Rod outer segment lipids in vitamin A-adequate and -deficient rats

Long term effects of diaminophenoxypentane in the rat retina: protection against light damage

The retinotoxic drug diaminophenoxypentane (DAPP) was administered to rats to determine its long term effects on rhodopsin levels, retinal morphology and the retina's susceptibility to damage from visible light. In rats given 2 intraperitoneal injections of DAPP at doses of 65 mg/kg body wt, there was a dramatic and sustained loss of rhodopsin. One wk later visual pigment levels were 41% lower than in comparable dark maintained rats injected with saline. Rhodopsin levels in the DAPP treated rats remained lower than in control animals for the 13 wk period of the study. Morphologically, the ROS of rats 1-2 wks after DAPP treatment exhibited some disorganization and shortening; the RPE was unremarkable. Seven wks after DAPP treatment an occasional focal area of damage was seen in the RPE. Similarly, focal areas of degeneration were seen in the outer nuclear layer between the rows of photoreceptor cells. As determined by photoreceptor cell nuclear counts, the retinotoxic effect of DAPP persisted long after drug administration. In the treated rats the loss of visual cell nuclei was 11% at 7 wks; it was 22% 13 wks after DAPP treatment. Immediately after exposure to intense visible light, damage was seen in both the photoreceptor cells and RPE of DAPP treated rats. However, the effects of exposure in the treated rats were less dramatic than in retinas from rats without DAPP treatment. Following a 2 wk dark recovery period, the DAPP treated rats had a normal appearing retinal morphology and an intact RPE layer. The retinas of rats without DAPP treatment showed extensive visual cell and RPE loss.(ABSTRACT TRUNCATED AT 250 WORDS)

Docosahexaenoic acid increases in frog retinal pigment epithelium following rod photoreceptor shedding

The vertebrate retina conserves docosahexaenoic acid (22:6n-3) during n-3 fatty acid deficiency. The mechanism of conservation is not known, although recycling of this fatty acid between the retinal pigment epithelium (RPE) and retina is one possibility. We examined the role of the RPE in conservation of 22:6n-3 by quantitating the fatty acids and phospholipid molecular species (PLMS) in frog RPE before and after light-stimulated shedding of rod outer segments (ROS). RPE cells were dissociated with brush agitation and purified by a discontinuous ficoll density gradient. One hour after the light-induced shedding of ROS, the phagocytosed ROS tip and opsin content of RPE had increased. Simultaneously, the levels of 22:6n-3 and 22:6(n-3)-containing PLMS were increased in the RPE. Within 8 hr following the shedding event, 22:6n-3 in the RPE had returned to the dark level. These findings indicate that the phagocytosed ROS tips contain 22:6n-3 and that the RPE metabolizes these ROS tips and eliminates 22: 6n-3 from the cell. Thus, the RPE is intimately involved in the metabolism of 22: 6n-3 in the retina. The recycling of 22: 6n-3 from the RPE to the retina is a possible means of conserving this important fatty acid in the retina.

Decreased docosahexaenoic acid levels in retina and pigment epithelium of frogs fed crickets

Whole retina, rod outer segments, and retinal pigment epithelium of frogs (Rana pipiens) fed crickets for more than 1 year had significantly lower levels of docosahexaenoic acid (22: 6n-3) than the same tissues of frogs fed crickets for less than 1 month. Decreases in 22:6n-3 levels in these tissues were compensated for by increases in the n-6 polyunsaturated fatty acids (PUFAs), primarily 22:5n-6. There were no changes in the levels of saturated, monoenoic, or dienoic acids. Analysis of diacyl phospholipid molecular species (PLMS) revealed decreases in both the 22:6(n-3)-containing dipolyenoic molecular species in phosphatidylethanolamine and phosphatidylserine, and the monopolyenoic molecular species in phosphatidylcholine. These PLMS were replaced by species containing 22:5n-6 or other n-6 PUFAs. Examination of fatty acid methyl esters of total lipids extracted from crickets revealed that less than 1 mol% fatty acids were of the n-3 family, while more than 30 mol% were of the n-6 family. Thus, frogs raised on an n-3-deficient diet have reduced levels of n-3 PUFA in their retinas, rod outer segments, and retinal pigment epithelium. Although such changes have been reported for mammals, this is the first report of the effects of n-3 deficiency on the lipids of amphibians.

Light damage in the rat retina: the effect of dietary deprivation of N-3 fatty acids on acute structural alterations

We investigated the effect of depleting membrane docosahexaenoic acid (DHA, 22:6n-3) content through dietary deprivation of n-3 fatty acids on the susceptibility of the photoreceptors and pigment epithelium cells to acute light-induced changes. Male Sprague-Dawley rats were raised throughout gestation, lactation and up to the age of 8 weeks on semi-purified diets containing either safflower oil (SFO, n-3 deficient diet) or soybean oil (SO) as the sole source of lipids. A third group was switched at weaning from safflower oil to soybean oil (SFO/SO). Rats were maintained on a 12 hr/12 hr light/dark cycle in which the light level at the front of the cages was 5-10 lx. Light damage was produced by exposing dark-adapted animals to diffuse white fluorescent light of 700-800 lx for 30 min followed by 90 min of darkness. In order to study recovery from light damage, additional groups of SFO and SO rats were returned to dim cyclic light for 27 hr following bright light exposure. DHA content in retinal phosphatidylethanolamine and phosphatidylcholine was 65-75% lower in rats fed SFO than in rats fed SO. The decrease was compensated for by an increase in 22:5n-6, the total content of polyunsaturated fatty acids (PUFA) being similar in both the SFO and SO groups. The SFO/SO rats had DHA levels similar to SO animals, but 22:5n-6 remained elevated resulting in a slightly higher level of total PUFA. Severe rod outer segment (ROS) membrane disruptions were seen following bright light exposure in rats on the SO and SFO/SO diets. The appearance of these disruptions did not change significantly during more than 24 hr in dim cyclic light. In contrast, there were virtually no acute ROS lesions in the SFO group. Furthermore, there was a strong light-elicited disk-shedding response in the SO rats but not in the other two groups. The pigment epithelium of the DHA deficient retinas showed a significantly greater accumulation of large lipid droplets in the dark-adapted state. Notably, whole retina rhodopsin levels were 15% higher in the SFO than in the SO group. These results indicate that depletion of retinal DHA reduces the susceptibility of the rod outer segments to acute light damage and at the same time may alter visual pigment photochemistry and other photoreceptor and pigment epithelium functions.

The rhodopsin content of the human eye

The content of rhodopsin in the human retina was determined using a radioimmunoassay which measures both rhodopsin and opsin. A value of 0.5 nmole of rod visual pigment per mg of extractable protein was obtained, corresponding to 3.94 +/- 0.17 (mean +/- SEM, n = 42) nmole per eye. This value is slightly higher than those previously reported which were based on spectral determinations measuring only native rhodopsin. No significant difference was seen in the population studied with respect to age or sex. Human rhodopsin was partially purified by affinity chromatography on concanavalin A, and some of its properties were studied. A comparison of the immunological reactivity of human, bovine, rat and chicken rhodopsin indicated similarity among the three mammalian species, while the chicken was some 50 fold less reactive.