Neuronal plasticity is a central theme of modern neurobiology, from cellular and molecular mechanisms of synapse formation in Drosophila to behavioural recovery from strokes in elderly humans. Although the methods used to measure plastic responses differ, the stimuli required to elicit plasticity are thought to be activity-dependent. In this article, we focus on the neuronal changes that occur in response to complex stimulation by an enriched environment. We emphasize the behavioural and neurobiological consequences of specific elements of enrichment, especially exercise and learning
In the present paper, we focus on neuronal changes that occur in response to complex stimulation by an enriched environment. An enriched environment is 'enriched' in relation to standard laboratory housing conditions. In general the 'enriched' animals are kept in larger cages and in larger groups with tunnels, nesting material and toys.
Exposure to an enriched environment has been found to elicit neuroanatomical and behavioural changes, such as enhanced dendritic arborization, gliogenesis, neurogenesis, and improved learning.
It is difficult to isolate the behavioural factors that are responsible for neural changes. Factors such as social interaction, learning and motor activity have been suggested to mediate neural consequences of the enriched environment.
Similar to the effects of environmental enrichment, voluntary exercise in a running wheel enhances the survival of newborn neurons in the dentate gyrus. In addition, both conditions improve learning and memory. Therefore effects of enrichment and exercise on behavioural, morphological and molecular changes in the brain are compared in the review.
Enrichment increases dendritic branching and synaptogenesis in cortex and hippocampus. It is not known if exercise results in similar changes.
Exercise enhances dentate gyrus long-term potentiation. Enrichment increases excitatory postsynaptic potential slopes in the dentate gyrus.
Both exercise and enrichment enhance production of certain growth factors and neurotransmitters in the hippocampus.
Two areas of neurobiological research that may benefit from use of the environmental enrichment and exercise models are mouse genetics and studies of recovery of function following brain and spinal injury or disease. For example, enrichment has been shown to enhance function in mice that have been engineered to overexpress Huntington's disease gene products. In addition, enrichment as well as exercise seem to be beneficial for conditions such as stroke and epilepsy.
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We thank the reviewers for their helpful comments on our paper. We appreciate the editorial assistance of M. L. Gage and the assistance of L. Kitabayashi in preparation of the figures. We are grateful for the continued support of the Christopher Reeve Paralysis Foundation, The Lookout Fund, The Parkinson's Disease Foundation and the National Institutes of Health.
- SHOLL RING ANALYSIS
A clear overlay with concentric rings at 20 μm intervals is centred over the cell body and the number of times the dendrites intersect the rings are counted.
- THETA RHYTHM
Neural activity with a frequency of 4–8 Hz.
- HEBB–WILLIAMS MAZE
A rectangular field with a start box and a goal box placed at opposite ends of the apparatus. Different configurations are obtained by placing barriers at different points of the field.
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