Olfactory Reaction In Insects

1. Introduction

1.1 Importance of olfaction in insects

Olfaction is a very important sensory mechanism for insects. It allows them to perform key tasks such as to smell, detect and localize various food sources, oviposition sites & mates, toxic chemicals and also it helps them to sense predators from a long distance away and so like most of the species, olfaction is of critical importance to insects and insects have a very sensitive olfactory mechanism. So the olfactory system in insects allows them to differentiate and discriminate between various odors. (Bill H. Hansson & Marcus C Stensmyr, Dethier). Insects perform all the olfactory tasks with the help of their antennae and maxillary palps. The size and shape

OSNs are surrounded by supporting cells. The Odorants and Pheromones marked in red are highly hydrophobic in nature. These molecules enter through the cuticular pores and become attached to the OBPs and PBPs (Pheromone binding proteins) which contain a hydrophobic pocket and thus protects it from getting degraded by degrading enzymes present in the sensilla lymph which is hydrophilic in nature. These OBPs are secreted by the tormogen and trichogen supporting cells and secreted in the sensilla lymph in a very high concentration. So OBPs are globular proteins which help to present odorants to the underlying receptor and one of their major function is to protect these molecules from degradation. Binding of odorants to the receptors activates either the ionotropic and / or the metabotropic pathway translating the chemical signal into electrical potential. (Sachse &

Although no relationship exists between the behavioral response profiles of OBPs and molecular response profiles of odorant receptors as shown by (Hallem et al) in 2004, Hallem and Carlson in 2006 showed that functional organization exists between electrophysiological response patterns of ORs and behavioral response patterns of OBPs. The odorants which are structurally similar activate common odorant receptors and behavioral response to these odorants can be suppressed by inhibition of some specific OBPs and this fact also holds true for the OBPs carrying same functional groups like aldehydes or ketones. Odorants which are functionally same but are expressed in different sensilla may interact with some common OBPs. Odorants with different functional groups that activate the same odorant receptor can also be affected by suppressed expression or inhibition of common specific OBPs (Hallem et al). Swarup et al in 2011 showed that OBPs are critically important for odor recognition. They used RNAi mediated suppression of OBP gene expression and noticed that suppressing a specific OBP alters the response of the fly to some odorants but not all the odorants and this is sex specific which means that suppressing OBP X will alter the fly’s response to Odorant X in males but the same will not be