Abstract
Insects possess one of the most exquisitely sensitive olfactory systems in the animal kingdom, consisting of three different types of chemosensory receptors: ionotropic glutamate-like receptors (IRs), gustatory receptors (GRs) and odorant receptors (ORs). Both insect ORs and IRs are ligand-gated ion channels, but ORs possess a unique configuration composed of an odorant-specific protein OrX and a ubiquitous coreceptor (Orco). In addition, these two ionotropic receptors confer different tuning properties for the neurons in which they are expressed. Unlike IRs, neurons expressing ORs are more sensitive and can also be sensitized by sub-threshold concentrations of stimuli. What is the mechanistic basis for these differences in tuning? We show that intrinsic regulation of Orco enhances neuronal response to odorants and sensitizes the ORs. We also demonstrate that inhibition of metabotropic regulation prevents receptor sensitization. Our results indicate that Orco-mediated regulation of OR sensitivity provides tunable ionotropic receptors capable of detecting odors over a wider range of concentrations, providing broadened sensitivity over IRs themselves.
Highlights
Insects, for which olfaction is of primary importance for survival [1,2] possess remarkable chemosensory capabilities
Insect odorant receptors (ORs) exhibit a unique configuration of heterodimers composed of an odorant-specific olfactory receptor protein (OrX) and a ubiquitous coreceptor (Orco) [7] which operate as ligand-gated ion channels [8,9]
We previously demonstrated that activators of phospholipase C (PLC) or protein kinase C (PKC) can stimulate OrX and a ubiquitous coreceptor (Orco) channel activity, while inhibition of PLC or PKC abolishes Orco sensitivity to cAMP [24]
Summary
For which olfaction is of primary importance for survival [1,2] possess remarkable chemosensory capabilities. The cellular and molecular mechanisms underlying the outstanding sensitivity of the insect olfactory system are not well understood. Insects are known to possess three different types of chemosensory receptors: odorant receptors (ORs), ionotropic glutamate-like receptors (IRs), and gustatory receptors (GRs) [4,5,6]. IRs are threetransmembrane proteins, whereas GRs and ORs are seventransmembrane proteins [5,6,7]. Insect odorant receptors (ORs) exhibit a unique configuration of heterodimers composed of an odorant-specific olfactory receptor protein (OrX) and a ubiquitous coreceptor (Orco) [7] which operate as ligand-gated ion channels [8,9]
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