Behavioral, Cognitive, and Biochemical Consequences of Early Life Stress in Later Life: Insights from an Animal Model

Adverse experiences during early life contribute to the development of psychiatric conditions in later life. In fact, young children who directly experience or witness traumatic event(s) during early life, a sensitive developmental period, are considered highly vulnerable to psychiatric disorders during adult life. Interestingly, not all children who experience traumatic events are equally at risk of developing later life psychiatric disorders. Some are resilient despite being exposed to the same risk factors, while others are susceptible. The relationship between early life trauma exposure and development of later life psychiatric symptoms is not fully understood, and the mechanistic basis for resilience is also not clear. Clinical and preclinical studies have suggested that defects in stress-adaptive mechanisms potentially contribute to etiology of later life psychiatric conditions. Preclinical data from our laboratory has indicated poor oxidative/antioxidative balance as a critical component of maladaptive stress responsiveness in rodents. Our published work has demonstrated that induction of psychological stress leads to behavioral and cognitive deficits in rats. These impairments correlate with an increase in oxidative stress markers in the periphery as well as in selected regions of the brain including the hippocampus, amygdala, and the prefrontal cortex. Moreover, heightened oxidative stress was associated with decreased levels of key antioxidant enzymes. It seems like that early life stress causes behavioral and cognitive deficits via an oxidative stress-mediated weakening of neuronal connections. The central hypothesis of this Dissertation is that the ability to acquire susceptibility or resistance to stress-induced behavioral and cognitive deficits resides in oxidative-antioxidative balance within the CNS. This balance is maintained by transcriptional and epigenetic mechanisms. Therefore, our long-term goal is to investigate a) the role of early life stress on behavior and cognition across different ages in rats, b) reveal resilience and susceptible phenotypes and c) to identify the role of oxidative mechanisms in the regulation of behavioral and cognitive function and resilience. We propose to utilize a comprehensive approach to address our goals. In Aim 1, the effect of induction of early life trauma was examined using a rat model of early-life stress on later life behaviors. Sprague Dawley (SD) rats were exposed to single prolonged stress (SPS) at postnatal day (PND) 25. Behavior tests to assess anxiety-like behavior, depression-like behavior, and learning and memory function were performed at different stages of development during PND 32, 60 and 90. Resilience and susceptibility phenotypes also were examined. In Aim 2 we examined the effect of early life stress on oxidative stress mechanisms as well as transcriptional and epigenetic regulation of specific genes that presumably control antioxidative capacity. We focused explicitly on Keap1-Nrf2 and NF-κB pathway. SD rats were exposed to SPS at PND25. One group of rats were sacrificed at PND32, the other group of rats was sacrificed at PND90. Blood was collected, and plasma was used to examine systemic markers of oxidative stress and physiological stress. Brains were harvested, and specific brain areas were isolated, and homogenates were prepared for conducting biochemical analysis to determine the effect of early life SPS on oxidative-antioxidant balance, and activation of redox-sensitive pathways such as Nrf2 and NF-κB pathways. Studies proposed in aim 1 revealed that rats exposed to SPS exhibited both anxiety- and depression-like behavior at PND32. Moreover, short-term (STM) but not long-term memory (LTM) was impaired. Rats exposed to SPS at PND60 exhibited anxiety- but not depression-like behavior. STM but not LTM was impaired. Rats exposed to SPS at PND90 exhibited fearful (as indicated by elevated plus maze test) but not an overall anxiety-like behavior (in light and dark test). These rats also displayed significant depression-like behavior with no changes in STM or LTM. Interestingly, when data was further analyzed, two subsets of PND90 rats exposed to SPS were identified, “susceptible”: with depression-like behavior and “resilient”: without depression-like behavior. Importantly, while resilient group expressed early signs of anxiety- (at PND32 and PND60) and depression-like behavior (at PND32), these behavioral deficits were absent at PND90. On the other hand, susceptible PND90 rats exposed to SPS expressed later onset of anxiety-like behavior (at PND60), while depression-like phenotype was evident only later on at PND90. At the biochemical level, SPS exposure at PND25 led to an increase in oxidative stress in specific regions of the brain (pre-frontal cortex), as indicated by the increased level of oxidative stress marker at PND32 and PND90. SPS exposure at PND25 also led to an initial increase in antioxidant enzyme expression at PND32 and a decrease in antioxidant enzyme expression at PND90. The increase in oxidative stress and the decrease in antioxidant enzymes at PND90 correlates with the depressive phenotypes in SPS rats at PND90. Further biochemical studies revealed a state of a compromised Nrf2 pathway and activated NF-κB pathway in the pre-frontal cortex (PFC) homogenates. The state of compromised Nrf2 pathway and activated NF-κB pathway was indicated by a decrease in the levels of Nrf2 and increased levels of NF-κB, as well as NF-κB-mediated increased levels of interleukin-6 (IL-6) and tumor necrosis factor α (TNFα) in PFC. In summary, our findings suggest that early life stress caused co-occurrence of anxiety and depression-like behavior at PND32 (mimics human early-adolescent period). This co-occurrence was lost at PND60 with a demonstration of anxiety- but not depression-like behavior. Later, depression but not the anxiety-like behavior was observed at PND90. It seems that behavioral adaptations occur at the critical PND60 stage (mimics human late-adolescent period) (Sengupta 2013), where behavioral and cognitive switching occurs, thereby, expressing susceptible and resilient phenotypes. Moreover, susceptible phenotype correlates with an increase in oxidative stress and a decrease in antioxidant enzymes in the emotion-regulating brain region of the PFC. The correlation between susceptible phenotype and increased oxidative stress markers suggests that the early life stress causes a buildup of oxidative stress, which negatively affects neuronal circuitry that contributes to depressive phenotypes. The increase in oxidative stress induced by early life stress activates NF-κB pathway, which triggers cellular inflammatory responses.Pharmacological and Pharmaceutical Sciences, Department o