The distribution of 5-hydroxydopamine (5-OHDA) and horseradish peroxidase (HRP) in the cochlea was examined by light and electron microscopy after perilymphatic perfusion. Reaction product of 5-OHDA was observed along the endolymphatic surface of the cochlear duct in both 10-minute and 30-minute perfusions, while that of HRP was observed only in the 30-minute perfusion. Both tracers were found in pinocytotic vesicles of Reissner's membrane, Claudius' cells, and Hensen's cells. No labeling was seen in tight junctions of the epithelial cells lining the scala media. The epithelial cells of the spiral prominence showed various degrees of degeneration in the 5-OHDA experiment. These morphologic findings suggest that 5-OHDA and HRP can enter the endolymphatic space from the perilymphatic space by pinocytosis and that the degree of transport depends on the size of the particles.
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