AVAILABILITY OF PHENYLALANINE AND TYROSINE FOR BRAIN NOREPINEPHRINE SYNTHESIS IN DEVELOPMENTALLY PROTEIN-MALNOURISHED RATS

Ontogenetic changes in brain phenylalanine, tyrosine and norepinephrine concentrations occurred in rats as sequelae of protein inadequacies incurred during prenatal and postnatal development. The malnourished pups, whose dams received a low protein diet (8% casein) starting 5 wk prior to conception, showed significantly elevated brain phenylalanine and norepinephrine, but not tyrosine, at most ages examined compared with offspring from dams fed a normal diet (25% casein) throughout the same time interval. Although the brain tyrosine concentrations of the malnourished offspring were somewhat lower than the normal pups, these decreases were found only at some ages and were restricted to certain brain regions (mainly the telencephalon and the cerebellum). Also, the peripheral availabilities of phenylalanine and tyrosine in the protein-deprived rats during the study (birth to age 30 days) displayed alterations which corresponded to their brain concentrations of these amino acids (increases of the former and decreases of the latter vs. the controls). Although the malnourished rats had lower plasma tyrosine concentrations than the normal pups, both groups of offspring consistently showed higher concentrations of this amino acid compared with their phenylalanine values throughout postnatal development. In the case of the deprived rats, the increased availability of the semiessential rather than the essential amino acid to their brains may explain, in part, why the amounts of tyrosine in their brain-stem regions were generally comparable to the normal animals. This, in turn, may indirectly account for the higher amine biosynthesis in the brains of the malnourished pups. The nutritional status with respect to the amount of dietary protein during prepartum and postpartum development has an important role in determining the substrate availability of tyrosine and/or phenylalanine for brain norepinephrine metabolism.