Processing of plant-associated odors by a subset of projection neurons in the antennal lobe of the female moth, Manduca sexta

Abstract

The olfactory system of the hawkmoth Manduca sexta is able to discriminate among odors and to mediate a behavioral response, such as feeding or oviposition in females. This dissertation research describes attempts to elucidate neural coding involved in the processing of odors within the first-order olfactory center of the brain, the antennal lobe (AL), in female moths. This study focused on a small and identifiable subset of projection neurons within the AL, which have their cell bodies grouped on the anterior surface of the AL (AC neurons). The subpopulation of about 14-20 neurons was characterized physiologically and morphologically. The anatomical features of AC neurons and their physiological tuning specificities to plant-associated odors were investigated by means of electrophysiological recording and staining techniques. All AC neurons identified were output neurons, each having a distinct uniglomerular dendritic branching pattern. The entire population of AC neurons, which sometimes exhibited a separation of its cell body group into two clusters (primary and satellite), projected into two small groups of glomeruli. One group was located dorsally and medially in the posterior part of the AL and probably consists of five glomeruli. The other group was situated dorsally and anteriorly and possibly consists of one uniquely identifiable glomerulus. Furthermore, the results indicate response specificity and a narrow receptive range of certain AC neurons in response to flowers of Solanaceous hostplants. Among the odors tested were nitrogen (N)-bearing compounds, monoterpenes, aromatics and green-leaf volatiles. The response profiles were divided into different response categories: "excited" (multiphasic), "inhibited", "mixed" and "no response to odors tested". The majority of AC neurons were spontaneously active and exhibited rhythmic and periodic bursting (burst frequency 1-3 Hz). Also, AC neurons showed low-amplitude, membrane oscillations. These oscillations, as well as the bursts, are not odor-induced. In summary, the data show that the organization of a subset of unique, ordinary AC glomeruli is spatially distinct and recognizable, and functionally significant for hostplant odor processing in the AL. The mechanisms underlying processing seem to include a rate code (encoding concentration) and possibly a complex spatial code (encoding quality).