The hypothalamo-pituitary-adrenal (HPA) axis plays a central role in promoting adaptations to acute stress, while over activity of this system may be involved in adverse effects on physiology and behavior. Glucocorticoids, the end-products of HPA axis activation, are key mediators in adaptive and maladaptive effects following acute and chronic stress. Previous research has focused on how chronic stress and elevated glucocorticoid exposure influences hippocampal structure and functioning in the rat. The prefrontal cortex, a brain region important for executive function, is involved in inhibiting the stress response but has also been shown to be affected by repeated stress exposure. The current set of experiments were designed to get a fuller picture of how chronic stress and elevated glucocorticoids impact prefrontal structure and function in the rat.
Chapter 2 investigates how differences in basal functioning on the HPA axis impact prefrontal structure and functioning. Aging has been shown to be accompanied by disruptions in circadian functioning, so aged and young animals were investigated for basal adrenocortical activity. Individual differences were found in both young and aged groups, and animals were partitioned into high or low HPA activity. Aged animals with high glucocorticoid (CORT) secretion showed significant dendritic spine loss in prefrontal neurons, when compared to aged animals with low CORT secretion and young animals. Using a delayed alternation task using a T maze, a prefrontal dependent task, we showed that aged animals with high CORT secretion show significant working memory impairments compared to all groups.
Chapter 3 investigates the role that glucocorticoids play in the restructuring of prefrontal structure following chronic stress. As previous work has shown that chronic stress leads to regressive alterations in dendritic spines in pyramidal neurons in the medial prefrontal cortex (mPFC), this study examined the capacity of sustained increases in circulating CORT alone to alter spine density and morphology in this region. A subset of rats were implanted with subcutaneous CORT pellets to provide continuous exposure to levels approximating the circadian mean or peak for 1, 2, or 3 weeks. Pyramidal neurons in the prelimbic area of mPFC were selected for intracellular dye filling followed by high resolution three-dimensional imaging and analysis of dendritic arborization and spine morphometry. Two or more weeks of peak CORT exposure resulted in apical dendritic retraction and dendritic spine loss, with thin spine subtypes showing the greatest degree of attrition. Finally, these alterations persisted following a 3-week washout period suggesting prolonged disruptions in HPA activity may be sufficient to induce enduring regressive structural alterations.
As the majority of studies investigating the stress response have focused on male rodents, Chapter 4 investigated sex differences in prefrontal structure and function following chronic stress, and CORT exposure in male and female rats. Adult male and female rats were exposed to two weeks of chronic variable stress (CVS) and then either tested on the delayed alternation task of the T-maze or perfused for dendritic spine analyses. Both males and females showed significant impairments on the working memory task following CVS exposure compared to non-stressed animals. CVS also resulted in significant spine loss in mPFC neurons in both males and females. As chapter 2 focused on the effects of sustained CORT exposure on structural reorganization of mPFC, female animals were implanted with subcutaneous CORT pellets and analyzed for mPFC structural alterations. Females were found to show similar effects as males in that they demonstrate decreased spine density on mPFC pyramidal neurons following 2-weeks of sustained high CORT exposure. Finally, in an attempt to generalize chronic stress effects on PFC, males and females were exposed to a repeated stress paradigm and analyzed for dendritic spine changes in PFC. Both males and females exposed to 3 weeks of repeated restraint show significant spine loss compared to non-stressed animals.
These results show that chronic stress and subsequent glucocorticoid exposure significantly alter PFC structure and function in both male and female rats. These data enhance our understanding of how both stress and CORT specifically alter dendritic spine density and morphology and how this may lead to changes in prefrontal function.