Pheromone Reception

Abstract

Insects are analytical chemists par excellence. They perceive the world through semiochemicals with inordinate sensitivity. A male moth, for example, can detect a “scent of woman,” i.e., a female-produced sex pheromone, even when the signal-to-noise ratio is very low. In a sense the antennae are “signal translators.” The chemicals signals are “translated” into the language of the brain (nerve impulses or spikes) by an array of sensilla mainly located on the antennae. This information is conveyed to the brain for further processing. Chemical ecologists utilize insect antennae as biosensors for the identification of pheromones and other semiochemicals. The insect olfactory system is also highly selective, able to discriminate natural pheromones from molecules with minimal structural changes. In some cases, one stereoisomer functions as an attractant sex pheromone and its antipode is a behavioral antagonist (inhibitory signal). The specificity of the olfactory system seems to be achieved by two layers of filters. The first level of discrimination is determined by odorant-binding proteins (OBPs) that assist the hydrophobic pheromones to cross an aqueous barrier and reach their receptors. Both OBP and odorant receptor (OR) contribute to the specificity of the cell response and lead to the remarkable selectivity of the insect olfactory system. The members of the OBP-gene family, encoding the encapsulins, form a large group with olfactory and non-olfactory proteins. While the functions of many members of the family are yet to be determined, there is solid evidence for the mode of action of OBPs. Pheromones (and other semiochemicals) enter the sensillar lymph through pore tubules in the cuticle (sensillar wall), are solubilized upon being encapsulated by odorant-binding proteins, and transported to the olfactory receptors. Bound pheromone molecules are protected from odorant-degrading enzymes. Upon interaction with negatively-charged sites at the dendritic membrane, the OBP-ligand complex undergoes a conformational change that leads to the ejection of pheromone. Direct activation of odorant receptors by odorant molecules initiates a cascade of events leading to the generation of spikes. Reverse chemical ecology is a new concept for the screening of attractants based on the binding ability of OBPs to test compounds.