Routes of excretion of neuronal lysosomal dense bodies after ventricular infusion of leupeptin in the rat: a study using ubiquitin and PGP 9.5 immunocytochemistry.

To determine the rate and routes of removal of lysosomal, lipofuscin-like dense bodies from neurons, the protease inhibitor, leupeptin, was infused into the lateral ventricle of rats for up to nine days. After seven days a number of animals were then allowed to recover. The formation and later disappearance of dense bodies was followed by morphology and immunocytochemistry. After 48 h of infusion lysosomal dense bodies in large numbers appeared in cortical, hippocampal and cerebellar neurons, which also showed increased ubiquitin immunoreactivity, as well as in other cell types. By 3-4 days ubiqutin-immunoreactive dense bodies were equally distributed between neurons and astroglia. After seven to nine days of infusion ubiquitin immunoreactive dense bodies filled neuronal perikarya, dendrites and expanded initial segments of many axons and were abundant in glial processes. All dense bodies studied by electron microscopy were ubiquitin immunoreactive. After four days of recovery dense bodies were markedly fewer in neuronal perikarya, and virtually all were now within glial processes. From 7 to 28 days of recovery, when most neurons appeared normal, lipofuscin bodies remained in axon initial segments and in reduced numbers in glial processes, particularly around blood vessels and beneath the pia of hippocampus and of cerebellar cortex. Thus, neurons probably have a steady passage of short lived proteins through the lysosomal excretory pathway. The observed temporal sequence of events on recovery suggests that secondary lysosomes probably pass rapidly from neuronal perikarya and dendrites to astrocytes and thus to the vascular bed or pia-arachnoid. The mechanism of cell-to-cell transfer is not clear from this study.