Food cravings tied to brain chemicals.

Frustrated by your inability to resist snacking on potato chips or grabbing an extra cookie or two... or three? Blame your brain, suggest neuroscientists.

At least two chemicals in the brain convey very precise - and compelling commands to the body about ingesting food, says Sarah E Leibowitz of Rockefeller University in New York City. Her studies in rats indicate that one of these chemicals, called neuropeptide Y, causes carbohydrate erarings, while the other, called galanin, seems to underlie a yen for fat.

The more of each the body produces, the stronger the drive to eat those particular food groups, she says.

Now, scientists can slow production of these messengers by suppressing the genes that control output, she and her colleagues report this week in Washington, D.C., at the annual meeting of the Society for Neuroscience.

"There are clearly specific chemicals for specific appetites," she says. Because of parallels between what happens in rats and what is known about cravings and these chemicals in people, she predicts that her studies will lead to new ways to treat eating disorders in humans.

Leibowitz' group gave rats small pieces of a specific "antisense" genetic material (SN: 6/5/93, p. 366) each day for four days. This synthetic DNA interfered with brain cells' ability to make neuropeptide Y, reducing stores by about 35 percent. As a result, the animals ate only about onethird their normal carbohydrate and fat calories, the team reports.

Other factors - such as hormones or the amount of glucose the cells are using-

also modulate neuropeptide Y production, Leibowitz says. She and her colleagues observed that when they gave the rats a chemical that blocked glucose use, two areas of the brain's hypothalamus - the arcuate nucleus and the suprachiasmatic nucleus - produced more neuropeptide Y. Adding chemicals that may enhance sugar use by cells tended to result in decreased neuropeptide Y, the group notes.

Leibowitz reports that, particularly in female rats, the "eat fat" message can increase over time. Her group raised 41 females, feeding them a mixture of fat, protein, and carbohydrate. After 65 days, the researchers could distinguish the rats that preferred fats - fat made up 38 percent of their food intake, compared to the 13 percent consumed by so-called low-fat rats, she says.

The group could trace that preference back to when the rats were 21 days old, possibly even 10 days old. "There's a critical period [near weaning] that is strongly predictive for their [adult] appetite for fat," Leibowitz adds. Fat preference increases sharply when the rats enter puberty. Those choosing high-fat diets had more galanin in their brains than the other rats, she notes.

These results indicate that changes in cravings for sugar and fat are linked to changes in the amounts of neuropeptide Y and galanin. "To show that they are so tightly linked is giving me hope that there is some definable simplicity [to food preferences]," says Leibowitz.

Other researchers are not so sure, especially about the control of fat urges. "In other people's hands, [galanin] is marginally specific for fat," comments David A. York of Louisiana State University in Baton Rouge.

Once researchers understand the neurochemical and endocrine signals that guide appetites for fat and sugar, "we can work out ways nutritionally and pharmacologically for dealing with that," Leibowitz asserts.

But controlling these two messengers may not be part of the solution. "The real problem with neuropeptide Y and galanin is that they occur all over the brain and they regulate multiple systems [such as water intake and sexual behavior]:' York says.

Also, "there must be at least 25 neuropeptides that affect food intake," he notes. "Undoubtedly. it's going to turn out to be a complex system."