The possibility of centrifugal projections to the retina in the rat

Ellipsoid Zone Change According to Glaucoma Stage Advancement

PURPOSE

To compare retinal photoreceptor ellipsoid zone (EZ) intensity between normal eyes and those with different stages of glaucoma.

DESIGN

Retrospective cross-sectional study.

METHODS

The study included 37 normal, 38 preperimetric glaucoma, 39 mild-to-moderate glaucoma (visual field [VF] mean deviation [MD]: -7.7 ± 2.0 dB), and 36 severe glaucoma eyes (VF MD: -17.8 ± 3.2 dB). The subjects underwent high-resolution horizontal and vertical line scans through the fovea by spectral-domain optical coherence tomography (SD-OCT). Image processing software was employed to quantify the intensity of the first and second hyperreflective bands corresponding with the external limiting membrane (ELM) and EZ. In order to account for the brightness variation among scans, the relative EZ intensity as the ratio of the second to first reflective band (EZ/ELM) was determined.

RESULTS

The relative EZ intensity in severe glaucoma eyes was significantly lower than in mild-to-moderate glaucoma eyes (2.46 ± 0.38 vs 3.15 ± 0.43, P < .001); also, it was lower in mild-to-moderate than in preperimetric glaucoma eyes (3.15 ± 0.43 vs 3.86 ± 0.44, P < .001). However, the comparison between preperimetric glaucoma and normal eyes showed no significant difference (3.86 ± 0.44 vs 4.06 ± 0.40, P = .751). In 75 glaucomatous eyes with VF defect, there was a significant correlation between relative EZ intensity and VF MD (r = 0.83 and P < .001).

CONCLUSIONS

According to SD-OCT, relative EZ intensity reduction occurs in the mild-to-moderate and severe glaucoma stages. These findings suggest, at least provisionally, that in the course of glaucoma progression, mitochondrial changes in the inner segments of photoreceptors occur. Further investigation is warranted to evaluate the potential clinical significance of EZ intensity reduction in glaucoma.

Long-term effect of retinal ganglion cell axotomy on the histomorphometry of other cells in the porcine retina

PURPOSE

To determine the effect of retinal ganglion cell axotomy on the thickness of inner plexiform, inner nuclear, and outer plexiform layers, as well as the densities of short- and middle-to-long-wavelength cones, in the porcine retina.

METHODS

Unilateral retinal ganglion cell axotomy was performed in seven domestic pigs by either surgical optic nerve section or peripapillary argon laser photocoagulation. Damage to the retinal vasculature was ruled out with fluorescein angiography. Histologic examination of the retinal tissue was performed nine months later. Cone densities were determined immunohistochemically with the anti-visual pigment antibodies COS-1 and OS-2. Image analysis of semithin retinal cross sections was used to measure the thickness of the retinal layers. The effect of axotomy was quantified by optic nerve axon counts and estimations of retinal ganglion cell counts. The data were compared between the eyes with axotomy and the contralateral normal eye using the nonparametric Wilcoxon rank sum test.

RESULTS

Treatment of the peripapillary retina with the argon laser resulted in a median decrease in axon counts and retinal ganglion cell density estimates of 31%. No optic nerve axons and cells resembling retinal ganglion cells were found in the eyes with transected optic nerves. There was no significant difference in either the thickness of any retinal layers or cone densities between axotomized and normal control eyes.

CONCLUSION

No signs of retrograde transsynaptic degeneration were observed in porcine retinas nine months after retinal ganglion cell axotomy.

Photoreceptor plasticity in the albino rat retina following unilateral optic nerve section

Rhodopsin content of the retina increased when an albino rat was moved to a lower intensity cyclic light, and decreased when moved to a higher intensity. Unilateral optic nerve section was employed to test if intact optic nerves are necessary to maintain regulation of rhodopsin when albino rats are switched between two intensities, 3 lx and 600 lx (12 hr/12 hr, cyclic light). The ability to regulate rhodopsin content was altered but not lost in the eye which had the optic nerve transected. This alteration was in the direction of incomplete up-regulation when the intensity change was from high to low, and was in the direction of excessive down-regulation when the intensity change was from low to high. This indicates that efferent fibers from the brain to the retina may be involved in the regulation of rhodopsin content. Using histological techniques, cell number and rod outer segment (ROS) length were measured: surgery alone had no effect on ROS length or outer nuclear layer thickness and in all cases ROS lengths were inversely related to habitat illuminance.

The effect of unilateral optic nerve section on retinal light damage in rats

The effect of unilateral optic nerve section on the susceptibility of rat photoreceptors to damage by constant light was studied. Optic nerves of albino rats were cut intracranially by a ventral approach, so as not to interfere with structures in the orbit of the eye, with the brain or with the blood supply to the retina. Two separate experiments were performed on unilaterally optic nerve sectioned rats from two different sources. One group was purchased from a commercial supplier and the other group was born and raised in 3 lux cyclic light in our laboratory. For 1-4 weeks after surgery they were exposed to either 1000 lux for 24 hr or 80 lux for 48 hr. Light damage was quantified by measuring the outer nuclear layer area remaining in histological sections through the vertical meridian of the retina. It was found that retinas with optic nerves cut suffered substantially less damage from light than did those with intact optic nerves. Sham operated rats suffered the same amount of damage as did the optic nerve intact retinas of rats with one nerve cut. The extent of protection was greatest in the region previously shown to be most susceptible to damage. The protection afforded by optic nerve section could not be explained on the basis of behavior or rhodopsin photochemistry. The possible role of heat-shock proteins and a neuromodulator is discussed.