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Abstract
Odorants inhibit as well as excite primary olfactory receptor neurons (ORNs) in many animal species. Growing evidence suggests that inhibition of mammalian ORNs is mediated by phosphoinositide (PI) signaling through activation of phosphoinositide 3-kinase (PI3K), and that canonical adenylyl cyclase III signaling and PI3K signaling interact to provide the basis for ligand-induced selective signaling. As PI3K is known to act in concert with phospholipase C (PLC) in some cellular systems, the question arises as to whether they work together to mediate inhibitory transduction in mammalian ORNs. The present study is designed to test this hypothesis. While we establish that multiple PLC isoforms are expressed in the transduction zone of rat ORNs, that odorants can activate PLC in ORNs in situ, and that pharmacological blockade of PLC enhances the excitatory response to an odorant mixture in some ORNs in conjunction with PI3K blockade, we find that by itself PLC does not account for an inhibitory response. We conclude that PLC does not make a measurable independent contribution to odor-evoked inhibition, and that PI3K is the primary mediator of PI-dependent inhibition in mammalian ORNs.
Highlights
The process of odor detection and identification begins when odorants bind to olfactory receptors (ORs) expressed in the cilia of primary olfactory receptor neurons (ORNs) in the olfactory epithelium (OE) to activate signal transduction
In the founding experiments leading to this line of investigation (Spehr et al, 2002), a few ORNs responded to an odor mixture only when phosphoinositide 3-kinase (PI3K) and phospholipase C (PLC) were blocked together, suggesting that the enzymes may act in concert as part of an inhibitory
If PLC contributes to odor-evoked inhibition in mammalian ORNs, at least one isoform of PLC should be present in the transduction compartment. mRNA encoding several isoforms of PLC can be found in the mouse OE (Szebenyi et al, 2014)
Summary
The process of odor detection and identification begins when odorants bind to olfactory receptors (ORs) expressed in the cilia of primary olfactory receptor neurons (ORNs) in the olfactory epithelium (OE) to activate signal transduction. There is evidence that odorants can activate PLC as well as PI3K in olfactory ciliary membranes (Vogl et al, 2000; Klasen et al, 2010), and isoforms of both enzymes have been detected at the level of the OE (Bruch et al, 1995; Brunert et al, 2010; Ukhanov et al, 2010; Szebenyi et al, 2014), in some cases in the olfactory cilia (Brunert et al, 2010; Ukhanov et al, 2010) These findings raise the possibility that PLC and PI3K both contribute to an inhibitory signaling branch of LiSS. We conclude that PLC by itself does not make a measurable contribution to odor-evoked inhibition, and that PI3K is the primary mediator of PI-dependent inhibition in mammalian ORNs
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