Answer: The trace amine-associated receptors (TAAR) are a set of receptors that respond to trace amines, monoamines, and psychostimulants such as amphetamine.
It was known that some amines that function in non humans are found at very trace amounts in humans. Some of these include tyramine, beta-phenylethylamine, tryptamine, and octopamine. In 2001, a study was published by Beth Borowsky and colleagues that characterized the receptors that respond to these trace amines in the journal Proceedings of the National Academy of Science (Trace amines: Identification of a family of mammalian G protein-coupled receptors). Although some receptors are more well characterized and more completely understood, the function of these trace amine-associated receptors are still a relative mystery.
Currently, we believe that there are 6 different TAARs in human, called hTAAR1, hTAAR2, hTAAR5, hTAAR6, hTAAR8, and hTAAR9. These TAARs are known to be expressed highly in the olfactory epithelium, where it is expected that they respond to trace amines in the air. These TAARs are not only found in the brain; some are expressed in the pancreas, liver, and gut.
Human trace amine-associated receptor 1
The hTAAR1 is a target of frequent investigation. It is known to signal to the cell using the G-proteins Gs and Gq.
Many neurotransmitter receptors are expressed embedded within the plasma membrane. These components react to chemicals and can change the internal activity of the cell.
The TAAR1 however, is a cytoplasmically expressed receptor. It exists inside the presynaptic intracellular milieu. For a trace amine ligand like tyramine or tryptamine to bind to the receptor, they must first enter the cell through the action of a membrane transport protein.
In general, activation of the TAAR1 in a presynaptic cell increases excitability of the neuron through signaling via adenylate cyclase and cAMP production.