Effects of an enriched environment on the release of dopamine in the prefrontal cortex produced by stress and on working memory during aging in the awake rat

The present study was designed to investigate the effects of a mild acute stress on the in vivo release of dopamine in the prefrontal cortex (PFC) during aging and whether housing animals in an enriched environment changes these effects. Behavioural parameters such as spontaneous motor activity (open-field) and working memory performance in a delayed alternation task (water T-maze) were also studied. Male Wistar rats (3 months of age) were housed during 3, 12, and 21 months (6, 15 and 24 months of age at the end of housing) in enriched or control conditions. After behavioural testing, animals were subdivided in two groups. In one of the groups BDNF protein levels were determined in PFC, hippocampus and amygdala. Rats of the second group were implanted with guide cannula in the PFC to perform microdialysis experiments and to monitor extracellular concentrations of dopamine. The release of dopamine in the PFC produced by handling stress (40 min) was significantly reduced in both enriched and control 24 months animals. However, the increases of dopamine produced by stress were significantly lower in enriched animals when compared to controls. Similarly, the increases of dopamine produced by perfusing K+ 100 mM into the PFC were also reduced by aging and environmental enrichment. Both spontaneous motor activity and working memory performance were significantly reduced by aging. Moreover, animals housed in an enriched environment did show a lower spontaneous motor activity at all ages studied, though they did not show any change in performing the working memory task, either in basal conditions or after an acute stress. The BDNF protein levels were increased by environmental enrichment in the hippocampus and amygdala, but not in the PFC. These results suggest that both environmental enrichment and aging reduces the activity of the mesocortical dopamine system.