Shrinkage and Loss of RGC Axons in a New Latex Bead Rat Model of High-Pressure Glaucoma

Abstract

Purpose:: Glaucoma is characterized by a progressive loss of vision due to death of retinal ganglion cells (RGCs). In high pressure glaucoma (HPG), loss of RGCs is associated with elevations in intraocular pressure (IOP). We sought to evaluate axon pathology and RGC loss in a rat model of HPG, where elevations in IOP could be induced and sustained by injections of latex beads into the anterior chamber.

Methods:: In adult Brown Norway rats, we injected fluorescent latex beads with a diameter of 15 um into the anterior chamber of one eye and saline into the other eye. Using a Tonopen, we measured subsequent changes in IOP of both eyes for up to 6 weeks. As a function of time and spatial location in the nerve, we measured axonal area and loss of axons in semi-thin sections of optic nerve approximately 1mm from the globe. Using basic histology we also examined corneal changes and inflammation in 6 um paraffin sections of whole eyes, as a function of IOP and time. In 15 um frozen sections, we used fluorescence microscopy to evaluate accumulation of latex beads in outflow canals.

Results:: A single injection of 15 um latex beads increased IOP up to 30% over a period of 2 weeks. Two injections of 15 um latex beads increased IOP 30-40% for 6 weeks. Accumulation of latex beads in outflow canals near the anterior chamber was apparent out to 6 weeks. Two weeks of elevated IOP did not induce significant loss of RGC axons in optic nerve, but did induce a 30% decrease in axonal area (p<.01). This decrease in area was uniform across the nerve, suggesting shrinkage of axons prior to loss. Four weeks of elevated IOP uniformly decreased the density and area of RGC axons by 35% and 31%, respectively. Significant corneal pathology was not noted until 5 weeks of elevated IOP, where the central corneal became acellular and the thickness increased as much as 60%. For all time points, we did not note the presence of migrating immune cells. For all analysis, comparisons were relative to saline-injected eyes.

Conclusions:: Our data establishes the use of latex bead injections as a viable means of stably increasing IOP to induce a loss of RGC axons in rats. These data also suggest that the latex bead model lacks systemic ocular inflammatory responses observed in other animal models of HPG.

Keywords: optic nerve • ganglion cells • intraocular pressure