Ontogenesis of NADPH-diaphorase activity in the olfactory bulb of the rat.

The enzyme NADPH-diaphorase, which has been shown to correspond to nitric oxide synthase, is present in discrete neuron populations in the olfactory bulb of the adult rat. The ontogenesis of NADPH-diaphorase activity was studied and compared with the ontogenesis of tyrosine hydroxylase containing cells from embryonic day E15 to postnatal day P30. In the main olfactory bulb, scanty NADPH-diaphorase reactive neurons were first present at E21 in an immature phenotype. The periglomerular positive cells increased in number and acquired their adult morphology in the postnatal period. No colocalization of tyrosine hydroxylase with NADPH-diaphorase was observed at any developmental stage studied. In the granule cell layer, a population of rather bipolar neurons transiently expressed NADPH-diaphorase from P3 to P15; a population of large multipolar cells permanently expressed NADPH-diaphorase from P3 to P30. In the accessory olfactory bulb, NADPH-diaphorase staining appeared in the granule cell layer at P3, and then in the granule cell projections towards the mitral cells. From E21 to P7, neural processes often seemed to contact blood vessels. Endothelial cells showed a diffuse and faint staining at all stages; moreover patches of high NADPH-diaphorase staining were transiently present on blood vessels from E15 to P7. The presence of both permanent and transient expression of NADPH-diaphorase during olfactory bulb genesis is discussed according to the hypotheses of the function of NO during development.