Nitric oxide-synthesizing perivascular nerves in the rat middle cerebral artery

Although perivascular nerves containing nitric oxide synthase (NOS) have been anatomically described for rat cerebral arteries, a dilator function for these nerves has eluded investigators when using isolated vessels. Rat middle cerebral arteries (MCAs) were isolated, pressurized, and electrically stimulated. The resting diameter of the MCAs after pressurization was 233 +/- 4 microns (n = 17) in one study. The MCAs showed a frequency-dependent dilation when stimulated. Maximum dilation (25-30% increase in diameter) occurred at a frequency of 8-16 Hz. Removal of endothelium or glibenclamide (10(-5) M), a blocker of ATP-sensitive potassium channels, had no effect on the dilations. The dilations were completely blocked with NG-nitro-L-arginine methyl ester (L-NAME) (10(-5) M), a general NOS inhibitor, and cold storage (24 h). The inhibition by L-NAME could be reversed by the addition of 10(-8) M L-arginine, the active precursor of NOS. Furthermore, 7-nitroindazole (10(-4) M), an inhibitor specific for the neuronal isoform of NOS, reduced the dilations by 43% (P < 0.05). Transections of nerve bundles originating from the sphenopalatine ganglia at the ethmoidal foramen blocked the dilations produced by electrical stimulations. We conclude that rat cerebral arteries have functionally intact perivascular nerves that dilate by releasing nitric oxide.