Sense of smell: ephaptic interactions in insects

8 June 2017

Our sense of smell affects our appetite, also for mosquitoes. Angela Tsang (Croucher Scholarship 2015) is studying the sense of smell in insects to find ways in minimising mosquito-borne disease.

Humans are visual creatures. We primarily experience our world through our sense of sight. Of course, hearing, smell, touch, and taste play a role in the way that we perceive the world around us, but no other sense is as central to our existence. For us, the smell of food may reinforce our visual perception of it. A sound may alert us, signaling that we should bring the source into our field of vision for further examination. For most other animals, however, vision is not the principle sense. Dogs, our constant companion, have 300 million scent receptors in their noses compared to our measly 6 million. A world perceived primarily through smell requires a leap in thought and understanding than can be impossible for us to comprehend.

Angela Tsang (Croucher Scholarship 2015) is studying the sense of smell in insects. She recalls, “Recently, I temporarily lost my sense of smell due to a cold. The taste of food was drastically affected and I greatly lost my appetite. I realized how much I depended on my sense of smell to enjoy food. A whiff of a loved one’s perfume can trigger profound memories. And the comforting scent of lavender can often soothe us and help us unwind.”

Perhaps only when losing the sense are we able to more deeply understand it. And that understanding can help with the shift in perspective necessary for studying something so foreign to our own experience.

While many predators rely on their sense of smell to track down and attack prey, most pose no true threat to humanity. One predator though, is responsible for more human deaths than any other.

Mosquitoes track the carbon dioxide released in our breath to locate and identify us as potential feeding sites. These mosquitoes may then transfer any number of diseases through their bites. Some, like malaria and yellow fever are dangerous, and responsible for hundreds of thousands of deaths each year.

Tsang hopes that through elucidating the mechanisms by which insects process olfactory information, methods for blocking their ability to track us may be found, potentially saving lives that would otherwise be lost to mosquito-borne disease.

In her lab, Tsang studies fruit flies as a model for mosquitoes. Fruit fly larvae, for example, don’t really rely on their visual systems at all. Instead, their sense of smell drives their ability to find and locate food. Mating behavior is also dependent on smell, as olfactory cues provide the triggers needed to stimulate reproduction. Pheromones tell the fruit flies when to court and these chemical signals are interpreted to signal the time for copulation. From birth to death, these insects rely on smell for all facets of their life.

Through her research into fruit flies, Tsang and her lab have discovered that the carbon dioxide detecting neurons in fruit flies can be inhibited through ephaptic interaction. Ephaptic interactions are a special class of neuronal communication, separate from the electrical and chemical signaling that is more commonly talked about.

In ephaptic interactions, nerve cells communicate directly with each other when packed tightly together and well-insulated.

“When I was in college, I was taught that nerve cells communicate by two methods: secreting chemicals or passing electrical signals through each other. So when I learned about ephaptic interactions, I felt like I uncovered the secret language that nerve cells use to speak to each other. It was exciting!” says Tsang.

And studying these lesser-known, and less well understood types of neuronal communication has paid off, as the discoveries made by her lab have shown that the neuronal inhibition that prevents fruit flies from detecting carbon dioxide can also be applied to mosquitoes.

By inhibiting the mosquitoes’ ability to detect carbon dioxide, we could greatly hinder their prey-seeking abilities. As carbon dioxide is a powerful arousal cue for a hungry mosquito, blocking out this trigger will help prevent mosquitoes frombeing able to locate their prey: us.

While this kind of application is certainly far down the line, Tsang’s research now is helping characterize the neurophysiology of olfactory system of insects as a whole, which will enable the kind of deeper inquiry necessary for developing an applied olfaction-mediated mosquito defense.

Tsang also highlights the importance of basic science. “The discovery of ephaptic interactions in the fly’s nose was not the result of new technologies,” she says, “but instead a more careful examination of the responses of nerve cells to odorants.

“In the past few decades, many basic science researchers have made important discoveries in the field of insect olfaction.”

Our understanding of olfaction itself also has a long way to come. While we are visually dominant creatures, smell adds “beauty and nuance that allows humans to interact more richly with each other and their environment,” says Tsang.

Our sense of smell is famous (or infamous) for its potent ability to stir up old memories and strong emotions. The power of smell could open up new paths in therapy and relaxation. Aromatherapy has been used across the world for millennia for its ability to excite or calm certain emotions within us.

“Studies in olfaction could also theoretically help humans extend our capacity to smell,” Tsang says. With a more powerful sense of smell, the way in which we perceive our world could shift dramatically. 

Angela Tsang is currently a PhD student at the University of California, San Diego, where she studies the neurobiology of the sensory system. She graduated summa cum laude from Arizona State University, with a Bachelor’s degree in Genetics, Cell and Developmental Biology. She is passionate about deciphering the neural basis of the sense of smell. She is working in Dr. Chih-Ying Su’s lab to study novel neural circuit mechanisms for olfactory sensory neurons (neurons that directly detect odors) to communicate with each other, ultimately modulating the sense of smell and odor-driven behavior.

To view Tsang’s Croucher profile, please click here