Distribution of intravenously injected protein tracers in peripheral ganglia of adult mice

Abstract

The distribution of intravenously administered horseradish peroxidase (HRP), cytochrome c, and ferritin was studied in peripheral ganglia of adult mice in order to detect any possible differences in their extra- and intracellular localization. There was a rapid extravasation of all tracers into the extracellular space of craniospinal and autonomic ganglia. Endothelial fenestrae were identified in vessels from all ganglionic types, and the present findings are in line with previous reports that leakage of tracers takes place through fenestrae and open intercellular junctions. A few minutes after intravenous injection, HRP and cytochrome c penetrated between the satellite cell processes and nerve cell bodies in trigeminal ganglia and were also present in the periaxonal spaces of unmyelinated axons. The concentration of ferritin in the perineuronal space was low even 4 h after injection. HRP and ferritin were rapidly taken up by macrophages, but only small amounts of cytochrome c were detected inside these cells. Accumulation and intracellular digestion in macrophages seem to be important mechanisms for elimination of foreign proteins from the extracellular space. Nerve cells took up small quantities of HRP by endocytosis but no ferritin or cytochrome c was observed in neurons. The possible causes for differences in uptake of tracers are discussed.