Penetration of glutamate into brain of 7-day-old rats

The permeability of the blood-brain barrier to glutamate was measured by quantitative autoradiography in brains of 7-day-old rats (average plasma glutamate 114 microM) and rats injected subcutaneously with glutamate (average plasma glutamate 2,670 microM). Measurements of glutamate permeability were initiated by the injection of [14C]glutamate into the inferior vena cava and the 7-day-old rats sacrificed at 1 minute to avoid the accumulation of [14C]glutamate metabolites in plasma. Glutamate entered the brain at a slow rate, with an average permeability-surface area product of 12 microl x min(-1) x g(-1), except in those areas known to have fenestrated capillaries. Thus, glutamate readily entered and accumulated in circumventricular organs where the radioactivity was localized. Although three areas with a blood-brain barrier, the cerebral cortex, the hypothalamus and the midbrain, of 7-day-old rats had permeabilities similar to adult rats, the other areas of the brain with a blood-brain barrier had a permeability about 1.5-1.9 times that of adult rats. The greater permeability of the brain of 7-day-old rats may reflect the degree of immaturity of the blood-brain barrier.