Insect Odorant Receptors Are Molecular Targets of the Insect Repellent DEET

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The insect repellent DEET is effective against a variety of medically important pests, but its mode of action still draws considerable debate. The widely accepted hypothesis that DEET interferes with the detection of lactic acid has been challenged by demonstrated DEETinduced repellency in the absence of lactic acid. The most recent hypothesis suggests that DEET masks or jams the olfactory system by attenuating electrophysiological responses to 1-octen-3-ol. Our research shows that mosquitoes smell DEET directly and avoid it. We performed single-unit recordings from all functional ORNs on the antenna and maxillary palps of Culex quinquefasciatus and found an ORN in a short trichoid sensillum responding to DEET in a dosedependent manner. The same ORN responded with higher sensitivity to terpenoid compounds. SPME and GC analysis showed that odorants were trapped in conventional stimulus cartridges upon addition of a DEET-impregnated filter paper strip thus leading to the observed reduced electrophysiological responses, as reported elsewhere. With a new stimulus delivery method releasing equal amounts of 1-octen-3-ol alone or in combination with DEET we found no difference in neuronal responses. When applied to human skin, DEET altered the chemical profile of emanations by a ''fixative'' effect that may also contribute to repellency. However, the main mode of action is the direct detection of DEET as indicated by the evidence that mosquitoes are endowed with DEET-detecting ORNs and corroborated by behavioral bioassays. In a sugar-feeding assay, both female and male mosquitoes avoided DEET. In addition, mosquitoes responding only to physical stimuli avoided DEET.DEET mode of action ͉ DEET-detecting neuron ͉ fixative effect of DEET ͉ mosquito olfaction ͉ surface-landing bioassay T he insect repellent DEET, N,N-diethyl-3-methylbenzamide, has been used for Ͼ50 years, with 200 million people using it worldwide to reduce their risk of vector-borne diseases (1) but its mode of action has yet to be elucidated. The report that DEET modulates the physiological response of lactic acid-sensitive olfactory receptor neurons (ORNs) in the antennae of the yellow fever mosquito, Aedes aegypti (2), led to the hypothesis that DEET may interfere with and inhibit the response of the olfactory system to a normally attractive chemical signal (3). This notion of ''jamming'' the olfactory system has been substantiated, on the one hand, by behavioral observations indicating that lactic acid per se is a mosquito attractant and suggesting that DEET inhibits attraction to lactic acid (4, 5) and by the recent report on DEET attenuation of mosquito response to 1-octen-3-ol (6). However, repellency solely by inhibition of lactic acid detection was challenged by indoor (4) and field experiments (7) demonstrating the repellent effect of DEET with carbon dioxide as the only attractant. Intrigued by this controversy and puzzled by the dichotomy between sensory physiology and behavioral observations, we undertook a multidisciplinary approach aimed at ...

Section: Resultscontrasting

confidence: 56%

mentioning

confidence: 90%

Mosquitoes smell and avoid the insect repellent DEET

Syed

Leal

2008

287

231

“…In addition, the larvae were observed to manifest modest positive PIs in response to the sources of heterocyclic aromatic compounds carvone and thiazole at 10 Ϫ5 dilutions but were indifferent when tested against lower dilutions (data not shown). Interestingly, the widely used insect repellent N,N-diethyl-m-toluamide (DEET), which has recently been shown to target olfactory pathways (15), consistently evoked dose-dependent and highly significant negative PIs at dilutions of 10 Ϫ4 or less (Fig. 2 J).…”

Section: Resultsmentioning

confidence: 99%

“…2 J and 4), which, based on the larval specific expression of AgOr40, may indicate the presence of additional DEET-sensitive AgORs in the adult. An alternative suggestion is that DEET may not act as a true behavioral repellent but rather by inhibiting the attraction to another compound, perhaps through antagonistic mechanisms at the molecular level (15,29).…”

Section: Discussionmentioning

confidence: 99%

The molecular and cellular basis of olfactory-driven behavior inAnopheles gambiaelarvae

Xia

Wang

Buscariollo

et al. 2008

140

211

The mosquito Anopheles gambiae is the principal Afrotropical vector for human malaria. A central component of its vectorial capacity is the ability to maintain sufficient populations of adults. During both adult and preadult (larval) stages, the mosquitoes depend on the ability to recognize and respond to chemical cues that mediate feeding and survival. In this study, we used a behavioral assay to identify a range of odorant-specific responses of An. gambiae larvae that are dependent on the integrity of the larval antennae. Parallel molecular analyses have identified a subset of the An. gambiae odorant receptors (AgOrs) that are localized to discrete neurons within the larval antennae and facilitate odor-evoked responses in Xenopus oocytes that are consistent with the larval behavioral spectrum. These studies shed light on chemosensory-driven behaviors and represent molecular and cellular characterization of olfactory processes in mosquito larvae. These advances may ultimately enhance the development of vector control strategies, targeting olfactory pathways in larval-stage mosquitoes to reduce the catastrophic effects of malaria and other diseases.malaria ͉ olfaction ͉ signal transduction ͉ odorant receptors S ensitivity and the ability to respond to a wide range of olfactory cues are essential for many behavioral processes that mediate the vectorial capacity of Anopheles gambiae and other diseasecarrying mosquitoes (1). Although there is a growing body of knowledge of the adult An. gambiae olfactory system, there is a paucity of information as to the molecular and cellular basis of olfaction in larval stages, in which it may be of potential importance in disease control. Paradoxically, despite being one of the historically most successful strategies for mosquito control (2) and prevention of human malaria, the targeting of mosquito larvae or larval habitats around human dwellings is sparsely implemented in Africa and other malaria-endemic regions (3). Furthermore, the simplicity of insect larval olfactory systems makes them excellent models to study olfactory signal transduction and coding. Indeed, the arbovirus vector mosquito Aedes aegypti expresses 24 odorant receptor (OR) genes in the larval antenna, 15 of which are larval specific (4). Elegant work in the Drosophila melanogaster model has detailed larval behavioral responses and characterized 25 ORs that are expressed in 21 olfactory receptor neurons (ORNs) in each of the two dorsal organs, which constitute the olfactory apparatus of the fly larva (5-7).In this study, we designed and used a simple olfactory-based assay to carry out an initial characterization of An. gambiae larval behavioral responses to a range of natural and synthetic chemical stimuli. Consistent with olfactory function, ablation of the larval antennae specifically eliminated these behavioral responses, and molecular approaches identified a set of larval AgOrs, which were in some cases larval specific and the transcripts of which were mapped to a distinctive population of ORNs within th...

“…It was originally thought to act by affecting the olfactory receptor neurons (ORNs) sensitive to lactic acid (3) and thus inhibit the mosquito's response to this, normally attractive, compound (4-6), but this view was challenged by the finding that DEET can function as a repellent even when other attractants are present (4,7). A recent investigation on Anopheles gambiae has suggested that the ORN for 1-octen-3-ol, a component of human sweat (8) that in combination with CO 2 acts as an attractant for this species (9,10), is blocked by DEET, and thus, in the presence of DEET, a higher concentration of the 1-octen-3-ol is required for the mosquitoes to be able to detect it (11). However, Syed and Leal (12) investigated this theory by using Culex quinquefasciatus and suggest that the reduction in response to the 1-octen-3-ol in this species is not because of a diminished response of the ORN but to interactions between the two compounds when DEET and 1-octen-3-ol are tested in the same cartridge.…”

mentioning

confidence: 99%

Behavioral insensitivity to DEET in Aedes aegypti is a genetically determined trait residing in changes in sensillum function

Stanczyk

Brookfield

Ignell

et al. 2010

123

N,N-Diethyl-m-toluamide (DEET) is one of the most effective and commonly used mosquito repellents. However, during laboratory trials a small proportion of mosquitoes are still attracted by human odors despite the presence of DEET. In this study behavioral assays identified Aedes aegypti females that were insensitive to DEET, and the selection of either sensitive or insensitive groups of females with males of unknown sensitivity over several generations resulted in two populations with different proportions of insensitive females. Crossing experiments showed the "insensitivity" trait to be dominant. Electroantennography showed a reduced response to DEET in the selected insensitive line compared with the selected sensitive line, and single sensillum recordings identified DEET-sensitive sensilla that were nonresponders in the insensitive line. This study suggests that behavioral insensitivity to DEET in A. aegypti is a genetically determined dominant trait and resides in changes in sensillum function.O ne of the most widely used and effective insect repellents available is the synthetic compound N,N-Diethyl-mtoluamide (DEET) (1). DEET was identified more than 50 years ago by a structure-activity study of synthetic compounds and, although a number of compounds with similar activity have been identified, their efficacy is often judged by comparison with DEET (2). The mode of action of DEET has not been elucidated fully. It was originally thought to act by affecting the olfactory receptor neurons (ORNs) sensitive to lactic acid (3) and thus inhibit the mosquito's response to this, normally attractive, compound (4-6), but this view was challenged by the finding that DEET can function as a repellent even when other attractants are present (4, 7). A recent investigation on Anopheles gambiae has suggested that the ORN for 1-octen-3-ol, a component of human sweat (8) that in combination with CO 2 acts as an attractant for this species (9, 10), is blocked by DEET, and thus, in the presence of DEET, a higher concentration of the 1-octen-3-ol is required for the mosquitoes to be able to detect it (11). However, Syed and Leal (12) investigated this theory by using Culex quinquefasciatus and suggest that the reduction in response to the 1-octen-3-ol in this species is not because of a diminished response of the ORN but to interactions between the two compounds when DEET and 1-octen-3-ol are tested in the same cartridge. Additionally, single sensillum recordings have identified an ORN that responds directly to DEET in C. quinquefasciatus (12) and Aedes aegypti (4), indicating that these mosquito species are actively detecting DEET rather than DEET interfering with an ORN detecting another compound.Despite the proven efficacy of DEET as a repellent, during laboratory and field trials it is common to find that a small proportion of mosquitoes are not repelled by the compound (13-15). Boeckh et al. (4) demonstrated that DEET reduced, but did not entirely eliminate, the approach of A. aegypti to host odors, and experiments with Dros...