Edition: U.S. / Global

How the Taste Bud Translates Between Tongue and Brain

Published: August 4, 1992

WHILE shrimp barbecued in a tangy marinade may provide your taste buds with a burst of simple delight, researchers who study the workings of individual taste cells have recently shown that those sensory organs are far from simple. Such a dish would send the taste buds into overdrive as they raced through a series of recently discovered biophysical reactions to process the complex seasonings even before you swallowed the first bite.

The information gleaned from taste-cell studies, mainly on animals like the mudpuppy and catfish, is expected to lead eventually to many tangible benefits. These include modifying people's "taste appetites" to aid in weight control, make dull, nutritious foods taste better, improve the taste perceptions and appetites of the elderly, produce tastier substitutes for sugars and salt, and develop drugs to counter taste disorders.

Contrary to long-held beliefs, the new studies reveal taste buds to be far more than simple conduits that immediately pass on information about sweet, sour, salty and bitter substances to the brain to tell you what you are eating and help you decide whether you want more. Rather, the research has shown that cells in the taste buds communicate with each other, actively accepting, rejecting and modifying taste stimuli through a complicated network of chemical and electrical signals before sending signals to the brain.

As scientists scramble to decipher those signals, they are finding that taste stimuli can affect the taste-bud cells in unexpected ways. The stimuli sometimes exert their effect by interacting with messenger molecules in the taste buds, and they sometimes directly stimulate tiny electrical currents within the cells.

For example, researchers at the Roche Institute of Molecular Biology in Nutley, N.J., recently published their identification of an important protein messenger in taste buds, gustducin, that is activated in response to all sweet and some bitter taste stimuli. Dr. Robert F. Margolskee and his colleagues at the institute said gustducin's role in taste buds was comparable to that of protein receptors called transducins in the eye. Transducins, which are far better studied messenger chemicals, help to translate the light that reaches the retina into messages to be sent to the brain. Gustducin, which is found only in taste buds, acts as an intermediary between the receptor molecule for sweet stimuli and a chain of subsequent steps, finally sending a message to the brain that something sweet has been tasted.

"Taste research has not been a high-priority item with our major funding agency, the National Institutes of Health," said Dr. Stephen D. Roper, a neurobiologist at Colorado State University in Fort Collins, "although this may change now that connections have been established between taste and the control of food intake."

As scores of studies recently summarized by Dr. Roper in The Journal of Neuroscience have shown, each of the four basic tastes impinges upon taste cells in its own way, setting off reactions that change the cells' electrical charge and trigger the release of chemical messengers that ultimately tell the brain what is being eaten.

Take, for example, the shrimp marinade. Each flavoring ingredient registers individually yet somehow gets integrated into a memorable taste sensation. The sodium in the soy-based marinade flows readily into taste cells through channels in their membranes; the sour-tasting lemon juice closes the channels that normally let potassium flow out of the cells; the honey is greeted by cell-surface receptors that carry sweet taste stimuli over the threshold of the cell membrane; the bitterness in the orange rind closes potassium channels and also may link up with receptors, setting off a chain reaction that ultimately releases calcium within the cell.

At the same time, that generous pinch of cayenne that gave the marinade its special kick is irritating sensory nerve fibers in the tongue and nose and, in the process, enhancing perception of the other taste stimuli.

But that is not all. Each taste cell, it turns out, does not act independently, picking up taste messages from the environment and sending them on to the brain. Rather, as Dr. Roper put it, the cells seem to "talk" to one another, passing taste information to and fro and modifying it in still-mysterious ways before it gets to the brain.

"It's like a complicated chord in a musical piece," Dr. Roper explained in an interview. "The chords -- bitter, sour, salt and sweet -- probably have multiple notes. We've got one of those notes, but the others are not yet known."

However, taste researchers now know that the end result is a series of micro-electric currents, induced by nerve-cell messenger chemicals or direct interaction of taste stimuli with taste-cell membranes, that tell the brain what the tongue has experienced less than a moment ago. The scientists' remaining tasks include deciphering the precise message each taste stimulus transmits, which would foster a more scientific approach to taste modifications than the "throw in a little of this or that" method now universally used by food producers, cooks and hapless diners.

Though often inextricably linked in the consumer's mind, taste and smell are distinctly different, independent sensory systems that, in yet unknown ways, interact upon reaching the brain. Anyone whose nose has ever become stuffy because of a cold knows that what people perceive as taste sensations is based only partly on the tongue.

The sense of smell, olfaction, represents 80 percent of flavor, Dr. Roper said. "Without a sense of smell, you would never know the flavor of the shrimp."

"But," he added quickly, "even if the olfactory system is destroyed, you can still sense the four basic tastes. You would sense that a jellybean is sweet, but you could not tell if the flavor is lemon or licorice." Anatomy of a Taste Bud

The focus of all this research, the taste cell, is really a modified skin cell inside the mouth. Tens of thousands of taste cells reside in the mouth, but most are concentrated on the tongue in clusters of about 100 cells each that make up the taste buds. At the center of each taste bud is a pore; tasty chemicals fall into the pore, stimulating the taste cells to start their sensory analysis.

There are two kinds of taste cells, receptor cells and basal cells. At the surface of the tongue, each taste receptor cell has a chemically sensitive tip that responds to taste stimuli in the mouth. These cells may "talk" back and forth in chemical-ese before passing their environmentally derived message on to the basal cells at the bottom of the taste bud, which may also talk back and forth with the receptor cells. Once the final message has been ironed out, the basal cells convey it to sensory nerve endings that carry the signal, which eventually is interpreted by the brain.

People differ markedly in the number and distribution of taste buds in their mouths, which may account in part for the discriminating palates of some and the insensitivity of others who (alas) are inclined to salt their food even before tasting it. The Mudpuppy Helps Out

The taste buds, in turn, are grouped in pink mushroom-shaped bumps called papillae, which can be seen by the naked eye on the surface of the tongue. While most papillae house only two to five taste buds, others in different parts of the tongue contain up to 250. This fact has fostered the simplistic belief that different tastes are perceived solely or most intensely in certain regions of the tongue, such as sweet tastes in the front and bitter in the back.

On the contrary, Dr. Roper and others say, most taste buds, regardless of location, appear to be sensitive to many taste qualities over a wide range of concentrations. But, he conceded, "we still don't know whether there are specific cells in the taste buds for specific tastes. Can every cell process each of the four taste stimuli, or does each cell specialize in a particular taste? This is the $64,000 question in taste research."

Much has been learned in the past four or five years about the workings of taste cells through research sponsored largely by the National Institutes of Health and, to some extent, by food and fragrance manufacturers. The cells appear to act as tiny microprocessors that are hardly, as Dr. Roper put it, "slaves to their environment."

The Colorado scientist and his colleagues work primarily with a foot-long freshwater lizard called a mudpuppy, which lives throughout the Mississippi River Valley and frequently gets snagged in fishing nets. Dr. Roper is delighted to work with the animal because its taste cells are about 100 times larger than those in people. Because the cells are so large, the Colorado researchers can expose individual taste cells to selected taste stimuli and easily record the cells' electrical responses.

Others, such as Dr. John Teeter and Dr. Joseph Brand at the Monell Chemical Senses Center in Philadelphia, study taste cells in animals like the American catfish, which is covered from the tips of the barbels of its lips to the end of its tail with thousands of external taste cells.

While different animals respond differently to taste stimuli, Dr. Roper explained, the cellular mechanics of taste perception are remarkably alike across species.

Until the recent studies, most scientists believed that each taste cell had highly specific receptor molecules on its surface that were sensitive to particular taste stimuli. The stimuli would, in effect, utter a chemical password to enter the cell and the message, as Dr. Roper put it, "got magically transmitted to the brain."

The new studies show that for most taste stimuli, other mechanisms are at work. The chemicals that transmit sensations of salty, sour and bitter tastes affect the workings of so-called ion channels, which normally let sodium into cells and potassium out. Because those ions are electrically charged, the change in their movements sets up a small electrical current in the taste cell. That, in turn, activates a synapse, a communication link between cells.

Synapses enable taste cells to communicate with their like-minded neighbors as well as with sensory nerve fibers. A host of messenger chemicals are involved in the transmission of taste signals, but the roles played by the various substances are not yet understood.

Dr. Roper suggested that a substance called serotonin may play a role in appetite and weight regulation at the level of the taste buds, as serotonin is known to do in the brain.

Dr. Brand of Monell said that a better understanding of the many chemicals and mechanisms involved in taste perception may help food scientists design artificial taste stimulants that are highly specific, like substances to stimulate flagging appetites or to satisfy desires for sweet or salty tastes without compromising health. Currently, he explained, efforts by the food industry to decrease salty and sweet ingredients in processed foods unfortunately emphasize the underlying bitterness that had been masked by the added sugar and salt. The ultimate goal, he said, would be to influence the taste-receptor system so it could take the bitter without the sweet.

Photo: In this scanning electron micrograph of one papilla of human tongue, arrow marks taste pore, where stimuli gain access to receptor cells. (Dr. Stephen D. Roper) Diagram: "Onions to Ions: Pathway From Tongue to Brain" New research has shown that cells in the taste buds are not just passive receptors of sweet, sour, salty and bitter, but actively accept, reject and modify taste stimuli through complex chemical and electrical signals. Visible bumps called papillas contain multiple taste buds; in each bud, taste cells surround a pore. The taste receptors Receptor taste cells have chemically sensitive tips; basal cells interact with them and work out a message. For example, some tastes affect the workings of ion channels. Ion movements change the cells' electrical charge, triggering chemical messengers to the brain. Recognition in the brain The end result of taste is a series of tiny electric signals to taste centers in the brain. There isno direct connection with the offactory bulb, in the center of smell processing, yet the sense of smell may represent 80 percent of flavor. The sensory centers interract in the brain in ways that are not yet known. (Sources: "Principles of Neural Science," Kandel, Schwartz and Jessell (Elsevier); Dr. Stephen D. Roper) (pg. C8)