New joint research from Northwestern University and Lund University could lead people to go into their backyard instead of the store at the start of this year’s mosquito season.
Because of its euphoric and hallucinogenic effects on cats, catnip is often used as an additive in cat toys and treats and has long been known for its strong repellant properties against insects, especially mosquitoes. Recent research shows that catnip compounds are at least as effective as synthetic insect repellants like DEET.
Until now, the mechanism that produced insects’ aversion to this common member of the mint family was unknown. In an article scheduled for March 4 in the journal Current Biology, a team of researchers from the universities of Northwestern and Lund reported that they found the underlying receptors that contribute to the mosquito’s aversive response.
“Catnip and its active ingredient nepetalactone have been used for thousands of years to ward off insect pests, at least since Pliny the Elder,” said Marcus C. Stensmyr, associate professor at Lund University and co-author. “But why catnip is so effective on such a wide range of insect species remains unknown.”
Traditional approaches to mosquito control have included insecticides, but they eliminate other insect species as collateral damage. Modern formulations of insect repellants such as DEET target mosquito’s odor and taste receptors, rendering the insect unable to read the chemical clues that point to human prey.
“We discovered that catnip and its active ingredient nepetalactone activate the stimulus receptor TRPA1, an ancient pain receptor found in animals as diverse as flatworms, fruit flies and humans,” said Marco Gallio, associate professor of neurobiology at the Weinberg College of Arts and Sciences. “We now think that catnip is so averse to so many species of insects because it activates this widespread stimulus receptor.”
In previous work, the Gallio Lab and others have shown that humans, insects, and many other animal species have some version of the transient receptor potential ankyrin 1 (TRPA1) ion channel, a protein best known as the “wasabi receptor,” and environmental irritants such as z pain and itching.
“What is particularly interesting is that, unlike wasabi or garlic compounds, which also activate these receptors in humans, catnip appears to selectively activate the insect receptor,” said Gallio. “This explains why people are indifferent to it and offers a serious benefit to its use as a repellent.”
Why cats are so attracted to catnip is a whole different story that isn’t entirely understood. Research shows that this may be due to an unusual interaction between one of the active ingredients in catnip and a molecular component in the reward system of the cat’s brain.
“Mosquitoes, especially those that carry disease, are becoming a bigger problem as climate change creates attractive conditions for them further north and south of the equator,” said Stensmyr. “Plant-derived compounds represent a new approach to insect repellant development because plants have long known how to protect themselves from insect pests.”
Gallio added that herbal repellants are often available at a much lower cost and are easier to obtain. Catnip accessibility could have a significant impact in developing countries, where mosquito-borne diseases are a major concern.
The Gallio laboratory in the northwest examines the sensory systems of the common laboratory fruit fly, Drosophila, including the mechanisms that control reactions to external temperature and pain. Lund’s Stensmyr laboratory is primarily focused on mosquitoes and other human disease insect vectors.
Researchers studied different types of insects to better understand how catnip and its active ingredient repel a wide range of insects without irritating humans.
To confirm their results, the team conducted a series of tests, including offering a blood meal to mosquitoes in a nylon sock dipped in catnip, experiments with a wind tunnel, and experiments in which volunteers put their hands in a cage with live mosquitoes or rub oil without protection of a catnip.
Gallio believes the mechanism they discovered also provides a proof of concept for developing next-generation repellants that use the same logic – selectively targeting the mosquito stimulus receptor.
“This is an entry point to study how this molecule acts on the receptor,” he said. “As soon as we understand the chemistry and the interaction with the receptor, we can develop even more powerful and targeted molecules.”
The team’s next project? Find out how to get rid of the cats that keep chasing them.
