Generic insect repellent detector from the fruit fly Drosophila melanogaster

Abstract

Background: Insect repellents are prophylactic tools against a number of vector-borne diseases. There is growing demand for repellents outperforming DEET in cost and safety, but with the current technologies R&D of a new product takes almost 10 years, with a prohibitive cost of $30 million dollar in part due to the demand for large-scale synthesis of thousands of test compounds of which only 1 may reach the market. R&D could be expedited and cost dramatically reduced with a molecular/physiological target to streamline putative repellents for final efficacy and toxicological tests.

Methodology: Using olfactory-based choice assay we show here that the fruit fly is repelled by not only DEET, but also IR3535 and picaridin thus suggesting they might have "generic repellent detector(s)," which may be of practical applications in new repellent screenings. We performed single unit recordings from all olfactory sensilla in the antennae and maxillary palps. Although the ab3A neuron in the wild type flies responded to picaridin, it was unresponsive to DEET and IR3535. By contrast, a neuron housed in the palp basiconic sensilla pb1 responded to DEET, IR3535, and picaridin, with apparent sensitivity higher than that of the DEET detectors in the mosquitoes Culex quinquefasciatus and Aedes aegypti. DmOr42a was transplanted from pb1 to the "empty neuron" and showed to be sensitive to the three insect repellents.

Conclusions: For the first time we have demonstrated that the fruit fly avoids not only DEET but also IR3535 and picaridin, and identified an olfactory receptor neuron (ORN), which is sensitive to these three major insect repellents. We have also identified the insect repellent-sensitive receptor, DmOr42a. This generic detector fulfils the requirements for a simplified bioassay for early screening of test insect repellents.

Figure 1. Repellency assay indicating avoidance of D. melanogaster to three insect repellents: DEET, IR3535, and picaridin.

(A) Male and female flies responded equally to DEET (N = 180 flies tested). Female flies avoid entering the food chambers treated with (B) IR3535 (N = 90 flies tested) and (C) picaridin (N = 90 flies tested). Data are from 9 independent trials for each test, with ten flies used in each trial.

Figure 2. Dose-dependent excitatory responses from a picaridin-sensitive ORN housed in an antennal basiconic sensillum ab3 on D. melanogaster antennae.

Hexane (control) responses were subtracted. (N = 7). Error bars are standard error of the mean (SEM).

Figure 3. Extracellularly recorded single unit responses from an ab3 sensillum.

Spontaneous activity (upper trace) and picaridin-induced excitatory response (lower trace) from the large amplitude neuron, ab3A. Source dose, 100 µg. Note the excitatory responses lasted beyond the stimulus period.

Figure 4. Excitatory responses from an ORN housed in the maxillary palp basiconic sensillum pb1A when challenged with three insect repellents: DEET, IR3535 and picaridin.

Two types of neurons, A and B, are identified on the basis of their spike amplitudes. The ORN with larger amplitude, A, is stimulated by the three insect repellents, whereas the neuron with smaller amplitude, B, was unresponsive. Source dose, 100 µg.

Figure 5. All three insect repellents induce dose-dependent excitatory responses in the pb1A ORN.

Hexane (control) responses were subtracted. (N = 7). Error bars are standard error of the mean (SEM).

Figure 6. Comparative DEET-elicited responses.

DEET-induced excitatory responses from Drosophila pbA1 ORN showed lower threshold than those recorded from the DEET-sensitive mosquito ORNs from the Southern House mosquito, Cx. quinquefasciatus and the yellow fever mosquito, Aedes aegypti , respectively. Error bars are standard error of the mean (SEM).

Figure 7. Repellent-sensitive ORN, pb1A, challenged with PMD stereoisomers.

(+)-PMD elicited higher response from the large spike neuron than (−)-PMD. Source dose, 100 µg.

Figure 8. Action potentials recorded from ab3 sensilla.

Δhalo flies showed spontaneous activity of neuron B, but not A, thus showing the ab3A is indeed “empty.” Lower traces were excitatory responses induced by DEET, IR3535, and picaridin and recorded from ab3 sensilla of Or42-expressing flies (w; Δhalo; UAS-DmOr42a/Or22a-GAL4).

Figure 9. Comparative responses of Or42a expressed in its native environment and in the “empty neuron.”

Ethyl butyrate (source dose, 10 µg) elicited higher responses from pb1 sensilla of wild type flies (top trace) than from the ab3 sensilla of Or42a-expressing flies (w; Δhalo; UAS-DmOr42a/Or22a-GAL4) (lower trace). SEM, standard error of the mean.

Figure 10. Dose-dependent responses from ab3A neuron of transgenic flies expressing Or42a (w; Δhalo; UAS-DmOr42a/Or22a-GAL4).

Test flies were challenged with DEET, IR3535, and picaridin (N = 15). Error bars are standard error of the mean (SEM).