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S. D. Crish, R. M. Sappington, D. J. Calkins; Axon Degeneration and Loss of Transport in a New Latex Bead Model of High-Pressure Glaucoma. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4352.
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We examined whether deficits in axonal transport and axon degeneration precede retinal ganglion cell somatic loss in a new latex bead injection (LBI) model of high-pressure glaucoma in the rat. We compared changes in neuronal tracer uptake and transport in bead-injected (experimental) eyes and vehicle-injected (control) eyes in the same animals.
We injected 15µm latex beads or vehicle in the anterior chamber of each eye of Brown Norway rats; IOP was measured with a Tonopen-XL. Twelve to 40 days later the animals received intravitreal injections of the neuronal tracer cholera toxin ß subunit conjugated to a fluorophore. Forty-eight hours after this injection the brains, optic nerves, and eyes were harvested.Fifty um frozen sections were taken of the brain in the coronal plane. Photomicrographs were quantitatively analyzed for amount and location of fluorescent label. Segments of optic nerve 1-2mm in length were taken proximal to the eye and at the optic chiasm, were embedded in Epon resin, and 1 µm cross-sections cut. The middle section of optic nerve was embedded in agarose and longitudinal vibratome sections were taken. Eyes were either embedded in paraffin and 6µm vertical sections cut or the retina was removed and flatmounted.
IOP increased on average about 30% in LBI eyes. Amount of tracer label in all retinal targets in the brain from experimental eye was significantly different (p<0.05) in all cases. About 65% of the label was lost in the superior colliculus and lateral geniculate nucleus when compared to the control eye. Examination of the longitudinal sections indicated that axonal transport continued through the entire length without any obvious gaps. In semithin sections of nerve, a 30% decrease in RGC axons was seen in sections near the chiasm but sections adjacent to the eye were normal compared to control. No statistically significant deficits in RGC numbers or retinal uptake was found in the experimental eyes.
Our findings support the hypothesis that LBI-induced increases in intraocular pressure results in axonal degeneration prior to RGC soma death. Also, these data suggest that the axon degenerates in a retrograde direction and that transport to the brain may be an early sign of injury in glaucoma.
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