The entry of acidic amino acids into brain and CSF during development, using in situ perfusion in the rat

Previous studies using the rapid single pass blood to tissue uptake of substances by the capillaries of the blood-brain barrier, have failed to show significant uptake of acidic amino acids. However, by the use of a bilateral in situ brain perfusion in neonatal and adult rats, extending the perfusion time to 30 min, the carrier-mediated uptake of aspartate and glutamate into brain and CSF has been demonstrated. The ratios of 14C-acidic amino acids in the brain and CSF to that in perfusate were measured and represented as Rbrain and RCSF respectively, after 30 min, neonatal (1-week-old) Rbrain values for both amino acids were approximately twice that of adults, while neonatal RCSF for aspartate and glutamate were 3 to 5 times that of the adult. In contrast, there was no significant entry of NMDA into either compartment for both adults and neonates. The transfer coefficient, Kin into brain and CSF was also measured in relation to stages of development. In general the Kin values for brain and CSF for aspartate and glutamate were higher in the younger age groups than the adult group (1 week > 2 week > 3 week > or = adult). In 1- and 2-week-old rats entry into CSF appears to be higher than that of brain, whereas for adults entry into the brain tissue was dominant.