1. In order to exclude the possibility of differences in maternal care which are known to result from typical methods of undernutrition during the suckling period, rat pups were reared artificially on different planes of nutrition away from their mothers.2. Artificial rearing was accomplished by fitting infant rats with a gastric cannula through which a milk substitute was infused intermittently. Rats were fed thus from 4 to 21 d on a high (ARHI) or a low (ARLO; 44%of ARHI level) plane of nutrition. Underfeeding of the ARLO group was continued till 25 d, after which all rats were given a good-quality pelleted diet nd lib. 3. Compared with mother-reared (MR) litter-mates, ARHI rats showed advanced eye-opening and, at 21 and 25 d, they resisted restraint more strongly.4. Growth in body-weight of ARHI and MR rats was similar but, when autopsied at 32 weeks, the ARHI rats were shorter (nose-rump length) and had lighter gastrocnemius muscles, adrenals and brains, but heavier epididymal-fat pads.5. ARLO rats had deficits at 32 weeks compared with ARHI rats in whole body, kidney and epididymal-fat-pad weights, and in tibia length.6. In a second experiment, ARHI and MR rats were killed at 21 d. All the differences found at 32 weeks were already present at 21 d. In addition, the ARHI pups had enlarged livers and intestines but shorter tibias.7. The milk substitute, which is one commonly used in such studies, has a low protein and high carbohydrate content compared with rats' milk. This difference probably caused the abnormal organ growth of ARHI rats.
Nutritional deprivation at different stages of development in rats was shown to cause reductions in some parameters of the cerebrum that could not be reversed by an extended period of adequate feeding. The deficits varied in magnitude, depending on whether undernutrition occurred during the suckling period alone or was combined with additional deprivation either before birth or after weaning. Whereas the weight, length, and width of the cerebrum were affected significantly by undernutrition at every age, effects on the thickness of the cortex and hippocampus were associated only with the combined pre- and postnatal deprivation. The deficits in weight, length, and cortical thickness could be modified later by housing in enriched and impoverished environments for 30 days. The previously undernourished rats responded similarly to environmental complexity regardless of the age at which they had been deprived, and their responses did not differ significantly from those of well fed controls. The degree to which enrichment can be said to have reduced deficits arising from undernutrition depends on the relative size of the nutritional and environmental effects on the particular parameters in question and on the choice of a "normal' baseline against which to assess recovery.
Rats were either undernourished or fed normally during the suckling period, then at 1 and 5 months of age littermates were housed in enriched or impoverished environments for 30 days. The undernutrition caused lasting reductions in the weight and size of the cerebrum and in parameters of the hippocampus, but significant deficits were not observed in the thickness and area of the occipital cortex. Differential housing differed from early undernutrition in that its largest effects were on cortical parameters, but the effects of the two conditions did partly overlap. This meant that some nutritionally induced deficits could be modified later by manipulating environmental complexity. The cerebral response of the previously undernourished rats to differential environments was not distinguishable from that of well-fed controls on the basis of the gross anatomical changes that were measured. Furthermore, no significant differences were found between the environmental effects in young and mature rats, although the latter's response tended to be somewhat less for most parameters.
To test the hypothesis that the motivational effects of neonatal undernutrition might conceal the detrimental effects on learning, we tested previously undernourished and normally nourished Sprague-Dawley rats on learning of a novel maze pattern under either latent learning (nonappetitive) or food-motivated conditions. Under the nonappetitive conditions, the previously undernourished rats learned significantly less than the normal controls, but when motivated for food, the undernourished rats performed as well as the controls. When learning performance measures are sensitive to motivation, differential motivation between undernourished and normal subjects must be controlled or eliminated.
After 1 month in enriched or impoverished environments, groups of rats were housed for an additional month in either the same or the opposite environment. The cross-over design allowed us to see whether or not the cerebral effects of differential environments are modified by subsequent housing conditions, and also whether or not such effects might be influenced by previous experience. Differential housing for 1 month was associated with significant alterations in the weight and length of the cerebrum, the thickness of the occipital cortex, and the relative number of neurons and oligodendrocytes, and no change in the ratio of astrocytes to neurons. The effects of enrichment were not reduced in animals that were previously impoverished. When impoverishment followed enrichment, certain effects appeared to diminish, notably, the increase in cortical thickness. Other effects, however, such as the increase in the ratio of oligodendrocytes to neurons, were very stable. The gross cerebral alterations associated with differential housing did not differ significantly after 2 as opposed to 1 month's exposure, but the effects on neuronal density and the ratio of oligodendrocytes to neurons in layers V and VI of the cortex were, paradoxically, significantly smaller with the longer duration. We suggest that this decrease is due to the continuation of processes induced in the first month of differential experience rather than to their dissipation.
Male rats were either undernourished or fed normally from birth to day 21, after which time food was made freely available. At 1 month of age littermate pairs from both nutritional groups were housed in either enriched or impoverished conditions for 30 days and then killed for brain measurements. Significant deficits due to undernutrition were observed in the weight and size of the cerebrum, but not in the thickness or area of the visual cortex. Although there were large differences of between 21 and 39% in the number of higher-order basal dendrites of layers II and III pyramidal cells, and of about 19% in the distal ring intersections, none except the fourth-order branches and intersections at 100 micron from the cell body approached statistical significance. Changes in cerebral weight and size also occurred as a result of differential housing, with the enriched rats showing increased values relative to their impoverished littermates. In contrast to the nutritional treatment, differential housing significantly affected cortical thickness and area, as well as basal dendritic branching of the pyramidal cells. Enriched rats had relative increases of 26% in the number of fifth-order branches and 45-80% in the number of distal ring intersections.
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