Amino acid supply to individual cerebral structures in awake and anesthetized rats

The movement of phenylalanine from plasma into various cerebral structures was examined in detail in normal alert rats as well as in rats anesthetized with nitrous oxide, halothane, and sodium pentobarbital. Radioactive phenylalanine was infused intravenously in such a way as to rapidly establish and maintain a trace concentration in arterial blood. The rat was decapitated after 1.5 min, the brain removed, frozen, and thin sections cut for quantitative autoradiography. The plasma concentrations of phenylalanine as well as other neutral amino acids that compete with phenylalanine for entry into brain were measured. Phenylalanine influx varied considerably between structures, ranging from 5 nmol.min-1.g-1 in the globus pallidus to 12 nmol.min-1.g-1 in the inferior colliculus. Anesthetics reduced phenylalanine influx, but this effect was primarily due to alterations in the circulating neutral amino acid pattern. The anesthetics had little, if any, direct effect on the neutral amino acid transport process itself.