Lifetime brain serotonin: regional effects of age and precursor availability

In the rat, regional brain serotonin levels which do not change from 2-30 months of age are increased at 36 months. Corresponding catecholamine levels progressively decrease. Feeding a diet restricted in the amino acid tryptophan (the precursor of serotonin) from weaning to two years of age markedly reduces serotonin levels in all brain regions and lowers norepinephrine levels in the cerebral hemispheres. Regional activity of synthesizing (tyrosine and tryptophan hydroxylases) and catabolizing enzymes (MAO-A) does not change markedly with age or dietary manipulation except for sporadic increases in tryosine hydroxylase activity in pair-fed animals. Returning the tryptophan-deficient animals to a normal diet produces a certain degree of rehabilitation the effectiveness of which varies with the function considered: Impaired brain serotonin levels recover moderately but remain lower than controls as late as 36 months, growth is never completely compensated, and norepinephrine levels show a rebound increase.