Interneuronal transfer of horseradish peroxidase associated with exo/endocytotic activity on adjacent membranes.

Transneuronal transport of horseradish peroxidase (HRP) was observed by electron microscopy following intracellular injection of the enzyme into single interneurons inhibiting the teleost Mauthner cell. Complex membrane infolding, associating protrusions of the stained cells with adjacent indentations of the postsynaptic neuron, favours the interpretation that transport of HRP is brought about by an active process of extrusion and consecutive micropinocytosis of HRP by the target cell.