Response of the Brain to Enrichment
Neural grafting to experimental neocortical infarcts improves behavioral outcome and reduces thalamic atrophy in rats housed in enriched but not standard environments.
THE EFFECTS OF ENRICHMENT ON THE CEREBRAL CORTEX
“What do we mean by “enrichment” for the rats who have served as the animal of choice for most of these studies? Thirty six Long-Evans rats were sorted into three experimental conditions using 12 animals in each group: 1) enriched 2) standard or 3) impoverished environments. All animals had free access to food and water and similar lighting conditions. Eventually, it was determined that animals maintained in their respective environments from the age of 30 days to 60 days developed the most extensive cerebral cortical changes. For the enriched environment, the 12 animals lived together in a large cage ( 70 x 70 x 46 cm) and were provided 5-6 objects to explore and climb upon (e.g., wheels, ladders, small mazes). The objects were changed two to three times a week to provide newness and challenge; the frequent replacement of objects is an essential component of the enriched condition. The combination of “friends” and “toys” was established early on by Krech as vital to qualify the experiential environment as “enriched.” For the standard environment, the animals were housed 3 to a small cage ( 20 x 20 x 32 cm) with no exploratory objects. For the impoverished environment, one animal remained alone in a small cage with no exploratory objects. The numbers of animals placed in these separate conditions were based on the manner in which the routine housing was established in the rat colony. Three rats in a cage has been considered standard for all experimental work over the decades. Since prior to these experiments no one had designed studies to examine brain changes in response to different environmental conditions, the decisions about what represented “impoverishment” and what represented “enrichment” was more arbitrarily than scientifically reasoned. After 30 days in their respective environments, all animals were anesthetized before the brains were removed for comparison among the three groups. Twenty micrometer frozen sections were cut and stained, and the thickness of the frontal, parietal and occipital cortices were measured. Results indicated clearly that the cortex from the enriched group had increased in thickness compared with that living in standard conditions, whereas, the brains from the impoverished group decreased compared to the standard. Because the nerve cells were farther apart in the enriched vs. the impoverished brains, it was thought that the major component of the brain changes due to enrichment had to do with alterations in the dendritic branching. With more detailed studies, the cortical thickness increases were found to be due to several factors, including increased nerve cell size, number and length of dendrites, dendritic spines, and length of postsynaptic thickening as measured on electron microscopic pictures of synapses. In the initial experiments designed to explore the impact of an enriched environment on the brain of post-weaned rats, only enriched and impoverished groups were used. Rats were maintained in their respective environments from 25 to 105 days of age because there were no available data on how long it would take to create chemical or structural changes in the cortex. Chemical and anatomical measurements taken from these animals showed significant differences between the two groups – in cortical thickness, cortical weight, acetylcholinesterase, cholinesterase, protein and hexokinase levels. In these initial experiments, however, it was not clear if the changes were due to enrichment or impoverishment because there were no standard conditions established as controls. Nonetheless, the differences in cortical thickness with this 80-day exposure to the two environmental conditions were not as great as during the 30-day exposure. Consequently, in subsequent experiments, the period of exposure to the experimental conditions was reduced from 80 days to 30 days, then 15 days, 7 days and finally to 4 days. At each of these intervals, animals from the enriched environment showed increases in cerebral cortical thickness in some areas but not in others. For example, in the male animals exposed for 80 days to enriched conditions, the somatosensory cortex did not show significant changes, whereas male animals exposed for 30 days did develop significant differences in the somatosensory cortex. The occipital cortex showed significant changes for both the 80- and the 30-day experiments, but, again, the differences were greater at 30 days than at 80 days.” Ref