Ontogenesis of neurons immunoreactive for nitric oxide synthase in rat forebrain and midbrain

We studied the immunohistochemical localization of neuronal nitric oxide sy nthase (nNOS) in the developing rat brain on embryonic days 13 (E13), 15 (E 15), 17 (E17) and 19 (E19) and postnatal days 0 (P0), 7 (P7) and 14 (P14). A few neurons positive for nNOS were first detected at E15 in the hypothala mus and pons. At E17, many positive cells became detectable in the thalamus . At E19, the positive cells in these three regions were rapidly increased in number, and a few positive neurons were also observed in such regions as the cerebral cortex and striatum. Positive cells in the hypothalamus tende d to locate ventrolaterally, Positive neurons, stained very intensely as in adult rats, were seen in the pedunculopontine tegmental nucleus, laterodor sal tegmental nucleus and parafascicular nucleus. Two weeks after birth, po sitive neurons of larger somata with many processes were distributed widely in the cerebral cortex and hippocampus. The present study indicates that, in the forebrain and midbrain, the distribution pattern of nNOS-containing neurons is fundamentally completed by E19. (C) 2001 Elsevier Science BY All rights reserved.