Biochemical characterization of nNOS during aging in the cortex, the hippocampus and the brainstem of SAMR1 and SAMP8. nNOS mRNA levels in arbitrary units (A.U.) (A), nNOS enzymatic activity in pmol of citrulline per minute per mg of protein (B) and Nitrites and nitrates (NOx) levels in nmol per mg of tissue (C) were detected in SAMR1 (solid line and open box) and SAMP8 (dotted line and dark box). Data are mean ± S.E.M) (n = 7 mice in each group). Statistics: significantly different from 2 months *P < 0.05, ** P < 0.01; or from 8 months aP < 0.05, bP < 0.01; significantly different from strain cP < 0.01.