The effects of environmental diversity on well-fed and previously undernourished rats: I. Body and brain measurements

Behavioral consequences of postnatal undernutrition and enriched environment during later life

In rat models, large litter groups during suckling are used in the study of undernutrition. Large litter sizes are known to promote alterations in memory processes and anxiety-like behavior. Nevertheless, the effect of large litter size on sexual behavior and the reproductive system is still unknown. Environmental enrichment has been reported to (EE) enhance behavior and to correct some of the alterations produced by postnatal undernutrition. We used the Elevated Plus Maze (EPN), Morris Water Maze (MWM), Object Recognition test (OR) and several parameters of sexual behavior to determine the effect of large litter size on rats exposed to enriched and non-enriched environments. Newborn Wistar rats of both sexes were assigned to be suckled under lactation conditions, in litters of 8 pups or 16 pups. The large litter size (16 pups) caused a reduction in weight gain during the lactation period. On PND 45, four experimental groups were established for both sexes: Well-nourished Non-enriched (WN); Well-nourished Enriched (WE); undernourished Non-enriched (UN); Undernourished Enriched (UE). On PND 90, the UN males spent more time in the open arms on EPM. On PND 100, the UE females increased the latency to find the platform in training days (D1-4) in MWM. On probe day (D5) the UE males spent more time in the target quadrants in MWM. On PND 110, irrespective of EE the large litter size had increased the exploration time in both groups (UN) and (UE) in OR test. On PND 120, the performance of sexual behavior was more evident by effect of EE irrespective of the litter size. In conclusion, the large litter size showed no effects on sexual behavior, in contrast, EE has a sharp influence on sexual behavior. Conversely, memory processes and anxiety-like behavior are altered by large litter size.

Effect of training and environment on brain morphology and behavior

Using defined rearing or training paradigms, environmental stimulation has been found to increase brain weight (especially forebrain), cortical thickness, the number of glial cells, the glia to neuron ratio, neuronal cell body and nucleus size, and to alter synaptic profiles by increasing dendritic branching, dendritic spine density and the number of discontinuous synapses. Examples will be given from both animal and human studies that document these profound changes. Controversy exists as to whether enriched environments and/or training can compensate for neural deficits produced earlier in life. Examples will be given from animal studies with induced cortical lesions and prenatal genetic neural anomalies that support a role for environmental manipulations ameliorating earlier central nervous system damage.

Response threshold to aversive stimuli in stimulated early protein-malnourished rats

Two animal models of pain were used to study the effects of short-term protein malnutrition and environmental stimulation on the response threshold to aversive stimuli. Eighty male Wistar rats were used. Half of the pups were submitted to malnutrition by feeding their mothers a 6% protein diet from 0 to 21 days of age while the mothers of the other half (controls) were well nourished, receiving 16% protein. From 22 to 70 days all rats were fed commercial lab chow. Half of the animals in the malnourished and control groups were maintained under stimulating conditions, including a 3-min daily handling from 0 to 70 days and an enriched living cage after weaning. The other half was reared in a standard living cage. At 70 days, independent groups of rats were exposed to the shock threshold or to the tail-flick test. The results showed lower body and brain weights in malnourished rats when compared with controls at weaning and testing. In the shock threshold test the malnourished animals were more sensitive to electric shock and environmental stimulation increased the shock threshold. No differences due to diet or environmental stimulation were found in the tail-flick procedure. These results demonstrate that protein malnutrition imposed only during the lactation period is efficient in inducing hyperreactivity to electric shock and that environmental stimulation attenuates the differences in shock threshold produced by protein malnutrition.

Effects of early protein malnutrition and environmental stimulation upon the reactivity to diazepam in two animal models of anxiety

In order to investigate the effects of early protein malnutrition and environmental stimulation upon the response to the anxiolytic properties of diazepam, two animal models of anxiety (elevated plus-maze and light-dark transition tests) were used. Rats were malnourished by feeding their dams a 6% protein diet during the lactation period (0-21 days of age) while well-nourished controls received a 16% protein diet. From 21 to 70 days of age all rats received a balanced lab chow diet. Environmental stimulation consisted of 3-min daily handling from birth to 70 days of age. Additional stimulation was provided from 21 to 70 days of age by rearing the rats in an enriched living cage. Eight groups of rats were studied in a 2 (malnourished or well-nourished) x 2 (stimulated or nonstimulated) x 2 (diazepam or vehicle) design. At 70 days of age, independent groups of rats treated with diazepam (2.5 mg/kg, IP) or vehicle were submitted to testing in the elevated plus-maze or light-dark transition procedures. The results showed that both diazepam and environmental stimulation reduced anxiety in the elevated plus-maze; stimulation changed the anxiolytic response to diazepam and the two diet conditions altered differentially the response to both pharmacological and stimulation procedures. These results suggest that environmental stimulation can affect differentially the behavioral response of malnourished and well-nourished rats treated with diazepam.

Effects of environmental enrichment on cortical depth and Morris-maze performance in B6D2F2 mice exposed prenatally to ethanol

Pregnant mice were fed a liquid diet with 25% of the calories as ethanol from day 5 to 17 of gestation; controls received equivalent amounts of diet with maltose-dextrin substituted isocalorically for the ethanol. Two male weanlings from each litter were assigned randomly to an enriched or isolated environmental condition. After 6 weeks in these environments measures of brain growth were obtained, including thickness of frontal, parietal, and occipital cortex (study 1), or their behavioral capabilities were assessed in a Morris water maze (study 2). Ethanol decreased birth weight (both studies), postweaning body weight (study 2), and brain weight (study 1), while the enriched animals in both studies were heavier. Ethanol decreased the thickness of the occipital cortex only. All groups demonstrated learning by showing a decrease in latency to locate the hidden platform over the 5 days of testing; this was supported by their spending most time in the target quadrant during the probe trial. The latencies of the enriched animals were shorter than the isolated; covariance analysis indicated that this was not due solely to their faster swimming speed.

Neuronal and synaptic measurements in the visual cortex of adult rats after undernutrition during normal or artificial rearing

It is possible that the reported effects of early life undernutrition on brain morphology may be due to alterations in mother-infant interactions and not directly to undernutrition. We have investigated this possibility by comparing artificially reared with mother-reared rats. Four groups of black-and-white hooded male rats were reared. These consisted of mother reared control (MRC), mother reared undernourished (MRU), artificially reared control (ARC) and artificially reared undernourished (ARU). Artificially reared rats were raised in isolation away from their mothers from 5 to 21 days of postnatal age. They were fitted with a gastric cannula through which 'milk' was infused automatically. The period of undernutrition lasted from 5 to 25 postnatal days, following which the animals were fed ad libitum until 312 days of age. Rats from each group were then killed by perfusion with buffered 2.5% glutaraldehyde. Pieces of visual cortex from each rat were postfixed in osmium tetroxide and embedded in resin. Stereological procedures at the light and electron microscopical levels were used to estimate the synapse-to-neuron ratios in cortical layers II to IV. Both MRC and ARC rats had about 7000 synapses per neuron. However, this ratio was about 8300 in MRU rats whilst it was only about 5000 in ARU animals. The rearing x nutrition interaction was statistically significant at the 0.1% level. These changes in the synapse-to-neuron ratio were mainly due to alterations in the numerical densities of the synapses rather than that of neurons. These results demonstrate that environmental isolation, as a result of artificial rearing procedures, and concurrent undernutrition during the first three weeks of postnatal life, interact with one another to produce marked morphological changes in the adult rat brain. However, environmental isolation was not, by itself, sufficient to cause permanent changes in interneuronal connectivity.

The effects of a 30 day period of environmental diversity on well-fed and previously undernourished rats: neuronal and synaptic measures in the visual cortex (area 17)

Black and white Lister hooded rats were undernourished from the 16th day of gestation until 25 postnatal days of age. These previously undernourished rats and a set of well-fed rats were later subjected to 30 days of environmental diversity, i.e., environmental enrichment or isolation. Two separate experiments were carried out. In experiment 1, the environmental diversity lasted from 85 to 115 days of age and in experiment 2, from 35 to 65 days of age. At the end of the period of environmental diversity, all rats were killed by perfusion with 2% phosphate-buffered glutaraldehyde. Small pieces of tissue from the right visual cortex were embedded in Spurr's resin. For each rat two blocks of resin-embedded tissue were randomly selected, and from these semithin sections (0.5 micron) were cut and stained with toluidine blue. Photomicrographs of cortical layers II and III were taken from these sections and used to estimate the numerical density of neurons. Ultrathin (ca. 70 nm) sections of the same region of the cortex were cut and stained with lead citrate. These sections were used to estimate the synaptic disc diameter and numerical density. Synapse-to-neuron ratios were calculated from the estimates of synaptic and neuronal numerical densities. In experiment 1, well-fed rats raised in enriched environments had a significantly smaller neuronal numerical density and a greater synaptic disc diameter than well-fed rats raised in an impoverished environment. In experiment 2, neither the well-fed nor previously undernourished rats showed significant effects of environmental treatment on any of the features studied. The statistical interaction between nutrition and environment was not significant for any of the features in either experiment.

The comparative effects of early-life undernutrition and subsequent differential environments on the dendritic branching of pyramidal cells in rat visual cortex

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.