Rapid and protracted phases of retinal ganglion cell loss follow axotomy in the optic nerve of adult rats

Abstract

To investigate the short- and long-term effects of axotomy on the survival of central nervous system (CNS) neurons in adult rats, retinal ganglion cells (RGCs) were labelled retrogradely with the persistent marker diI and their axons interrupted in the optic nerve (ON) by intracranial crush 8 or 10 mm from the eye or intraorbital cut 0.5 or 3 mm from the eye. Labelled RGCs were counted in flat-mounted retinas at intervals from 2 weeks to 20 months after axotomy. Two major patterns of RGC loss were observed: (1) an initial abrupt loss that was confined to the first 2 weeks after injury and was more severe when the ON was cut close to the eye; (2) a slower, persistent decline in RGC densities with one-half survival times that ranged from approximately 1 month after intraorbital ON cut to 6 months after intracranial ON crush. A small population of RGCs (approximately 5%) survived for as long as 20 months after intraorbital axotomy. The initial loss of axotomized RGCs presumably results from time-limited perturbations related to the position of the ON injury. A persistent lack of terminal connectivity between RGCs and their targets in the brain may contribute to the subsequent, more protracted RGC loss, but the differences between intraorbital cut and intracranial crush suggest that additional mechanisms are involved. It is unclear whether the various injury-related processes set in motion in both the ON and the retina exert random effects on all RGCs or act preferentially on subpopulations of these neurons.

Axon-Regenerating Retinal Ganglion Cells in Adult Rats Synthesize the Cell Adhesion Molecule L1 but Not TAG-1 or SC-1

“…The number of RGCs in grafted rats backfilled with HRP, and thereby identified as RGCs with regenerating axons (about 4%), roughly corresponds to the number of axon-regenerating RGCs determined in earlier studies (4.7%, Villegas-Pérez et al, 1992;2%, Schaden et al, 1994) where other retrograde tracers were used. HRP is not a quantitative marker but can be combined with in situ hybridization, and thus allows a good estimate of the number of RGCs regenerating axons and producing mRNA.…”

“…4. The topographic inhomogeneity of RGC numbers [with the temporal retina possessing roughly four times as many RGCs as the nasal retina (Villegas-Pérez et al, 1992;Bähr et al, 1992)] was disregarded because our study focuses on mRNA-producing RGCs in grafted rats (see below), which occur in low numbers throughout the retina. Compared to RGC counts in the literature (VillegasPérez et al, 1988(VillegasPérez et al, , 1992Bähr et al, 1992) it appears that most or perhaps all RGCs in the normal adult retina produce L1 mRNA.…”

Animal models of optic nerve diseases

“…[42][43][44] One-half of them will degenerate 4 days after optic nerve axotomy, and it rises to about 70% in 1 week and to more than 95% in 2 weeks.…”

“…The intra-orbital technique with forceps is the one most commonly used. 44,49 With the aid of a binocular operating microscope, the conjunctiva is incised and the optic nerve is exposed. The crush injury is inflicted for 1 s by a cross-action forceps 2-3 mm from the eyeball, taking special care not to interfere with the blood supply.…”

Apoptotic cell death induced by optic nerve lesion in the neonatal rat

“…A lesion to the optic nerve has long been known to induce the death of most axotomized retinal ganglion cells both in the developing (Miller and Oberdorfer, 198 1;Beazley et al, 1987;Harvey and Robertson, 1992) and adult rat (Misantone et al, 1984;Villegaz-Perez et al, 1992) although the process is much more rapid in the young animal. The retrograde morphological alterations induced by the axotomy in the developing animal have been described, and include the appearance of pyknotic nuclei, characterized by a densely stained nucleus, and the presence of phagocytic cells often engulfing the degenerating cells (Miller and Oberdorfer, 198 1;Beazley et al, 1987).…”