RESUMO
Insects have developed remarkable adaptations to effectively interact with plant secondary metabolites and utilize them as cues to identify suitable hosts. Consequently, humans have used aromatic plants for centuries to repel mosquitoes. The repellent effects of plant volatile compounds are mediated through olfactory structures present in the antennae, and maxillary palps of mosquitoes. Mosquito maxillary palps contain capitate-peg sensilla, which house three olfactory sensory neurons, of which two are mainly tuned to either carbon dioxide or octenol - two animal host odorants. However, the third neuron, which expresses the OR49 receptor, has remained without a known ecologically-relevant odorant since its initial discovery. In this study, we used odorant mixtures and terpenoid-rich Cannabis essential oils to investigate the activation of OR49. Our results demonstrate that two monoterpenoids, borneol and camphor, selectively activate OR49, and OR9-expressing neurons, as well as the MD3 glomerulus in the antennal lobe. We confirm that borneol repels female mosquitoes, and knocking out the gene encoding the OR49 receptor suppresses the response of the corresponding olfactory sensory neuron. Importantly, this molecular mechanism of action is conserved across culicine mosquito species, underscoring its significance in their olfactory systems.
RESUMO
Mosquitoes are the deadliest of all combined insects and animals affecting millions and killing hundreds or thousands of people each year. Existing protection methods however are limited and include volatile compounds that actively repel mosquitoes such as N,N-Diethyl-meta-toluamide (DEET) or different essential oils such as geraniol and citronella. Most are odorous compounds and require organic solvents for dispersion. This work investigates the barrier properties of cellulose nanocrystals (CNCs). CNCs are known to self-assemble in strong, transparent, chemical barrier films. They are fully bio-based, and their surface chemistry is ideal for aqueous dispersion of many compounds. This work saw a significant 80% decrease in feeding on human skin when a thin CNC coat was applied. The effect was further confirmed by artificial feeding on Aedes aegypti wherein CNC appears to act as a chemical camouflage to the many cues sought by the insects. The combined effect of CNC with indole reduced egg laying post exposure to mammalian blood close to null with 99.4% less eggs as compared to control. The chemical barrier effect was assessed through a simple headspace experiment showing that the same CNC coat blocked the passage of ammonium hydroxide vapor, a commonly used mosquito attractant, when applied on a filter paper membrane.
RESUMO
BACKGROUND: Mosquitoes are responsible for disease transmission worldwide. They possess the ability to discriminate between different ecological resources, including nectar sources, animal hosts and oviposition sites, a feature mediated by their olfactory system. Insect repellents, such as N,N-diethyl-meta-toluamide (also called DEET), have been shown to activate and inhibit mosquito odorant receptors, resulting in behavioral modulation. This and other repellents currently available for personal protection against mosquitoes are topically applied to the skin and operate at a short range. In our search for potential long-range inhibitors of attractants to human hosts, we have hypothesized that the shared chemical similarities between indole and DEET may confer the former with the ability to block odorant receptor function and inhibit human host attraction in a similar way as DEET. METHODS: We used the two-electrode voltage clamp system to assay Xenopus laevis oocytes as a platform to compare the pharmacological effect of commercially available insect repellents and indole on the Aedes aegypti (R)-1-octen-3-ol receptor, OR8, a receptor involved in the decision-making of female mosquitoes to identify human hosts. We also conducted arm-in-a-cage and wind-tunnel bioassays to explore the effect of indole on human host-seeking female Aedes aegypti mosquitoes. RESULTS: Our results demonstrate that indole inhibited the Aedes aegypti (R)-1-octen-3-ol receptor OR8. In our arm-in-a-cage assay, 1 M of DEET reduced mosquito visits on average by 69.3% while the same indole concentration achieved 97.8% inhibition. This effect of indole on flight visits was dose-dependent and disappeared at 1 µM. In the flight tunnel, indole elicited on average 27.5% lower speed, 42.3% lower upwind velocity and 30.4% higher tortuosity compared to the control. CONCLUSIONS: Indole significantly inhibits OR8 activation by (R)-1-octen-3-ol, mosquito visits to a human hand and long-range human host-seeking. The volatility of indole may be leveraged to develop a novel insect repellent in the context of personal mosquito protection.
