Developmental Changes in the Neutral α‐Amino Acid Transport Systems of Rat Brain Over the First Three Weeks After Birth

: Transport of seven different amino acids into brain slices increased as donor rats aged from 1 to 6 days. Uptakes of 2‐aminoisobutyric acid, 2‐(methyl‐amino)isobutyric acid, and L‐alanine then decreased by day 14, while uptakes of other amino acids continued to increase or remained fairly constant. Neutral α‐amino acid transport systems were characterized by measuring inhibition of uptakes and kinetics for representative amino acids at different ages. Results indicate that 2‐ami‐noisobutyrate and 2‐(methylamino)isobutyrate used only one (and the same) system in brain slices from 6‐day‐old rats, with characteristics of system A (the major sodium‐dependent system in most mammalian cells). They used at least two systems at ages 1, 14, and 23 days, but, of these, only at 1 day did they use the same systems in the same proportions. Alanine and leucine used more than one system at all four ages, and somewhat different combinations than used by each other or by 2‐aminoisobu‐tyrate or 2‐(methylamino)isobutyrate. Their transport characteristics showed they used mostly system ASC (a sodium‐dependent system distinguished from A) and/or system L (sodium‐independent). We conclude that system A increases as the brain ages from 1 to 6 days and declines thereafter. System L probably increases with aging from 1 to 23 days.