Analysis of olfactory transduction mechanisms in the behaving hawkmoth Manduca sexta
Insect odor transduction and encoding is still under debate. In insect olfactory receptor neurons (ORNs) an ubiquitous Orco receptor is suggested to multimerize with olfactory receptors (ORs) to form an odor-gated ion channel. The functional implications of this ionotropic mechanism are under debate. In the hawkmoth Manduca sexta we propose a new hypothesis which suggests that next to metabotropic cascades leaky OR-Orco ion channels are employed in temporal encoding of pheromone stimuli. They are suggested to generate subthreshold membrane potential oscillations which control spontaneous activity. Thereby, threshold and time course of odor transduction are determined. We hypothesize that Orco functions as a hormone-dependent pacemaker current, regulated via changes in the intracellular Ca2+ and cyclic nucleotide concentrations dependent on the behavioral state of the insect. Here, we examine pheromone-dependent transduction cascades in the hawkmoth during different behavioral states. Focus is to challenge our new hypothesis of the role of MsexOrco in temporal encoding of pheromones. With patch clamp studies, pharmacology, and Ca2+-imaging in a heterologous expression system as well as with primary cell cultures of ORNs in vitro we examine the respective contribution of ionotropic mechanisms and metabotropic cascades. With ELISAs and tip recordings of intact sensilla of M. sexta in situ we further challenge our new hypothesis and analyze whether dependence on different behavioral contexts and physiological states is based upon circadian changes in cyclic nucleotide levels.