Neuronal NADPH-diaphorase has been proved to be nitric oxide synthase itself. In this study, we investigated distribution and origins of NADPH-diaphorase-containing nerve fibers in the cerebral vessels in the rat. Adult male Sprague-Dawley rats were divided into 4 groups. Nasociliary nerves were transected bilaterally in group 1. In group 2, intracranial branches of the sphenopalatine ganglion were transected bilaterally. In group 3, both of these structures were transected. The remaining animals were served as control (group 4). Two weeks after the above procedures, they were perfused with paraformaldehyde and glutaraldehyde. The pial arteries and superior cervical, trigeminal, internal carotid, otic and sphenopalatine ganglia were dissected. All specimens were processed for NADPH-diaphorase histochemistry. Numerous NADPH-diaphorase-containing nerve fibers with varicosities forming plexuses were observed in the circle of Willis and its branches. Relatively thick nerve bundles were noted in the anterior half of the circle of Willis. They are most abundant in the internal ethmoidal artery. Approximately 5% of such fibers in anterior half of the circle of Willis disappeared in group 1, 90% in group 2, and no fibers were seen to remain in group 3. NADPH-diaphorase reaction was positive in the neurons of sphenopalatine, otic trigeminal and internal carotid ganglia. Among these ganglia, the reaction was prominent in sphenopalatine, otic and internal carotid ganglia. In summary: (1) NADPH-diaphorase-containing nerve fibers distribute to the circle of Willis and its branches. The density is higher in the anterior half of the circle of Willis than its posterior half; (2) NADPH-diaphorase activity is observed in the parasympathetic and sensory cranial ganglia which are known to innervate the cerebral vessels; and (3) the main source of cerebrovascular NADPH-diaphorase-containing nerve fibers is the sphenopalatine ganglion.
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