Temporally staggered glomerulus development in the moth Manduca sexta

Spatial odor map formation, development, and possible function in a nocturnal insect

An odor plume is composed of fine filamentous structures interspersed by clean air. Various animals use bilateral comparison with paired olfactory organs for detecting spatial and temporal features of the plume. American cockroaches are capable of locating a sex pheromone source with one long antenna spanning 5 cm, so-called unilateral odor sampling. This capability stems from an antennotopic map in which olfactory sensory neurons located proximo-distally in the antenna send axon terminals proximo-distally in a given glomerulus, relative to axonal entry points. Multiple output neurons (projection neurons) utilize this spatial map in the pheromone-receptive glomerulus. Here, I summarize neuronal underpinnings of receptive field formation, development, and how this intraglomerular spatial map can be utilized for odor localization.

Two types of sensory proliferation patterns underlie the formation of spatially tuned olfactory receptive fields in the cockroach Periplaneta americana

In the cockroach Periplaneta americana, to represent pheromone source in the receptive space, axon terminals of sex pheromone-receptive olfactory sensory neurons (pSNs) are topographically organized within the primary center, the macroglomerulus, according to the peripheral locations of sex pheromone-receptive single walled (sw)-B sensilla. In this study, we sought to determine when and where pSNs emerge in the nymphal antenna. We revealed two different pSN proliferation patterns that underlie the formation of topographic organization in the macroglomerulus. In nymphal antennae, which lack sw-B sensilla, pSNs are identified in the shorter sensilla, termed sw-A sensilla. Because new sw-A sensilla emerge on the proximal antenna at every molt, topographic organization in the macroglomerulus must be formed by adding axon terminals of newly emerged pSNs to the lateral region in the macroglomerulus at each molt. At the final molt, a huge number of new sw-B sensilla appeared throughout the whole antenna. Sw-B sensilla in the proximal part of the adult antenna were newly formed during the last instar stage, whereas those located in the distal antenna were transformed from sw-A sensilla. This transformation was accompanied by an increase in the number of pSNs. Axon terminals of newborn pSNs in new sw-B sensilla were recruited to the lateral part of the macroglomerulus, whereas those of newborn pSNs in transformed sw-B sensilla were recruited to the macroglomerulus according to the sensillar location. These mechanisms enable an increase in sensitivity to sex pheromone in adulthood while retaining the topographic map formed during the postembryonic development.

Anisometric brain dimorphism revisited: Implementation of a volumetric 3D standard brain in Manduca sexta

Lepidopterans like the giant sphinx moth Manduca sexta are known for their conspicuous sexual dimorphism in the olfactory system, which is especially pronounced in the antennae and in the antennal lobe, the primary integration center of odor information. Even minute scents of female pheromone are detected by male moths, facilitated by a huge array of pheromone receptors on their antennae. The associated neuropilar areas in the antennal lobe, the glomeruli, are enlarged in males and organized in the form of the so-called macroglomerular complex (MGC). In this study we searched for anatomical sexual dimorphism more downstream in the olfactory pathway and in other neuropil areas in the central brain. Based on freshly eclosed animals, we created a volumetric female and male standard brain and compared 30 separate neuropilar regions. Additionally, we labeled 10 female glomeruli that were homologous to previously quantitatively described male glomeruli including the MGC. In summary, the neuropil volumes reveal an isometric sexual dimorphism in M. sexta brains. This proportional size difference between male and female brain neuropils masks an anisometric or disproportional dimorphism, which is restricted to the sex-related glomeruli of the antennal lobes and neither mirrored in other normal glomeruli nor in higher brain centers like the calyces of the mushroom bodies. Both the female and male 3D standard brain are also used for interspecies comparisons, and may serve as future volumetric reference in pharmacological and behavioral experiments especially regarding development and adult plasticity. J. Comp. Neurol. 517:210-225, 2009. (c) 2009 Wiley-Liss, Inc.

Functional and topographic segregation of glomeruli revealed by local staining of antennal sensory neurons in the honeybee Apis mellifera

In the primary olfactory center of animals, glomeruli are the relay stations where sensory neurons expressing cognate odorant receptors converge onto interneurons. In cockroaches, moths, and honeybees, sensory afferents from sensilla on the anterodorsal surface and the posteroventral surface of the flagellum form two nerves of almost equal thicknesses. In this study, double labeling of the two nerves, or proximal/distal regions of the nerves, with fluorescent dyes was used to investigate topographic organization of sensory afferents in the honeybee. The sensory neurons of ampullaceal sensilla responsive to CO2, coelocapitular sensilla responsive to hygrosensory, and thermosensory stimuli and coeloconic sensilla of unknown function were characterized with large somata and supplied thick axons exclusively to the ventral nerve. Correspondingly, all glomeruli innervated by sensory tract (T) 4 received thick axonal processes exclusively from the ventral nerve. Almost all T1-3 glomeruli received a similar number of sensory afferents from the two nerves. In the macroglomerular complexes of the drone, termination fields of afferents from the two nerves almost completely overlapped; this differs from moths and cockroaches, which show heterogeneous terminations in the glomerular complex. In T1-3 glomeruli, sensory neurons originating from more distal flagellar segments tended to terminate within the inner regions of the cortical layer. These results suggest that some degree of somatotopic organization of sensory afferents exist in T1-3 glomeruli, and part of T4 glomeruli serve for processing of hygro- and thermosensory signals.