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

Presumed retinal lead poisoning: a case report

PURPOSE

To describe a case of presumed retinal lead poisoning.

METHODS

Clinical examination, optical coherence tomography, fundus autofluorescence, fluorescein angiography, and electroretinography were used to study a 42-year-old male with the complaint of bilateral reduced vision following systemic lead poisoning.

RESULTS

The fundus examination showed venous tortuosity, as well as macular atrophy, and pigmentary changes in his both eyes. Optical coherence tomography revealed retinal thinning, outer retinal and retinal pigment epithelium atrophy, as well as foveal schitic changes. Blue autofluorescence showed moderately hypoautofluorescence in peripapillary area of both eyes. Fluorescein angiogram showed a leopard-like pattern of hypo- and hyperfluorescence in the posterior pole. Electroretinogram showed a moderate reduction in photopic and scotopic responses.

CONCLUSIONS

The most probable diagnosis of this case is early onset retinal lead poisoning.

Gene-chemical interactions in the developing mammalian nervous system: Effects on proliferation, neurogenesis and differentiation

The orderly formation of the nervous system requires a multitude of complex, integrated and simultaneously occurring processes. Neural progenitor cells expand through proliferation, commit to different cell fates, exit the cell cycle, generate different neuronal and glial cell types, and new neurons migrate to specified areas and establish synaptic connections. Gestational and perinatal exposure to environmental toxicants, pharmacological agents and drugs of abuse produce immediate, persistent or late-onset alterations in behavioral, cognitive, sensory and/or motor functions. These alterations reflect the disruption of the underlying processes of CNS formation and development. To determine the neurotoxic mechanisms that underlie these deficits it is necessary to analyze and dissect the complex molecular processes that occur during the proliferation, neurogenesis and differentiation of cells. This symposium will provide a framework for understanding the orchestrated events of neurogenesis, the coordination of proliferation and cell fate specification by selected genes, and the effects of well-known neurotoxicants on neurogenesis in the retina, hippocampus and cerebellum. These three tissues share common developmental profiles, mediate diverse neuronal activities and function, and thus provide important substrates for analysis. This paper summarizes four invited talks that were presented at the 12th International Neurotoxicology Association meeting held in Jerusalem, Israel during the summer of 2009. Donald A. Fox described the structural and functional alterations following low-level gestational lead exposure in children and rodents that produced a supernormal electroretinogram and selective increases in neurogenesis and cell proliferation of late-born retinal neurons (rod photoreceptors and bipolar cells), but not Müller glia cells, in mice. Lisa Opanashuk discussed how dioxin [TCDD] binding to the arylhydrocarbon receptor [AhR], a transcription factor that regulates xenobiotic metabolizing enzymes and growth factors, increased granule cell formation and apoptosis in the developing mouse cerebellum. Alex Zharkovsky described how postnatal early postnatal lead exposure decreased cell proliferation, neurogenesis and gene expression in the dentate gyrus of the adult hippocampus and its resultant behavioral effects. Bernard Weiss illustrated how environmental endocrine disruptors produced age- and sex-dependent alterations in synaptogenesis and cognitive behavior.

Low-level human equivalent gestational lead exposure produces supernormal scotopic electroretinograms, increased retinal neurogenesis, and decreased retinal dopamine utilization in rats

BACKGROUND

Postnatal lead exposure in children and animals produces alterations in the visual system primarily characterized by decreases in the rod-mediated (scotopic) electroretinogram (ERG) amplitude (subnormality). In contrast, low-level gestational Pb exposure (GLE) increases the amplitude of scotopic ERGs in children (supernormality).

OBJECTIVES

The goal of this study was to establish a rat model of human equivalent GLE and to determine dose-response effects on scotopic ERGs and on retinal morphology, biochemistry, and dopamine metabolism in adult offspring.

METHODS

We exposed female Long-Evans hooded rats to water containing 0, 27 (low), 55 (moderate), or 109 (high) ppm of Pb beginning 2 weeks before mating, throughout gestation, and until postnatal day (PND) 10. We measured maternal and litter indices, blood Pb concentrations (BPb), retinal Pb concentrations, zinc concentrations, and body weights. On PND90, we performed the retinal experiments.

RESULTS

Peak BPb concentrations were < 1, 12, 24, and 46 microg/dL in control, low-, moderate- and high-level GLE groups, respectively, at PNDs 0-10. ERG supernormality and an increased rod photoreceptor and rod bipolar cell neurogenesis occurred with low- and moderate-level GLE. In contrast, high-level GLE produced ERG subnormality, rod cell loss, and decreased retinal Zn levels. GLE produced dose-dependent decreases in dopamine and its utilization.

CONCLUSIONS

Low- and moderate-level GLE produced persistent scotopic ERG supernormality due to an increased neurogenesis of cells in the rod signaling pathway and/or decreased dopamine utilization, whereas high-level GLE produced rod-selective toxicity characterized by ERG subnormality. The ERG is a differential and noninvasive biomarker of GLE. The inverted U-shaped dose-response curves reveal the sensitivity and vulnerability of the developing retina to GLE.

Nerve conduction, visual evoked responses and electroretinography in tunnel workers previously exposed to acrylamide and N-methylolacrylamide containing grouting agents

The study examines possible persisting effects on the peripheral nervous system and visual system in tunnel workers previously exposed to acrylamide and N-methylolacrylamide during grouting work. We compared neurophysiological function in 44 tunnel workers previously exposed during grouting operations (2-10 years post exposure), with 49 tunnel workers with no history of exposure to acrylamide. Nerve conduction velocities (NCV), distal delay, F-response and amplitude in median and ulnar nerves of the right arm, peroneal, sural and tibial nerves of the right leg, visual evoked response (VER) and electroretinography (ERG) were measured. VER and ERG were also performed in 24 subjects more recently exposed to acrylamide grout (16 months post exposure). Exposure to acrylamide containing grouts was assessed by questionnaires. A statistically significant reduction in the mean sensory NCV of the sural nerve (p=0.005), as well as a non-significant reduction of sural amplitude was found in the previously exposed group compared to the control group. VER latencies to the onset of the occipital potential (N75) were prolonged in both exposed groups compared to the control group (p<0.05). ERG 30 Hz flicker amplitude was reduced in the recently exposed group compared to the referents (p<0.05). The results indicate slight subclinical, but persistent toxic effects in the sural nerve and the visual system in tunnel workers exposed to N-methylolacrylamide and acrylamide during grouting operations.

Homology of assessment of visual function in human and animal models

To connect animal models with human neurobehavioral evaluations, it is necessary to understand the level of homology present between tests administered across species. This paper identifies four different levels of homology of assessment based on identity of measurement, function, and underlying neural substrate. These are discussed using detailed examples from toxicology of the visual system, with additional examples from tests of motor and cognitive function. This should provide a framework for considering both animal to human extrapolation and human to animal extrapolation, that is, how to import human experimental epidemiology findings into the lab for further work investigating mechanisms of toxicity. Designing neurobehavioral or sensory evaluations that permit easier extrapolation between human and animal models is necessary if we are to develop testing strategies that take advantage of mechanistic information at whole animal, in vitro, proteonomic, or genomic levels.

Postnatal development of flicker sensitivity in guinea pigs

BACKGROUND: The retinal response to flickering stimuli (steady state ERG) recruits many retinal elements and is a sensitive indicator of early retinal dysfunction. This study reports the post-natal maturation of the steady state ERG response in guinea pigs. METHODS: The steady state ERG response to flickering stimuli (0.6 to 20 Hz) was recorded from dark adapted (more than 12 hrs) English Shorthair guinea pigs (n = 7) using flashes that produced rod and cone dominated responses. Temporal sensitivity functions and critical fusion frequencies (CFF) were derived over a range of ages from postnatal day (PND) 1 to 45. RESULTS: Guinea pig rod and cone temporal sensitivity functions show shape characteristics and CFF similar to humans. Furthermore, the post-natal development of the guinea pig temporal characteristics is also similar to that of humans - they are present at birth and mature rapidly post-natally. The time-course of CFF maturation is similar for rod and cone mediated responses. CONCLUSIONS: These data show that the temporal response and its maturation in the guinea pig retina is similar to that in humans. Therefore, we propose that the guinea pig is a particularly useful animal model to study retinal disease in early childhood.

Lead-induced alterations in retinal cGMP phosphodiesterase trigger calcium overload, mitochondrial dysfunction and rod photoreceptor apoptosis

Lead exposure results in the selective apoptotic loss of rods and bipolar cells. During and following developmental lead exposure rod/retinal cGMP phosphodiesterase expression and activity are delayed in onset and decreased, [Ca2+] is elevated, and mitochondrial ATP synthesis is decreased. In vitro studies, using retinas incubated in Ca2+ and/or Pb2+, demonstrate that rods selectively die by apoptosis, retinal mitochondrial ATP synthesis is decreased, mitochondrial cytochrome c is released and caspase activity is increased. These results suggest that lead-induced rod and bipolar cell apoptosis is triggered by Ca2+ and Pb2+ overload due to altered cGMP phosphodiesterase activity and that mitochondrial alterations play a central role in this process.

Effect of a cGMP-specific phosphodiesterase inhibitor on retinal function

Multiple forms of phosphodiesterase have been reported in many tissues. Phosphodiesterase 6, a cGMP-specific phosphodiesterase, is described as a photoreceptor cell-specific phosphodiesterase. Phosphodiesterase 6 is known to play a crucial role in visual function. A novel phosphodiesterase inhibitor, GF248 (5["(propoxy),7'(4-morpholino)-phenacyl],[1-methyl-3 propyl]pyrazolo[4,3d]pyrimidin-7-one), has been described to be a very potent cGMP-specific phosphodiesterase inhibitor. In the present study, we compared the potency of GF248 and other known cGMP-specific phosphodiesterase inhibitors on phosphodiesterase 5 and phosphodiesterase 6. GF248 displayed an IC50 of 2 and 5 nM for phosphodiesterase 5 and phosphodiesterase 6, respectively. Thereafter, we assessed the effect of GF248 on retinal function, using an ex vivo model of isolated retina electroretinogram recording. Exposure of retina to GF248 resulted in a dose-dependent decrease in electroretinogram amplitude (PIII and b-waves), with no marked modification of PIII and b-wave implicit time. Among other phosphodiesterase inhibitors, DMPPO (1,3-dimethyl-6-(2-propoxy-5-methanesulfonylamidophenyl)pyrazol ol[3,4d]-pyrimidin-4-(5H)-one) and dipyridamole, cGMP-specific phosphodiesterase inhibitors, and IBMQ (1-isobutyl-3-methylimidazol[1,5a]quinoxalin-4-(5H)one), a nonselective phosphodiesterase inhibitor, altered retinal function but less potently than GF248, consistent with their in vitro phosphodiesterase 6 inhibition. Phosphodiesterase 3- and phosphodiesterase 4-selective inhibitors, cilostamide and rolipram, respectively, did not affect retinal function at 10 micromol l(-1). Our conclusion from these data is that GF248, a potent phosphodiesterase 6 inhibitor, could interfere with visual transduction by cGMP accumulation.

Lead exposure alters the drug metabolic activity and the homeostasis of essential metal ions in the lenticular system of the rat

Potential lead exposure to the eyes as a result of the use of traditional cosmetic Kohl in Asia, Africa and the Middle East has been a subject of recent debate to the scientific community. In continuation of our earlier work we therefore examine in the present study, the drug metabolic activity and the homeostasis of essential metal ions in the lenticular system of adult rats exposed to long term low level lead (lead acetate 0.1% w/v). The results of our investigation demonstrate that long term low level lead exposure impaired the phase I & phase II metabolic activity of the lenticular system when assessed by aminopyrine demethylase, benzo[a]pyrene hydroxylase, aniline hydroxylase and UDP glucuronyl transferase (UDPGT), glutathione S-transferase (GST), respectively. A more pronounced decrease (55%) in GST was noticed compared to UDPGT, aminopyrene demethylase, benzo[a]-pyrene hydroxylase and aniline hydroxylase (20-30%). Increased lead concentration in the lenticular system of the rats as monitored by atomic absorption spectroscopy resulted in a significant decrease (15-35%) in the levels of Ca, Cu, Zn and Fe, along with a progressive loss in body weight. Respective increase in blood lead level was also monitored parallel to increase in lenticular lead concentration at different time points in lead treated rats. The present investigation, therefore, demonstrates that long term low level lead exposure to rats results in a profound impairment in the homeostasis of essential metal ions, lenticular drug metabolizing enzymatic activity and significant loss in body weight when compared to untreated control rats. Whether such a decrease in these functions reflects an inhibition of protein synthesis at transcriptional/post transcriptional levels or gene regulation at molecular level remains to be established.

Molecular mechanism of the lead-induced inhibition of rod cGMP phosphodiesterase

Retinal cGMP hydrolysis is inhibited following developmental and in vitro lead exposure. To determine whether Pb2+ directly inhibits the rod-specific cGMP phosphodiesterase (PDE) and to examine the kinetic mechanism of this inhibition, purified bovine rod cGMP PDE was assayed at varying [cGMP], [Mg2+] and [Pb2+]. Increasing [cGMP] or [Mg2+] shifted the Pb2+ curves leftward or rightward revealing increased or decreased potency of Pb2+ (nM to pM range), respectively. A 1/velocity vs. 1/mg2+ plot revealed that picomolar [Pb2+] competitively inhibited PDE relative to millimolar [Mg2+]. These novel findings reveal that Pb2+ binds at the Mg2+ site but with 4-6 log units higher affinity, thus preventing cGMP hydrolysis. These results may have implications for other enzymes using Mg2+ as a co-factor and suggest that Mg2+ may be useful for reversing the PDE inhibition by Pb2+.

Oxygen consumption in the rat outer and inner retina: light- and pharmacologically-induced inhibition

Biochemical, physiological and histological data have established that 55-65% of retinal mitochondria are located in the photoreceptor inner segments and suggested that photoreceptors have at least a two-fold greater oxygen consumption (QO2) than the remaining inner retina. QO2 in isolated whole rat retina (QWR), outer retina (QOR) and inner retina (QIR) was measured during dark and rod-saturating light adaptation. The effects of function-specific chemical agents on QWR, QOR and QIR during dark and light adaptation were determined. In addition, the oxidation-reduction (redox) potential of cytochrome a3 of whole, outer and inner retina was measured during dark and light adaptation. During dark adaptation, the mean QWR was 1.62 mumol O2 (mg dry wt)-1 hr-1 and whole retinal level of reduced cytochrome a3 was 19%. They decreased by 24% and 37% during light adaptation, respectively. To determine QOR and QIR during dark and light adaptation, the outer retina was pharmacologically-isolated from inner retina using L-2-amino-4-phosphonobutyric acid plus kynurenic acid (APB/Kyn). Experiments in the presence or absence of APB/Kyn revealed that: (i) QOR, but not QIR, of the dark-adapted retina was decreased 37% during light adaptation, (ii) the outer and inner retina consumed 65% and 35% of the QWR during dark adaptation, respectively, and 54% and 46% of the QWR during light adaptation, respectively, (iii) the level of reduced retinal cytochrome a3 in the outer, but not inner, retina was decreased 34% during light adaptation, (iv) during light adaptation, the rate of QO2 was equal in the outer and inner retina, and (v) the effects of APB/Kyn were reversible. These results establish that the mean rate of QIR and retinal cytochrome a3 are unchanged during dark or light adaptation. In addition, they suggest that QOR:QIR in the rat may be modeled using a 65%:35% model during DA and a 55%:45% model during LA. All the function-specific agents--IBMX, lead, diltiazem, ouabain, CO2+ plus Mg2+ and verapamil--significantly decreased QWR during dark and light adaptation. A more detailed analysis revealed that IBMX and lead each selectively reduced (> or = 90%) QOR during dark adaptation whereas CO2+ plus Mg2+ and verapamil each selectively reduced (> or = 93%) QIR during dark and light adaptation. These results are consistent with the known pharmacological sites and mechanisms of these agents. Additional experiments determined that the IBMX- and lead-induced inhibition of QOR during dark adaptation resulted, either wholly or partially, from the influx of extracellular Ca2+. During dark adaptation in Ca(2+)-free medium: (i) QWR and QOR increased while QIR was unchanged, (ii) QOR was not decreased in the presence of IBMX and (iii) QOR was only partially decreased in the presence of lead.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of low-level lead on retinal ganglion sustained and transient cells in developing rats

Neonatal rats were exposed to lead from parturition to weaning via the milk of dams drinking 0.2% lead acetate solutions. The alterations in the excitability and temporal response properties of retinal ganglion cells in adult rats (90 days) following developmental lead exposure were studied. The results of this investigation demonstrated that the lead exposure in neonatal rats caused an increase in excitability, and a decrease in optimal temporal frequency, bandwidth at half amplitude, temporal resolution, and response phase of the retinal ganglion cells in adult rats. Compared with the sustained cells, the transient cells had a much greater alteration in excitability and temporal response properties.

Retinal ischemia induced by occlusion of both common carotid arteries in rats as demonstrated by electroretinography

In 2 models of reduced cerebral blood flow-permanent occlusion of the vertebral arteries plus transient occlusion of the common carotid arteries (4VO) and transient clamping of the common carotid arteries (BCCA)-the acute effects on the electrical function of the retina were monitored by recording the photopic electroretinogram. During both 4VO and BCCA the amplitude of the b-wave was reduced. Within 30 min of reperfusion after 4VO and after BCCA the b-wave had fully recovered. In contrast, the a-wave was not affected by either treatment. The data suggest that occlusion of common carotid arteries leads to retinal ischemia and might represent a useful model of amaurosis fugax.

Direct zinc binding to purified rhodopsin and disc membranes

Using the radionuclide 65Zn, we have demonstrated the direct binding of zinc to purified rhodopsin. 65Zn is eluted with detergent-solubilized rhodopsin from concanavalin A columns and remains bound to the visual pigment through a subsequent gel-filtration step. Zinc binding to purified disc membranes is highly specific and, of the ions tested, copper is the best competitor. Equilibrium-dialysis experiments indicate that zinc binding to detergent-solubilized forms of rhodopsin may increase on bleaching the photopigment. These results may have important implications for studies that indicate that zinc plays a role in retinal degeneration and normal photoreceptor physiology.

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.

Electroretinogram b-wave implicit time and b/a wave ratio as a function of intensity in central retinal vein occlusion

The ability of electroretinogram (ERG) b-wave implicit time and b/a wave ratio to predict iris neovascular response was analyzed as a function of stimulus intensity over a 3.6 log unit intensity range in 39 patients with central retinal vein occlusion (CRVO). Predictive power for CRVO patients was evaluated using ROC area at intensities of 1.23, 1.83, 2.43, and 3.03 effective log quanta/rod, where reliable data for both parameters were obtainable from most patients. The relative predictive power of b-wave implicit time and b/a wave ratio were shown to vary with stimulus intensity. The predictive power of b-wave implicit time, as measured by ROC area, declined to below significance at high intensity (above 1.83 log quanta/rod), while b/a wave ratio performed best at middle intensities (1.83 and 2.43 log quanta/rod) and not as well at high and low intensities. Further analysis of statistical behavior of both ERG parameters was obtained from the t statistic. Insight into the mechanism influencing predictive power of b-wave implicit time was derived from measurements on normal adults and CRVO patients with response data taken at high intensities. These results suggest that an optimal stimulus intensity range can be found for these ERG parameters in the evaluation of CRVO.

The comparative developmental neurotoxicity of lead in humans and animals

The effects of lead on neurobehavioral development have been extensively investigated in humans as well as animals. This valuable lode of research findings offers a basis for comparing the developmental neurobehavioral toxicity of lead across species and for assessing the validity of animal models of developmental neurotoxicity. Comparisons of human and animal findings suggest that the greatest qualitative similarities involve relatively complex behavioral processes such as cognition and learning. Quantitative comparisons based on dose-response relationships for these endpoints are difficult to make because the relationships are sometimes nonmonotonic (U-shaped) and because blood lead levels may not be directly comparable between species. However, the lowest levels of exposure at which developmental neurobehavioral effects have been observed are similar: 10-15 micrograms/dl in children, less than 15 micrograms/dl in primates, and less than 20 micrograms/dl in rodents. Although the convergence between animal and human findings for other neurobehavioral endpoints is not as striking, sensory-evoked potentials and communicative processes offer two promising areas for continued investigation and cross-species comparison.

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.

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)

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.