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The Effect of Environmental Enrichment on the Number of NADPH-d Positive Interneurons in the Dentate Gyrus of the Rat Dorsal Hippocampus
Exposure to an enriched environment has been shown to be beneficial to brain structure and cognition by preserving neuronal integrity and strengthening the functioning and plasticity of neural circuits . These benefits derive from the added spatial, social, and sensory complexities in an enriched environment . It has recently been established that physical exercise and enriched environments stimulate adult neurogenesis  and differentiation in dentate granule cells (DGCs)  in the dentate gyrus of the hippocampus. However, possible changes in other cell types, particularly interneurons, has remained elusive. Interneurons are an integral regulator in neurotransmission in the hippocampus. Damage to hippocampal interneurons have serious implications and lead to a decline in cognitive abilities . Our study sought to elucidate possible changes in a subclass of interneurons affected by exposure to an enriched environment (EE) and enriched changing (EC) environment. The topographic arrangement of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) positive interneurons was studied in the dentate gyrus of the rat dorsal hippocampus. We observed an increase in the number of NADPH-d positive neurons found in both enrichment groups. The elevated level of NADPH-d activity was represented uniformly across all six layers of the dentate gyrus and was most significant in the expected granular cell layer (GRCL) and infragranular zone (infraGRZ). The overall results suggest that the brain has the ability to adapt to increased amounts of sensory stimulation. These changes highlight a new mechanism of physiological homeostasis; with an increased demand for energy from new-born DGCs and increased input, the brain will regulate its energy expenditure via an increase in either the activity of existing interneurons, or the actual number of interneurons. Further research will involve observing the differences to the ventral hippocampus, CA1, and CA3 regions, and the effects on other specific subpopulations of interneurons.