From freshly baked cookies to rotten eggs, an Italian restaurant to a fish market, our strong reactions to the odors emanating from different foods is really quite amazing. The sight of a moldy Tupperware dish does not disgust us as much as its stench; likewise, the sensation of chewing a warm chocolate chip cookie does not excite us as much as does its mouthwatering aroma. We may be repulsed by the smell of a particular food item at one point, only to find it much more attractive later on when we are hungry. But how does our brain use smell to categorize a food as “good” or “bad”? How do we distinguish among these different scents? And what exactly are the biological underpinnings to our sense of smell?
While there is still much to be learned about the human olfactory system in regards to food, researchers have made great progress in understanding the basic mechanisms involved by studying the brains of fruit flies. A recent study published in The Journal of Neuroscience has highlighted the role played by a certain type of protein, dNPF (Drosophila Neuropeptide F), in fruit flies’ sense of smell and their resulting reaction to certain foods.
dNPF is a peptide deemed as the fruit fly-equivalent to human’s Neuropeptide Y—the expressions of both of these peptides are associated with “graded” responses to odors from particular foods. Essentially, in the case of fruit flies, smells from different sources trigger different chemical signals in the nervous system. These chemical signals act to induce dNPF production in a small number of neurons located in the fly’s “mushroom body” (MB), a brain region associated with sensory memory. The higher the dNPF production in the MB, the more appealing a fruit fly finds that particular food. If a fly is hungry, then the production of dNPF in response to any food odor is heightened significantly.
The critical role dNPF plays in the perception of smell was made clear when scientists began manipulating dNPF-producing neurons (called “Kenyon cells”) in the fruit fly’s MB. By artificially stimulating production of dNPF, researchers could make fruit flies perceive a “pleasant” food odor. Likewise, inhibiting dNPF production would prevent the flies from recognizing and responding to any odor at all.
These Kenyon cells are limited in number but incredibly powerful in their ability to distinguish between different smells. In fact, hundreds of thousands of distinct odors are recognized by as few as 25 Kenyon cells. The human equivalents of these cells (Neuropeptide Y-producing neurons) are so specific and selective in the signals they transmit that individual odors come to be strongly associated with certain memories and sensations. Indeed, humans seem to attribute more emotional connections to our sense of smell than to any other sense, especially in regards to food. Whether from a homemade loaf of banana bread, a grandmother’s traditional holiday dish, or a muffin from a best friend’s bakery, the mere aroma of a food item has the power to unleash a wealth of memories and unconscious responses within us.
As research on the causes of obesity and over-eating across the globe progresses, studying the intersections between our olfactory and nervous systems will take on a whole new level of importance. Is there a certain gene that causes some individuals to produce more Neuropeptide Y than others, and thus make them more attracted to particular food odors? How is the activity of our human Kenyon cell equivalents linked to our drive to eat? Our sense of satiety? Further research will no doubt illuminate these connections, and possibly provide us with methods of combating the ongoing obesity epidemic.