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.
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