A light and electron microscopic study of the intraneuronal transport of horseradish peroxidase and wheat germ agglutinin-peroxidase conjugates in the rat visual system.

The ability of horseradish peroxidase (HRP) and the lectin wheat germ agglutinin (WGA) covalently conjugated with HRP to label retrogradely dorsal lateral geniculate nucleus (dLGN) neurons, subsequent to injections of either marker into rat striate cortex, was assessed by counting labelled neurons in the dLGN. Rats injected with either marker in concentrations ranging from 0.1 to 100 micrograms/microliter of HRP either free or coupled to WGA were perfused 24 h later and their brains incubated using the chromagen tetramethyl benzidine. At high concentrations (2-100 micrograms/microliters), comparable numbers of labelled neurons were observed in the dLGN but at low concentrations (0.1-1.0 micrograms/ microliters), WGA-HRP labelled 2-5 times more dLGN neurons than did unconjugated HRP. The sugar N-acetylglucosamine, and free WGA added in excess to WGA-HRP, abolished the retrograde labelling of dLGN neurons. In additional rats, which received striate cortex injections of 100 micrograms/microliters of either free HRP or HRP coupled to WGA, the injection site was studied with electron microscopy after survivals of 30 min to 24 h. Similar organelles in neuronal perikarya, dendrites and axons were labelled by both markers, with the exception that only rats injected with WGA-HRP had labelled GERL in some of their neurons in striate cortex. It was concluded from these studies that: (1) WGA-HRP is a more sensitive retrograde marker than free HRP at low concentrations in the rat visual system; (2) WGA-HRP binds specifically to moieties with terminal N-acetylglucosamine; and (3) WGA-HRP, but not free HRP, is localized to neuronal GERL of striate cortex subsequent to endocytosis.