Structural and signaling mechanisms of TAAR1 enabled preferential agonist design.

Trace amine-associated receptor 1 (TAAR1) senses a spectrum of endogenous amine-containing metabolites (EAMs) to mediate diverse psychological functions and is useful for schizophrenia treatment without the side effects of catalepsy. Here, we systematically profiled the signaling properties of TAAR1 activation and present nine structures of TAAR1-Gs/Gq in complex with EAMs, clinical drugs, and synthetic compounds. These structures not only revealed the primary amine recognition pocket (PARP) harboring the conserved acidic D3.32 for conserved amine recognition and "twin" toggle switch for receptor activation but also elucidated that targeting specific residues in the second binding pocket (SBP) allowed modulation of signaling preference. In addition to traditional drug-induced Gs signaling, Gq activation by EAM or synthetic compounds is beneficial to schizophrenia treatment. Our results provided a structural and signaling framework for molecular recognition by TAAR1, which afforded structural templates and signal clues for TAAR1-targeted candidate compounds design.

Ancient and Nonuniform Loss of Olfactory Receptor Expression Renders the Shark Nose a De Facto Vomeronasal Organ.

Cartilaginous fishes are renowned for a keen sense of smell, a reputation based on behavioral observations and supported by the presence of large and morphologically complex olfactory organs. At the molecular level, genes belonging to the four families coding for most olfactory chemosensory receptors in other vertebrates have been identified in a chimera and a shark, but it was unknown whether they actually code for olfactory receptors in these species. Here, we describe the evolutionary dynamics of these gene families in cartilaginous fishes using genomes of a chimera, a skate, a sawfish, and eight sharks. The number of putative OR, TAAR, and V1R/ORA receptors is very low and stable, whereas the number of putative V2R/OlfC receptors is higher and much more dynamic. In the catshark Scyliorhinus canicula, we show that many V2R/OlfC receptors are expressed in the olfactory epithelium in the sparsely distributed pattern characteristic for olfactory receptors. In contrast, the other three vertebrate olfactory receptor families are either not expressed (OR) or only represented with a single receptor (V1R/ORA and TAAR). The complete overlap of markers of microvillous olfactory sensory neurons with pan-neuronal marker HuC in the olfactory organ suggests the same cell-type specificity of V2R/OlfC expression as for bony fishes, that is, in microvillous neurons. The relatively low number of olfactory receptors in cartilaginous fishes compared with bony fishes could be the result of an ancient and constant selection in favor of a high olfactory sensitivity at the expense of a high discrimination capability.

Discovery of a Novel Chemo-Type for TAAR1 Agonism via Molecular Modeling.

The search for novel effective TAAR1 ligands continues to draw great attention due to the wide range of pharmacological applications related to TAAR1 targeting. Herein, molecular docking studies of known TAAR1 ligands, characterized by an oxazoline core, have been performed in order to identify novel promising chemo-types for the discovery of more active TAAR1 agonists. In particular, the oxazoline-based compound S18616 has been taken as a reference compound for the computational study, leading to the development of quite flat and conformationally locked ligands. The choice of a "Y-shape" conformation was suggested for the design of TAAR1 ligands, interacting with the protein cavity delimited by ASP103 and aromatic residues such as PHE186, PHE195, PHE268, and PHE267. The obtained results allowed us to preliminary in silico screen an in-house series of pyrimidinone-benzimidazoles (1a-10a) as a novel scaffold to target TAAR1. Combined ligand-based (LBCM) and structure based (SBCM) computational methods suggested the biological evaluation of compounds 1a-10a, leading to the identification of derivatives 1a-3a (hTAAR1 EC50 = 526.3-657.4 nM) as promising novel TAAR1 agonists.

Vasoconstrictor Effects of Trace Amine-Associated Receptor Agonists in the Isolated Perfused Rat Kidney.

INTRODUCTION: Endogenous trace amines such as tryptamine and 3-iodothyronamine (T1AM) are present in mammalian tissues at very low concentrations. They produce their actions by activating surface G protein-coupled receptors known as trace amine-associated receptors (TAARs). OBJECTIVE: The study was designed to investigate the possible vasoconstrictor effects of tryptamine, T1AM, and the selective TAAR1 agonist RO5263397 in isolated perfused rat kidney. METHODS: Renal vascular reactivity experiment using male Wistar Kyoto (WKY, n = 76) and spontaneously hypertensive rats (SHRs, n = 81) were used in this study. RESULTS: Tryptamine (1011-106 mole), T1AM (1011-106 mole), and RO5263397 (1011-106 mole) increased perfusion pressure in preparations from WKY rats and SHRs in a dose-dependent manner. EPPTB, a selective TAAR1 antagonist (10-6 M), significantly (p < 0.05) reduced agonist-induced increase in perfusion pressure in both WKY rats and SHRs, suggesting a role for TAAR1 activation in these responses. The vasoconstrictor responses in both groups were significantly reduced by L-type dihydropyridine calcium channel blocker, Rho-kinase, and protein kinase C (PKC) inhibitors suggesting the involvement of extracellular calcium and enhanced calcium sensitization. Reactive oxygen species (ROS) scavenger TEMPO significantly inhibited the agonist-induced increase in perfusion pressure in preparations from SHRs but not WKY. CONCLUSION: The TAARs enhanced agonist-induced increase in perfusion causing vasoconstriction in the kidney involves the influx of extracellular calcium, increased calcium sensitization, and ROS in SHRs animals only.

Evolution of Brain-Expressed Biogenic Amine Receptors into Olfactory Trace Amine-Associated Receptors.

The family of trace amine-associated receptors (TAARs) is distantly related to G protein-coupled biogenic aminergic receptors. TAARs are found in the brain as well as in the olfactory epithelium where they detect biogenic amines. However, the functional relationship of receptors from distinct TAAR subfamilies and in different species is still uncertain. Here, we perform a thorough phylogenetic analysis of 702 TAAR-like (TARL) and TAAR sequences from 48 species. We show that a clade of Tarl genes has greatly expanded in lampreys, whereas the other Tarl clade consists of only one or two orthologs in jawed vertebrates and is lost in amniotes. We also identify two small clades of Taar genes in sharks related to the remaining Taar genes in bony vertebrates, which are divided into four major clades. We further identify ligands for 61 orphan TARLs and TAARs from sea lamprey, shark, ray-finned fishes, and mammals, as well as novel ligands for two 5-hydroxytryptamine receptor 4 orthologs, a serotonin receptor subtype closely related to TAARs. Our results reveal a pattern of functional convergence and segregation: TARLs from sea lamprey and bony vertebrate olfactory TAARs underwent independent expansions to function as chemosensory receptors, whereas TARLs from jawed vertebrates retain ancestral response profiles and may have similar functions to TAAR1 in the brain. Overall, our data provide a comprehensive understanding of the evolution and ligand recognition profiles of TAARs and TARLs.

In Vitro Comparison of Ulotaront (SEP-363856) and Ralmitaront (RO6889450): Two TAAR1 Agonist Candidate Antipsychotics.

BACKGROUND: Trace amine-associated receptor-1 (TAAR1) agonists have been proposed as potential antipsychotics, with ulotaront and ralmitaront having reached clinical trials. While ulotaront demonstrated efficacy in a recent Phase II trial, a corresponding study studies of ralmitaront failed to show efficacy as a monotherapy or as an adjunct to atypical antipsychotics. In addition to TAAR1 agonism, ulotaront is a partial agonist at the serotonin 1A receptor (5-HT1AR). However, little is known about ralmitaront. METHODS: We compared ulotaront and ralmitaront at TAAR1, 5-HT1AR, and dopamine D2 using luciferase complementation-based G protein recruitment, cAMP accumulation, and G protein-coupled inward rectifier potassium channel activation assays. RESULTS: Ralmitaront showed lower efficacy at TAAR1 in G protein recruitment, cAMP accumulation, and GIRK activation assays. Moreover, ralmitaront lacked detectable activity at 5-HT1AR and dopamine D2. CONCLUSIONS: Compared with ulotaront, ralmitaront shows lower efficacy and slower kinetics at TAAR1 and lacks efficacy at 5-HT1AR. These data may be relevant to understanding differences in clinical profiles of these 2 compounds.

Untargeted metabolomics in the aqueous humor reveals the involvement of TAAR pathway in glaucoma.

Understanding the metabolic dysfunctions and underlying complex pathological mechanisms of neurodegeneration in glaucoma could help discover disease pathways, identify novel biomarkers, and rationalize newer therapeutics. Therefore, we aimed to investigate the local metabolomic alterations in the aqueous humor and plasma of primary glaucomatous patients. This study cohort comprised primary open-angle glaucoma (POAG), primary angle-closure glaucoma (PACG), and cataract control groups. Aqueous humor and plasma samples were collected from patients undergoing trabeculectomy or cataract surgery and subjected to high-resolution mass spectrometry (HRMS) analysis. Spectral information was processed, and the acquired data were subjected to uni-variate as well as multi-variate statistical analyses using MetaboAnalyst ver5.0. To further understand the localized metabolic abnormalities in glaucoma, metabolites affected in aqueous humor were distinguished from metabolites altered in plasma in this study. Nine and twelve metabolites were found to be significantly altered (p < 0.05, variable importance of projection >1 and log2 fold change ≥0.58/≤ -0.58) in the aqueous humor of PACG and POAG patients, respectively. The galactose and amino acid metabolic pathways were locally affected in the PACG and POAG groups, respectively. Based on the observation of the previous findings, gene expression profiles of trace amine-associated receptor-1 (TAAR-1) were studied in rat ocular tissues. The pharmacodynamics of TAAR-1 were explored in rabbits using topical administration of its agonist, ß-phenyl-ethylamine (ß-PEA). TAAR-1 was expressed in the rat's iris-ciliary body, optic nerve, lens, and cornea. ß-PEA elicited a mydriatic response in rabbit eyes, without altering intraocular pressure. Targeted analysis of ß-PEA levels in the aqueous humor of POAG patients showed an insignificant elevation. This study provides new insights regarding alterations in both localized and systemic metabolites in primary glaucomatous patients. This study also demonstrated the propensity of ß-PEA to cause an adrenergic response through the TAAR-1 pathway.

Ulotaront: review of preliminary evidence for the efficacy and safety of a TAAR1 agonist in schizophrenia.

Ulotaront is a trace amine-associated receptor 1 (TAAR1) agonist in Phase 3 clinical development for the treatment of schizophrenia. Ulotaront was discovered through a unique, target-agnostic approach optimized to identify drug candidates lacking D2 and 5-HT2A receptor antagonism, while demonstrating an antipsychotic-like phenotypic profile in vivo. The mechanism of action (MOA) of ulotaront is thought to be mediated by agonism at TAAR1 and serotonin 5-HT1A receptors. Ulotaront has completed two Phase 2 trials (4-week acute study and 26-week open-label extension) which led to Breakthrough Therapy Designation from the US Food and Drug Administration for the treatment of schizophrenia. In the double-blind, placebo-controlled, acute study, ulotaront was associated with significant (p < 0.001) improvement in Positive and Negative Syndrome Scale (PANSS) total score (effect size [ES]: 0.45), with improvements vs. placebo also observed across secondary endpoints. Post-hoc analyses of the acute trial revealed additional evidence to support the effect of ulotaront on negative symptoms. In the 4-week study, ulotaront was well-tolerated, with an incidence of adverse events (AEs) numerically lower compared to placebo (45.8% vs. 50.4%; with a number needed to harm [NNH] for individual ulotaront AEs all > 40). The open-label extension demonstrated further improvement across schizophrenia symptoms and confirmed the tolerability of ulotaront, with a 6-month completion rate of 67%. Based on current data, ulotaront shows potential to be a first-in-class TAAR1 agonist for the treatment of schizophrenia with a safety and efficacy profile distinct from current antipsychotics.

Residues of ractopamine, a livestock feed additive, in meat might alleviate misuse of cocaine, nicotine, methamphetamine, and morphine.

Background: Substance misuse brings tremendous harm to global health. Strategies for the treatment and prevention of drug addiction are in urgent need. Aim: Trace amine-associated receptor 1 (TAAR1) widely distributed in the central nervous system has been identified as a hopeful target in the management of certain substance abuse. Discovery of food ingredients that act on TAAR1 might help health care providers develop chemoprevention for substance misuse disorders. Methods: Animal experiments clearly demonstrated the capability of TAAR1 agonists in attenuating addictive behavior regarding cocaine, nicotine, methamphetamine, and morphine. Ractopamine, a livestock feed additive used in the United States for over 20 years, has proven to be a full TAAR1 agonist. Literature review and internet web database survey were performed to see if ractopamine residues in meat could affect substance addiction behavior. Results: Integrating all available epidemiologic studies revealed that the prevalence of cocaine, nicotine, methamphetamine, and opioid misuse showed steadily downward or stable trends coincidently during the same time period of ractopamine use in the United States. Conclusion: A hypothesis is thus raised here that ractopamine residues in meat might have contributed secretly to the smoothened prevalence curves of cocaine, nicotine, methamphetamine, and opioids addiction.

Ractopamine residues in meat might reduce the risk of type 2 diabetes.

Background: The overall prevalence of diabetes in the world has risen substantially in the past several decades, so have complications and mortalities associated with it. Aim: Prevention strategies for diabetes thus become an urgent public health need for reducing the burden of diabetes. Methods: Ractopamine, a ß1/2-adrenergic receptor agonist, has been approved for use in finishing swine, cattle, and turkey in countries where meat exporting brings tremendous economic benefits. This leanness enhancer is recently found to be a full agonist at trace amine-associated receptor 1 also. A thorough literature review was performed to assess possible effects of ractopamine on glucose metabolism. Results: Activating ß-adrenoceptor could lead to glucose-lowering effects independent of insulin while activation on trace amine-associated receptor 1 induces an incretin-like signaling on insulin-secreting pancreatic ß-cells. Conclusion: Accordingly, it is hypothesized that long-term consuming meat containing ractopamine might lower the risk of type 2 diabetes.

T1AM/TAAR1 System Reduces Inflammatory Response and ß-Amyloid Toxicity in Human Microglial HMC3 Cell Line.

Microglial dysfunction is one of the hallmarks and leading causes of common neurodegenerative diseases (NDDs), including Alzheimer's disease (AD) and Parkinson's disease (PD). All these pathologies are characterized by aberrant aggregation of disease-causing proteins in the brain, which can directly activate microglia, trigger microglia-mediated neuroinflammation, and increase oxidative stress. Inhibition of glial activation may represent a therapeutic target to alleviate neurodegeneration. Recently, 3-iodothyronamine (T1AM), an endogenous derivative of thyroid hormone (TH) able to interact directly with a specific GPCR known as trace amine-associated receptor 1 (TAAR1), gained interest for its ability to promote neuroprotection in several models. Nevertheless, T1AM's effects on microglial disfunction remain still elusive. In the present work we investigated whether T1AM could inhibit the inflammatory response of human HMC3 microglial cells to LPS/TNFα or ß-amyloid peptide 25-35 (Aß25-35) stimuli. The results of ELISA and qPCR assays revealed that T1AM was able to reduce microglia-mediated inflammatory response by inhibiting the release of proinflammatory factors, including IL-6, TNFα, NF-kB, MCP1, and MIP1, while promoting the release of anti-inflammatory mediators, such as IL-10. Notably, T1AM anti-inflammatory action in HMC3 cells turned out to be a TAAR1-mediated response, further increasing the relevance of the T1AM/TAAR1 system in the management of NDDs.

Expression Pattern of Trace Amine-Associated Receptors during Differentiation of Human Pluripotent Stem Cells to Dopaminergic Neurons.

Trace amine-associated receptors (TAARs), which were discovered only in 2001, are known to be involved in the regulation of a spectrum of neuronal processes and may play a role in the pathogenesis of a number of neuropsychiatric diseases, such as schizophrenia and others. We have previously shown that TAARs also have interconnections with the regulation of neurogenesis and, in particular, with the neurogenesis of dopamine neurons, but the exact mechanisms of this are still unknown. In our work we analyzed the expression of TAARs (TAAR1, TAAR2, TAAR5, TAAR6, TAAR8 and TAAR9) in cells from the human substantia nigra and ventral tegmental areas and in human pluripotent stem cells at consecutive stages of their differentiation to dopaminergic neurons, using RNA sequencing data from open databases, and TaqMan PCR data from the differentiation of human induced pluripotent stem cells in vitro. Detectable levels of TAARs expression were found in cells at the pluripotent stages, and the dynamic of their expression had a trend of increasing with the differentiation and maturation of dopamine neurons. The expression of several TAAR types (particularly TAAR5) was also found in human dopaminergic neuron-enriched zones in the midbrain. This is the first evidence of TAARs expression during neuronal differentiation, which can help to approach an understanding of the role of TAARs in neurogenesis.

Striking Neurochemical and Behavioral Differences in the Mode of Action of Selegiline and Rasagiline.

Selegiline and rasagiline are two selective monoamine oxidase B (MAO-B) inhibitors used in the treatment of Parkinson's disease. In their clinical application, however, differences in L-dopa-sparing potencies have been observed. The aim of this study was to find neurochemical and behavioral explanations for the antiparkinsonian effects of these drugs. We found that selegiline possesses a dopaminergic enhancer effect: it stimulated the electrically induced [3H]dopamine release without influencing the resting [3H]dopamine release from rat striatal slices in 10-10-10-9 mol/L concentrations. Rasagiline added in 10-13 to 10-5 mol/L concentrations did not alter the resting or electrically stimulated [3H]dopamine release. Rasagiline (10-9 mol/L), however, suspended the stimulatory effect of selegiline on the electrically induced [3H]dopamine release. The trace amine-associated receptor 1 (TAAR1) antagonist EPPTB (10-8-10-7 mol/L) also inhibited the stimulatory effect of selegiline on [3H]dopamine release. The effect of selegiline in its enhancer dose (5.33 nmol/kg) against tetrabenazine-induced learning deficit measured in a shuttle box apparatus was abolished by a 5.84 nmol/kg dose of rasagiline. The selegiline metabolite (-)methamphetamine (10-9 mol/L) also exhibited enhancer activity on [3H]dopamine release. We have concluded that selegiline acts as an MAO-B inhibitor and a dopaminergic enhancer drug, and the latter relates to an agonist effect on TAAR1. In contrast, rasagiline is devoid of enhancer activity but may act as an antagonist on TAAR1.

Trace amine-associated receptor 1 regulation of Kv1.4 channels in trigeminal ganglion neurons contributes to nociceptive behaviors.

BACKGROUND: Trace amines, such as tyramine, are endogenous amino acid metabolites that have been hypothesized to promote headache. However, the underlying cellular and molecular mechanisms remain unknown. METHODS: Using patch-clamp recording, immunostaining, molecular biological approaches and behaviour tests, we elucidated a critically functional role of tyramine in regulating membrane excitability and pain sensitivity by manipulating Kv1.4 channels in trigeminal ganglion (TG) neurons. RESULTS: Application of tyramine to TG neurons decreased the A-type K+ current (IA) in a manner dependent on trace amine-associated receptor 1 (TAAR1). Either siRNA knockdown of Gαo or chemical inhibition of ßγ subunit (Gßγ) signaling abrogated the response to tyramine. Antagonism of protein kinase C (PKC) prevented the tyramine-induced IA response, while inhibition of conventional PKC isoforms or protein kinase A elicited no such effect. Tyramine increased the membrane abundance of PKCθ in TG neurons, and either pharmacological or genetic inhibition of PKCθ blocked the TAAR1-mediated IA decrease. Furthermore, PKCθ-dependent IA suppression was mediated by Kv1.4 channels. Knockdown of Kv1.4 abrogated the TAAR1-induced IA decrease, neuronal hyperexcitability, and pain hypersensitivity. In a mouse model of migraine induced by electrical stimulation of the dura mater surrounding the superior sagittal sinus, blockade of TAAR1 signaling attenuated mechanical allodynia; this effect was occluded by lentiviral overexpression of Kv1.4 in TG neurons. CONCLUSION: These results suggest that tyramine induces Kv1.4-mediated IA suppression through stimulation of TAAR1 coupled to the Gßγ-dependent PKCθ signaling cascade, thereby enhancing TG neuronal excitability and mechanical pain sensitivity. Insight into TAAR1 signaling in sensory neurons provides attractive targets for the treatment of headache disorders such as migraine.

Convergent olfactory trace amine-associated receptors detect biogenic polyamines with distinct motifs via a conserved binding site.

Biogenic amines activate G-protein-coupled receptors (GPCRs) in the central nervous system in vertebrate animals. Several biogenic amines, when excreted, stimulate trace amine-associated receptors (TAARs), a group of GPCRs in the main olfactory epithelium, and elicit innate behaviors. How TAARs recognize amines with varying numbers of amino groups is largely unknown. We reasoned that a comparison between lamprey and mammalian olfactory TAARs, which are thought to have evolved independently but show convergent responses to polyamines, may reveal structural determinants of amine recognition. Here, we demonstrate that sea lamprey TAAR365 (sTAAR365) responds strongly to biogenic polyamines cadaverine, putrescine, and spermine, and shares a similar response profile as a mammalian TAAR (mTAAR9). Docking and site-directed mutagenesis analyses show that both sTAAR365 and mTAAR9 recognize the two amino groups of cadaverine with the conserved Asp3.32 and Tyr6.51 residues. sTAAR365, which has remarkable sensitivity for cadaverine (EC50 = 4 nM), uses an extra residue, Thr7.42, to stabilize ligand binding. These cadaverine recognition sites also interact with amines with four and three amino groups (spermine and spermidine, respectively). Glu7.36 of sTAAR365 cooperates with Asp3.32 and Thr7.42 to recognize spermine, whereas mTAAR9 recognizes spermidine through an additional aromatic residue, Tyr7.43. These results suggest a conserved mechanism whereby independently evolved TAAR receptors recognize amines with two, three, or four amino groups using the same recognition sites, at which sTAAR365 and mTAAR9 evolved distinct motifs. These motifs interact directly with the amino groups of the polyamines, a class of potent and ecologically important odorants, mediating olfactory signaling.

Lamprey possess both V1R and V2R olfactory receptors, but only V1Rs are expressed in olfactory sensory neurons.

The sense of smell employs some of the largest gene families in the genome to detect and distinguish a multitude of different odors. Within vertebrates, 4 major olfactory receptor families have been described; of which, only 3 (OR, TAAR-like, and V1R) were found already in lamprey, a jawless vertebrate. The forth family (V2R) was believed to have originated later, in jawed vertebrates. Here we have delineated the entire vomeronasal receptor repertoire in 3 lamprey species. We report the presence of 6 v1r and 2 v2r genes in Lethenteron camtschaticum, arctic lamprey, and Lampetra fluviatilis, river lamprey (6 and 1, respectively, in sea lamprey, Petromyzon marinus). Three v1r genes but no v2r genes were found to be expressed in olfactory sensory neurons in the characteristic sparse expression pattern. Our results show the olfactory function of some V1Rs already in lamprey and, unexpectedly, an early origin of the V2R family in the shared ancestor of jawed and jawless vertebrates. However, lamprey v2r genes appear not to have acquired an olfactory function yet, thus dissociating the evolutionary origin of the family from the onset of a function as olfactory receptor.

Migraine signaling pathways: amino acid metabolites that regulate migraine and predispose migraineurs to headache.

Migraine is a common, debilitating disorder for which attacks typically result in a throbbing, pulsating headache. Although much is known about migraine, its complexity renders understanding the complete etiology currently out of reach. However, two important facts are clear, the brain and the metabolism of the migraineur differ from that of the non-migraineur. This review centers on the altered amino acid metabolism in migraineurs and how it helps define the pathology of migraine.

Discovery of Novel Trace Amine-Associated Receptor 5 (TAAR5) Antagonists Using a Deep Convolutional Neural Network.

Trace amine-associated receptor 5 (TAAR5) is a G protein-coupled receptor that belongs to the TAARs family (TAAR1-TAAR9). TAAR5 is expressed in the olfactory epithelium and is responsible for sensing 3-methylamine (TMA). However, recent studies showed that TAAR5 is also expressed in the limbic brain regions and is involved in the regulation of emotional behaviour and adult neurogenesis, suggesting that TAAR5 antagonism may represent a novel therapeutic strategy for anxiety and depression. We used the AtomNet® model, the first deep learning neural network for structure-based drug discovery, to identify putative TAAR5 ligands and tested them in an in vitro BRET assay. We found two mTAAR5 antagonists with low to submicromolar activity that are able to inhibit the cAMP production induced by TMA. Moreover, these two compounds also inhibited the mTAAR5 downstream signalling, such as the phosphorylation of CREB and ERK. These two hits exhibit drug-like properties and could be used to further develop more potent TAAR5 ligands with putative anxiolytic and antidepressant activity.

Trace Amine Associate Receptor 1 (TAAR1) as a New Target for the Treatment of Cognitive Dysfunction in Alzheimer's Disease.

Worldwide, approximately 27 million people are affected by Alzheimer's disease (AD). AD pathophysiology is believed to be caused by the deposition of the ß-amyloid peptide (Aß). Aß can reduce long-term potentiation (LTP), a form of synaptic plasticity that is closely associated with learning and memory and involves postsynaptic glutamate receptor phosphorylation and trafficking. Moreover, Aß seems to be able to reduce glutamatergic transmission by increasing the endocytosis of NMDA receptors. Trace amines (TAs) are biogenic amines that are structurally similar to monoamine neurotransmitters. TAs bind to G protein-coupled receptors, called TAARs (trace amine-associated receptors); the best-studied member of this family, TAAR1, is distributed in the cortical and limbic structures of the CNS. It has been shown that the activation of TAAR1 can rescue glutamatergic hypofunction and that TAAR1 can modulate glutamate NMDA receptor-related functions in the frontal cortex. Several lines of evidence also suggest the pro-cognitive action of TAAR1 agonists in various behavioural experimental protocols. Thus, we studied, in vitro, the role of the TAAR1 agonist RO5256390 on basal cortical glutamatergic transmission and tested its effect on Aß-induced dysfunction. Furthermore, we investigated, in vivo, the role of TAAR1 in cognitive dysfunction induced by Aß infusion in Aß-treated mice. In vitro data showed that Aß 1-42 significantly decreased NMDA cell surface expression while the TAAR1 agonist RO5256390 promoted their membrane insertion in cortical cells. In vivo, RO5256390 showed a mild pro-cognitive effect, as demonstrated by the better performance in the Y maze test in mice treated with Aß. Further studies are needed to better understand the interplay between TAAR1/Aß and glutamatergic signalling, in order to evaluate the eventual beneficial effect in different experimental paradigms and animal models. Taken together, our data indicate that TAAR1 agonism may provide a novel therapeutic approach in the treatments of disorders involving Aß-induced cognitive impairments, such as AD.

Enhancer Regulation of Dopaminergic Neurochemical Transmission in the Striatum.

The trace amine-associated receptor 1 (TAAR1) is a Gs protein-coupled, intracellularly located metabotropic receptor. Trace and classic amines, amphetamines, act as agonists on TAAR1; they activate downstream signal transduction influencing neurotransmitter release via intracellular phosphorylation. Our aim was to check the effect of the catecholaminergic activity enhancer compound ((-)BPAP, (R)-(-)-1-(benzofuran-2-yl)-2-propylaminopentane) on neurotransmitter release via the TAAR1 signaling. Rat striatal slices were prepared and the resting and electrical stimulation-evoked [3H]dopamine release was measured. The releaser (±)methamphetamine evoked non-vesicular [3H]dopamine release in a TAAR1-dependent manner, whereas (-)BPAP potentiated [3H]dopamine release with vesicular origin via TAAR1 mediation. (-)BPAP did not induce non-vesicular [3H]dopamine release. N-Ethylmaleimide, which inhibits SNARE core complex disassembly, potentiated the stimulatory effect of (-)BPAP on vesicular [3H]dopamine release. Subsequent analyses indicated that the dopamine-release stimulatory effect of (-)BPAP was due to an increase in PKC-mediated phosphorylation. We have hypothesized that there are two binding sites present on TAAR1, one for the releaser and one for the enhancer compounds, and they activate different PKC-mediated phosphorylation leading to the evoking of non-vesicular and vesicular dopamine release. (-)BPAP also increased VMAT2 operation enforcing vesicular [3H]dopamine accumulation and release. Vesicular dopamine release promoted by TAAR1 evokes activation of D2 dopamine autoreceptor-mediated presynaptic feedback inhibition. In conclusion, TAAR1 possesses a triggering role in both non-vesicular and vesicular dopamine release, and the mechanism of action of (-)BPAP is linked to the activation of TAAR1 and the signal transduction attached.