NADPH-diaphorase (neuronal nitric oxide synthase) activity in the cerebral blood vessels was investigated by light and electron microscopic histochemistry to elucidate the sites of nitric oxide production. Networks of adventitial nerves containing NADPH-diaphorase were distributed in all parts of the circle of Willis. However, NADPH-diaphorase activity in adventitial nerves was much sparser in the region of the posterior cerebral artery, and absent in the pial arteries smaller than 100 μm in diameter. Endothelial cells were intensely stained in arteries and arterioles. These results support the hypothesis that vascular tone is regulated by nitric oxide, which is derived from endothelial cells and adventitial nerves.
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