Researchers decipher our ability to taste

Scientists explore brain mechanisms that may control what and how much we eat.

By Beth Azar
Monitor staff

If left to their own devices in a natural environment, animals, including most people, will select a proper diet over time, an ability researchers call 'body wisdom,' said Thomas Scott, PhD.

The ability has a lot to do with brain mechanisms involved in reinforcing behavior and taste?mechanisms that researchers are beginning to decipher by studying animals.

At a symposium on ingestive behavior, Scott, of the University of Delaware, joined Bart Hoebel, PhD, of Princeton University, to describe some brain mechanisms they believe help control what and how much we eat.

The symposium was part of the 'Mind, brain and behavior' series organized by APA divisions 3 (Experimental) and 6 (Behavioral Neuroscience and Comparative Psychology) and sponsored in part by the National Science Foundation at APA?s 1998 Annual Convention.

A process of push and pull

Like all biological mechanisms, eating is a process controlled by chemical signals deep within the brain. Researchers have long known that the neurotransmitter dopamine reinforces eating?cells in the area of the brain called the nucleus accumbens release dopamine during meals, thereby reinforcing our desire to eat more of those foods. Now they believe they?ve found another neurotransmitter, acetylcholine, which appears to inhibit eating, said Hoebel.

He and his colleages focused on acetylcholine because while morphine increases dopamine in the brain, it lowers acetylcholine.

Also, rats try to avoid situations that result in increased acetylcholine, said Hoebel. For example, if researchers pair saccharin with elevated acetylcholine levels, rats learn to avoid eating saccharin.

Acetylcholine also inhibits normal eating, said Hoebel. When he and his colleagues raise rats? acetylcholine levels during a meal, the animals stop eating. And, if they destroy the neurons that respond to acetylcholine, the rats overeat. Even in a natural eating setting, acetylcholine peaks when an animal is satiated, Hoebel said. 'A meal begins with an increase in dopamine,' he said. Then, as the animal fills up, acetylcholine is released to inhibit eating.

Body wisdom

But we all know that there?s more to a meal than simply filling up. Most humans would say that taste is more important than even nutrients. In fact, taste is extremely important for ensuring we eat what?s nutritious and not what?s poisonous, said Scott.

'If we look for a unifying principle for the taste system, for how taste is organized, [it] is physiological welfare,' said Scott. 'In the same sense that wavelength underlies color and frequency underlies pitch, physiological welfare organizes and underlies the span of taste experiences that we have.'

When we put something in our mouths, information about its taste enters the brain and triggers four basic mechanisms:

? Reflexes that trigger the muscles in the mouth to swallow or to reject the item.

? Reflexes that trigger the digestive system to begin the digestive process. For example, when a person eats something sweet, the body immediately releases insulin to prime the body for digestion.

? Neural pathways that identify what?s in the mouth.

? Neural pathways that provide a sense of pleasure or disgust.

This system provides us with a basic wisdom about which foods are good and which are bad, said Scott. But it can also be tailored to a person?s physiological makeup and immediate needs, he said.

This realization that experience can modify the taste system comes in large part from studies in animals, said Scott. He and his colleagues measure animals? reactions to different flavors by recording the electrical activity in rats? brains while they eat. When the rats eat something bitter?the taste of most poisons?the brain shows an immediate burst of activity. But when they eat sugar, there?s no burst of activity, just a moderate increase over time.

If the researchers condition rats to dislike the taste of sodium saccharin?a sweet substance?by pairing it with a substance that makes them sick, they can change how the brain reacts to sodium saccharin. These rats show a sharp initial burst of activity that looks much like the pattern seen when the rats ingest small concentrations of the poison quinine.

The sodium saccharin switches from being acceptable to being rejected, said Scott. These findings indicate that animals, including humans, 'have the capacity to tailor this global species-wide system to our own individual needs,' he said.

The system can also change over time based on immediate needs, he said. When he and his colleagues deprived rats of sodium, they began to accept salt as readily as sugar and the salt received a brain response similar to when the rats ate sugar.

These changes to the taste system don?t require the types of extraordinary experiences Scott and his colleagues imposed on their rats. They can occur in the course of a regular day?s eating, said Scott. To study this they took brain recordings from monkeys as they ate their fill of sugar. As the monkeys became full and their eating slowed, the response of cells in the primary taste cortex?the area Scott believes identifies the actual taste of the food?remained the same. But the response of cells in the secondary taste cortex?the area that identifies food as pleasurable?declined. 'The brain?s saying, ?It?s still sweet, but I don?t like it any more,?' Scott said.

'At one level or another, taste and the processes it drives guide our daily eating and the process of accepting things at one time and rejecting them at another,' concluded Scott. 'This system is tailored to guide your physiological welfare from birth to death.'

But if the taste system is so tuned in to what?s good for us, why is it that today we assume that the better something tastes, the worse it is for us, asked Scott. The answer is that the taste system evolved in a very different time, when food was scarce. Now we live in a society in which all the things our taste system evolved to crave are available in vast quantity. And the taste system goads us to over-consume items that are good for us only in small quantities.