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Is life sweeter for the mouse on the right, who lacks a normal leptin receptor?

Sweet Tooth Soured by Leptin

A hormone called leptin whetted the appetites of obesity researchers several years ago when it was shown to influence how much mice eat. Now a study shows that, in addition to mediating metabolism, leptin actually changes how food tastes.

Leptin, discovered in 1994, is produced by fat cells and is thought to signal the hypothalamus that the body has enough energy. Mice with a certain mutation in the leptin receptor become obese. The mice are also more sensitive to sweet flavors, leading neurobiologist Yuzo Ninomiya of Kyushu University in Fukuoka, Japan, to wonder if leptin has a direct impact on taste buds.

To test leptin's power over taste, Ninomiya's team measured the neural responses of nerves in the tongues of mice. When they injected normal mice with leptin, the nerve activity in response to sweets was dampened by about 35%. In mutant mice with faulty leptin receptors, injections of leptin had no impact on the taste buds' sensitivity. The researchers also found molecular evidence that leptin is active in taste buds: Normal mice carried messenger RNA for the leptin receptor in their taste neurons, and molecular tags showed that the receptor protein was also there, the researchers report in the 19 September issue of the Proceedings of the National Academy of Sciences.

Although the link between metabolism and taste sensitivity is exciting, says neurobiologist Stephen Roper of the University of Miami, Florida, it's not clear how taste changes what you eat. Psychobiologist Michael Tordoff of the Monell Chemical Senses Center in Philadelphia agrees, pointing out that lower sensitivity to sweets might lead a person to eat less sugar because it tastes bad--or more sugar, because they can't get enough.

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The Monell Chemical Senses Center