As unhealthy as it is, sugar is a hard habit to kick, not least because it's delicious. Normally, taste receptors in the mouth pick up the sweetness and tell us to keep eating, but researchers have now found one receptor that does the opposite – for fruit flies, anyway. A pair of neurons in the throat has been found to tell the insects when they've had enough sugar, a find which could potentially lead to a new way to combat sugar cravings in humans.

Hailing from Yale, the researchers stumbled onto the discovery while investigating IR60b, a gene that was only being expressed in nerve cells in the fruit flies' throats. Its function a mystery, the team tested IR60b using optogenetics, a technique that allows scientists to switch certain genes on and off using pulses of light.

The green fluorescence marks where the IR60b genes are being expressed, in the fruit fly's throat(Credit: Yale University)

To their surprise, when the researchers turned off either the gene or the nerve cells themselves, the flies gorged themselves on more sucrose than they normally would. By the same token, when the nerve cells were triggered, the insects generally ate less, which suggests that IR60b plays a key role in regulating their sugar intake. That said, it wasn't an instant off-switch: it takes some time for the "genetic brake" to activate, which allows the flies a little time to enjoy their meal before they decide enough is enough.

"This was very surprising because most sugar-sensing taste cells promote eating, but these are doing just the opposite," says John Carlson, senior author of the study.

The next step for the researchers is to see whether mammals like us might use a similar system for regulating our sugar intake. They're hopeful this may be the case because Carlson points out that humans and fruit flies very often share similarities when it comes to the control of fundamental processes such as eating. If so, it could give us a new weapon in the fight against obesity.

The research was published in the journal eLife.

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