Bitter receptors

Humans perceivebitter substances with the help of so-called bitter taste receptors(G-protein coupled hTAS2 receptors - hTAS2R - h stands for human, TAS for "taste"). Basically, bitter taste receptors are supposed to warn against the consumption of toxic substances.

The bitter receptors are encoded by bitter receptor genes located on chromosome 12. 25 human bitter receptor genes are known.
Like other taste receptors (sweet, sour, salty, umami) they are also found on the tongue. They are taste receptor cells specialized in bitter substances and are organized in the taste buds on the tongue. Bitter receptors therefore occur in different combinations in one and the same (taste) receptor cell in humans.
Although they react specifically to the different bitter compounds, they ultimately activate the same receptor cell. Thus, regardless of which individual receptor is activated, only one uniform signal is transmitted to the brain.
This special constellation leads to the fact that humans (including other mammals) recognize only one single quality as "bitter". Although he is able to distinguish the intensity of bitter, he cannot separate the different bitter substances in taste.

There is sufficient evidence for the ubiquitous presence of bitter substance receptors in the organism. They are not only distributed on the tongue as taste receptors. Rather, they seem to be integrated in a complex signal transduction system, which is involved in the control of central metabolic and immunological functional circuits via endogenous bitter substance receptors. Thus in the glucose, lipid and energy metabolism. Furthermore, corresponding bitter substance receptors influence the tone of smooth muscles (bronchial tract and urinary bladder), alter immunological reactions (in the respiratory tract or digestive tract), influence central thyroid functions and gastro-intestinal absorption. The function of bitter substance receptors in the skin has not yet been conclusively clarified, nor that of the central nervous system.

Since the bitter substance receptors were supposed to warn against the consumption of toxic substances, it was evolutionary important to recognize as many bitter substances as possible, but not their differences.

1. Clark AA et al (2015) TAS2R bitter taste receptors regulate thyroid function. FASEB J 29:164-172.
2. Doggrell SA (2011) Bitter taste receptors as a target for bronchodilation. Expert Opinion Ther Targets 15:899-902.
3. Dotson CD et al (2008) Bitter taste receptors influence glucose homeostasis. PLoS One 3: e3974.
4. Jeon TI et al (2008) SREBP-2 regulates well peptide secretion through intestinal bitter taste receptor signaling in mice. J Clin Invest 118:3693-3700.
5. Kook JH et al (2016) Increased expression of bitter taste receptors in human allergic nasal mucosa and their contribution to the shrinkage of human nasal mucosa. Clin Exp Allergy 46:584-601.
6. Lee RJ et al (2014) Sinonasal solitary chemosensory cells "taste" the upper respiratory environment to regulate innate immunity. At J Rhinol Allergy 28:366-373.
7. Liggett SB (2013) Bitter taste receptors on airway smooth muscle as targets for novel bronchodilators. Expert Opinion Ther Targets 17:721-731.
8. Manson ML et al (2014) Bitter taste receptor agonists mediate relaxation of human and rodent vascular smooth muscle. Eur J Pharmacol 740:302-311
9. Shaik FA et al (2016) Bitter taste receptors: Extraoral roles in pathophysiology. Int J Biochem Cell Biol 77(Pt B):197-204.
10. Singh N et al.(2011) Functional bitter taste receptors are expressed in brain cells. Biochem Biophys Res Commun 406:146-151.