Abstract
Many insect behaviors are driven by olfaction, making insect olfactory receptors (ORs) appealing targets for insect control. Insect ORs are odorant-gated ion channels, with each receptor thought to be composed of a representative from a large, variable family of odorant binding subunits and a highly conserved co-receptor subunit (Orco), assembled in an unknown stoichiometry. Synthetic Orco directed agonists and antagonists have recently been identified. Several Orco antagonists have been shown to act via an allosteric mechanism to inhibit OR activation by odorants. The high degree of conservation of Orco across insect species results in Orco antagonists having broad activity at ORs from a variety of insect species and suggests that the binding site for Orco ligands may serve as a modulatory site for compounds endogenous to insects or may be a target of exogenous compounds, such as those produced by plants. To test this idea, we screened a series of biogenic and trace amines, identifying several as Orco antagonists. Of particular interest were tryptamine, a plant-produced amine, and tyramine, an amine endogenous to the insect nervous system. Tryptamine was found to be a potent antagonist of Orco, able to block Orco activation by an Orco agonist and to allosterically inhibit activation of ORs by odorants. Tyramine had effects similar to those of tryptamine, but was less potent. Importantly, both tryptamine and tyramine displayed broad activity, inhibiting odorant activation of ORs of species from three different insect orders (Diptera, Lepidoptera and Coleoptera), as well as odorant activation of six diverse ORs from a single species (the human malaria vector mosquito, Anopheles gambiae). Our results suggest that endogenous and exogenous natural compounds serve as Orco ligands modulating insect olfaction and that Orco can be an important target for the development of novel insect repellants.
Highlights
Insects have positive and negative impacts on humans, in terms of health, economy, and food stores
As the “pharmacology” of synthetic Orco agonists and antagonists has expanded, it has become clear that Orco subunits from disparate insect species have very similar sensitivities to known Orco ligands[27,31,32,33,35]. This suggested to us that the binding site for Orco ligands may serve as a modulatory site for compounds endogenous to the insects or may be a target of exogenous compounds, such as those generated by plants
We found tryptamine to be a potent Orco antagonist with broad activity at Orco subunits from different species
Summary
Insects have positive and negative impacts on humans, in terms of health, economy, and food stores. Based on the VUAA1 structure, several additional synthetic Orco agonists and a larger, more diverse series of synthetic Orco antagonists have been identified[31,32,33] Several of these Orco antagonists were shown to inhibit odorant activation of ORs through a noncompetitive mechanism[31,32,33]. As the “pharmacology” of synthetic Orco agonists and antagonists has expanded, it has become clear that Orco subunits from disparate insect species have very similar sensitivities to known Orco ligands[27,31,32,33,35] This suggested to us that the binding site for Orco ligands may serve as a modulatory site for compounds endogenous to the insects or may be a target of exogenous compounds, such as those generated by plants. Our findings suggest a role for Orco as a modulatory site common to all insect ORs and support the development of Orco-directed compounds that can be used to manipulate insect behavior
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