MOLECULAR CHARACTERISTICS OF PIAL MICROVESSELS OF THE RAT OPTIC-NERVE- CAN PIAL MICROVESSELS BE USED AS A MODEL FOR THE BLOOD-BRAIN-BARRIER

Pial microvessels have commonly been used in studies of the blood-brai n barrier because of their relative accessibility. To determine the va lidity of using the pial microvessel as a model system for the blood-b rain barrier, we have extended the comparison of pial and cerebral mic rovessels at the molecular level by a partial characterization of the glycocalyx of pial endothelial cells, in view of the functional import ance of anionic sites within the glycocalyx. Rat optic nerves were fix ed by vascular perfusion. Anionic sites on the endothelium were labell ed with cationic colloidal gold by means of post- and pre-embedding te chniques. The effects of digestion of ultrathin sections on subsequent gold labelling was quantified following their treatment with a batter y of enzymes. Biotinylated lectins, viz. wheat germ agglutinin and con canavalin A with streptavidin gold, were employed to identify specific saccharide residues. The results demonstrate that the luminal glycoca lyx of pial microvessels is rich in sialic-acid-containing glycoprotei ns. Neuraminidase, which is specific for N-acetylneuraminic (sialic) a cid, and papain (a protease with a wide specificity) significantly red uce cationic colloidal gold binding to the luminal endothelial cell pl asma membrane. Wheat germ agglutinin (with an affinity for sialic acid ) binds more to the luminal than abluminal plasma membrane, whereas co ncanavalin A, which binds mannose, binds more to the abluminal surface . Similar results have been obtained for cerebral cortical endothelial cells. With respect to these molecular characteristics, therefore, th e pial and cortical microvessels appear to be the same. However, since the two vessel types differ in other respects, caution is urged regar ding the use of pial microvessels to investigate the blood-brain barri er.