Abstract
Purpose: :
Horseradish peroxidase (HRP) has been widely used as a neural tracer in the visual system for over thirty years. Most previous studies have focused on visual system development, retinal ganglion cell (RGC) labeling and RGC axon projections in the visual cortex. Little work has been devoted to the process of HRP transport in optic nerve axons. The aim of this work was to determine the time course of HRP axonal transport in the optic nerve of normal adult rat.
Methods: :
A sponge soaked with 20% HRP was placed over the superior colliculi (SC) in adult Brown Norway rats (n=22). Animals were sacrificed at 4, 8, 12, 14 and 18 hours, as well as 1, 2, 3, 5, 7 and 9 days. Retinas and the whole length of optic nerves (from the optic nerve head to optic chiasm) were removed and fixed. The optic nerves were longitudinally sectioned. HRP was visualized in the retina and optic nerve sections using tetramethylbenzidine (TMB) as the chromogen (Mesulam, 1982), and examined by light microscopy.
Results: :
HRP labeling was observed over the entire length of the optic nerve at 4 hours after application to the SC. The density of labeling increased over the entire length of the nerve with time (8 and 12 hours). However, by 18 hours, only a few HRP positive axons could be found in the optic nerve, and the density of labeling in the portion of nerve proximal to the nerve head was greater than in the chiasm. At 1 day, there was no detectable HRP in the optic nerve. RGCs were first labeled evenly in the whole retina at 8 hours and remained labeled up to 7 days. HRP positive RGCs disappeared in the retina at 9 days after the application to the SC.
Conclusions: :
Our results demonstrate that HRP is a useful tracer for assessing fast axonal transport in the optic nerve.
Keywords: retina • superior colliculus/optic tectum • microscopy: light/fluorescence/immunohistochemistry