Date Published: August 11, 2010
Publisher: Public Library of Science
Author(s): Jonathan D. Bohbot, Joseph C. Dickens, Mark A. Frye. http://doi.org/10.1371/journal.pone.0012138
Abstract: DEET, 2-undecanone (2-U), IR3535 and Picaridin are widely used as insect repellents to prevent interactions between humans and many arthropods including mosquitoes. Their molecular action has only recently been studied, yielding seemingly contradictory theories including odorant-dependent inhibitory and odorant-independent excitatory activities on insect olfactory sensory neurons (OSNs) and odorant receptor proteins (ORs).
Here we characterize the action of these repellents on two Aedes aegypti ORs, AaOR2 and AaOR8, individually co-expressed with the common co-receptor AaOR7 in Xenopus oocytes; these ORs are respectively activated by the odors indole (AaOR2) and (R)-(−)-1-octen3-ol (AaOR8), odorants used to locate oviposition sites and host animals. In the absence of odorants, DEET activates AaOR2 but not AaOR8, while 2-U activates AaOR8 but not AaOR2; IR3535 and Picaridin do not activate these ORs. In the presence of odors, DEET strongly inhibits AaOR8 but not AaOR2, while 2-U strongly inhibits AaOR2 but not AaOR8; IR3535 and Picaridin strongly inhibit both ORs.
These data demonstrate that repellents can act as olfactory agonists or antagonists, thus modulating OR activity, bringing concordance to conflicting models.
Partial Text: The exact modes of action and molecular targets of the active ingredients found in insect repellents are poorly understood. Addressing this gap in our knowledge has become an urgent matter in order to understand how to improve the effectiveness of repellents and to develop a novel generation of olfactory disruptive compounds. Currently, most insect repellent products include the active ingredients N,N-diethyl-3-methylbenzamide (DEET), Insect Repellent 3535 (IR3535), and more recently Picaridin and 2-undecanone (2-U) (Fig. 1). In the current study, we investigate the molecular action of these repellents on two isolated odorant receptors (ORs) of the yellow fever mosquito Aedes aegypti.
We studied the actions of insect repellents DEET, 2-U, IR3535 and Picaridin on the activities of two Aedes aegypti ORs, AaOR2 and AaOR8, in the absence and presence of odorants specific to these ORs, indole (OR2) and octenol (OR8). In all cases, the ORs were expressed in Xenopus oocytes along with their common obligate co-receptor AaOR7. In the absence of odorant, DEET activated AaOR2 but not AaOR8, while 2-U activated AaOR8 but not AaOR2; neither receptor was activated by IR3535 or Picaridin. In the presence of odor, DEET strongly inhibited odorant-induced responses of AaOR8 but only slightly inhibited AaOR2, while 2-U strongly inhibited odorant-induced responses of AaOR2 but only slightly inhibited AaOR8; both receptors were equally and strongly inhibited by IR3535 or Picaridin. The observed OR activation by DEET and 2-U is consistent with previous physiological reports of adult OSNs and a molecular study of a larval OR. DEET alone activated two OSNs in the short blunt tipped sensilla (A-2) of Ae. aegypti. 2-U acted as a mosquito attractant  and activated mosquito OSNs including an OSN sensitive to octenol , . DEET alone also activated a larval OR and affected larval behavior in An. gambiae.