Neuronal nitric oxide synthase (NOS), visualized immunohistochemically or with NADPH diaphorase histochemistry, is transiently expressed in discrete areas of the developing rat nervous system. In the brain transient NOS expression occurs in the cerebral cortical plate. At E15-E19, the majority of cells in the plate stain, with their processes extending through the corpus striatum to the thalamus. This staining decreases after birth and vanishes by the 15th postnatal day. Neurons in olfactory epithelium also express NOS from E15 till early postnatal life. In embryonic sensory ganglia virtually all neuronal cells are NOS positive, whereas by early adulthood only 1 % express NOS. By contrast to these areas of transient NOS expression, in other neuronal sites NOS staining appears after cell bodies cease dividing and cells extend processes, and the staining persists in adult life. The transient expression of neuronal NOS may reflect a role in developmental processes such as programmed cell death.
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