The effects of undernutrition during early life on the rat optic nerve fibre number and size-frequency distribution

Interrelationships of undernutrition and neurotoxicity: food for thought and research attention

The neurotoxic actions of chemical agents on humans and animals are usually studied with little consideration of the subject's nutritional status. States of protein-calorie, vitamin and/or mineral undernutrition are associated with a range of neurodevelopmental, neurological and psychiatric disorders, commonly with involvement of both the central and the peripheral nervous system. Undernutrition can modify risk for certain chemical-induced neurologic diseases, and in some cases undernutrition may be a prerequisite for neurotoxicity to surface. In addition, neurologic disease associated with undernutrition or neurotoxicity may show similarities in clinical and neuropathological expression, especially in the peripheral nervous system. The combined effects of undernutrition and chemical neurotoxicity are most relevant to people with low incomes who experience chronic hunger, parasitism and infectious disease, monotonous diets of plants with neurotoxic potential (notably cassava), environmental pollution from rapid industrial development, chronic alcohol abuse, or prolonged treatment with certain therapeutic drugs. Undernutrition alone or in combination with chemical exposure is also important in high-income societies in the setting of drug and alcohol abuse, old age, food faddism, post-bariatric surgery, and drug treatment for certain medical conditions, including cancer and tuberculosis. The nutritional demands of pregnancy and lactation increase the risk of fetal and infant undernutrition and chemical interactions therewith.

Comparative anatomy of the optic nerve head and inner retina in non-primate animal models used for glaucoma research

To judge the information of experimental settings in relation to the human situation, it is crucial to be aware of morphological differences and peculiarities in the species studied. Related to glaucoma, the most important structures of the posterior eye segment are the optic nerve head including the lamina cribrosa, and the inner retinal layers. The review highlights the differences of the lamina cribrosa and its vascular supply, the prelaminar optic nerve head, and the retinal ganglion cell layer in the most widely used animal models for glaucoma research, including mouse, rat, rabbit, pig, dog, cat, chicken, and quail. Although all species show some differences to the human situation, the rabbit seems to be the most problematic animal for glaucoma research.

Myelination changes in the rat optic nerve after prenatal exposure to methamphetamine

The use of psychostimulants during adolescence and early adult life has increased in recent years. It is known that these substances affect the sensory systems, and the optic nerve has been shown to be a target tissue. This work was conducted to evaluate the effects of prenatal exposure to methamphetamine (MA) on the developmental pattern of the rat optic nerve. Pregnant female rats were given 5 mg/kg body weight/day MA, s.c., in 0.9% saline from gestational days 8 to 22. The control group was injected with an isovolumetric dose of 0.9% saline. Animal model parameters, such as gestational body weight evolution, food intake and pups parameters were registered. The offspring were sacrificed at postnatal days (PND) 7, 14 and 21. Morphometric analyses were performed at light and electron microscopic levels on optic nerve cross sections; parameters measured included optic nerve diameter and area, axonal density, total number of axons and myelin thickness. Myelin basic protein (MBP) was measured by western blotting in optic nerve samples at PND14 and PND21. The animal model parameters, such as maternal and pup weight, showed no significant differences between MA and control groups. Optic nerve diameter was smaller at PND7 in the male MA group and in both male and female MA groups at PND21. The mean cross-sectional area was smaller at PND14 in the male MA group and in both male and female groups at PND21. The total number of myelinated axons did not vary between groups at any of the studied ages. The myelin thickness of the axons in MA-treated females was thinner when compared with the respective control group at PND21. No other differences were found concerning myelin thickness. There was a reduction of MBP protein expression in MA-injected females at PND14 and PND21. The combined results suggest that prenatal exposure to MA affects the myelination process.

Age related optic nerve axonal loss in adult Brown Norway rats

The effect of age on the number and morphology of optic nerve axons in adult Brown Norway rats (5-31 months old) (n=29) was examined using transmission electron microscopy (TEM). By manually counting every axon in areas representing 60% of the optic nerve cross-section, we found a significant negative correlation between age and axon count (R(2)=0.18, P<0.05). However, when the oldest animals were omitted, the relationship was no longer statistically significant. Simultaneously, the proportion of spontaneously degenerating axons increased at an exponential rate (R(2)=0.79, P<0.05), with significantly more degeneration in the 31-month group than in 5-month-old animals (ANOVA, P<0.05). This study demonstrates, using quantitative TEM methods, that optic nerve axonal numbers are relatively constant throughout the majority of the adult life of the Brown Norway rat, an increasingly popular strain for glaucoma research. Total axonal loss with aging is substantially less than that reported for other strains. The reduction in axonal numbers and the rate of axonal degeneration do not appear significantly altered until the last few months of life, failing to support some studies that have concluded that optic nerve axon loss in adult rats is linear. However, they do agree with other studies in the rat, and a similar study performed in non-human primate eyes, that concluded that aging changes in the optic nerve and retina follow a complex pattern. Therefore, the impact of animal age must be considered when modeling the course and pathophysiology of experimental glaucomatous optic nerve damage in rats.

Exposure of rats to a high but not low dose of ethanol during early postnatal life increases the rate of loss of optic nerve axons and decreases the rate of myelination

Visual system abnormalities are commonly encountered in the fetal alcohol syndrome although the level of exposure at which they become manifest is uncertain. In this study we have examined the effects of either low (ETLD) or high dose (ETHD) ethanol, given between postnatal days 4-9, on the axons of the rat optic nerve. Rats were exposed to ethanol vapour in a special chamber for a period of 3 h per day during the treatment period. The blood alcohol concentration in the ETLD animals averaged approximately 171 mg/dl and in the ETHD animals approximately 430 mg/dl at the end of the treatment on any given day. Groups of 10 and 30-d-old mother-reared control (MRC), separation control (SC), ETLD and ETHD rats were anaesthetised with an intraperitoneal injection of ketamine and xylazine, and killed by intracardiac perfusion with phosphate-buffered glutaraldehyde. In the 10-d-old rat optic nerves there was a total of approximately 145,000-165,000 axons in MRC, SC and ETLD animals. About 4% of these fibres were myelinated. The differences between these groups were not statistically significant. However, the 10-d-old ETHD animals had only about 75,000 optic nerve axons (P < 0.05) of which about 2.8 % were myelinated. By 30 d of age there was a total of between 75,000-90,000 optic nerve axons, irrespective of the group examined. The proportion of axons which were myelinated at this age was still significantly lower (P < 0.001) in the ETHD animals (approximately 77 %) than in the other groups (about 98 %). It is concluded that the normal stages of development and maturation of the rat optic nerve axons, as assessed in this study, can be severely compromised by exposure to a relatively high (but not low) dose of ethanol between postnatal d 4 and 9.

Stereology of nerve cross sections

Quantitative morphological data from nerve cross sections are of great value in experimental, developmental, and pathological studies. Efficient unbiased stereological techniques to estimate the total numbers and absolute size distributions of both myelinated and unmyelinated axons from a small sample of axons are described and evaluated. Axon numbers are estimated with the fractionator technique. Axon areas are estimated with the 2D-nucleator and the point counting technique for myelinated and unmyelinated axons, respectively. The axon perimeter is estimated by use of a line grid, and the myelin sheath thickness by direct orthogonal measurements in uniform, random locations. The empirically evaluated variance of the number estimates of myelinated axons was approximately approximately 0.05 when approximately 200 axons were counted. The empirical variance of area estimates obtained with a four-way 2D-nucleator was approximately 0.10 and contributed less than 0.5% to the observed total variance of axon areas within an animal. A light microscope modified for stereology with computer assisted stereological test systems and a motorised stage facilitate the acquisition of data for the myelinated axons, which is of primary interest in most studies. The unmyelinated axons are evaluated at the more time consuming electron microscopical level.

Demyelination in the central nervous system mediated by an anti-oligodendrocyte antibody

The factors responsible for the major demyelinating disease of the central nervous system (CNS), multiple sclerosis, are poorly defined. Although T-cell-mediated immune responses play a pivotal role in establishing the inflammatory response, humoral factors also may be critical in disease progress. We have isolated a mouse monoclonal antibody (mAb 2B10) that recognizes a cell-surface molecule expressed exclusively by rat oligodendrocytes, the cells responsible for the formation and maintenance of CNS myelin. In cultures of neonatal rat spinal cord, mAb 2B10 specifically mediated oligodendrocyte cell death in the absence of complement. In the current study, mAb 2B10-producing hybridoma cells were implanted into adult rat brain ventricles, and the effect of mAb 2B10 on CNS cytoarchitecture was examined. In the optic nerves of mAb 2B10-treated animals, there was significant focal myelin degeneration near the optic chiasm. Axons in the myelin degenerate regions were largely healthy. There was no significant infiltration of hematopoietic-derived cells into the affected regions, but microglia were activated focally and phagocytosed the collapsed myelin. This study demonstrates that an antibody directed against myelin-forming cells induces CNS demyelination and supports the hypothesis that autoantibodies may play a role in CNS demyelinating diseases.

A morphological and morphometric analysis of the optic nerve in the hypothyroid rat

This investigation was designed to morphologically evaluate the effects of hypothyroidism on the development of myelin and axons in the rat optic nerve. Four pups from each group of normal and propylthiouracil-induced hypothyroid rats were sacrificed at 14, 21, 28, and 35 postnatal days. Optic nerves were studied by both light and electron microscopes. The hypothyroid animals had significantly reduced body and brain weights compared to those of their age-matched controls. In the hypothyroid animals, the cross-sectional area of the optic nerve, the fiber density, and fiber occupancy were significantly diminished compared to those of the controls. The mean individual fiber size was unaffected. However, the relationship between the total axonal area to myelin thickness was similar in the control and experimental groups, implying that the feedback mechanism between myelinating cells and axons was not affected by hypothyroidism. Thus, this study indicates that the principal insult of neonatal hypothyroidism results in a delay in myelin acquisition of myelinated fibers, resulting in diminished cross-sectional area of the optic nerve, fiber density, and fiber occupancy.

An animal model of prophylactic cranial irradiation: histologic effects at acute, early and delayed stages

Wistar rats (body wt. 200 g) were subjected to a fractionated course of radiation similar to that used in prophylactic brain irradiation for small cell carcinoma of the lung (2000 cGy in 5 fractions over 5 days with 60Co). Effects of this regimen were assessed by histologic examination of brain sections at 1 week, 1 month and 6 months post-irradiation. With conventional stains there were no apparent differences between control and irradiated brains at any of the post-irradiation intervals. Immunohistochemistry for neurotransmitter synthetic enzymes tyrosine hydroxylase and glutamate decarboxylase, as well as histochemistry for acetylcholinesterase, failed to uncover any changes in the irradiated animals. Immunohistochemistry for glial fibrillary acidic protein, an astrocyte marker, also showed no differences in the irradiated groups. However, an antibody against a major histocompatibility complex, class II antigen (OX-6) revealed a microglial response in grey and white matter beginning at 1 month and increasing up to the 6 month post-irradiation interval. The neuroanatomical basis for this microglial response was suggested by the results of silver stains for nerve axons, which revealed axonal loss in striatal white matter bundles in a pattern implicating vascular insufficiency.

Calibers and microtubules of nerve fibers: differential effect of undernutrition in developing and adult rats

Sural nerves of 9-week-old rats undernourished since birth, and of adult rats food-restricted for 27 and 48 days, were studied to explore the effect of severe undernutrition on the caliber and microtubules of axons in growing and non-growing animals. In 9-week-old undernourished rats, the number and caliber of myelinated fibers were normal while the cross-sectional area of non-medullated fibers was 29% smaller than controls. By contrast, in adult undernourished rats the cross-sectional area of myelinated fibers was affected sooner and to a greater extent (-28%) than that of non-medullated fibers (-23%). Regardless of age, in both controls and in undernourished rats non-medullated fibers of equal caliber had similar microtubular content. The same was found in 3-microns myelinated axons. These findings indicate that food restriction affects proportionately caliber and microtubules of axons. It is proposed that the anatomy of the axon is in a dynamic equilibrium and that microtubules participate in the specification of the axonal caliber.

Functional development of the visual system in normal and protein-deprived rats. VIII. Post-natal development of optic nerve axons

Protein deprivation results in a persistent impairment of transmission of impulses from the specific cortical projection within the visual cortex. In order to evaluate changes in subcortical structures during pre-weaning development, a study was made on number, calibre and myelination of optic nerve axons in control (C) and protein-deprived (PD) rats 5, 12, 20 and 30 days of age. Protein deprivation was induced by giving rat mothers a diet containing 7% protein by weight (control diet 14%, during gestation and lactation. The cross-sectional area of the nerve was measured on a digitizer. Between 3000 and 4000 axons collected from 7-13 field areas sampled by a random, systematic procedure from a cross-shaped area of the nerve were counted and measured. Between 5 and 12 days after birth, the number of axons was reduced by 50% in C rats. The total number of optic nerve axons was not significantly different in PD compared to C rats, indicating that protein deprivation does not affect the formation or naturally occurring nerve cell death of retinal ganglion cells. At all ages examined there were significant reductions in the number of axons larger than 0.52 micron as well as the number of myelinated axons. The rates of growth/maturation and myelination of axons in C rats and PD rats suggested that the reductions seen in PD rats up to 20 days of age may represent a developmental delay of approximately 4 days. At 30 days, a delay or a distortion of development may present. The retarded development of optic nerve axons is discussed in relation to delays, distortions and deficits during visual system maturation in protein-deprived rats.

Undernutrition during early adult life significantly affects neuronal connectivity in rat visual cortex

Male black and white Hooded Lister rats were undernourished for 29 days during early adult life. Undernourished rats had 30% more synapses per neurone in the visual cortex than matched controls. It is suggested that undernutrition may cause a delay in the normal decline of this ratio.

A quantitative morphological study of interstrain variation in the developing rat optic nerve

Groups of pigmented (Black and White Hooded Lister) and albino (Sprague Dawley) rats were killed at 7, 15, and 25 postnatal days of age. Their optic nerves were embedded in resin suitable for both light and electron microscopy. Quantitative stereological procedures were used to estimate total fibre number and the degree of myelination in the optic nerves at the various ages. At 7 days of age, both albino and pigmented rats had about 220,000 optic nerve fibres. By 15 days, both strains showed a reduction of some 30,000 fibres. This fibre loss continued in both strains after 15 days of age, but more rapidly in the pigmented strain. At 25 days of age, pigmented rats had 72,371 +/- 7,244 fibres, whilst albino rats had 102,681 +/- 4,138 fibres (P less than .01). More than 99.5% of optic nerve axons in both strains were unmyelinated at 7 days of age. By 25 days of age about 90% of all remaining fibres were myelinated in both strains. The mean diameter of the myelinated axons was estimated to be between 0.59 and 0.65 micron in all animals, there being no significant age or strain differences. In contrast, the mean diameter of nonmyelinated axons increased significantly with age. This increase was greater for albino than pigmented rats, such that by 25 days of age the respective values were 0.49 micron and 0.42 micron (P less than .05).

A quantitative morphological study of the optic nerve of pre-weanling rats

A quantitative histological estimation of the optic nerve of pre-weanling rats has revealed that the total fiber number decreases rapidly between 7 and 25 days of age.

Myelination of the rat optic nerve during postnatal undernourishment and recovery: a morphometric analysis

Newborn rats were undernourished from the second postnatal day through 20 days of age and weaned to a diet of laboratory chow ad libitum. Optic nerve development was examined by various light and electron microscopic techniques at 14, 21, 35 and 63 days of age. The degree of undernourishment achieved resulted in body growth lag comparable to results obtained in our previous studies. Although cellularity (cells per photomicrograph area) of the oligodendroglia was unaffected, there was an apparent significant relative reduction in the total number of myelinated fibers by 21 days of age, as determined by light microscopic sampling. Ratios of unmyelinated-to-myelinated fibers were thus estimated by electron microscopy, and results indicated an early increase in the ratio (14 days). Either as the result of catch up in a developmental lag, or as a result of possible restorative effects of rehabilitation, these differences were significant by 35 days of age. The relationship between axon circumference and numbers of myelin lamellae was determined by regression analysis, which revealed a significant reduction in numbers of lamellae over axons of all sizes at 14 days. By 21 days, only fibers in the size range of 1-2 micron of circumference showed a difference, and by 35 days there were no significant differences. These results all indicate that there is a significant myelin reduction in optic nerve of the undernourished rat.