Abnormalities of the electroretinogram and visual-evoked potential in vitamin E deficient rats

Vitamin E and neurological function

The clinical, neuropathological and electrophysiological evidence that vitamin E (alpha-tocopherol) is essential for normal neurological function will be reviewed. The possible reasons why neural tissues should be particularly affected by a deficiency of this fat-soluble vitamin and the mechanism(s) involved will be considered.

The effects of different levels of all-rac- and RRR-α-tocopheryl acetate (vitamin E) on visual function in rats

OBJECTIVE

A rat model of vitamin E (alpha-tocopherol) deficiency with similar "clinical", electrophysiological and neuropathological abnormalities to that seen in man was used to investigate the effects of various dietary intakes of synthetic (all-rac-) and natural (RRR-) alpha-tocopheryl acetate (alphaTA) on visual function.

METHODS

Longitudinal measurements of the electroretinogram (ERG) and visual evoked potentials (VEPs) were made monthly for 14 months in 9 groups of rats (n=12/group) receiving different amounts and types of alphaTA. The animals were then killed for biochemical analyses.

RESULTS

(1) The first significant abnormalities of both the ERG and VEP were found after 8 months of deficiency. (2) A diet containing 1.25 mg/kg of alphaTA provided marginal protection. (3) The biological activity of all-rac- was approximately 75% of RRR-alphaTA. (4) The concentration of free malondialdehyde (a measure of lipid peroxidation) was significantly increased in all tissues, including the eye, from deficient compared to control rats.

CONCLUSIONS

These results are consistent with alpha-tocopherol deficiency causing increased lipid peroxidation leading to abnormal visual function but could also be explained by more specific but undefined function(s) of alpha-tocopherol in the eye.

Vitamin E deficiency in dogs with retinal pigment epithelial dystrophy

The role of vitamin E deficiency in the development of retinal pigment epithelial dystrophy was investigated in 11 cocker spaniels and four other dogs. The concentration of alpha-tocopherol was measured by high performance liquid chromatography in plasma samples obtained from the affected dogs and from 28 ophthalmoscopically normal, healthy control dogs. The mean (sd) plasma alpha-tocopherol concentration in the normal dogs was 20.2 (7.1) microg/ml, compared with 1.14 (0.67) microg/ml in the 11 affected cocker spaniels. The difference between the two groups remained highly significant when the alpha-tocopherol concentrations were expressed relative to the concentrations of the plasma lipids cholesterol and triglycerides. Low plasma concentrations of alpha-tocopherol were observed in the four affected dogs of other breeds, but the finding was not so consistent. The plasma lipid concentrations were normal in the affected dogs. The deficiency of alpha-tocopherol in the affected dogs appeared to be primary, because there was no clinical, biochemical or pathological evidence of underlying disease, or any indication of a dietary deficiency which might have contributed to the low concentrations of alpha-tocopherol.

Friedreich-like ataxia with retinitis pigmentosa caused by the His101Gln mutation of the alpha-tocopherol transfer protein gene

The alpha-tocopherol transfer protein (alpha-TTP) is a cytosolic liver protein that is presumed to function in the intracellular transport of alpha-tocopherol, the most biologically active form of vitamin E. We studied 4 unrelated patients with autosomal recessive Friedreich-like ataxia who had isolated vitamin E deficiency. A point mutation was identified in all of them at position 101 of the gene for alpha-TTP, where histidine (CAT) was replaced with glutamine (CAG). Three of the 4 patients developed retinitis pigmentosa subsequent to the onset of ataxia. Neurological symptoms included ataxia, dysarthria, hyporeflexia, and decreased proprioceptive and vibratory sensations. Electrophysiological and pathological examinations showed that the cardinal sites affected were the central axons of dorsal root ganglion cells and the retina, with minor involvement of the peripheral sensory nerve, optic nerve, and pyramidal tract. The vitamin E tolerance test performed showed that the absorption of vitamin E was normal but that its decrease from the serum was accelerated. Oral administration of vitamin E appeared to halt the progression of visual and neurological symptoms. We propose a new treatable syndrome of Friedreich-like ataxia and retinitis pigmentosa caused by a defect in the alpha-TTP gene.

Endogenous ascorbate regenerates vitamin E in the retina directly and in combination with exogenous dihydrolipoic acid

Vitamin E (alpha-tocopherol) is the major lipid-soluble antioxidant of retinal membranes whose deficiency causes retinal degeneration. Its antioxidant function is realized via scavenging peroxyl radicals as a result of which phenoxyl radicals of alpha-tocopherol are formed. Our hypothesis is that alpha-tocopherol phenoxyl radicals can be reduced by endogenous reductants in the retina, providing for alpha-tocopherol recycling. The results of this study demonstrate for the first time that: (i) endogenous ascorbate (vitamin C) in retinal homogenates and in rod outer segments is able to protect endogenous alpha-tocopherol against oxidation induced by UV-irradiation by reducing the phenoxyl radical of alpha-tocopherol, (ii) in the absence of ascorbate, neither endogenous nor exogenously added glutathione (GSH) is efficient in protecting alpha-tocopherol against oxidation; (iii) GSH does not substantially enhance the protective effect of ascorbate against alpha-tocopherol oxidation; (iv) exogenous dihydrolipoic acid (DHLA), although inefficient in direct reduction of the alpha-tocopherol phenoxyl radical, is able to enhance the protective effect of ascorbate by regenerating it from dehydroascorbate. Thus, regeneration of alpha-tocopherol from its phenoxyl radical can enhance its antioxidant effectiveness in the retina. The recycling of alpha-tocopherol opens new avenues for pharmacological approaches to enhance antioxidants of the retina.

Nigrostriatal function in vitamin E deficiency: clinical, experimental, and positron emission tomographic studies

Four patients with vitamin E deficiency and sensory ataxia were studied using [18F]dopa positron emission tomography. The 2 most disabled patients, who had severe and prolonged vitamin E deficiency due to abetalipoproteinemia, showed reduced [18F]dopa uptake in both putamen and caudate. Putaminal uptake was in a similar range to that seen in Parkinson's disease. Studies of [3H]mazindol binding in the striatum of vitamin E--deficient rats indicated a reduced number of dopamine terminals, which was most severe in ventrolateral striatum. These observations suggest that severe and prolonged vitamin E deficiency results in loss of nigrostriatal nerve terminals, and support the hypothesis that oxidative stress may contribute to the etiology of Parkinson's disease.

Alpha-tocopherol concentrations of the nervous system and selected tissues of adult dogs fed three levels of vitamin E

The effects of dietary vitamin E levels on tissue alpha-tocopherol (alpha-T) concentrations in different parts of the nervous system are largely unknown. Therefore, we measured the alpha-T contents of nervous and other tissues obtained from beagle dogs fed for two years a vitamin E-deficient diet (-E, 0.05 +/- 0.02 mg vitamin E/kg diet, n = 2), a vitamin E-supplemented diet (+E, 114 +/- 14 mg/kg, n = 2), or a standard chow diet (En, 74 +/- 6 mg/kg, n = 3). Brain regions and spinal cords of +E dogs contained about double the alpha-T concentrations of En dogs, and about 10-fold those of -E dogs. The various brain regions of -E dogs, compared with En dogs, retained 12-18% of the alpha-T concentrations, with the exception of the caudal colliculus, which retained 48%. Peripheral nerve alpha-T concentrations in +E dogs (67 ng/mg wet weight) were nearly 5-fold higher than in En dogs (13.4 +/- 5.9 ng/mg) and 80-fold higher than in -E dogs (0.8 ng/mg). Within each dietary group, the lowest alpha-T concentrations in the central nervous system (CNS) were in the spinal cord. Peripheral nerves were the most susceptible to vitamin E repletion or depletion: in +E dogs, nerves contained higher concentrations of alpha-T than most brain regions; in En dogs, they contained similar concentrations; but in -E dogs, they contained less alpha-T than most brain regions. Muscles and other tissues of -E dogs retained from 1 to 10% of En values.(ABSTRACT TRUNCATED AT 250 WORDS)