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.