Electrophysiological responses of olfactory receptor neurons to stimulation with mixtures of individual pheromone components

Abstract

Multicomponent pheromone systems are the rule in many species. As our knowledge about the number and kinds of different chemical compounds actually employed in the communication system of a particular species has increased, so too has our appreciation for the level of neurobiological complexity that must underlie these capabilities. The supposition that mixtures are differentially processed in the nervous system arises most easily when biologically relevant materials are evaluated, either singly or in multicomponent blends, with modern behavioral assay techniques. It is becoming increasingly clear that this increase in the chemical and behavioral complexity of a particular communication system must be paralleled by an increase in the efficiency of the physiological mechanisms employed for the neural encoding of behaviorally relevant odor compounds and blends. Here we review several studies that have examined the electrical activity elicited in primary olfactory receptor neurons when they are stimulated with mixtures of odorants. Particular attention is given to the responses elicited in a subset of the individual pheromone-sensitive sensilla on the antennae of male cabbage looper moths (Trichoplusia ni). Electrophysiological responses to single- and multiple-component stimuli, each drawn from among the seven known behaviorally active compounds for this insect, were obtained at several different stimulus intensities. Both (Z)7-dodecenyl acetate and (Z)7-dodecenol were effective stimuli for both of the receptor neurons found in one of the two classes of pheromone-sensitive sensilla, even at relatively low stimulus intensities (0.0005 micrograms). Dodecyl acetate, although behaviorally active, did not significantly excite either of these receptor neurons. However, when mixed with either of the unsaturated components, it significantly enhanced the receptor neuron's response to its appropriate parent compound only in the middle range of stimulus intensities. A mixture of all three components did not show this enhancement and at the middle range of intensities actually elicited reduced responses when compared to those elicited by appropriate amounts of any of the one- and two-component stimuli evaluated. Thus, some blends elicited electrical responses from primary olfactory receptor neurons that were not readily predicted from a knowledge of the receptor neurons' response to individual components.