"There is no doubt in my mind that sweet receptors evolved with the sole purpose of identifying highly caloric food sources and bitter receptors to warn the organism of of noxious stimuli," says Charles Zuker
Photo: Barbara Ries
Even before all the details are worked out, Zuker says that researchers can use TR1 and TR2 to begin probing other mysteries of taste. One of the biggest mysteries is how information received by cells in the tongue is transmitted to the brain. "Do you have, for example, coded lines of information, where there's a sweet line, a bitter line, a sour line, and a salty line? Or do you have mixed lines?" Zuker asks. And if the lines are mixed, how does the brain sort out signals so it knows whether you've just tasted lemon meringue or chocolate cream pie?
Because TR1 and TR2 are expressed in some cells and not others, scientists can use them to mark and study particular cells. "We can ask, for example, if all the cells that express one of these molecules are sending information via the same fibers to the brain, or are mixed with other cells that are different," says Zuker. "By doing this type of experiment, you can immediately begin to answer whether you have individual lines or mixed lines."
Probing differences among cells also could help researchers find out just how many taste receptors there are. After an exhaustive search, Zuker's and Ryba's group turned up only two, but that doesn't mean there aren't more. "There are zillions of sweet substances, and there are zillions of bitter substances that are all very different from one another," observes Catherine Dulac, an HHMI investigator at Harvard University who wrote about the Zuker team's findings in the February 1999 issue of Neuron. Carbohydrates, amino acids, proteins, inorganic salts and a variety of artificial sweeteners all can taste sweet, Dulac points out. "It would be difficult to conceive that only two receptors could accommodate this huge variety."
Zuker agrees, but thinks it's just as unlikely that thousands of different taste receptors will be found. "There is no doubt in my mind that sweet receptors evolved with the sole purpose of identifying highly caloric food sources and bitter receptors to warn the organism of noxious stimuli, essentially saying, 'This is bad news, get the hell away from it!'" So what's important to survival, he says, is the ability to distinguish between bitter and sweet, not to make fine distinctions within each taste category.
From the perspective of a foraging mammal, "I just want to know that something is bad news; I don't need to discriminate it from other bad news," explains Zuker. "So I strongly believe that the repertoire of taste receptors is likely to be way smaller than the olfactory universe, where you have a thousand receptors. But at the other extreme, I think one is too few." And without doing the research, he adds, there's really no way to predict how many taste receptors may turn up.
