Mapping CO2 sensitive neurons in a model preparation

The ability of animals to detect volatile molecules in their environment is universal, and in an evolutionary perceptive, ancient. In many species a contact with the chemical world through the sense of smell is crucial for survival. Typical for the olfactory sensory system is the presence of sub-systems that process distinct odor stimuli. In the present study, the anatomy of the carbon dioxide sensory system in the moth Helicoverpa armigera was investigated. By applying fluorescent dye to the labial pit organ the total axonal assembly of first order neurons was mapped. Further, synapsin immunostaining allowed for detection of central neuropil areas. In accordance with previous reports, this double staining confirmed that the carbon dioxide sensitive neurons project to three main areas of the moth central nervous system: 1) one large glomerulus in each antennal lobe, 2) the subesophageal zone, and 3) the ventral nerve cord. In addition to mapping the target region of labial-palp axons passing in the ventral nerve cord −, which is documented for the first time −, the high resolution confocal images obtained here revealed several novel findings. These include an arrangement of several sub-tracts targeting the labial pit organ glomerulus and an extensive pattern of axon terminals in the subesophageal zone including projections in the antennal mechanosensory and motor center. Based on the anatomical organization of these pathways, as well as unique qualities typifying carbon dioxide as an olfactory cue, putative functions of this system is discussed.