title: In situ tip-recordings found no evidence for an Orco-based ionotropic mechanism of pheromone-transduction in Manduca sexta.
abstract: The mechanisms of insect odor transduction are still controversial. Insect odorant receptors (ORs) are 7TM receptors with inverted membrane topology. They colocalize with a conserved coreceptor (Orco) with chaperone and ion channel function. Some studies suggest that insects employ exclusively ionotropic odor transduction via OR-Orco heteromers. Other studies provide evidence for different metabotropic odor transduction cascades, which employ second messenger-gated ion channel families for odor transduction. The hawkmoth Manduca sexta is an established model organism for studies of insect olfaction, also due to the availability of the hawkmoth-specific pheromone blend with its main component bombykal. Previous patch-clamp studies on primary cell cultures of M. sexta olfactory receptor neurons provided evidence for a pheromone-dependent activation of a phospholipase Cβ. Pheromone application elicited a sequence of one rapid, apparently IP3-dependent, transient and two slower Ca(2+)-dependent inward currents. It remains unknown whether additionally an ionotropic pheromone-transduction mechanism is employed. If indeed an OR-Orco ion channel complex underlies an ionotropic mechanism, then Orco agonist-dependent opening of the OR-Orco channel pore should add up to pheromone-dependent opening of the pore. Here, in tip-recordings from intact pheromone-sensitive sensilla, perfusion with the Orco agonist VUAA1 did not increase pheromone-responses within the first 1000 ms. However, VUAA1 increased spontaneous activity of olfactory receptor neurons Zeitgebertime- and dose-dependently. We conclude that we find no evidence for an Orco-dependent ionotropic pheromone transduction cascade in M. sexta. Instead, in M. sexta Orco appears to be a slower, second messenger-dependent pacemaker channel which affects kinetics and threshold of pheromone-detection via changes of intracellular Ca(2+) baseline concentrations.
authors.affiliation: Department of Animal Physiology, University of Kassel, Kassel, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Nolte
authors.forename: Nico W
authors.initials: NW
authors.lastname: Funk
authors.forename: Latha
authors.initials: L
authors.lastname: Mukunda
authors.forename: Petra
authors.initials: P
authors.lastname: Gawalek
authors.forename: Achim
authors.initials: A
authors.lastname: Werckenthin
authors.forename: Bill S
authors.initials: BS
authors.lastname: Hansson
authors.forename: Dieter
authors.initials: D
authors.lastname: Wicher
authors.forename: Monika
authors.initials: M
authors.lastname: Stengl
mesh_terms: D000200:Action Potentials; D000818:Animals; D020013:Calcium Signaling; D057809:HEK293 Cells; D006801:Humans; D019476:Insect Proteins / Q000378:metabolism*; D007473:Ion Channels / Q000378:metabolism*; D018402:Manduca / Q000166:cytology / Q000502:physiology*; D018034:Olfactory Receptor Neurons / Q000502:physiology; D010675:Pheromones / Q000494:pharmacology / Q000502:physiology*; D018035:Receptors, Odorant / Q000378:metabolism; D012903:Smell; D013864:Thioglycolates / Q000494:pharmacology; D014230:Triazoles / Q000494:pharmacology
chemical_list: D019476:Insect Proteins; D007473:Ion Channels; C572612:N-(4-ethylphenyl)-2-((4-ethyl-5-(3-pyridinyl)-4H-1,2,4-triazol-3-yl)thio)acetamide; D010675:Pheromones; D018035:Receptors, Odorant; D013864:Thioglycolates; D014230:Triazoles