Permeability of intercellular junctions in brain epithelia and endothelia to exogenous amine: cytochemical localization of extracellular 5-hydroxydopamine

Summary

Intraventricular perfusion of 5-hydroxydopamine (5-OH-DA) leads to its accumulation on ventricular cell surfaces and in the extracellular space of most periventricular brain regions. Thus, using a cytochemical method for the selective demonstration of monoamines at the ultrastructural level, a highly electron-dense reaction product could be observed on the surface of ependymal cells (including cilia and microvilli) and choroid plexus epithelial cells (including their characteristic club-shaped microvilli). In most brain regions (caudate nucleus, hypothalamus, subcommissural organ, median eminence and floor of the fourth ventricle) the reaction product was not confined to the ventricular surface but was also observed within clefts between adjacent cells and extended up to endothelial cells of capillaries in the periventricular zone; the only exception in this respect was the choroid plexus whose intercellular clefts were devoid of electron-dense material. The accumulation of extracellular 5-OH-DA, in contrast to its known intraneuronal localization in storage granules, was reserpine resistant.

These results support the conclusion that 5-OH-DA permeates gap junctions between ependymal cells but is excluded by tight junctions which form continuous belts, e.g. between choroid plexus epithelial cells and capillarly endothelial cells. The presence of 5-OH-DA within clefts between tanycytes of the median eminence implies that their tight junctions are ‘leaky’ or macular (discontinuous) and do not provide a complete barrier to the penetration of the amine. Tight junctions of the choroid plexus epithelium and capillary endothelium seem to prevent small molecules, such as transmitters, from passing from blood to cerebrospinal fluid (CSF) and blood to brain as well as in the reverse direction.