Nitric oxide: a modulator, but not a mediator, of neurovascular coupling in rat somatosensory cortex.

We investigated the role of nitric oxide (NO)/cGMP in the coupling of neuronal activation to regional cerebral blood flow (rCBF) in alpha-chloralose-anesthetized rats. Whisker deflection (60 s) increased rCBF by 18 +/- 3%. NO synthase (NOS) inhibition by N(omega)-nitro-L-arginine (L-NNA; topically) reduced the rCBF response to 9 +/- 4% and resting rCBF to 80 +/- 8%. NO donors [S-nitroso-N-acetylpenicillamine (SNAP; 50 microM), 3-morpholinosydnonimine (10 microM)] or 8-bromoguanosine 3', 5'-cyclic-monophosphate (8-BrcGMP; 100 microM)] restored resting rCBF and L-NNA-induced attenuation of the whisker response in the presence of L-NNA, whereas the NO-independent vasodilator papaverine (1 mM) had no effect on the whisker response. Basal cGMP levels were decreased to 35% by L-NNA and restored to 65% of control by subsequent SNAP superfusion. Inhibition of neuronal NOS by 7-nitroindazole (7-NI; 40 mg/kg ip) or soluble guanylyl cyclase by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 100 microM) significantly reduced resting rCBF to 86 +/- 8 and 92 +/- 10% and whisker rCBF response to 7 +/- 4 and 12 +/- 3%, respectively. ODQ reduced tissue cGMP to 54%. 8-BrcGMP restored the whisker response in the presence of 7-NI or ODQ. We conclude that NO, produced by neuronal NOS, is a modulator in the coupling of neuronal activation and rCBF in rat somatosensory cortex and that this effect is mainly mediated by cGMP. L-NNA-induced vasomotion was significantly reduced during increased neuronal activity and after restoration of basal NO levels, but not after restoration of cGMP.