NET) (Gozal et al., 2014). Specifically, it was shown that addition of β-PEA, tyramine, or tryptamine induced locomotor like activity (LLA) firing patterns of these neurons in the presence of N-Methyl D-Aspartate. Temporally, it was found that the trace amine induction of LLA is delayed compared to serotonin and norepinephrine induced LLA, indicating the target site for the trace amines is not located on the plasma membrane and could perhaps be intracellular. Importantly, blocking of SLC22A with pentamidine abolished trace amine induced LLA, indicating that trace amine induced LLA does not act on receptors found on the plasma membrane but requires their transport to the cytosol by SLC22A for induction 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.
Moreover, in ADRA2A/TAAR1 hetero-oligomers, the capacity of NorEpi to stimulate Gi/o signaling is reduced by co-stimulation with 3-T1AM. The present study therefore points to a complex spectrum of signaling modification mediated by 3-T1AM at different G protein-coupled receptors.
Interaction of TAAR1 with D2R altered the subcellular localization of TAAR1 and increased D2R agonist binding affinity.
Other biogenic amines are present in the central nervous system at very low concentrations in the order of 0.1–10 nm, representing <1% of total biogenic amines (Berry, 2004). For these compounds, the term 'trace amines' was introduced. Although somewhat loosely defined, the molecules generally considered to be trace amines include para-tyramine, meta-tyramine, tryptamine, β-phenylethylamine, para-octopamine and meta-octopamine (Berry, 2004) (Figure 2).
[AMPH and METH] promote DA release from synaptic storage vesicles into the cytoplasm (Partilla et al., 2006) and then into the extracellular space via DAT with an EC50 of 25 nM and a Ki uptake in rat synaptosomes of 34 nM (Rothman et al., 2001). These are drug concentrations approximately 20-30 fold lower than the EC50 values Reese et al. (2007) calculated for eliciting an in vitro functional response from rTAAR1 (0.8 μM). However, experienced METH users can typically consume gram quantities of drug per day (Kramer et al., 1967) and achieve peak blood concentrations of 100 μM (Derlet et al., 1989; Baselt, 2002; Peters et al., 2003). [...] The extracellular free concentration of METH surrounding relevant human dopaminergic synapses in the brain presumably is the relevant pharmacodynamic parameter, at least in part, underlying the desirable effects of the drug. Although this value is not known with certainty for humans METH serum levels typically represent one tenth of what is found in rat brain (Riviere et al., 2000). Consequently, when considered together with the human forensic evidence the in vitro results of Reese et al. (2007) are consistent with the interpretation that in vivo the human TAAR1 is likely to be a mediator of at least some of METH's effects.
In vitro functional studies showed agonist activity of solriamfetol at human, mouse, and rat TAAR1 receptors. hTAAR1 EC50 values (10–16 μM) were within the clinically observed therapeutic solriamfetol plasma concentration range and overlapped with the observed DAT/NET inhibitory potencies of solriamfetol in vitro. TAAR1 agonist activity was unique to solriamfetol; neither the WPA modafinil nor the DAT/NET inhibitor bupropion had TAAR1 agonist activity.
Several series of substituted phenylethylamines were investigated for activity at the human TAAR1 (Table 2). A surprising finding was the potency of phenylethylamines with substituents at the phenyl C2 position relative to their respective C4-substituted congeners. In each case, except for the hydroxyl substituent, the C2-substituted compound had 8- to 27-fold higher potency than the C4-substituted compound. The C3-substituted compound in each homologous series was typically 2- to 5-fold less potent than the 2-substituted compound, except for the hydroxyl substituent. The most potent of the 2-substituted phenylethylamines was 2-chloro-β-PEA, followed by 2-fluoro-β-PEA, 2-bromo-β-PEA, 2-methoxy-β-PEA, 2-methyl-β-PEA, and then 2-hydroxy-β-PEA.The effect of β-carbon substitution on the phenylethylamine side chain was also investigated (Table 3). A β-methyl substituent was well tolerated compared with β-PEA. In fact, S-(–)-β-methyl-β-PEA was as potent as β-PEA at human TAAR1. β-Hydroxyl substitution was, however, not tolerated compared with β-PEA. In both cases of β-substitution, enantiomeric selectivity was demonstrated.In contrast to a methyl substitution on the β-carbon, an α-methyl substitution reduced potency by ~10-fold for d-amphetamine and 16-fold for l-amphetamine relative to β-PEA (Table 4). N-Methyl substitution was fairly well tolerated; however, N,N-dimethyl substitution was not.
The EC50 of (+)-AMPH to release DA via DAT is 25 nM, with Ki uptake values in rat synaptosomes for this neurotransmitter of 34 nM at the DAT, concentrations approximately 20- to 30-fold lower than the EC50 values we calculated for eliciting an in vitro functional response from rTAAR1 (0.8 μM). Chronic METH abusers can typically consume gram quantities of drug per day (Kramer et al., 1967). [...] plasma concentrations of both drugs can reach into the high-micromolar range (Drummer and Odell, 2001; Baselt, 2002; Peters et al., 2003). Although the extracellular free concentration of METH around relevant human dopaminergic synapses presumably involved in producing desirable CNS effects is not known with certainty, in rats METH serum levels are typically 1/10 what is found in brain (Riviere et al., 2000). Forensic evidence indicates that experienced METH users can typically achieve peak blood concentrations of 100 μM (Baselt, 2002; Peters et al., 2003). Both isomers of METH were full agonists of h-rChTAAR1 over a range of EC50 values from 3.5 to ~15 μM, concentrations often exceeded in the blood of human METH addicts (Derlet et al., 1989). If TAAR1s, whether expressed in the CNS or periphery, are exposed to such concentrations, it is possible they could become functionally activated. [...] Given that the EC50 values for METH and AMPH activation of the h-rChTAAR1 in vitro are well below the concentrations frequently found in addicts, we suggest that TAAR1 might be a potential mediator of some of the effects of AMPH and METH in humans, including hyperthermia and stroke.
Another feature that distinguishes [synthetic cathinones (SCs)] from amphetamines is their negligible interaction with the trace amine associated receptor 1 (TAAR1). Activation of this receptor reduces the activity of dopaminergic neurones, thereby reducing psychostimulatory effects and addictive potential (Miller, 2011; Simmler et al., 2016). Amphetamines are potent agonists of this receptor, making them likely to self‐inhibit their stimulating effects. In contrast, SCs show negligible activity towards TAAR1 (Kolaczynska et al., 2021; Rickli et al., 2015; Simmler et al., 2014, 2016). [...] It is worth noting, however, that for TAAR1 there is considerable species variability in its interaction with ligands, and it is possible that the in vitro activity of [rodent TAAR1 agonists] may not translate into activity in the human body (Simmler et al., 2016). The lack of self‐regulation by TAAR1 may partly explain the higher addictive potential of SCs compared to amphetamines (Miller, 2011; Simmler et al., 2013).
That TAAR1 signaling is coupled to the inhibition of VTA DA neuron firing was a surprising finding. [...] earlier data suggested that TAAR1 activation leads to inhibition of DA uptake by DAT, promotion of DA efflux by DAT and DAT internalization which presumably would augment extracellular DA levels, whereas inhibition of DA neuronal firing rate would likely decrease extracellular DA levels, [...] this mismatch in expectations immediately attracted the attention of those trying to determine whether TAAR1 is a METH/AMPH and DA receptor in vivo as it is in vitro (Bunzow et al., 2001; Wainscott et al., 2007; Xie and Miller, 2009; Panas et al., 2012).
Interestingly, the concentrations of amphetamine found to be necessary to activate TAAR1 are in line with what was found in drug abusers [3, 51, 52]. Thus, it is likely that some of the effects produced by amphetamines could be mediated by TAAR1. Indeed, in a study in mice, MDMA effects were found to be mediated in part by TAAR1, in a sense that MDMA auto-inhibits its neurochemical and functional actions [46]. Based on this and other studies (see other section), it has been suggested that TAAR1 could play a role in reward mechanisms and that amphetamine activity on TAAR1 counteracts their known behavioral and neurochemical effects mediated via dopamine neurotransmission.
β-Keto-analogue cathinones also exhibited approximately 10-fold lower affinity for the TA1 receptor compared with their respective non-β-keto amphetamines. [...] Activation of TA1 receptors negatively modulates dopaminergic neurotransmission. Importantly, methamphetamine decreased DAT surface expression via a TA1 receptor-mediated mechanism and thereby reduced the presence of its own pharmacological target (Xie and Miller, 2009). MDMA and amphetamine have been shown to produce enhanced DA and 5-HT release and locomotor activity in TA1 receptor knockout mice compared with wild-type mice (Lindemann et al., 2008; Di Cara et al., 2011). Because methamphetamine and MDMA auto-inhibit their neurochemical and functional effects via TA1 receptors, low affinity for these receptors may result in stronger effects on monoamine systems by cathinones compared with the classic amphetamines.
On the other hand, HTS approaches [100] followed by structure-activity optimization allowed for the discovery of the hTAAR1 antagonist RTI-7470-44, endowed with a species-specificity preference over mTAAR1 (Figure 11A) [99]. RTI-7470-44 displayed good blood–brain barrier permeability, moderate metabolic stability, and a favorable preliminary off-target profile. In addition, RTI-7470-44 increased the spontaneous firing rate of mouse ventral tegmental area (VTA) dopaminergic neurons and blocked the effects of the known TAAR1 agonist RO5166017. [...] Figure 11. (A) Chemical structures of the available hTAAR1 agonists: EPPTB [98], RTI-7470-44 [99], and 4c [33], [...] RTI-7470-44: hTAAR1 IC50 = 0.0084 μM, mTAAR1 IC50 = 1.190 μM.
While synthetic and endogenous hTAAR1 agonists have been developed (for a detailed review9), some with therapeutic potential for schizophrenia and substance abuse disorders,1,10–13 the discovery of TAAR1 antagonists remains surprisingly difficult. In the literature, the Hoffman-La Roche antagonist, [EPPTB], is the only fully characterized TAAR1 antagonist (Figure 1). [...] In the literature, a few additional antagonists have been identified (Cmpd 3, ET-54, ET-78),17–18 but all have weak potencies and none have been fully characterized at human, rat, and mouse TAAR1, making potency comparisons difficult (Figure 1; Cmpd 3: hTAAR1 IC50 = 9 μM, ET-54: rTAAR1 IC50 = 3 μM, ET-78: rTAAR1 IC50 = 4 μM).
A few additional antagonists (Cmpd 3, ET-92, ET-78)46,47 have been identified in the literature, but none have been fully characterized and all suffer from potency issues (Cmpd 3: hTAAR1 IC50 = 9 µM, ET-92: rTAAR1 IC50 = 3 µM, ET-78: rTAAR1 IC50 = 4 µM). The discovery of our antagonists is groundbreaking, as our compounds are the most potent hTAAR1 antagonists identified to date (hTAAR1 IC50 = 206 and 281 nM).
VMAT2 is the CNS vesicular transporter for not only the biogenic amines DA, NE, EPI, 5-HT, and HIS, but likely also for the trace amines TYR, PEA, and thyronamine (THYR) ... [Trace aminergic] neurons in mammalian CNS would be identifiable as neurons expressing VMAT2 for storage, and the biosynthetic enzyme aromatic amino acid decarboxylase (AADC). ... AMPH release of DA from synapses requires both an action at VMAT2 to release DA to the cytoplasm and a concerted release of DA from the cytoplasm via "reverse transport" through DAT.
The active form of the drug has a central nervous system stimulating activity by the primary inhibition of DAT, NET, trace amine-associated receptor 1 (TAAR1) and vesicular monoamine transporter 2 (SLC18A2), among other targets, therefore regulating the reuptake and release of catecholamines (primarily NE and DA) on the synaptic cleft. ... LDX can also promote the increase of DA in the synaptic cleft by activating protein TAAR1, which produces the efflux of monoamine NTs, mainly DA, from storage sites on presynaptic neurons. TAAR1 activation leads to intracellular cAMP signalling that results in PKA and PKC phosphorylation and activation. This PKC activation decreases DAT1, NET1 and SERT cell surface expression, intensifying the direct blockage of monoamine transporters by LDX and improving the neurotransmission imbalance in ADHD.
Despite the challenges in determining synaptic vesicle pH, the proton gradient across the vesicle membrane is of fundamental importance for its function. Exposure of isolated catecholamine vesicles to protonophores collapses the pH gradient and rapidly redistributes transmitter from inside to outside the vesicle. ... Amphetamine and its derivatives like methamphetamine are weak base compounds that are the only widely used class of drugs known to elicit transmitter release by a non-exocytic mechanism. As substrates for both DAT and VMAT, amphetamines can be taken up to the cytosol and then sequestered in vesicles, where they act to collapse the vesicular pH gradient.
inhibition of firing due to increased release of dopamine; (b) reduction of D2 and GABAB receptor-mediated inhibitory responses (excitatory effects due to disinhibition); and (c) a direct TA1 receptor-mediated activation of GIRK channels which produce cell membrane hyperpolarization.
AMPH also increases intracellular calcium (Gnegy et al., 2004) that is associated with calmodulin/CamKII activation (Wei et al., 2007) and modulation and trafficking of the DAT (Fog et al., 2006; Sakrikar et al., 2012). ... For example, AMPH increases extracellular glutamate in various brain regions including the striatum, VTA and NAc (Del Arco et al., 1999; Kim et al., 1981; Mora and Porras, 1993; Xue et al., 1996), but it has not been established whether this change can be explained by increased synaptic release or by reduced clearance of glutamate. ... DHK-sensitive, EAAT2 uptake was not altered by AMPH (Figure 1A). The remaining glutamate transport in these midbrain cultures is likely mediated by EAAT3 and this component was significantly decreased by AMPH
AMPH and METH also stimulate DA efflux, which is thought to be a crucial element in their addictive properties [80], although the mechanisms do not appear to be identical for each drug [81]. These processes are PKCβ– and CaMK–dependent [72, 82], and PKCβ knock-out mice display decreased AMPH-induced efflux that correlates with reduced AMPH-induced locomotion [72].
Gs-mediated cAMP production was initially considered as the hallmark of TAAR1 activation, but we have now evidence that TAAR1 can also activate inward rectifying potassium channels and the β-arrestin 2 pathway, probably by Gs-independent pathways.
Gs-mediated cAMP production was initially considered as the hallmark of TAAR1 activation, but we have now evidence that TAAR1 can also activate inward rectifying potassium channels and the β-arrestin 2 pathway, probably by Gs-independent pathways.
Amphetamines have at least three mechanisms of action: (1) they are transported by the monoamine transporters DAT and NET, thus competing with those neurotransmitters and decreasing their reuptake in the synapse; (2) They also cause trace amine-associated receptor 1 (TAAR1) to phosphorylate DAT. The phosphorylated DAT is either internalized into the presynaptic neuron and ceases transport or inverses the efflux of dopamine; (3) they enter the presynaptic monoamine vesicle and cause efflux of neurotransmitters off the vesicle, which in turn augments the efflux towards the synapse. These mechanisms are more studied and established for dopamine neurotransmission, but are thought to occur similarly for norepinephrine.
The mechanism of efflux of monoamines in the synapse is due to the activation of TAAR1 by TAs or drugs belonging to the amphetamine class which increases the level of cAMP (cyclic adenosine monophosphate) followed by an increase in the level of PKA (protein kinase A) and PKC (protein kinase C) phosphorylation. This reverses the monoamine transport by reversing the direction of monoamine transporters.
The study further showed that amphetamine (AMPH) activated TAAR1 by interacting with G13 and GS α-subunits to increase RhoA and PKA activity, respectively (Underhill et al. 2021).
TAAR1 is an intracellular G protein-coupled receptor (GPCR), which has activity that promotes the endocytosis of both DAT and the excitatory amino acid transporter 3 (EAAT3). DAT endocytosis, together with the aforementioned mechanisms, contributes to the enhancement of the dopaminergic neurotransmission
AMPHs also reduce VTA DA neuron firing. Reduced VTA DA neuron firing is generally expected to decrease the amount of DA in the synapse, whereas reduced DA uptake has the opposite effect. Thus, it is necessary to understand the effects of TAAR1 agonists on both release and uptake to discern their net effect on synaptic DA. ... AMPH-induced DA release (as measured by release V max ) was dependent on TAAR1. Of note, we also found that AMPH did not induce DA efflux in TAAR1 -/- cells with lower concentrations of DA. This agrees with previous reports that the largest component of AMPH-induced DA efflux is TAAR1-dependent, whereas TAAR1-independent AMPH-induced efflux is detectable only at micromolar DA concentrations. ... Overall, comparison of the effects of 3 investigational TAAR1 agonists and the TAAR1 agonist AMPH illustrated the unique pharmacology of each at DAT, in contrast to their comparable effect on decreasing VTA DA neuron firing. AMPH, despite decreasing surface expression of DAT and DA uptake, induced efflux of DA. Notably, all of these effects were TAAR1-dependent, consistent with prior reports.
The dysregulation of TA levels has been linked to several diseases, which highlights the corresponding members of the TAAR family as potential targets for drug development. In this article, we focus on the relevance of TAs and their receptors to nervous system-related disorders, namely schizophrenia and depression; however, TAs have also been linked to other diseases such as migraine, attention deficit hyperactivity disorder, substance abuse and eating disorders [7,8,36]. Clinical studies report increased β-PEA plasma levels in patients suffering from acute schizophrenia [37] and elevated urinary excretion of β-PEA in paranoid schizophrenics [38], which supports a role of TAs in schizophrenia. As a result of these studies, β-PEA has been referred to as the body's 'endogenous amphetamine' [39]
Although the functional role of trace amines in mammals remains largely enigmatic, it has been noted that trace amine levels can be altered in various human disorders, including schizophrenia, Parkinson's disease, attention deficit hyperactivity disorder (ADHD), Tourette syndrome, and phenylketonuria (Boulton, 1980; Sandler et al., 1980). It was generally held that trace amines affect the monoamine system indirectly via interaction with plasma membrane transporters [such as plasma membrane dopamine transporter (DAT)] and vesicular storage (Premont et al., 2001; Branchek and Blackburn, 2003; Berry, 2004; Sotnikova et al., 2004). ...Furthermore, DAT-deficient mice provide a model to investigate the inhibitory actions of amphetamines on hyperactivity, the feature of amphetamines believed to be important for their therapeutic action in ADHD (Gainetdinov et al., 1999; Gainetdinov and Caron, 2003). It should be noted also that the best-established agonist of TAAR1, β-PEA, shared the ability of amphetamine to induce inhibition of dopamine-dependent hyperactivity of DAT-KO mice (Gainetdinov et al., 1999; Sotnikova et al., 2004).Furthermore, if TAAR1 could be proven as a mediator of some of amphetamine's actions in vivo, the development of novel TAAR1-selective agonists and antagonists could provide a new approach for the treatment of amphetamine-related conditions such as addiction and/or disorders in which amphetamine is used therapeutically. In particular, because amphetamine has remained the most effective pharmacological treatment in ADHD for many years, a potential role of TAAR1 in the mechanism of the "paradoxical" effectiveness of amphetamine in this disorder should be explored.
changes in trace amines, in particular PE, have been identified as a possible factor for the onset of attention deficit/hyperactivity disorder (ADHD) [5, 27, 43, 78]. PE has been shown to induce hyperactivity and aggression, two of the cardinal clinical features of ADHD, in experimental animals [100]. Hyperactivity is also a symptom of phenylketonuria, which as discussed above is associated with a markedly elevated PE turnover [44]. Further, amphetamines, which have clinical utility in ADHD, are good ligands at trace amine receptors [2]. Of possible relevance in this aspect is modafanil, which has shown beneficial effects in ADHD patients [101] and has been reported to enhance the activity of PE at TAAR1 [102]. Conversely, methylphenidate, which is also clinically useful in ADHD, showed poor efficacy at the TAAR1 receptor [2]. In this respect it is worth noting that the enhancement of functioning at TAAR1 seen with modafanil was not a result of a direct interaction with TAAR1 [102].More direct evidence has been obtained recently for a role of trace amines in ADHD. Urinary PE levels have been reported to be decreased in ADHD patients in comparison to both controls and patients with autism [103-105]. Evidence for a decrease in PE levels in the brain of ADHD patients has also recently been reported [4]. In addition, decreases in the urine and plasma levels of the PE metabolite phenylacetic acid and the precursors phenylalanine and tyrosine have been reported along with decreases in plasma tyramine [103]. Following treatment with methylphenidate, patients who responded positively showed a normalization of urinary PE, whilst non-responders showed no change from baseline values [105].
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