Chronic lead administration in neonatal rats: electron microscopy of the retina

Retinal toxicity of heavy metals and its involvement in retinal pathology

The objective of the present review is to discuss epidemiological evidence demonstrating the association between toxic metal (Cd, Pb, Hg, As, Sn, Ti, Tl) exposure and retinal pathology, along with the potential underlying molecular mechanisms. Epidemiological studies demonstrate that Cd, and to a lesser extent Pb exposure, are associated with age-related macular degeneration (AMD), while the existing evidence on the levels of these metals in patients with diabetic retinopathy is scarce. Epidemiological data on the association between other toxic metals and metalloids including mercury (Hg) and arsenic (As), are limited. Clinical reports and laboratory in vivo studies have shown structural alterations in different layers of retina following metal exposure. Examination of retina samples demonstrate that toxic metals can accumulate in the retina, and the rate of accumulation appears to increase with age. Experimental studies in vivo and in vitro studies in APRE-19 and D407 cells demonstrate that toxic metal exposure may cause retinal damage through oxidative stress, apoptosis, DNA damage, mitochondrial dysfunction, endoplasmic reticulum stress, impaired retinogenesis, and retinal inflammation. However, further epidemiological as well as laboratory studies are required for understanding the underlying molecular mechanisms and identifying of the potential therapeutic targets and estimation of the dose-response effects.

Alpha lipoic acid attenuates the long-term effects of lead exposure in retinal ischemic injury mouse model

Lead (Pb) exposure is reported to be unsafe for humans. There have been several studies documenting acute and chronic Pb toxicity on the organ systems. New studies suggest that early-life exposure to such environmental toxins may increase the susceptibility to late-onset degenerative disorders. We aimed to examine the long-term effects of early-life postnatal exposure of Pb on retinal degeneration. Pb exposure (200 ppm) was provided either at postnatal day 1 through lactation (early-life exposure) or at 7th week of age (adulthood exposure) directly through drinking water for 20 days. The Pb-treated mice were followed till 20 weeks of age. At 20th week, ischemia/reperfusion (I/R) injury was induced in these mice by pterygopalatine artery ligation. Further, alpha lipoic acid (ALA) was administered to examine its neuroprotective effects against retinal damage. Histological and molecular analysis revealed that Pb-treated mice had greater retinal damage after I/R injury as compared to untreated or ALA treated mice, suggesting that ALA protects the early-life Pb exposure and its consequent impact on later life. The elevated levels of glial derived neurotrophic factor (GDNF) and ciliary neurotrophic factor (CNTF) and reduced levels of glial fibrillary acidic protein (GFAP) upon ALA pre-treatment suggest that it probably exerts anti-inflammatory effects via upregulation of neurotrophic factors.

Distribution of perospirone to melanin in pigmented rats

Following the administration of [Carbonyl-14C]perospirone ([CO-14C]perospirone) and [isothiazolyl-3-14C]perospirone ([TH-14C]perospirone) to male-pigmented rats at a single oral dose of 10 mg/kg, serum radioactivity reached maxima of 499 ng eq./g and 446 ng eq./g, respectively, at 1 hour, then decreased rapidly to below 2 ng eq./g at 168 hours, and radioactivity in eyeballs reached maxima of 576 ng eq./g and 3449 ng eq./g, respectively, at 24 hours, then decreasing slowly to become 120 ng eq./g and 314 ng eq./g at 26 weeks. Apparent elimination half-lives of radioactivity in eyeballs were 197 and 126 days for [CO-14C]perospirone and [TH-14C]perospirone, respectively. With radioluminograms, radioactivity was detected specifically in the uvea, reaching maxima at 168 hours for [CO-14C]perospirone and at 24 hours for [TH-14C]perospirone. In addition to unchanged compound, many metabolites were found in serum and eyeballs, but those were also detectable in SD albino rats and cynomolgus monkeys. Among radioactive components in eyeballs after administration of [CO-14C]perospirone, ratios of ID-15036 and MX11 followed by unchanged compound and MX9 are high. After administration of [TH-14C]perospirone, ID-11614 accounted for nearly half of the radioactivity. Values decreased in line with radioactivity in eyeballs over time and 26 weeks only a few polar metabolites could be detected.

Binding of drugs to eye melanin is not predictive of ocular toxicity

Ocular melanin is found in the uveal tract and in the pigmented epithelial layer of the retina. Many structurally and pharmacologically unrelated drugs from different therapeutic classes bind to melanin. Examples include numerous drugs acting on the central nervous system, beta-blockers, beta-agonists, antimalarial drugs, sympathomimetic amines, and antibiotics. The critical factors are the acid/base status and the lipophilicity of the molecule. In all cases, there are no direct consequences of drug-melanin binding. Drug-related toxic effects on the retina described in humans and animals are unrelated to melanin binding: melanin binding and retinal toxicity are two separate entities, the latter being related to the intrinsic toxicity of the compound rather than its ability to bind. Chloroquine and phenothiazines are often used as examples of drugs with retinal toxicity linked to melanin binding. In both cases however, experimental data show that the toxic mechanism is unrelated to binding. Melanin binding has also been found to be protective against the ocular toxicity of some drugs. In conclusion, we believe that potential ocular toxicity of future drugs can be assessed adequately by conducting well-designed toxicology studies, and using nonpigmented rodents in addition to pigmented nonrodent species remains fully justified. Binding of drugs to eye melanin is not predictive of ocular toxicity.

Developmental lead exposure selectively alters the scotopic ERG component of dark and light adaptation and increases rod calcium content

Electrophysiological studies have established that lead exposure produces selective rod deficits. The present electroretinographic (ERG) and correlated calcium experiments examined whether low-level or moderate-level developmental lead exposure (peak blood lead of 19 and 59 micrograms/dl, respectively) altered dark adaptation and/or light adaptation. Developmental lead exposure produced long-term dose-response (1) decreases in the slope of the increment threshold function only at scotopic adapting backgrounds, (2) decreases only in the sensitivity of the rod phase of dark adaptation and (3) increases in the calcium content of rod outer segments which may partially mediate the ERG results. The relevance and applicability of these data to lead-exposed children have yet to be established.

Effects of chronic developmental lead exposure on monkey neuroanatomy: visual system

The effects of lead on specific areas of the visual system were examined in two groups of monkeys (Macaca fascicularis). The first group (N = 3) received 2000 micrograms Pb/kg/day from infancy onward, while the second group (N = 4) received 25 micrograms Pb/kg/day from birth onward. Monkeys were killed at approximately 6 years of age. Areas of the visual system, including optic nerve, lateral geniculate nucleus, and primary area V1 and one visual projection area V2, were examined by a combination of light and electron microscopy and Golgi impregnation. No effect of lead on optic nerve was identified, nor were there consistent differences in the lateral geniculate nucleus. Within areas V1 and V2, the neuronal volume density was significantly reduced in the high dose compared to the low dose group. Moreover, analysis of the dendritic arborization by Golgi analysis revealed a relative decrease in the number of arborizations among pyramidal neurons in both areas V1 and V2. These data suggest that lead exposure beginning during the early postnatal period may result in changes in cytoarchitecture in visual areas V1 and V2.

Rods are selectively altered by lead: II. Ultrastructure and quantitative histology

Electroretinographic and cyclic nucleotide metabolism studies have established that low-level lead exposure during early postnatal development results in long-term selective rod deficits. To determine whether there was a corresponding selective rod photoreceptor cell degeneration we examined retinas of adult rats exposed to low-level lead during development using light and electron microscopy. In all retinal regions, a rod but not cone cell degeneration was observed. Overall, 20% of the rod cells were lost. Moreover, two specific regional differences were found. Degeneration was much greater in the inferior (-25%) than superior (-15%) retina and greater in the posterior (-22%) than peripheral (-17%) retina. The latter pattern indicates a central-peripheral gradient of degeneration. Total retinal thickness decreased 15-20%, which reflects cell loss in the outer and inner nuclear layers. Ultrastructurally, the most obvious lead-induced alterations were swollen and disorganized rod outer segments and large accumulations of beta-glycogen particles in rod photoreceptor mitochondria. Glycogen accumulations were heaviest in rod inner segment mitochondria followed by rod axon and synaptic terminal mitochondria. Possible cellular mechanisms of action responsible for these lead-induced retinal alterations include an inhibition of retinal cyclic GMP phosphodiesterase and the resultant elevation of cyclic GMP, an inhibition of intermediary metabolism, and/or an alteration in calcium metabolism. In addition, the thinning of the inner nuclear layers could be due to transneuronal degeneration. As noted in our preceding paper, the first possibility has been demonstrated in rats similarly exposed to lead. These quantitative histological results, in combination with the ERG and biochemical results in the preceding paper, demonstrate that low-level lead exposure during early postnatal development produces long-term selective rod functional deficits and degeneration.

Rods are selectively altered by lead: I. Electrophysiology and biochemistry

In vitro studies have demonstrated that lead selectively and reversibly depresses the rod photoreceptor component of the electroretinogram (ERG). To determine if low-level lead exposure during early postnatal development produced long-term selective rod deficits, we examined rod and cone ERG functions and cyclic GMP and cyclic AMP metabolism in adult control and lead-exposed rats. A-wave and b-wave voltage-log intensity and latency-log intensity functions, generated from single-flash ERGs in fully dark-adapted rats, revealed that low-level lead exposure during early postnatal development caused a 23- and 18% decrease in maximum amplitude, a 1.0- and 0.5 log unit decrease in absolute sensitivity and a mean latency increase of 47- and 29%, respectively. Additional ERG experiments, using scotopically balanced stimuli and scotopic and photopic flicker fusion frequency functions, also demonstrated selective rod deficits. Cone ERGs, elicited by 30-Hz white flashes in the presence of a white background adapting light, were similar in control and lead-exposed rats. Lead exposure during early postnatal development caused cGMP levels in dark-adapted and light-adapted retinas to increase 40- and 25%, respectively, above controls whereas cyclic AMP levels remained unchanged. Light-activated cyclic GMP phosphodiesterase (cGMP-PDE) was inhibited 40% while guanylate cyclase activity was unchanged. The retinal lead concentration was 10(-6) M at the end of exposure (day 21) while at the time of ERG testing and biochemical analysis it was 10(-7) M. In vitro studies with adult control retinas incubated with 10(-9)-10(-4) M lead revealed a dose-response inhibition (10-40%) of cGMP-PDE between 10(-6)- and 10(-4) M lead and stimulation of guanylate cyclase (20-158%) only above 10(-4) M lead, indicating that cGMP-PDE is more sensitive to the direct effects of lead than the synthetic cGMP enzyme. These in vitro cyclic nucleotide metabolism results are similar to those we observed in vivo and both are consistent with the observed ERG changes. The selective rod-mediated amplitude, sensitivity and temporal deficits and the lack of effect on the cone ERGs clearly demonstrate that low-level lead exposure during early postnatal development causes a long-term selective disruption of rat rod photoreceptors. The relevance and applicability of these data to subclinical pediatric lead poisoning has yet to be established.

The effect of postnatal exposure to lead on the electroretinogram in young rats

The morphological and electrophysiological effects on the retina resulting from lead exposure were assessed in neonatal rats. The electroretinogram (ERG), light and electron microscopy and morphometry of the retina were evaluated at 9, 11 and 13 days of age. The lead concentration in the blood was on average between 276 and 311 micrograms/100 ml in the different age groups. The brain concentration of lead varied on average between 102 and 104 micrograms/100 g. A reduction in amplitude, a prolonged latency and an elevated threshold sensitivity of the a-wave was observed in the 11 days old lead-exposed pups. The 13 days old lead-fed rats showed depressed amplitudes and delayed peak times of both the a- and b-waves. The latency of the a-wave was also longer in the lead-treated 13 days old animals. No ERG was recorded at the age of 9 days in neither the lead-exposed or control animals. No significant change in the morphology of the retina was found. We propose that lead exposure seem to induce a change primarily in the electrophysiological function of the photoreceptors. There was no evidence for a direct retardation of the development of the retina by the administration of lead.

Studies on developmental alterations in the electroretinogram in rats after post-natal exposure to lead

The effects of post-natal exposure to lead on the development of the electroretinogram in rats were studied. Newborn rats were fed with lead acetate by gastric intubation and weight and age-matched control rats were given sodium acetate in a similar way. At 15 and 26 days of age the lead concentrations in blood were on average 298 micrograms/100 ml and 80 micrograms/100 ml, respectively. The brain concentrations of lead were on average 248 micrograms/100 mg in the 15-day-old lead-fed animals and 244 micrograms/100 mg in the 26-day-old ones. Lead produced a transient depression of the post-natal development of the electroretinogram. A decrease in the amplitudes of the a- and b-waves, as well as an increase in the peak times of these potentials were found in the 15-day-old animals. The ERGs of the 26-day-old animals did not differ from controls. There were no morphological changes of the retina.