Distribution of immunoreactivity for γ‐aminobutyric acid in the salamander olfactory bulb

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Abstract
Intrinsic neurons provide inhibitory synaptic input to mitral (and tufted) output cells within several laminae of the olfactory bulb. In rodents, the two main types of intrinsic neurons are granule and periglomerular cells, both of which contain γ‐aminobutyric acid (GABA). In the present study, immunocytochemical techniques were used to determine whether intrinsic neurons in the salamander olfactory bulb might also contain GABA.With the aid of two antisera to different GABA‐conjugates, immunoreactivity for GABA was localized within the olfactory bulb laminae. In the glomerular layer, periglomerular cells, which were strongly immunoreactive, were concentrated in clusters along the border with the olfactory nerve layer. Dendrites of the cells encircled nearby glomeruli and were presumably a primary source of intraglomerular processes that were also stained. In the subglomerular region and external plexiform layer, relatively few immunoreactive cells were observed, most of which appeared to be periglomerular and tufted cell types with glomerular dendrites. Throughout the external plexiform and mitral cell layers, however, a dense matrix of spiny processes and puncta was stained, outlining large, unstained dendrites derived from the large, unstained cell bodies of mitral cells. The spiny processes and puncta appeared to be derived from granule cells, which were the most abundant immunoreactive cells in the bulb. Granule cell bodies filled the granule cell layer.In tissue fixed with 0.1–0.2% glutaraldehyde, staining in the olfactory bulb laminae was blocked by preadsorption of the two antisera with glutaraldehyde‐conjugated GABA‐bovine serum albumin. The staining therefore appeared to be specific for fixed GABA. The distribution of GABA immunoreactivity in the salamander olfactory bulb suggests that several types of GABAergic interneurons that branch in the glomerular, external plexiform, and mitral cell layers may influence the activity of mitral (and tufted) output cells.