EX-VIVO MEASUREMENT OF BRAIN-TISSUE NITRITE AND NITRATE ACCURATELY REFLECTS NITRIC-OXIDE SYNTHASE ACTIVITY IN-VIVO

The ex vivo tissue concentration of nitrite and nitrate (NOx) was foun d to correlate closely with the activity of nitric oxide synthase (NOS ; EC 1.14.13.39) in various brain regions, Systemic administration of the nonselective NOS inhibitor N-omega-nitro-L-arginine (L-NA) at dose s that completely inhibited both central and peripheral NOS, depleted whole-brain and CSF NOx by up to 75% but had no effect on plasma NOx. Selective inhibition of central NOS by intracerebroventricular adminis tration of L-NA methyl ester produced similar decreases in levels of w hole-brain NOx. A residual concentration of NOx of 10-15 mu M remained in all brain regions even after complete inhibition of brain NOS. Bra in NOx content decreased rapidly and in parallel with the inhibition o f brain NOS. The ex vivo measurement of levels of brain NOx was found to reflect the in vivo efficacy of several different types of NOS inhi bitor: L-NA, N-omega-monomethyl-L-arginine, and 7-nitroindazole. Intra peritoneal administration of the NOS substrate L-arginine increased br ain NOx concentrations by up to 150% of control values. These results demonstrate that the ex vivo measurement of levels of brain tissue NOx is a rapid, reliable, and straightforward technique to determine NOS activity in vivo. This method can be used to assess both the regional distribution and the degree of inhibition of NOS activity in vivo.