Comprehensive morphological identification and GABA immunocytochemistry of antennal lobe local interneurons in Bombyx mori

The insect antennal lobe (AL) is the structural and functional analog of the olfactory bulb of mammals, in which odor information is spatially and/or temporally represented by functional glomerular units. Local interneurons (LNs) play critical roles through intra- and interglomerular communication to shape the output from the AL to higher brain centers; however, the function and even the components of LNs are unclear. We have used morphological and immunocytochemical approaches to examine LNs in the silkworm moth, Bombyx mori. First, we comprehensively analyzed the morphological variation of LNs. One hundred fifty-three AL LNs were intracellularly stained, analyzed in three dimensions with a confocal microscope, and subdivided into five morphological types based on differences in the arborization region in the AL and dendritic profiles within the glomeruli. Two global multiglomerular types arborized in the macroglomerular complex (MGC) and in most ordinary glomeruli, and the other three oligoglomerular types innervated some ordinary glomeruli with or without the MGC. Second, we performed double-labeling of Lucifer Yellow staining of a single LN combined with gamma-aminobutyric acid (GABA) immunocytochemistry. The two global multiglomerular types and two of the oligoglomerular types were GABA-immunoreactive, but the third oligoglomerular type, which innervates the MGC and some ordinary glomeruli, included some GABA-immunonegative neurons, suggesting the existence of a non-GABAergic subtype. These results suggest that the complex neural circuits of the AL are composed of several morphologically different types of LNs, most of which are inhibitory.