Iodoacetic acid, but not sodium iodate, creates an inducible swine model of photoreceptor damage.

Our purpose was to find a method to create a large animal model of inducible photoreceptor damage. To this end, we tested in domestic swine the efficacy of two chemical toxins, known to create photoreceptor damage in other species: Iodoacetic Acid (IAA) and Sodium Iodate (NaIO(3)). Intravenous (IV) administration of NaIO(3) up to 90 mg/kg had no effect on retinal function and 110 mg/kg was lethal. IV administration of IAA (5-20 mg/kg) produced concentration-dependent changes in visual function as measured by full-field and multi-focal electroretinograms (ffERG and mfERG), and 30 mg/kg IAA was lethal. The IAA-induced effects measured at two weeks were stable through eight weeks post-injection, the last time point investigated. IAA at 7.5, 10, and 12 mg/kg produce a concentration-dependent reduction in both ffERG b-wave and mfERG N1-P1 amplitudes compared to baseline at all post-injection times. Comparisons of dark- and light-adapted ffERG b-wave amplitudes show a more significant loss of rod relative to cone function. The fundus of swine treated with ≥10 mg/kg IAA was abnormal with thinner retinal vessels and pale optic discs, and we found no evidence of bone spicule formation. Histological evaluations show concentration-dependent outer retinal damage that correlates with functional changes. We conclude that NaIO(3,) is not an effective toxin in swine. In contrast, IAA can be used to create a rapidly inducible, selective, stable and concentration-dependent model of photoreceptor damage in swine retina. Because of these attributes this large animal model of controlled photoreceptor damage should be useful in the investigation of treatments to replace damaged photoreceptors.

Anatomical evidence of photoreceptor degeneration induced by iodoacetic acid in the porcine eye.

Iodoacetic acid (IAA) induces photoreceptor (PR) degeneration in small animal models, however, eye size and anatomic differences detract from the usefulness of these models for studying retinal rescue strategies intended for humans. Porcine eyes are closer in size to human eyes and have a rich supply of rod and cones. This study investigated whether IAA also produced PR degeneration in the porcine retina, whether the damage was preferential for rods or cones, and whether IAA induced remodeling of the inner retina. Pigs were given a single i.v. injection of IAA and were euthanized 2-5 weeks later. Eyes were enucleated and immersed in fixative. Forty-six eyes were studied: Control (n = 13), and from pigs that had received the following IAA doses: 5.0 mg/kg (n = 7); 7.5 mg/kg (n = 10); 10.0 mg/kg (n = 6); 12.0 mg/kg (n = 6). Tissue was retrieved from four retinal locations: 8 mm and 2 mm above the dorsal margin of the optic disc, and 2 mm and 8 mm below the disc, and was processed for conventional histology, immunohistochemistry, and transmission electron microscopy. At 5.0 mg/kg IAA produced mild, variable cell loss, but remaining cells exhibited normal features. At doses above 5.0 mg/kg, a dose-dependent reduction was observed in the length of PR inner and outer segments, and in the number of PR nuclei. Specific labeling revealed a massive dropout of rod cell bodies with relative sparing of cone cell bodies, and electron microscopy revealed a reduction in the number of PR synaptic terminals. Mild dendritic retraction of rod bipolar cells and hypertrophy of Müller cell stalks was also observed, although the inner nuclear layer appeared intact. The porcine IAA model may be useful for developing and testing retinal rescue strategies for human diseases in which rods are more susceptible than cones, or are affected earlier in the disease process.

Long-term cellular and regional specificity of the photoreceptor toxin, iodoacetic acid (IAA), in the rabbit retina.

This study investigated the anatomical consequences of a photoreceptor toxin, iodoacetic acid (IAA), in the rabbit retina. Retinae were examined 2 weeks, 1, 3, and 6 months after systemic IAA injection. The retinae were processed using standard histological methods to assess the gross morphology and topographical distribution of damage, and by immunohistochemistry to examine specific cell populations in the retina. Degeneration was restricted to the photoreceptors and was most common in the ventral retina and visual streak. In damaged regions, the outer nuclear layer was reduced in thickness or eliminated entirely, with a concomitant loss of immunoreactivity for rhodopsin. However, the magnitude of the effect varied between animals with the same IAA dose and survival time, suggesting individual differences in the bioavailability of the toxin. In all eyes, the inner retina remained intact, as judged by the thickness of the inner nuclear layer, and by the pattern of immunoreactivity for protein kinase C-alpha (rod bipolar cells) and calbindin D-28 (horizontal cells). Müller cell stalks became immunoreactive for glial fibrillary acidic protein (GFAP) even in IAA-treated retinae that had no signs of cell loss, indicating a response of the retina to the toxin. However, no marked hypertrophy or proliferation of Müller cells was observed with either GFAP or vimentin immunohistochemistry. Thus the selective, long lasting damage to the photoreceptors produced by this toxin did not lead to a reorganization of the surviving cells, at least with survival as long as 6 months, in contrast to the remodeling of the inner retina that is observed in inherited retinal degenerations such as retinitis pigmentosa and retinal injuries such as retinal detachment.

Disrupted myelin and axon loss in the anterior commissure of the aged rhesus monkey.

This study assesses the effects of age on the composition of the anterior commissure of the rhesus monkey. The anterior commissures of nine young (5-10 years), five middle-aged (15-20 years), and eight old (25-35 years) monkeys were examined by light and electron microscopy. In all, 90-95% of the nerve fibers in the anterior commissure are myelinated. With age, the structure of the myelin sheaths of some nerve fibers is altered. Some of the axons also show signs of degeneration and this leads to a loss of nerve fibers. Thus, in young and the middle-aged monkeys the mean number of myelinated nerve fibers in the anterior commissure is 2.2 x 10(6), while in the old monkeys the mean is 1.2 x 10(6). Increasing age is correlated with a reduction in the number of myelinated nerve fibers in the anterior commissure, an increase in the frequency of structural alterations in myelin sheaths, and an increase in the frequency of occurrence of degenerating axons. However, the number of myelinated nerve fibers is the only variable that correlates with cognition: in monkeys 5-20 years of age the fewer the number of nerve fibers the poorer the cognitive performance, as measured by our Cognitive Impairment Index (CII). The most common neuroglial cells in the anterior commissure are oligodendrocytes. They account for 86% of all neuroglial cell profiles, while astrocytes account for 9%, and microglial cells for 5% of profiles. There is no apparent change with age in the total numbers of neuroglial cells, although as they age each of the neuroglial cell types acquires some inclusions in their cytoplasm. The data, together with those from previous studies, support the concept that in aging there is a ubiquitous loss of myelinated nerve fibers from the brain and that fiber loss is preceded by alterations in the structure of many of the myelin sheaths.

Effects of age on the glial cells in the rhesus monkey optic nerve.

The optic nerve is a circumscribed white matter tract consisting of myelinated nerve fibers and neuroglial cells. Previous work has shown that during normal aging in the rhesus monkey, many optic nerves lose some of their nerve fibers, and in all old optic nerves there are both myelin abnormalities and degenerating nerve fibers. The present study assesses how the neuroglial cell population of the optic nerve is affected by age. To address this question, optic nerves from young (4-10 years) and old (27-33 years) rhesus monkeys were examined by using both light and electron microscopy. It was found that with age the astrocytes, oligodendrocytes, and microglia all develop characteristic cytoplasmic inclusions. The astrocytes hypertrophy and fill space vacated by degenerated nerve fibers, and they often develop abundant glial filaments in their processes. Oligodendrocytes and microglial cells both become more numerous with age, and microglial cells often become engorged with phagocytosed debris. Some of the debris can be recognized as degenerating myelin, and in general, the greater the loss of nerve fibers, the more active the microglial cells become.