Mapping Central Olfactory Pathways in a Model Preparation - Double Staining of Second Order Olfactory Neurons

Abstract

The sense of smell is the evolutionary oldest sense, and the ability to detect and respond to chemical signals is the primary window for perceiving the external world for most animal species. In this study, the tobacco budworm moth, Heliothis virescens (Lepidoptera: Noctuidae), was used as a model organism to study the neural system linked to olfaction. The second order level of the olfactory pathway in this species includes three main tracts conveying information from the antennal lobe to higher brain areas. Two of the three parallel tracts are relatively well described in moths; thus, the most prominent one, the medial antennal-lobe tract, is known to innervate the calyces of the mushroom bodies and the lateral horn whereas the medio-lateral tract projects directly to the lateral horn without making any connection to the calyces. The third tract, the lateral one, on the other hand, is less well described. In particular, the portion of lateral-tract axons forming a direct connection with the calyces has not been systematically studied in any moth species so far. The main aim of the current project was therefore to map the fraction of lateral-tract neurons linked to the calyces. The data were obtained by visualizing the second-order neurons via mass-staining. Both retrograde (from the antennal lobe) and anterograde staining (from the calyces) were used, plus a combination of the two techniques. Most of the preparations were double-labelled (simultaneously stained from the antennal lobe and the calyces). The results, comprising high-resolution confocal images, showed that most of the neurons in the lateral antennal lobe tract terminate in the lateral horn. At most, six neurons connecting this tract to the calyces were found. Thus, in moths, the connection between the antennal lobe and the calyces is primarily made up by projection neurons confined to the medial antennal lobe tract.