NADPH‐diaphorase activity in the olfactory system of the hamster and rat
Authors: Davis Barry J. 
Published: January 2005

A comparative analysis of nicotinamide adenine dinucleotide phosphate (NADPH)‐diaphorase activity in the olfactory bulb was conducted in the hamster and rat. The distribution and morphological features of NADPH‐stained neurons were compared to those of glutamic acid decarboxylase‐like (GAD‐LI) and tyrosine hydroxylase‐like (TH‐LI) immunoreactive somata in order to relate NADPH‐staining to neuronal classes with specific biochemical properties.

Intense NADPH‐staining was located in primary nerve fibers of the accessory and main olfactory systems, producing dense staining of individual glomeruli. The entire vomeronasal nerve and all glomeruli were stained in the accessory olfactory bulb, but olfactory nerve and glomerular staining were restricted to the dorsal half of the main olfactory bulb.

The glomerular layer of the main olfactory bulb of both animals contained numerous small NADPH‐stained neurons. The range of somal areas of these neurons was relatively narrow and averaged about 60 μm2 (ca. 8 × 11 μm). Most neurons possessed ovoid somata and monoglo‐merular intraglomerular dendrites. Previous Golgi studies indicate that such features characterize periglomerular cells. The somal areas of GAD‐LI somata in the glomerular layer overlapped that of the NADPH‐stained neurons, providing additional evidence that these neurons are probably periglomerular cells. The range of somal areas of TH‐LI somata in the glomerular layer was broader and included both small and large neurons that usually possessed intraglomerular dendritic tufts. The smaller TH‐LI somata corresponded in size to both the NADPH‐stained and GAD‐LI somata, suggesting an interrelationship among periglomerular cells, GAD‐LI, TH‐LI, and NADPH‐diaphorase activity. The larger TH‐LI somata were probably external tufted cells.

In the external plexiform layer of the hamster, oriented NADPH‐stained neurons were observed that possessed an intraglomerular dendrite. These neurons appeared to be middle tufted cells. Lightly stained and smaller neurons were occasionally seen in the mitral body and internal plexiform layers, corresponding in somal area and morphological features to those of type III granule cells. No internal tufted or mitral cells were stained. The largest NADPH‐stained neurons were located in the inner half of the granule cell layer and were classified as Golgi cells. Their somata averaged 125 μm2 (ca. 10 × 17 μm).

Many NADPH‐stained neurons were observed in all subdivisions of the anterior olfactory nucleus, the anterior hippocampal rudiment, anterior and posterior levels of the piriform cortex, and the vertical and horizontal limbs of the diagonal band of Broca, all of which are known to provide centrifugal inputs to the olfactory bulb. However, fiber staining was restricted to the inner half of the granule cell layer, indicating limited central influences.

In combination with immunocytochemistry, NADPH histochemistry has demonstrated stronger biochemical ties between periglomerular and external tufted cells than previously described, and points to possible shared physiological properties of periglomerular cells and certain types of external tufted cells.