ENVIRONMENTAL-INFLUENCES ON THE CENTRAL-NERVOUS-SYSTEM AND THEIR IMPLICATIONS FOR THE AGING RAT

Two methods of providing environmental stimulation that were introduce d in the 1950s have guided much research on neurobehavioural plasticit y. These are neonatal handling and environmental enrichment. Neonatal handling has been shown to permanently affect behaviour and endocrine responses. Recently this manipulation has been shown to have important influences on the aging individual, protecting the hippocampus from a ge-related dysfunction and neuronal loss. These effects are mediated, in part, by keeping glucocorticoid levels low. This has been character ised by, among other things, elevated expression of glucocorticoid rec eptors in the hippocampus. Earlier studies have failed to present conv incing evidence for differences in hormonal variables between animals housed in enriched and impoverished environments, and have not consist ently reported changes in the hippocampus. Recent data from our labora tories have shown that adult animals housed in enriched environments h ad, like neonatally-handled rats, higher expression of the gene encodi ng glucocorticoid receptors in the hippocampus. Taken together with th e induction of NGF and NGFIA gene expression in the hippocampus of enr iched animals, these results implicate genes encoding transcription fa ctors and glucocorticoid receptors in the cascade of events leading to environmentally induced cerebral changes. In addition, these results suggest that environmental enrichment in adulthood, like neonatal hand ling, may have the potential to protect the aging hippocampus from glu cocorticoid neurotoxicity.