The effect of age upon the entry of some amino acids into the brain, and their incorporation into cerebral protein.

The way in which the influx of amino acids into the brain changes as the age of rats increases from one to 25 weeks is reported. Most of the amino acids have a very high rate of influx during the first weeks of life, which falls progressively until an adult is reached by about 8 to 10 weeks of age. The high influx in early life is due to carrier-mediated transport systems which act more rapidly in the young than in the adult. These systems can be saturated by raised levels of amino acid in the blood, both in young and adult animals. The high influx in early life is necessary to provide abundant supplies of amino acids for cerebralprotein formation during the period of rapid brain growth. In adult life, influx declines to a level that is adequate to support the synthesis of cerebral protein, which is still being continuously broken down and renewed at a remarkably high rate.