Trace amine-associated receptors: ligands, neural circuits, and behaviors.

Trace amine-associated receptors (TAARs) are G Protein-Coupled Receptors that function as vertebrate olfactory receptors. Like odorant receptors, TAARs constitute an ever-evolving sensory subsystem, with individual TAARs recognizing particular chemicals and some evoking stereotyped behaviors. Several TAARs mediate aversion or attraction towards volatile amines that include the mouse odor trimethylamine, the predator odor 2-phenylethylamine, and the death-associated odor cadaverine. TAAR-expressing sensory neurons achieve monoallelic receptor expression, use canonical olfactory signaling molecules, and target a dedicated olfactory bulb region. In mouse, TAAR4 and TAAR5 are encoded by adjacent genes and localize to adjacent glomeruli, yet mediate opposing behaviors. Future studies are needed to understand how TAAR-expressing sensory neurons engage higher-order neural circuits to encode odor valence.