131 The brain‐barrier system to plasma proteins in the developing rat

Whereas the existence of a brain-barrier system to plasma proteins in the adult brain is generally accepted, the cerebrovascular permeability to plasma proteins in the developing brain remains a controversy. Physiological studies employing circulating radioactive-labelled albumin have suggested that the pemieability is relatively higher in the neonatal brain. However, motphological studies have not been able to confirm these data. We have adapted a protocol for detecting plasma proteins in adult rat CNS (Moos & Hoyer, J. Histochem. Cytochem., 1996) to examine the distribution of plasma proteins in the neonatal rat brain. We also examined the distribution of exogenous plasma proteins injected i.p. or stereotaxically in the ventricles. Endogenous albumin, IgG and transferrin and i.p.-injected exogenous plasma proteins were detected both extraand intracellularly to a higher degree than that seen in the adult CNS. Both neurons and glial cells were labelled. Intraventricularly injected plasma proteins were distributed much more evenly in the brain parenchyma than seen in the adult brain; there, they were unidirectionally transpoaed out of the brain from the cerebrospinal fluid (CSF) thus only displaying immunoreactivity near the ventricles. Since brain capillaries consist of non-fenestrated endothelial cells with adjacent tight junctions in both neonatal and adult brain, the apparent higher permeability to plasma proteins in the developing brain may be explained by a higher degree of transcytotic activity in the endothelial cells transporting proteins from blood to brain. The more even distribution of plasma proteins in the CNS parenchyma of the developing brain likely reflects the lower CSF flow-rate in the developing brain, which leaves CSF-proteins a possibility to diffuse out from the ventricles and into the brain parenchyma. A NOVEL OLIGODENDROCYTE PROTEIN WITH HIGH SIMILARITY AT ITS 5’-END TO A GLUTAMATE-BINDING DOMAIN