Trace amine-associated receptors (TAARs), sometimes referred to as trace amine receptors (TAs or TARs), are a class of G protein-coupled receptors that were discovered in 2001.[1][2] TAAR1, the first of six functional human TAARs, has gained considerable interest in academic and proprietary pharmaceutical research due to its role as the endogenous receptor for the trace amines phenethylamine, tyramine, and tryptamine – metabolic derivatives of the amino acids phenylalanine, tyrosine and tryptophan, respectively – ephedrine, as well as the synthetic psychostimulants, amphetamine, methamphetamine and methylenedioxymethamphetamine (MDMA, ecstasy).[3][4][5][6][7][8] In 2004, it was shown that mammalian TAAR1 is also a receptor for thyronamines, decarboxylated and deiodinated relatives of thyroid hormones.[5] TAAR2–TAAR9 function as olfactory receptors for volatile amine odorants in vertebrates.[9]
The following is a list of the TAARs contained in selected animal genomes:[10][11]
Six human trace amine-associated receptors (hTAARs) – hTAAR1, hTAAR2, hTAAR5, hTAAR6, hTAAR8, and hTAAR9 – have been identified and partially characterized. The table below contains summary information from literature reviews, pharmacology databases, and supplementary primary research articles on the expression profiles, signal transduction mechanisms, ligands, and physiological functions of these receptors.
Ulotaront / SEP 363856 is a TAAR1 agonist in phase 3 clinical trials for schizophrenia and earlier trials for Parkinson's Disease psychosis. The medicine has obtained Breakthrough designation from the US FDA.[30][31][32]
Roles for another receptor are supported by TAAR5-independent trimethylamine anosmias in humans [32]. ... Several TAARs detect volatile and aversive amines, but the olfactory system is capable of discarding ligand-based or function-based constraints on TAAR evolution. Particular TAARs have mutated to recognize new ligands, with almost an entire teleost clade losing the canonical amine-recognition motif. Furthermore, while some TAARs detect aversive odors, TAAR-mediated behaviors can vary across species. ... The ability of particular TAARs to mediate aversion and attraction behavior provides an exciting opportunity for mechanistic unraveling of odor valence encoding.
TAAR2 and TAAR9 Two of the trace amine receptors are inactivated in a portion of the human population. There is a polymorphism in TAAR2 (rs8192646) producing a premature stop codon at amino acid 168 in 10–15% of Asians. TAAR9 (formerly TRAR3) appears to be functional in most individuals but has a polymorphic premature stop codon at amino acid 61 (rs2842899) with an allele frequency of 10–30% in different populations (Vanti et al., 2003). TAAR3 (formerly GPR57) and TAAR4 (current gene symbol, TAAR4P) are thought to be pseudogenes in man though functional in rodents (Lindemann et al., 2005).
Importantly, three ligands identified activating mouse Taars are natural components of mouse urine, a major source of social cues in rodents. Mouse Taar4 recognizes β-phenylethylamine, a compound whose elevation in urine is correlated with increases in stress and stress responses in both rodents and humans. Both mouse Taar3 and Taar5 detect compounds (isoamylamine and trimethylamine, respectively) that are enriched in male versus female mouse urine. Isoamylamine in male urine is reported to act as a pheromone, accelerating puberty onset in female mice [34]. The authors suggest the Taar family has a chemosensory function that is distinct from odorant receptors with a role associated with the detection of social cues. ... The evolutionary pattern of the TAAR gene family is characterized by lineage-specific phylogenetic clustering [26,30,35]. These characteristics are very similar to those observed in the olfactory GPCRs and vomeronasal (V1R, V2R) GPCR gene families.
Primary Transduction MechanismsComments: TAAR2 is found to be coexpressed with Gα proteins. However, the transduction pathway of TAAR2 is yet to be determined.
While mice produce gender-specific amounts of urinary TMA levels and were attracted by TMA, this odor is repellent to rats and aversive to humans [19], indicating that there must be species-specific functions. ... Furthermore, a homozygous knockout of murine TAAR5 abolished the attraction behavior to TMA [19]. Thus, it is concluded that TAAR5 itself is sufficient to mediate a behavioral response at least in mice. ... Whether the TAAR5 activation by TMA elicits specific behavioral output like avoidance behavior in humans still needs to be examined.
We show that [human TAAR5] responds to the tertiary amine N,N-dimethylethylamine and to a lesser extent to trimethylamine, a structurally related agonist for mouse and rat TAAR5 (Liberles and Buck, 2006; Staubert et al., 2010; Ferrero et al., 2012).
Tissue Distribution Kidney, amygdala, hippocampus; Species: Human; Technique: RT-PCR ...Human brain tissues (with the level of expression descending from hippocampus, substantia nigra, amygdala, frontal cortex to basal ganglia), human fetal liver. Not detected in the cerebellum or placenta.; Species: Human; Technique: RT-PCR
Tissue Distribution Comments ... No expression of TAAR9 was detected by RT-PCR in the Grueneberg ganglion [2]. TAAR9 expression was not detected by Northern blot analysis in thalamus, amygdala, midbrain, hippocampus, putamen, caudate, frontal cortex, pons, prostate, stomach, heart, bladder, small intestine, colon or uterus [4].