Microperoxidase uptake into the rat choroid plexus epithelium

Abstract

Uptake, transport, and diffusion of the heme-peptide tracer microperoxidase (MP; MW 1900) in the rat choroid plexus were studied. MP was perfused ventriculo—cisternally or intravenously. The plexuses were fixed by ventriculo—cisternal perfusion of aldehydes, and tissue sections were incubated in a diaminobenzidine—H 2 O 2 medium containing imidazole at pH 8.8. After intravenous administration MP was found in the connective tissue of the plexus. Basement membranes were heavily labeled, while intercellular spaces between epithelial cells exhibited only a little MP. Some micropinocytic uptake of MP into vacuoles and lysosomes of the epithelial cells occurred. After ventriculo-cisternal perfusion of MP, it was micropinocytozed and incorporated into numerous vacuoles, multivesicular bodies, and dense bodies. Labeling of elongated cisterns (ER cisterns, or cisterns formed by fusion of endocytic vesicles) also occurred. Apical tight junctions between choroid plexus epithelial cells were never penetrated by MP. The intercellular spaces between epithelial cells, the connective tissue between epithelium and endothelium, and the blood vessels were, in general, devoid of MP, although diffusion of MP between ependymal cells into the extracellular space of the neuropil close to the base of the plexus was pronounced, as evaluated from the density of reaction product. No MP could be located in the kidney tubules of rats exposed ventriculo—cisternally to MP. The observations indicate that the choroid plexus is involved in absorption of molecules—also smaller than proteins—from the cerebrospinal fluid (csf). No significant diffusion of MP via ependyma and neuropil (the “functional leak”) into the plexus occurred, and no support of a transepithelial vesicular csf-to-blood transport of MP was obtained. This discrepancy in relation to previous horseradish peroxidase experiments is discussed.