Morphological Analysis of Axonal Loss in TNF––Induced Optic Nerve Degeneration and Nicotinamide Adenine Dinucleotide

Abstract

Purpose: : To examine the distribution of axon size in TNF–α–induced optic nerve degeneration and retinal ganglion cell death. To evaluate whether nicotinamide adenine dinucleotide (NAD) prevents axonal degeneration.

Methods: : Rats were euthanized at 1 day, 1, 2 weeks, 1 or 2 months after PBS or TNF–α (1–10 ng) intravitreal injection. Morphological analysis of axons in optic nerve was performed using image processing software. Fluorogold retrograde labeling of RGCs was performed 5 days before the intravitreal injection. The effects of NAD on TNF–α–induced optic nerve degeneration were determined by axon number counting.

Results: : Morphological analysis of optic nerve axons showed that the decrease in axon number was more prominent with axon diameters less than 2 µm from 2 weeks to 2 months after TNF–α injection. There was no significant difference in axon numbers between PBS and TNF–α–treated eyes with axon diameter larger than 2 µm. Fluorogold labeling analysis showed no significant difference in RGCs numbers between PBS and TNF–α–treated eyes 2 weeks after injection despite significant optic nerve axonal losses in TNF–α–treated eyes. However, a significant decrease in RGCs number was observed at 2 months after TNF–α injection. Pretreatment of NAD significantly prevented TNF–α–induced axonal loss in a dose–dependent manner. This protective effect was predominantly observed in smaller axons.

Conclusions: : TNF–α causes optic nerve axonal degeneration, especially of smaller fibers and with delayed loss of retinal ganglion cell bodies. NAD may be neuroprotective to this effect and holds the potential for the treatment of some optic neuropathies.

Keywords: neuro-ophthalmology: optic nerve • neuroprotection • ganglion cells