Washington, March 14 : Rockefeller University researchers have come up with an explanation of how a widely used bug repellent called DEET works, something that experts have failed to discern in about 50 years.
The researchers have shown that DEET - invented by the US Department of Agriculture and the Army to protect soldiers from disease-transmitting insects - acts like a chemical cloak to mask human odours that blood-feeding insects find attractive.
"For all these years, there were a lot of theories but no consensus on how DEET worked. Does it smell bad to mosquitoes or does it blind them to odours? It was a great unsolved problem," says Leslie Vosshall, head of the Laboratory of Neurogenetics and Behavior.
Mosquitoes are strongly attracted to odours in human breath and sweat, including carbon dioxide, lactic acid and an alcohol-based compound called 1-octen-3-ol. They use different receptors within their olfactory system, among others, to detect human odours.
DEET simply interferes with the proper functioning of odorant receptors, making the hunt for a tasty meal all the more difficult. However, this interference is selective.
With a view to seeing DEET's effect on different odorant receptors, the research team recorded the electrical activity of cells in the mosquito olfactory system while exposing the insects to the chemical. They found that DEET only shuts down those receptors that work in tandem with a smell coreceptor called Or83b, which is present in all insects.
In their study report, the researchers said that while DEET shuts down the receptor pairs that detect 1-octen-3-ol and two other sweaty odors, it does not affect the lone receptor that detects carbon dioxide because it does not require Or83b to function.
"Each receptor complex has different properties. And the idea is that DEET is acting on the uniqueness of this complex," says Vosshall.
For their study, the researchers used fruit fly mutants that lacked the coreceptor because mosquitoes lacking it had not been genetically engineered till that time.
While normal flies avoid a vial treated with DEET, the researchers found that flies without the coreceptor ventured into the vials, suggesting that Or83b is required to detect this potent chemical.
Thereafter, the researchers proved that DEET specifically affected the receptor/coreceptor as a unit by isolating the RNA of each, and injecting both into a frog egg. They observed that DEET inhibited the odorant receptor/coreceptor complex even in this environment, which was isolated from the olfactory system.
Vosshall says that DEET does not shut down the entire olfactory system as it targets the coreceptor complex rather than the coreceptor alone.
"Instead, it seems to shut down strategically different parts of it. It just shuts down enough of these receptors to confuse the mosquito or blind it to the odors it finds attractive," she says.
The new study, published in the journal Science Express, attains significance as it raises the possibility of improving the repellent properties of DEET, and making it a safer chemical.
"We now know how DEET works, and this is the first step in making significantly better insect repellents," says Vosshall.