Rhodopsin content and electroretinographic sensitivity in light-damaged rat retina

Structural and Functional Change in Albino Rat Retina Induced by Various Visible Light Wavelengths

The effects of visible light, from short to long wavelengths, on the retina were investigated functionally and histologically. The left eyes of Sprague-Dawley albino rats (6-weeks old, n = 6 for each wavelength) were exposed to seven narrow-band wavelengths (central wavelengths, 421, 441, 459, 501, 541, 581, and 615 nm) with bandwidths of 16 to 29 nm (half bandwidth, ±8-14.5 nm) using a xenon lamp source with bandpass filters at the retinal radiant exposures of 340 and 680 J/cm2. The right unexposed eyes served as controls. Seven days after exposure, flash electroretinograms (ERGs) were recorded, and the outer nuclear layer (ONL) thickness was measured. Compared to the unexposed eyes, significant reductions in the a- and b-wave ERG amplitudes were seen in eyes exposed to 460-nm or shorter wavelengths of light. The ONL thickness near the optic nerve head also tended to decrease with exposure to shorter wavelengths. The decreased ERG amplitudes and ONL thicknesses were most prominent in eyes exposed to 420-nm light at both radiant exposures. When the wavelengths were the same, the higher the amount of radiant exposure and the stronger the damage. Compared to the unexposed eyes, the a- and b-waves did not decrease significantly in eyes exposed to 500-nm or longer wavelength light. The results indicate that the retinal damage induced by visible light observed in albino rats depends on the wavelength and energy level of the exposed light.

Chemical Exacerbation of Light-induced Retinal Degeneration in F344/N Rats in National Toxicology Program Rodent Bioassays

Retinal degeneration due to chronic ambient light exposure is a common spontaneous age-related finding in albino rats, but it can also be related to exposures associated with environmental chemicals and drugs. Typically, light-induced retinal degeneration has a central/hemispherical localization whereas chemical-induced retinal degeneration has a diffuse localization. This study was conducted to identify and characterize treatment-related retinal degeneration in National Toxicology Program rodent bioassays. A total of 3 chronic bioassays in F344/N rats (but not in B6C3F1/N mice) were identified that had treatment-related increases in retinal degeneration (kava kava extract, acrylamide, and leucomalachite green). A retrospective light microscopic evaluation of the retinas from rats in these 3 studies showed a dose-related increase in the frequencies of retinal degeneration, beginning with the loss of photoreceptor cells, followed by the inner nuclear layer cells. These dose-related increased frequencies of degenerative retinal lesions localized within the central/hemispherical region are suggestive of exacerbation of light-induced retinal degeneration.

Brief exposure to damaging light causes focal recruitment of macrophages, and long-term destabilization of photoreceptors in the albino rat retina

PURPOSE

To characterize the long-term spatiotemporal features of light-mediated retinal degeneration.

METHODS

Sprague-Dawley rats were exposed to 1000 lux for 24 h, then kept in dim light (5 lux), for up to 56 days. Animals were killed at 0, 3, 7, 28, and 56 days post-exposure, and retinas were prepared for immunohistochemistry. Outer nuclear layer (ONL) thickness and TUNEL labeling were used to quantify photoreceptor death. Antibodies to opsins, glial fibrillary acidic protein (GFAP), fibroblast growth factor-2 (FGF-2), and ED1 were used to assess the retina.

RESULTS

At 0 days post-exposure, we detected photoreceptor death 2 mm superior to the optic disc (the "hotspot"), and ED1-positive macrophages in the retinal vasculature and underlying choroid. By 3 days, the ONL was thinner and there was gliosis in the outer retina, where ED1 positive macrophages were also present. Few ED1 positive cells remained at 28 days. At 56 days, there were TUNEL-positive nuclei in the penumbra, and increased FGF-2, and GFAP expression by Müller cells (MCs). In inferior retina, outer segment length was initially reduced, but recovered to near-normal by 28 days.

CONCLUSIONS

Short exposure to damaging light destabilizes the retina adjacent to a hotspot of degeneration, so that the damaged region expands in size over time. Recruitment of macrophages is associated with the early phase of damage, but not with the longer term photoreceptor loss in the penumbra. Features of the focal and progressive retinal damage in this model are reminiscent of the progression of age-related macular degeneration (AMD).

Quantitative relationship of the scotopic and photopic ERG to photoreceptor cell loss in light damaged rats

In order to use the ERG to track the effects of potential photoreceptor rescue treatments, we have compared retinal histology to the ERG in light damage. Male albino CD rats (40) were purchased at 7 weeks of age and reared in 50 lx cyclic light until 8 week old. They were exposed to a range of light intensities using white fluorescent light (1000, 1500, 2000, 2500 or 3000 lx) for 24 or 48 hr (n = 5 per group). Controls remained in dim cyclic light. Seven days after exposure, dark and light adapted ERGs were recorded from threshold up to 200 cd m-2 using 50 ms Ganzfeld white light stimuli. The STR, and scotopic and photopic b-wave thresholds and amplitudes were measured. After recording the ERG, the eyes were removed from the animals in each of the five 48 hr light exposed groups and control group for histological measurements. These included: (1) outer nuclear layer width in rod photoreceptor cell number (cell count) and micrometers, and (2) outer + inner segment layer width along the vertical meridian in the inferior retina. The product of cell count and outer + inner segment length was calculated. All histological measures showed a statistically significant linear relationship to light exposure intensity (P < 0.0001): r2 = 0.94 (cell count), 0.90 (outer nuclear layer width), 0.77 (outer + inner segment length). The log of the scotopic b-wave threshold and log amplitude showed a significant linear correlation to all histological parameters (P < 0.0001) and there was no significant difference between b-wave threshold and amplitude for any one of the histology measures used. However, overall, log b-wave threshold was significantly better correlated to histology P < 0.02. Only log b-wave amplitude showed a significant increase in variability in light damaged retinas (P < 0.02). The b-wave threshold intensity increased 0.33 log cd m-2 and the maximum amplitude decreased 0.23 log microV with each 10% decrease in cell number in the outer nuclear layer. The sensitivity of the scotopic threshold response, which originates from third order neurons, changed much more slowly with cell loss, than did the b-wave (P < 0.0005) and was well fit by a linear relationship to cell loss. The increase in photopic b-wave threshold was not significant for a cell loss of less than 70-80%. Neither the photopic or scotopic b-wave could be reliably recorded with more than 80% cell loss, but the scotopic threshold response remained. Both the scotopic and photopic ERG showed similar waveform changes near the threshold, including loss of the positive going b-wave and the predominance of a negative going response. Outer nuclear layer cell counts in this study showed the same relationship to log b-wave threshold elevation, as has been previously shown for whole retinal rhodopsin content in light damage, indicating that regional histology measurements can be good indicators of overall cell survival. Both the b-wave threshold and amplitude can be reliably used to track photoreceptor cell loss due to the damaging effects of constant light, but the scotopic threshold response may be more useful in severe damage.

The visual input stage of the mammalian circadian pacemaking system: II. The effect of light and drugs on retinal function

Acute light pulses as well as long-term light exposure may not only modulate photoreceptive properties, but also induce reversible or irreversible damage to the retina, depending on exposure conditions. Illuminance levels in laboratory animal colonies and manipulations of lighting regimens in circadian rhythm research can threaten retinal structure and physiology, and may therefore modify zeitgeber input to the central circadian system. Given the opportunity to escape light at any time, the nocturnal rat self-selects a seasonally varying "naturalistic skeleton photoperiod" that protects the eyes from potential damage by nonphysiological light exposures. Both rod rod-segment disk shedding and behavioral circadian phase shifts are elicited by low levels of twilight stimulation. From this vantage point, we hypothesize that certain basic properties of circadian rhythms (e.g., Aschoff's rule and splitting) may reflect modulation of retinal physiology by light. Pharmacological manipulations with or without the addition of lighting strategies have been used to analyze the neurochemistry of circadian timekeeping. Drug modulation of light input at the level of the retina may add to or interact with direct drug modulation of the central circadian pacemaking system.

The visual input stage of the mammalian circadian pacemaking system: I. Is there a clock in the mammalian eye?

Threads of evidence from recent experimentation in retinal morphology, neurochemistry, electrophysiology, and visual perception point toward rhythmic ocular processes that may be integral components of circadian entrainment in mammals. Components of retinal cell biology (rod outer-segment disk shedding, inner-segment degradation, melatonin and dopamine synthesis, electrophysiological responses) show self-sustaining circadian oscillations whose phase can be controlled by light-dark cycles. A complete phase response curve in visual sensitivity can be generated from light-pulse-induced phase shifting. Following lesions of the suprachiasmatic nuclei, circadian rhythms of visual detectability and rod outer-segment disk shedding persist, even though behavioral activity becomes arrhythmic. We discuss the converging evidence for an ocular circadian timing system in terms of interactions between rhythmic retinal processes and the central suprachiasmatic pacemaker, and propose that retinal phase shifts to light provide a critical input signal.

Age-related changes in retinal sensitivity, rhodopsin content and rod outer segment length in hooded rats following low-level lead exposure during development

Electroretinographic, morphometric and cyclic nucleotide metabolism studies in adult hooded rats have established that low-level lead exposure during early postnatal development (postnatal days 0-21) causes long-term selective rod deficits and degeneration. To determine if this same low-level lead exposure during early postnatal development produces immediate and/or long-term alterations in retinal sensitivity we examined ERG b-wave threshold responses in dark-adapted control and lead-exposed rats at 1-, 3- and 12 months of age. In addition, to determine possible sites and mechanisms of action responsible for the observed decreases in retinal sensitivity we analyzed the rhodopsin content per eye, the lambda max of rhodopsin and rod outer segment (ROS) length in superior and inferior posterior retina at 1-, 3- and 12 months of age. Relative to adult (3-month-old) controls whose log threshold was arbitrarily set at 0 log units, the mean log relative threshold in control rats was 0.4 log units at 1 month of age and 0.2 log units at 1 yr of age. In contrast, the mean log relative threshold in lead-exposed rats was 1.3 log units at 1 month of age and 1.1-1.2 log units at 3- and 12 months of age. Thus, compared with controls, retinal sensitivity in lead-exposed rats was decreased approx. 1 log unit at all ages examined. The rhodopsin content per eye in control rats increased 13% between 1- and 3 months of age, reaching an adult value of 1.99 nmol per eye, and then decreased 8% by 1 yr of age. In contrast, the eyes from lead-exposed rats contained 30-34% less rhodopsin at all ages examined. No change in the lambda max of rhodopsin was observed in the retinas from the lead-exposed rats. In both controls and lead-exposed rats, the developmental changes in log b-wave relative threshold were paralleled by linear increases and decreases in rhodopsin content per eye such that a log-linear relation between retinal sensitivity and rhodopsin content per eye existed between 1- and 12 months of age. The developmental changes in the superior and inferior retinal ROS length were similar in control and lead-exposed rats: ROSs were at their adult length at 1 month of age and then slightly decreased by 1 yr of age.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in absolute threshold with light-induced retinal damage

Albino rats were trained on a mixed signalled reinforcement procedure designed to yield repeated absolute visual thresholds within each session. After threshold stability animals were exposed 12 hours per night to 1000 lux of light froma cool-white fluorescent source. Log threshold rose as an approximate linear function of exposure time to a maximum of 2.0 log units above baseline after 36 cumulative hours of exposure. Light and electron microscopic analysis of the irradiated retinas revealed vesiculated photoreceptor outer segments, with varying degrees of vacuolation of the inner segments and pyknosis of photoreceptor nuclei depending on exposure time. The various combinations of retinal pathology suggests that damage to photoreceptor outer segments and inner segments interact to jointly affect psychophysical thresholds in light-induced retinal damage.