DEVELOPMENTAL TRANSITIONS IN UPTAKE OF AMINO ACIDS BY SYNAPTOSOMAL FRACTIONS ISOLATED FROM RAT CEREBRAL CORTEX

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Abstract
Developmental changes in mechanisms of synaptosomal amino acid transport have been studied in rat cerebral cortex. Well‐defined changes over an age continuum could be observed in both the rates of amino acid accumulation and the effects of Na+ on the accumulation. The uptakes of five amino acids (threonine, serine and valine in Na+‐free medium, aspartic acid and proline in Na+‐containing medium) increased progressively with the age of the animal, whereas the uptakes of leucine and arginine (in Na+‐free medium) decreased steadily. The uptake of serine or threonine by synaptosomal fractions prepared from newborn rats was markedly dependent on the presence of Na+in the incubation media. Na+exerted progressively less effect on the accumulation process with continuing postnatal development and to some extent inhibited uptake by fractions obtained from rats older than about 15 days. Na+significantly enhanced the accumulation of glycine in fractions from newborn and adult rats, but had only a slight effect in fractions prepared from 12 to 17‐day old rats. A detailed study of the accumulation of glycine indicated that the synaptosomal transport of this amino acid proceeded by two independent systems, one of which was totally dependent on external Na+and the and adult animals than in fractions from 12 to 17‐day‐old rats, wheras the Na+‐independent system was most active during this latter period of development. The decline in the Na+‐independent accumulation of glycine from about the 15th day to adulthood was characterized by a decrease in the Vmax. and an increase in the Km.