Age-related increase in presynaptic noradrenergic markers of the rat cerebral cortex.

Age-related alterations in the noradrenergic innervation of the cerebral cortex were investigated in young (3-4 months) and aged (28-29 months) Fischer-344 rats. The concentration of noradrenaline and the activity of tyrosine hydroxylase, the probable rate-limiting step in catecholamine biosynthesis, were significantly higher in the aged cerebral cortex, but no differences in L-3,4-dihydroxyphenylalanine decarboxylase activity were observed. The specific high-affinity uptake of noradrenaline by homogenates of the aged cerebral cortex exhibited higher maximal uptake, but lower affinity, than those of young rats. These results suggest increased presynaptic noradrenergic activity in the cerebral cortex of aged Fischer-344 rats.