In a paper on salt taste in flies, published recently in the journal Chemical Senses, Anupama Dahanukar, a professor of molecular, cell and systems biology at UC Riverside, and her former graduate students report they have identified a mechanism in flies that suppresses high salt intake.
High concentrations of dietary salt are harmful to health. Like most animals, the fruit fly is attracted to foods that have low concentrations of salt but shows strong taste avoidance of high salt foods. Indeed, the fly gustatory (or taste) system has evolved many mechanisms, including an inhibitory mechanism described in this study, to prevent the fly from ingesting high salt. Flies also use at least two different mechanisms to detect and avoid other types of aversive chemicals.
The research paper is titled “An inhibitory mechanism for suppressing high salt intake in Drosophila.” The study was funded by grants from the UCR Agricultural Experimental Station and United States Department of Agriculture National Institute of Food and Agriculture.
In the following Q&A, Dahanukar and coauthors, Manali Dey and Anindya Ganguly, explain their findings. Dey is now a postdoctoral researcher at UC Irvine. Ganguly is a postdoctoral researcher at UC Santa Barbara.
Q: What have you found about salt taste in flies?
Dahanukar: Various studies have shown that salt activates multiple classes of taste neurons in the fruit fly, including Gr64f sweet-sensing neurons that promote food acceptance and two other types of neurons, Gr66a bitter and Ppk23 high salt, that drive food rejection. Our study demonstrates that NaCl (salt) elicits a dose-dependent response in Gr64f taste neurons, with high activity at low salt levels but reduced activity at high salt levels. Moreover, high salt inhibits the sugar response of Gr64f neurons independently of their salt taste response. The inhibition of Gr64f neuron activity correlates with feeding suppression in the presence of salt, and this effect persists even when the high salt taste neurons are genetically silenced. Other salts exhibit similar effects on sugar response and feeding behavior. Overall, our research uncovers a mechanism within the appetitive Gr64f neurons that can discourage the consumption of potentially harmful salts.
Q: How does this affect humans?
Dey: Salts offer the vital micronutrients required for healthy body processes. But consuming too much salt can be bad for you. High salt intake has been associated with an increased risk of autoimmune illness, gastrointestinal cancer, hypertension, and osteoporosis in animals, for instance. It has been demonstrated that consuming large amounts of salt is harmful for flies as well. As a result, larger salt concentrations are often rejected by animals, even though most of them prefer food with low salt concentrations.
In mice a study showed that intake of high concentrations of salts can also cause obesity but the exact mechanism by which this condition has been induced are unknown. This study showed how presence of high salt in food changes the sweet sensitivity in flies and it can help us to understand if and how high salt consumption might be related to sweet consumption and related weight gain.
Q: What can be said about salt taste in other species?
Ganguly: Salt taste seems to be important for all sorts of insects, including those that feed on plants as well as those that feed on blood. Flies, mosquitos and butterflies can all taste salt. Most vertebrates enjoy the same basic tastes as humans do: sweet, salty, sour, bitter and umami. Dolphins and whales, though, are an exception. According to new research, these animals only have salt taste.
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