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Hans Merzendorfer

Every one of us has experienced a sleepless night courtesy of an annoying buzzing mosquito trying to bite us. What presumably prevents us most from sleeping is the helpless admission that the mosquito will achieve its aim a soon as we doze off. They do so even in complete darkness, attracted by our body odors. However, we are not completely at the mercy of our tormentors. Some of us will fight before surrender by taking up an insect repellent. Most of them contain N,N-diethyl-m-toluamide (DEET), a compound that efficiently wards off many biting insects. The common view is that the deterring effect of DEET is because it smells bad to the insects. Recently, however, the research team of Leslie Vosshall from the Rockerfeller University in New York reported in Science that this view is wrong.

The attraction that we exert on female mosquitoes is largely odor mediated. With specialized sensory organs called sensilla they sniff human body emanations such as lactic acid in our sweat or CO2 and octenol in our breath. The insect's sensilla harbor olfactory sensory neurons that fire electrical impulses in response to a particular attractant. By measuring these odor-evoked electrical impulses at a sensillum of the Anopheles mosquito, Vosshall's team at first examined the effect of DEET on the sensory neuron responses to octenol and CO2. While DEET had no effect on CO2-evoked responses, it inhibited firing of neurons responding to octenol.

As neurobiology is much better understood in fruit flies than in mosquitos, the scientists switched to carrying out behavioral experiments in Drosophila. They established an assay where the flies were given the choice of two different trap vials to enter. After setting up different experimental conditions they recorded the number of flies in each vial. If both vials were loaded with food, the flies took the bait and distributed themselves equally between them. However, if one of the two baited vials was treated with 10% DEET, a common concentration in repellent sprays, the flies avoided the DEET-treated vial. Surprisingly, the flies were happy to enter the DEET-scented vial when no food was present. It appeared as if the flies couldn't smell the food in the presence of DEET rather than just being warded off.

Then the scientists went further, testing whether the DEET effect involves odor receptors known to respond to particular smells including octenol. For this purpose they engineered frog egg cells producing various types of Drosophila and Anopheles odor receptors on their cell surface. When these eggs were exposed to different human body odors, the scientists could record inward cation currents indicating that the receptor had `smelled' the scent. In the presence of DEET a subset of the tested receptors, including the Anopheles octenol receptor, were far less responsive to the odor, as the measured currents were significantly reduced. DEET was blocking the ability of the Anopheles octenol receptor to detect the tell-tale human scent.

Vosshall and her colleagues have shown that DEET is not a repellent in the real sense but prevents the mosquito from smelling us. Their nifty trick of using frog egg cells as a sort of `midget nose' may help to identify new compounds that protect us even more effectively from mosquitoes, including those transmitting serious diseases.