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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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Idiosyncratic Responses to Various Tastants

Taste responses vary among individuals. For example, many people (about 30–40% of the U.S. population) cannot taste the bitter compound phenylthiocarbamide (PTC) but can taste molecules such as quinine and caffeine that also produce bitter sensations. Indeed, humans can be divided into two groups with quite different thresholds for bitter compounds containing the N—C=S group found in PTC. The difference between these individuals is the presence of a single autosomal gene (Ptc) with a dominant (tasters) and a recessive (nontasters) allele. Interestingly, people who are extremely sensitive to PTC or its analogues, called “supertasters,” have more taste buds than normal and tend to avoid certain foods such as grapefruit, green tea, and broccoli, all of which contain bitter-tasting compounds. Thus, an individual's genetic makeup with respect to taste receptors has implications for diet, and even health.

In the same vein, a number of quite different compounds taste sweet to humans. These include saccharides (glucose, sucrose, and fructose), organic anions (saccharin), amino acids (aspartame, or Nutrasweet®), l-phenyalanine methyl ester, and proteins (monellin and thaumatin). People can distinguish among different sweeteners, and some find that saccharin has a bitter-tasting component. One reason for such discriminability is that some of these compounds activate separate receptors. For example, saccharides activate cAMP pathways, whereas nonsaccharide sweeteners such as amino acids activate IP3 pathways (see next section). Thus, the perceptual experience of “sweet” encompasses much more than the taste of sucrose, can be elicited by various sensory transduction mechanisms, and may generate sensory qualities different from those generated by sucrose.

Taste sensitivity for salt also relies on a number of mechanisms. Not all salts, or even all monovalent chloride salts, activate the same pathway. Psychophysical studies have shown that amiloride, a diuretic that blocks Na+ entry through amiloride-sensitive Na+ channels, decreases the taste intensity of NaCl and LiCl, but not KCl. Although LiCl, like NaCl, tastes salty, it cannot be used as a salt Na+ substitute because of its profound effects on the central nervous system (it is used clinically to treat manic-depressive disorders). Sodium succinate, NH4Cl, and CsCl do not taste exclusively salty. Indeed, CsCl has a bitter or salty-bitter taste that probably arises from the inhibition of K+ channels. Additional evidence for a distinct receptor for NaCl comes from developmental studies. Infants up to 4 months old can distinguish between water and sucrose (and lactose), water and acid, and water and bitter tastants, but they cannot distinguish between water and a 0.2 M NaCl solution. Thus, either the receptor for Na+ has not yet been expressed, or, if expressed, it is not yet functional. Infants between the ages of 4 and 6 months, however, can discriminate between NaCl solutions and water, and children can detect the full salty taste of NaCl at about 4 years of age.

Sour taste is produced by relatively high concentrations of acid. At the same H+ activity, weak organic acids such as tartaric acid and citric acid exhibit distinctly different tastes from HCl, whereas strong inorganic acids such as HCl, HNO3, and H2SO4 have similar tastes. The accompanying anion influences the activity of gustatory responses to acids by virtue of its ability to diffuse across the tight junctions between taste cells, thus creating a variety of sour tastes.

In short, the taste system uses many mechanisms to distinguish among the various chemicals placed in the mouth. The four words “sweet,” “sour,” “salty,” and “bitter” do not accurately or fully describe tastes, which encompass a far more subtle range of sensations and may mean different things to different people.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK10852

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