Cytochrome oxidase staining reveals functionally important activity bands in the olfactory epithelium of newborn rat

We used cytochrome oxidase (CytOx) staining intensity, which is correlated with neuronal functional activity, to evaluate maturity and functionality of newborn rat olfactory epithelium (OE) and olfactory receptor neurons (ORNs). Nasal olfactory tissue of neonatal rats was stained with CytOx and analyzed qualitatively and quantitatively. Results revealed that newborn OE shows six differentially stained horizontal bands. Bands run parallel to the OE surface and were categorized as very light, medium or darkly stained. A narrow and pale Band 1 overlapped with horizontal basal cells. Next, a wide and lightly stained Band 2 was observed that coincides with the globose basal cell layer and immature ORNs, deep in OE. Next apically, a medium-staining Band 3 overlapped with ORN perikarya. Closer to the surface, a medium to light Band 4 was discerned where dendrites of mature ORNs normally occur. This band was interrupted with lighter areas due to the presence of supporting cells nuclei. Next, a superficial but dark Band 5 occurred, populated by the apical portions of ORN dendrites and their ciliated knobs and by supporting cell apices; mitochondria in apices of supporting cells contribute predominantly to dense staining of this Band 5. Apical to Band 5, a thin and fairly light Band 6 was observed which overlaps with the mucus layer that contains part of the ORN knobs, their cilia and supporting cell microvilli. Along the length of ORN dendrites, apical segments just below the ORN knobs, and wide basal segments showed a darker staining than the middle segments implying “microzones” of higher neural activity within the most apical and basal regions of dendrites. Our findings agree with ultrastructural studies showing a presence of mitochondria in knobs, basal portions of ORN dendrites and in OE supporting cell apices, suggesting that apical regions of both olfactory and supporting cells near the surfaces are metabolically most active, in odorant detection, signal processing, and detoxification, the latter for supporting cells.