In hungry flies, sense of smell grows keener

When fruit flies are hungry, they become especially attuned to the scent of their next good meal, according to a report in the April 1st issue of Cell, a Cell Press publication. That sensory and behavioural shift can be traced to insulin and to a neuropeptide already familiar in humans for its effects on a brain region that controls appetite.

'As humans, we sometimes forget that feeding behaviour has two components,' said Jing Wang of the University of California, San Diego. 'First you have to go out and hunt for food.' Actually eating that food is secondary.

Wang says that because of our unusual circumstances of abundance, most of the study of appetite has focused on the latter part of that equation and the role of a brain region in mammals known as the hypothalamus. But as our experiences suggest, it's also clear that the sense of smell has an important place in controlling appetite. When we are hungry, the fragrance of food becomes even more delectable. Likewise, food that smells especially good is also especially hard to resist.

In the new study, Wang and his colleagues set out to study what happens to flies' sense of smell after they were starved for a few hours. Under those conditions, they found that global insulin levels and local levels of short neuropeptide F work in concert to affect the sensitivity of a select group of odour-sensitive neurones.

Insulin in flies works in essentially the same way as it does in humans, Wang said. It controls the amount of sugar in the circulation. 'When a fly is hungry, insulin drops dramatically. This tells the olfactory neurone to change its sensitivity.'

That change is controlled on certain neurones through an increase in the activity of the gene encoding the neuropeptide F receptor. With more receptor on their surfaces, those neurones grow increasingly sensitive to the neuropeptide and to the odour cues they are designed to pick up. As their name implies, fruit flies forage on rotten fruits, Wang says, and it is the neurones that pick up the vinegary scent of fruit decay that are affected.

The shift in sensitivity reaches its peak in about four hours, a fact that Wang said he found intriguing in part because it mirrors the typical spacing of our meals - breakfast, lunch and dinner. After all, not all animals are episodic feeders like we are. Some animals eat all the time and others eat only very infrequently. It remains to be seen how this system might be different in other species according to that variation in meal times.

Wang says they also aren't sure what the findings in flies will mean for understanding human appetite and eating behaviour. 'On the surface, it appears that the nose may suppress appetite when insulin is abundant, but we don't know.' It's also possible that insulin resistance might cause this system to malfunction, leaving an individual generally more sensitive to the odour cues associated with food.

And, of course, when it comes to humans and food, obesity is always top of mind.

'One problem for us is that food is abundant,' Wang said. But, he adds, the other problem may be that food today is simply too good. It is packed with ingredients that can be almost irresistible. If there is a way to adjust our sensitivity to the smell of that food through this newly discovered system of appetite control, it might ultimately be possible to adjust our eating habits up or down by those means.