EFFECTS OF DIET-INDUCED HYPERTHREONINEMIA .2. TISSUE AND EXTRACELLULAR AMINO-ACID LEVELS IN THE BRAIN

Growing rats were fed graded levels of threonine (Thr. 0.4, 0.8, and 3 .3 g/100 g diet). Free amino acid content was measured in plasma and b rain, Extracellular amino acid levels were measured by microdialysis i n brain slices. Large quantities of dietary Thr (3.3 g/100 g) raised p lasma and brain Thr and glycine (Gly) levels. Brain and spinal cord ex tracellular levels of Thr were also raised, whereas the other amino ac id levels remained unchanged. A moderate level of dietary Thr (0.8 g/1 00 g) raised plasma Thr and Gly levels and brain Thr but not Gly level . The diet raised cortical Thr extracellular levels but did not modify the levels of the other amino acids, including glutamate (Glu) and as partate (Asp). These data suggest that brain neurochemical processes i nvolving Gly, Glu, and Asp are safeguarded in rats fed high Thr levels .