Environmental enrichment influences survival rate and enhances exploration and learning but produces variable responses to the radial maze in old rats.

Wistar rats at 7 (mature), 16 (aging), and 22 (old) months of age spent 70 days in normal laboratory (Social), impoverished (Isolated) or dynamic Enrichment cages. The Enriched cage emphasized spatial re-arrangements of significant items, and the learning of new routes. Subsequently, Enriched rats at all ages entered a novel environment and escaped from a bright light with significantly shorter latencies than rats from either of the other environments. Mature, aging and some of the old Enriched rats also significantly outperformed their Isolated and Social counterparts in the radial maze. However old Enriched and Isolated animals showed significant variability in relation to the measure of the proportion reaching criterion on this task, and a significantly lower proportion than of old Social rats reached criterion. Enriched rats had a significantly higher survival rate than Social and Isolated animals. These findings are discussed in terms of learning efficiency and behaviors that conserve energy and thereby enhance survival.

Lead retention in blood and brain after preweaning low-level lead exposure in the rat.

Newborn rats of the albino Wistar strain were exposed to lead from birth to 20 days of age through mothers milk, from dams which were fed diets containing 0, 0.25, 0.5 or 1.0% powdered lead. Subsequent determination of tissue lead revealed a direct relationship between the lead levels in both blood and brain of the pups and the lead dosage to which they were indirectly exposed via the dams' milk. Lead retention in both tissues was still evident at 100 days of age, with the relative elevation of lead levels being an order of magnitude higher in brain than in blood. There were no obvious signs of lead intoxication in the pups, apart from mild growth retardation in the group with the highest lead burden. However there was a significant retardation in behavioral development observed on two of four measures which were employed. It was concluded that brief exposure to low lead levels in infancy can have long lasting consequences in the brain and in behavior.

Cue significance and response regulation in 3- to 6-year-old children's learning of multiple choice discrimination tasks.

The learning of multiple choice discrimination tasks of 3-, 4-, 5-, and 6-year-old children was compared using the components of learning of cue significance and response regulation. No significant differences between age groups were found in the occurrence of cue significance learning, although the younger children's cue significance responses were less accurately related to task solution. On the two easier tasks, the component of response regulation improved significantly over the age range of 3-5 years. On the two more difficult tasks 6-year-olds' response regulation was significantly better than that of 4- and 5-year-olds. In a second experiment involving two different training procedures, 5-year-olds performed better than 4-year-olds. Only for 5-year-olds was performance with the procedure emphasizing response regulation superior to performance with the procedure emphasizing cue significance. These results are discussed in terms of an association between development of response regulation and maturation of hippocampal-prefrontal cortex systems.

Low level lead effects on activity under varying stress conditions in the developing rat.

The study was designed to determine whether lead ingestion by nursing rats would affect the way offspring reacted to the stress inducing properties of the test environment both as juveniles and mature rats. Dams were exposed to diets with 0.0, 0.2, 0.4 or 1.0 percent by weight metallic lead. Mean blood-lead levels of pups at weaning were 4, 25, 36 and 55 micrograms/100 ml of blood respectively. The stress factor was varied by (1) changing the test apparatus, i.e., forcing rats to occupy an open field or allowing the animal to be a free agent in the start box of a T-maze; (2) testing rats under a longitudinal and a cross-sectional experimental design to vary familiarity with the apparatus; and (3) comparing behavior in the presence or absence of noise. Reactivity was assessed by examining the inter-and intra-session pattern of ambulations and defecations. Analysis of data revealed that lead treated rats demonstrated the greater response to stress. This response was generally dose related although recovery was dependent upon the test applied and measures taken. The findings provide a conceptual framework to account for varied results across previous studies.

Disruption of passive avoidance learning in the rat by electrical stimulation of the hippocampus.

The effect of electrical stimulation to discrete regions of the dorsal hippocampus on passive avoidance was examined in five experiments. Rats stepped from a brightly lit, white compartment to a dark compartment. Following footshock in the dark compartment, step-through latencies were recorded 1 min, 24 hr, and 48 hr after footshock. When given 20 step-through trials prior to footshock, dentate-stimulated animals exhibited lack of passive avoidance with particularly short latencies at 1 min, and CA1 rats showed reduced latencies compared with latencies of cortical and nonoperated controls. In Experiment 2, rats given stimulation in the same regions performed the avoidance task without prior step-through experience. All groups showed less passive avoidance than the control group in Experiment 1, and there were no significant differences between groups. With a single step-through trial before the footshock trial, longer avoidance latencies were recorded, but again groups did not differ significantly. Dentate-implanted animals, given 20 prior exposure trials but with stimulation at different stages of the task sequence, demonstrated a passive avoidance deficit at 1 min after footshock. The results are discussed in terms of the generation of expectancy that the black compartment was a safe place and the effects of brain stimulation on the expectancy, with particular reference to Vinogradova's (1975) theory of hippocampal function.

Malnutrition and group brain- and body-weight variability: data in search of a hypothesis.

Chronic malnutrition, which may be induced by a number of different experimental manipulations, appears, from a survey of the literature, to lead to greater relative whole-brain-weight variability in affected groups compared with controls; there is a tendency for the same pattern to emerge in the case of body-weight variability. A hypothesis which might throw some light on this general trend is proposed.

Lead induced alterations in maternal behavior and offspring development in the rat.

The study was designed to determine whether lead ingestion by nursing rats would affect dam-pup interaction. Dams were exposed to diets with 0.0, 0.2, 0.4 or 1.0 percent by weight metallic lead. Mean blood-lead levels of pups at weaning were 3, 19, 40 and 57 micrograms/100 ml of blood respectively. Maternal behavior was assessed by videotaping behavior for complete 24 hour periods on alternate days from birth to weaning. Analysis of data revealed that lead affected dams nursed for longer periods than normal and offspring were slower to explore their environment. It was concluded that altered maternal behavior was related to delays in pup development and the functional isolation of experimental pups from their environment may be the antecedent to altered behavior later in maturity.

Early rearing conditions and within-group behavioral variability.

Rats were maintained in enriched or isolated conditions for periods ranging from 10 to 51 days postweaning in four experiments. Animals were subsequently run in a simple maze in acquisition and extinction trials. A measure of variability, the coefficient of variation, showed a tendency for enriched groups to be more homogeneous in their performance, especially after 30 days differential rearing, a finding analogous to some of those in the differential rearing-brain changes literature. Some additional material suggests that these findings may have some generality.

Cortical depth changes in enriched and isolated mice.

The occipital cortical depth was determined in laboratory mice at both 14 and 20 days of age and after various periods of postweaning exposure to enrichment or isolation. The depth was found to be maximal at 20 days of age. It declined thereafter, irrespective of environment, but the isolate cortical depth decreased faster than the enriched. The postweaning depth of the occipital cortex appears to be determined by an inevitable age-related decrease whose rate of decline may be attenuated by sensory stimulation. The postweaning cortical depth may reflect the extent of cortical neuronal development and associated metabolic activity.

The acetic acid component of lead acetate: its effect on rat weight and activity.

Suckling rats were exposed to one of three solutions, 2.6 x 10(-3) M lead acetate, 5 x 10(-3) M acetic acid or water, from parturition until the pups were 18 days old. Male offspring from dams on acetic acid demonstrated above normal preweaning body weights and were significantly less active than normals in the open field by day 44. The preweaning body weights and ambulation of offspring from dams on lead acetate fell between the results for normal and acetic acid rats. These results indicate that the role of the acetic acid component cannot be discounted when lead acetate is the compound utilized in behavioral studies aimed at examining an effect by lead on laboratory rats.

Enrichment-isolation, cortex length and the rank order effect.

In a previous paper we have proposed a developmental theory to account for the neurological changes which result from differntially rearing animals in either enrichment or isolation. On the basis of brain weight measurements we suggested that the primary cause of the differential development could be traced to retarded neurological growth in the isolated animals. The present 9 studies test the generality of this theory by applying it to the cortex length changes induced in rats by differential rearing periods of between 18 and 160 days. In the light of this new data the theory has been revised to the extent that the developmental ceiling for the dependent variable is now considered to change with age instead of being fixed. Two major consequences of this revision are as follows. Firstly that the environmentally induced changes in cortical development are seen as persistent. Secondly that the Rank Order Effect is shown to be transitory phenomenon which exists only when some, but not all of the isolate values have been restricted in their development.

A developmental theory of environmental enrichment.

The differential brain development induced by sensory enrichment or deprivation is most apparent in rats with low brain weights. These differences are hypothesized to represent the retarded development of environment-dependent neurons in the isolated animals.

Brightness discrimination learning under conditions of cue enhancement by rats with lesions in the amygdala or hippocampus.

Three groups of rats, one with amygdala lesions, one with hippocampal lesions and a control group were trained on a brightness discrimination task under one of three different conditions, enhancement of the negative cue, enhancement of the positive cue or a non-enhanced condition. Animals with amygdala lesions showed retarded learning compared with normal animals and those with hippocampal lesions under the positive cue enhancement condition. Under the negative cue enhancement condition animals with hipocampal lesions were significantly handicapped compared with the other two groups. Results are discussed in relation to the Douglas and Pribram concept of a reciprocal linking of the amygdala and hippocampal systems in discrimination learning with the amygdala functioning as a reinforce register system and the hippocampus as an error evaluation system.

Functional differentiation within the neostriatum of the rat using electrical (blocking) stimulation during discrimination learning.

Sixteen rats, eight with bipolar electrodes implanted bilaterally in the anterodorsal head of the caudate-putamen and eight with similar electrodes in the posteroventral caudate-putamen, learned a spatial and a form discrimination task and their reversals while receiving "continuous" stimulation. Rats receiving stimulation to the anterodorsal caudate-putamen were imparied on spatial reversal learning compared with the posteroventral group. On form discrimination reversal the posteroventral group were impaired compared with the anterodorsal group. This dissociation is related to the particular cortical neostriatal projection system for the region stimulated and demonstrates a behavioral differentiation in rat neostriatum comparable with that observed in the monkey.

Delayed alternation learning under electrical (blocking) stimulation of the caudate nucleus in the cat.

Of 9 cats with electrodes implanted in the dorsolateral caudate nucleus, 4 had previously reached criterion on a delayed alternation task. Tested under alternating conditions of stimulation and nonstimulation, these 4 animals showed a significant performance decline with stimulation. When a marker was placed on one response panel, performance under stimulation did not decline. Stimulation did not affect the learning of a visual discrimination. The remaining 5 cats were trained on delayed alternation with alternating stimulation and nonstimulation conditions. Under stimulation, performance remained near chance levels. These animals were tested on position learning and reversal, and under unilateral stimulation. The results support the view that the caudate nucleus and prefrontal cortex mediate separate aspects of delayed response behavior.