Archive | 2019
Responses of Insect Olfactory Neurons to Single Pheromone Molecules
Abstract
The status of our understanding of the molecular processes underlying olfactory reception in insects was summarized by Wicher (Progress in molecular biology and translational science, vol 130. Elsevier, New York, pp 37–54, 2015; see also Chap. 4) and recently by Stengl (Chemosensory transduction in arthropods. In: Byrne JH (ed) Oxford handbooks online. The Oxford handbook of invertebrate neurobiology. Oxford University Press, pp 1–42. https://doi.org/10.1093/oxfordhb/9780190456757.013.15, 2017) and Wicher and Grosse-Wilde (Chemoreceptors in evolution. In: Kaas J (ed) Evolution of nervous systems 2e. Elsevier, Oxford, pp 245 -255, 2017). The present chapter adds an overdue review of studies dealing with the responses of moth antennal olfactory neurons (nerve cells) to single impacts of airborne pheromone molecules. Weak pheromone stimuli elicit “elementary receptor potentials” (ERPs) which consist of one or several “bumps”, transient negative deflections of the resting trans-epithelial potential recorded from the tips of single trichoid sensilla, i.e. olfactory mini-organs on insect antennae. In the male silkmoth Bombyx mori a bump may elicit one, seldom two or three nerve impulses, but up to five impulses in the sphingid moth Manduca sexta. According to behavioral, electrophysiological and radiometric studies, the ERPs are elicited by single pheromone molecules. The analysis of the neuro-electrical circuit of moths sensilla revealed that the average bump amplitude (of about 0.5 mV) reflects an increase of the membrane conductance of an olfactory neuron by about 30 pS. The observation of several sublevels of bump amplitudes in B. mori suggest either varying degrees of opening of a single ion channel or varying numbers of superimposed openings of smaller channels. At weak stimulus intensities ion channels might be directly gated by the odor molecule-receptor interaction. At higher intensities intracellular signaling might be responsible for diminished channel opening that causes widening the range of the pheromone dose-response and adaptation (reduced responsiveness) after strong stimuli. In B. mori the temporal characteristics of the responses to single pheromone molecules were used to calculate the apparent residence time of the pheromone molecule at the receptor molecule, in the range of 100 ms.