W. Keith O'Steen

Retinal and optic nerve serotonin and retinal degeneration as influenced by photoperiod

Abstract The environmental light stimulus, which controls neuroendocrine events associated with reproductive cycles, also influences the serotonin cyclicity in the central nervous system. A possible relationship between photoperiod and serotonin in the retina has been determined by liquid scintillation spectrometry after intraocular injection of tritium-labelled amine precursor, 5-hydroxytryptophan (3H·5-HTP), into adult female rats kept for varying time periods in either constant light, constant darkness, or a cyclic photoperiod of 14 hours of light and 10 hours of darkness. Constant illumination suppressed retinal and optic nerve serotonin stores after 16 days, but had no effect after 4 or 30 days exposure. Radioactivity in the retina and optic nerves was reduced after 30 days of constant darkness. Exposure to constant illumination under ordinary laboratory conditions for 4, 16, and 30 days resulted in a reduction in the thickness of the retina, accompanied by degenerative changes in the photoreceptor cells. Terminal segments of these cells fragmented during early exposure periods and were absent, first in the posterior, and later, in the more peripheral areas of the retina after 16 and 30 days of constant light. Neither the cyclic photoperiod nor continuous darkness initiated degeneration in the retina. The results demonstrate that serotonin is influenced by photoperiod fluctuations and suggest that the biogenic amine might be related to light-induced changes in neuroendocrine function.

Photically evoked responses in the visual system of rats exposed to continuous light

Abstract The influence of environmental light on the cyclicity of brain amines and neuroendocrine function of animals is most likely mediated through the retina and higher visual centers, and constant illumination modifies some aspects of this cyclicity. The photoreceptor cells in the retina of albino rats selectively degenerate after exposure to continuous, low intensity, fluorescent illumination at normal body and room temperatures, while cyclic light has no effect. Damage to the outer segments of these cells can be detected during the first 4 days of constant light exposure. By 14 days of exposure, most of the receptor cells are destroyed, especially in the posterior part of the retina. After 30 days of exposure, normal receptors are completely absent. The pigment cell layer and the innermost layers of the retina are not damaged by this treatment. The functional integrity of the visual system of rats with degenerated photoreceptors was assessed by examining the characteristics of photically evoked potentials in the optic tract, lateral geniculate nucleus, and visual cortex. Evoked responses in the visual system of rats that had been exposed to 4 days of light varied according to the recording site and were typically of low amplitude and had long latencies of onset. Only responses of low amplitude and with long latencies were recorded after 14 days of exposure, and no photically evoked potentials could be recorded from the optic tract and lateral geniculate nucleus. Similar responses were recorded in the visual system of rats exposed to 30 days of light, although, more commonly, no potentials could be evoked by flash stimuli. The limited retinal degeneration described, resulting from exposure of rats to low intensity illumination, provides a model for examining visual function in the absence of photoreceptor cells in the retina, but without apparent damage to other cellular components of the system.

Black-white and pattern discrimination in rats without photoreceptors

Abstract The discrimination performance of rats whose retinas lacked receptor cells was compared with the performance of normal, control rats. Retinal degeneration was produced by exposing animals to 30 days of constant light at 18 ft-c intensity. Both control rats and those with retinal degeneration were trained and tested in a T-maze on a black-white discrimination and two pattern discriminations. In the pattern tasks, the discriminanda consisted of targets with alternating black-and-white stripes oriented either horizontally or vertically. In the easier pattern discrimination the black-and-white stripes were 0.8 cm wide and in the more difficult pattern task they were 0.4 cm wide. The general finding was that rats without receptor cells were not impaired on any of the discrimination tests used in this experiment, but could readily perform at high levels on both black-white and pattern tasks. They could not only retain a visual habit learned prior to the degeneration, but could learn new discriminations at rates indistinguishable from control animals. The results strongly suggest that retinal elements other than receptor cells are sensitive to light.

Photoreceptor degeneration after exposure of rats to incandescent illumination

SummaryThe retina of the albino rat undergoes degenerative changes when exposed to low intensity incandescent light. The retinal degeneration is limited specifically to the photoreceptor cells, and the pigment epithelium is unaffected. Early changes in the receptors included fragmentation of the inner and outer segments and pyknosis of the receptor cell nuclei. Phagocytic cells invaded and occupied the central retinal area of degeneration, between the receptor layer and the pigment epithelium, in the 4 and 5 day exposure periods. They were absent centrally after 14 and 30 days of exposure, but were present at these time periods in the peripheral retina, where photoreceptor destruction was still in progress. The destruction of photoreceptor cells, including the receptor and outer nuclear layers of the retina, by incandescent light progressed at a slightly reduced rate as compared to that after exposure to fluorescent light of the same intensity. These experiments indicate that exposure to either low intensity incandescent or fluorescent light will cause a selective degeneration of retinal photoreceptor cells, and therefore provide an easily reproducible model for the study of retinal structure and function in the absence of the receptors.

The origin of spontaneous, rhythmic potentials in the visual system of rats

Abstract Spontaneous, rhythmic potentials recorded from the visual system of intact rats were compared with those recorded from enucleated rats and from rats with retinal degeneration. Three kinds of rhythmic potentials were recorded in intact rats anesthetized with pentobarbital or phenobarbital. The most predominant potential had a frequency of 10–25 cycle/sec and could be recorded from the optic tract (OT), lateral geniculate nucleus (LGN), and visual cortex (VC). A second kind of potential had a frequency of 100–150 cycle/sec and was most commonly found in the OT and LGN. The third kind of potential had a frequency of 0.3–0.5 cycle/sec and was restricted almost exclusively to the VC. The exact frequency and shape of the rhythmic potentials were found to depend upon the level of anesthesia and, to some degree, upon the lighting conditions present at the time of recording. No rhythmic potentials were found in enucleated rats. In rats with moderate retinal degeneration, spontaneous, rhythmic potentials were abolished or markedly disrupted. In rats with severe retinal degeneration, spontaneous, rhythmic potentials were completely absent. These results indicate that rhythmic, oscillatory potentials in the visual system of the rat originate in the retina.

Altered response latencies on visual discrimination tasks in rats with damaged retinas

Abstract The discrimination performance of rats whose retinas have no apparent receptor cells was compared with the performance of normal, control rats. Retinal degeneration was produced by exposing animals to 30 days of constant illumination at 18 ft-c (184 lux) intensity. Both control rats and those with retinal degeneration were trained and tested in a T-maze on a black-white discrimination. In the pattern task, the discriminanda consisted of targets with alternating black-and-white stripes (0.4 cm wide) oriented either horizontally or vertically. For each animal two measures were used to assess visual performance: the percentage of correct responses to the visual targets and the time taken to make a choice (latency of response). The results showed that latency of response was a more sensitive indicator of retinal damage than was the percentage of correct responses made on the visual task. The results confirmed the findings in previous studies that rats could not only retain a visual habit learned prior to the retinal degeneration, but could learn new discriminations after the retinas were damaged. The extent to which experimental design factors and other variables may influence the kind of results obtained in experiments with retinally damaged rats was discussed.