BACKGROUND Nitric oxide (NO) has been reported to play an important role in isoflurane-induced cerebral hyperemia in vivo. In the brain, there are two constitutive isoforms of NO synthase (NOS), endothelial NOS (eNOS), and neuronal NOS (nNOS). Recently, the mutant mouse deficient in nNOS gene expression (nNOS knockout) has been developed. The present study was designed to examine the role of the two constitutive NOS isoforms in cerebral blood flow (CBF) response to isoflurane using this nNOS knockout mouse. METHODS Regional CBF (rCBF) in the cerebral cortex was measured with laser-Doppler flowmetry in wild-type mice (129/SV or C57BL/6) and nNOS knockout mice during stepwise increases in the inspired concentration of isoflurane from 0.6 vol% to 1.2, 1.8, and 2.4 vol%. Subsequently, a NOS inhibitor, N omega-nitro-L-arginine (L-NNA), was administered intravenously (20 mg/kg), and 45 min later, the rCBF response to isoflurane was tested again. In separate groups of wild-type mice and the knockout mice, the inactive enantiomer, N omega-nitro-D-arginine (D-NNA) was administered intravenously in place of L-NNA. Brain NOS activity was measured with radio-labeled L-arginine to L-citrulline conversion after treatment with L-NNA and D-NNA. RESULTS Isoflurane produced dose-dependent increases in rCBF by 25 +/- 3%, 74 +/- 10%, and 108 +/- 14% (SEM) in 129/SV mice and by 32 +/- 2%, 71 +/- 3%, and 96 +/- 7% in C57BL/6 mice at 1.2, 1.8, and 2.4 vol%, respectively. These increases were attenuated at every anesthetic concentration by L-NNA but not by D-NNA. Brain NOS activity was decreased by 92 +/- 2% with L-NNA compared with D-NNA. In nNOS knockout mice, isoflurane increased rCBF by 67 +/- 8%, 88 +/- 12%, and 112 +/- 18% at 1.2, 1.8, and 2.4 vol%, respectively. The increase in rCBF at 1.2 vol% was significantly greater in the nNOS knockout mice than that in the wild-type mice. Administration of L-NNA in the knockout mice attenuated the rCBF response to isoflurane at 1.2 and 1.8 vol% but had no effect on the response at 2.4 vol%. CONCLUSIONS In nNOS knockout mice, the cerebral hyperemic response to isoflurane is preserved by compensatory mechanism(s) that is NO-independent at 2.4 vol%, although it may involve eNOS at 1.2 and 1.8 vol%. It is suggested that in wild-type mice, eNOS and nNOS contribute to isoflurane-induced increase in rCBF. At lower concentrations (1.2 and 1.8 vol%), eNOS may be involved, whereas at 2.4 vol%, nNOS may be involved.