In the world of animals, there are no ingredient lists or expiration dates to base decisions on. For many animals, distinguishing between bitter and sweet tastes can mean the difference between eating something poisonous or safe, according to Doruk Savaş PhD’24, currently a postdoctoral fellow at Harvard Medical School.
Scientists previously thought that to make the lifesaving distinction, the two populations of neurons activated by the different tastes acted in circuits completely separate from each other. But a recent study from the Barnea Lab — under Professor of Ophthalmology, Visual Science and Neuroscience Gilad Barnea — has identified a pair of neurons in fruit flies where a single neuron can modulate feeding response to both bitter and sweet tastes.
The lab studied gustatory circuits in fruit flies — neural pathways from the tongue to the brain — using a mapping tool called trans-Tango created by the lab to trace how neurons responded to the different flavors.
“Sweet usually signals caloric value, so that we’re drawn to it, whereas bitter is the opposite because it might be signaling potential toxic compounds,” Savaş said in an interview with The Herald.
Gustatory receptor neurons are thought to be responsible for sensing taste and communicating with the central nervous system about food quality — guiding decision-making — according to the paper. The study suggests an overlap between the neurons that can process sweet and bitter signals, Savaş said.
Savaş identified a pair of neurons — called subesophageal leucokinin, or SELK, neurons — as allowing for the dual response to the contrasting signals.
Co-author Meet Zandawala — an assistant professor at University of Nevada, Reno in the Department of Biochemistry and Molecular Biology — said in an interview with The Herald that the cells receive “receives direct information about whether the food is bitter or sweet, integrates it and, depending on the kind of input it receives, it can release one of two different chemicals.”
When processing a bitter taste, the neurons released the specific neuropeptide, leucokinin, and when processing a sweet taste, they released a different chemical, the neurotransmitter acetylcholine.
“Depending on which chemical it releases, it drives opposite behaviors,” Zandawala said: The flies feed when they encounter sweet food and avoid food when it is bitter.
The lab also used feeding assays — to analyze how the flies’ feeding changed in response to different foods and controlling of the SELK neurons — and calcium imaging — a process that involves opening up the fly’s equivalent of a skull, inserting a microscope and using fluorescent proteins to visualize neuronal activity.
Molly Stanley, an assistant professor of biology at the University of Vermont specializing in the neuroscience behind feeding behavior, said she and other members of her lab were “really excited” by the study.
“It really progresses the field forward,” Stanley said in an interview with The Herald. To Stanley, the paper demonstrated two main things: Two different tastes can communicate through the same neuron, and different activation levels and types of neurotransmitters in a neuron can lead to opposite outputs.
Stanley noted that she would be curious to see how the cells and molecules identified by the study would react to tastes other than just sweet and bitter, such as salty.
“We’re sure that there are other pathways,” Zandawala said, adding that he believes there is an “entire network” that scientists are “slowly trying to ease … apart.” The researchers are currently working with other neuropeptides in the same pathway to observing what impact they have on feeding behaviors.
“This is really beautiful to see all of this come together so nicely to answer this really kind of foundational neurobiology question,” Stanley said.
Angel Lopez is a senior staff writer covering Science and Research. He’s a first-year student from Tyler, Texas and planning to study neuroscience and literary arts. In his free time, you can find him playing ping pong, listening to music, or reading.
