Donald M. Allen

The Role of Microglia in Diabetic Retinopathy

There is growing evidence that chronic inflammation plays a role in both the development and progression of diabetic retinopathy. There is also evidence that molecules produced as a result of hyperglycemia can activate microglia. However the exact contribution of microglia, the resident immune cells of the central nervous system, to retinal tissue damage during diabetes remains unclear. Current data suggest that dysregulated microglial responses are linked to their deleterious effects in several neurological diseases associated with chronic inflammation. As inflammatory cytokines and hyperglycemia disseminate through the diabetic retina, microglia can change to an activated state, increase in number, translocate through the retina, and themselves become the producers of inflammatory and apoptotic molecules or alternatively exert anti-inflammatory effects. In addition, microglial genetic variations may account for some of the individual differences commonly seen in patients susceptibility to diabetic retinopathy.

Photic control of the proportions of two visual pigments in a fish.

Abstract Photic control of the proportion of visual pigments in a cyprinid fish was investigated experimentally by exposing groups of fish to various light regimes. Light treatment increased the proportion of the retinene2 pigment by causing additional formation of visual pigment. This response was mimicked by adding thyroxine to the water. Photoperiod was the most important controlling factor, but light intensity and spectral quality were also important. All controlling factors act together to determine a steady state level of retinene2 visual pigment, regardless of the initial percentage. Both the significance of these findings in view of seasonal changes and the possible mechanisms involved are discussed.

Solar pruning of retinal rods in albino rainbow trout.

Morphology of the central retina and scotopic visual sensitivity were compared in juvenile albino and normally pigmented rainbow trout living under natural and reduced daylight. Outdoor albinos avoided exposing their eyes to direct sunlight, whereas normals were indifferent to it. After 4 months outdoors (approximately 10,000 lux in albinos, approximately 100,000 lux in normals), albinos had severely truncated or missing rod outer segments (ROS) and some missing rod ellipsoids, but normal numbers of photoreceptor nuclei and fully intact cones. Albino estimated ROS volume was only 7.1% of normal in July, but increased to 20% by the following February, mainly via an increase in numbers of ROS. However, in albinos moved indoors October 7 and exposed to 10-30 lux ambient daylight, both the number and length of ROS increased significantly, with estimated ROS volume reaching 95% of normal by 34 days. Albinos generally had more phagosomes (approximately 3 x normal) and more macrophages (approximately 2 x normal) in their outer retina. An optomotor reflex was used to define the effect of ROS volume on the ability to respond visually during dark adaptation. In July, albinos and normals from outdoor raceways (3 months) or indoor raceways (35 days) showed equal sensitivity after first being placed in darkness, but after 1 h in darkness, outdoor albinos with 6% of normal ROS volume were 2.0 log units less sensitive than indoor or outdoor normals, whereas indoor albinos with 53% of normal ROS volume were only 0.7 log units less sensitive. This verifies that most rod cell bodies of albino trout can persist without functional ROS in indirect sunlight, and can regrow functional outer segments in dim daylight. This finding is distinct from the extensive retinal light damage observed in albino rats exposed to more moderate cyclic light, in which entire rod cells degenerate early on.

Visual pigment mixtures and scotopic spectral sensitivity in rainbow trout

SynopsisChanges in scotopic visual sensitivity of rainbow trout,Salmo gairdneri, were induced by thyroxine and/or temperature treatments. The spectral sensitivities of these fish were correlated directly with changes in the absorption spectrum of a mixture of the two rod visual pigments, rhodopsin and porphyropsin. In contrast to earlier reports, these data for trout are more consistent with the hypothesis that labile binary visual pigment mixtures are useful in making short- or long-term adjustments for vision in a variable photic environment.

Photic history modifies susceptibility to retinal damage in albino trout

Albino vertebrates exposed to intense light typically lose photoreceptors via apoptosis, and thus serve as useful models of retinal degeneration. In contrast, albino rainbow trout exposed to intense light maintain populations of rod and cone nuclei despite substantial damage to rod outer segments (ROS). The aim of this study was to differentiate between two hypotheses that could account for this divergent result: (1) trout rod nuclei remain intact during light damage, or (2) rod nuclei die but are replaced by cell proliferation. A further aim was to examine whether photic history modulates retinal damage, as in rodents. Albino and normally pigmented trout were moved from defined photic regimes into full daylight, while some were not moved to serve as protected controls. ROS were always maintained in pigmented fish and in albinos protected from full daylight. In albinos exposed to full daylight, ROS were removed over most of the central retina, whereas rod nuclei were maintained in the outer nuclear layer over 10 days. Pyknotic and TUNEL-labeled rod nuclei were abundant in affected albinos at all time-points tested. Rod death occurred without a decrease in the number of rod nuclei, confirming that proliferation must be replacing cells. Indeed a transient increase in proliferation was observed in retinal progenitors of albinos receiving 5 days of damaging light. This proliferative response was decreased with further damage. Cones remained intact even in areas where rod nuclei had degenerated. Pretreatment with light of moderate versus low intensity light affected the cell death and proliferative responses, and the ectopic localization of rod opsin. We conclude that apoptotic demise of rods, but not cones, occurred during light damage in retinas of albino trout and proliferative responses have a limited a capacity to replace lost rods.

Thiourea does not block visual pigment responses to prolactin in trout

Thyroid hormones, injected or added to the water. increase the proportion of porphyropsin in the mixture of rhodopsin and porphyropsin visual pigments extracted from the retinas of rainbow trout, Sulmo guirdneri (Jacquest and Beatty. 1972). Similar doses of throxine elevate the serum T* level in brook trout, Salwlinus jbntinalis (Eales 1974), and in rainbow trout (Allen, 1977). Mammalian thyroid-stimulating hormone (TSH) is also effective in elevating serum T, in brook trout (Chan and Eales, 1975) and in increasing porphyropsin in kokanee satmon. Oncorhyncus nerka, and rainbow trout (Beatty, 1972; Jacquest and Beatty, 1972). Therefore, the pituitaryTSH-thyroid system may mediate interconver~ons of rhodopsin and porphyropsin. Prolactin. whose function is osmoregulation in euryhaline species of teleosts is firmly established (see Bern and Nicoll. 1969; Ball, 1969; Lam, 1972), may also induce interconversions of rhodopsin and parphyropsin in migrating salmonids (Cristy, 1974). Mammalian prolactin, given to rainbow trout in doses effective in osmoregulation replacement therapy, increased the per cent porphyropsin with or without presence of thyroxine in the water (Cristy, 1974). Apparently, prolactin can also stimuIate thyroid activity in Anquih (Olivereau, 19681 and in a cichlid fish. Pterophyllum scnlure (Osewofd and Fiedler, 1968). This raised the question of whether prolactin’s effect on the visual pigments is direct, or mediated through the pituitary-TSH-thyroid axis. We investigated this question by testing the effects of prolactin injections on serum Th and per cent

Computer-assisted analysis of visual pigments

Abstract Computing methods have been applied to the analysis and graphing of partial bleaching experiments. The programs are designed to handle automatically a broad range of experiments with different visual pigments. Individual and total difference spectra, scaled in either absorbance or percentage units, are drawn by a graphic plotter. Whether the retinal extracts are homogeneous or contain a mixture of visual pigments can be decided by inspection of the plotted difference curves. The absorbance peaks of single pigments are estimated by fitting the data to visual pigment nomograms. If a retinal extract contains a mixture of two known visual pigments, their proportions are calculated. Interpretation gains in precision from the fact that many more data points are taken than was possible heretofore. The number of partial bleaching experiments that can be performed and fully analyzed is greatly increased.

Effects of Tamoxifen Versus Raloxifene on Retinal Capillary Endothelial Cell Proliferation

PURPOSE Endothelial cell proliferation in angiogenesis is active in conditions such as cancers and diabetic retinopathy. Tamoxifen (T) and raloxifene (R) have been compared in numerous studies as a prophylaxis for breast cancer, and T is used to treat breast cancer. T, unlike R, has been linked to an increase in uterine cancers, thrombo-embolic events, and cataract. The purpose of our study was to evaluate the efficacies of T and R in reducing estrogen-induced retinal capillary endothelial cell proliferation. METHODS Rhesus monkey retinal capillary endothelial cells (ATCC RF/6A) were used to assay cell proliferation when treated with 0.0, 0.1, 1.0, and 10.0 nM 17 β estradiol (E2) for 24 and 48 h. Viable cells were counted using a Neubauer hemocytometer with a trypan blue exclusion method to determine the number of viable cells. Cell counts were also performed using 1.0 nM E2 with 0.01, 0.1, 1.0, and 10.0 nM concentrations of either T or R. Cell medium, collected at 24 h, was evaluated for vascular endothelial growth factor and pigment epithelium-derived factor. RESULTS Viable cells were significantly greater in cultures treated with 1.0 or 10.0 nM E2, compared to cells treated with 0.0 or 0.1 nM E2 both at 24 and 48 h. Viable cell counts were reduced significantly in cultures treated with 0.1, 1.0, or 10.0 nM T or R in addition to the 1.0 nM E2. Cell counts were not significantly different when comparing equal concentrations of T and R, that is, 1.0 nM E2+1 nM T or R. Vascular endothelial growth factor and pigment epithelium-derived factor protein/10,000 cells was reduced by 1.0 nM E2, but returned to higher levels with the introduction of T and R to growth media. CONCLUSIONS T and R showed similar potency in inhibiting estrogen-induced retinal capillary endothelial cell proliferation. Considering drug safety profiles, our results, when extended to animals and humans, suggest that R is preferable to T in treating angiogenic retinal diseases. Further studies on the signaling mechanism of estrogen-induced endothelial cell proliferation may lead to new treatment strategies in the treatment of ocular angiogenic diseases.

A Comparison of Light-Induced Rod Degeneration in Two Teleost Models

Albino rainbow trout, Oncorhynchus mykiss, are resistant to light damage, losing only their rod outer segments (ROS) in full daylight (10,000 to 100,000 lux) at 11°C. 1 To compare light damage in albino trout with light damage in albino rodents, we analyzed central retinal structure in albino trout exposed to full daylight, indoor dim day-light (30 lux-30 days or longer) and strong constant light (3000 lux). In albinos remaining outdoors or placed in constant light, ROS volume was reduced but the number of photoreceptor nuclei did not decline. Albinos placed in dim daylight re-grew ROS to 60% or more of normal volume but when returned to outdoor raceways lost most of their ROS volume within 5 days. Outdoor albinos placed in dim daylight replaced ROS much more slowly. In neither case was there a change in number of photoreceptor nuclei. In affected albinos there is apparently little rod cell death during the initial phases of light insult to ROS or thereafter. This confirms that most rod cells with ROS damage survive and retain capacity to re-grow ROS, and any which undergo apoptosis are replaced by cells derived from rod progenitors.

The Role of Sex Hormones in Diabetic Retinopathy

Jeffery G. Grigsby1, Donald M. Allen2, Richard B. Culbert3,4, Gerardo Escobedo4, Kalpana Parvathaneni1, Brandi S. Betts1 and Andrew T.C. Tsin1 1Department of Biology, University of Texas at San Antonio, San Antonio, Texas 2Department of Biology, University of Texas of the Permian Basin, Odessa, Texas 3Department of Surgery, Texas Tech School of Medicine, Midland, Texas 4Premier Retina Specialists, Midland, Texas 1, 2,3,4United States of America