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

•TAARs are vertebrate olfactory receptors and form an ever-evolving sensory subsystem.•Some TAARs detect volatile amines that evoke aversion or attraction behavior.•Natural TAAR ligands include odors from carnivores, male mice, and rotting tissue.•TAARs provide a valuable model for studying neural circuits that encode odor valence.

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.