Background: Chronic exposure to highly stressful situations is detrimental to the wellbeing of both body and brain. Chronic stress, especially during the unique developmental timeframe of adolescence and early adulthood, can have adverse effects on both behavioral and metabolic outcomes later in life. Chronic exposure to stress dysregulates the hypothalamic-pituitary-adrenal (HPA) axis causing an increase in systemic inflammation resulting in increased risk of neurodegenerative disorders, such as Alzheimer’s disease. Chronic adolescent stress in rodents alters glucose transporters in the brain, suggesting implications in metabolism and cognitive impairment. This project aimed to assess the impact of chronic stress and chronic low-level inflammation on behavior and synaptosomal metabolism.
Methods: Male (n = 31) and female (n = 32) C57Bl/6 mice underwent chronic repeated predation stress or daily handling for two rounds of 15 consecutive days of exposure during the adolescent and early adult timeframes. Subsequently, mice were exposed to repeated lipopolysaccharide (LPS; 7.5 x 105 EU/kg) or saline injections every third day for eight weeks. Exploratory and social behaviors were assessed in the open field and social interaction tests prior to examination of learning and memory with the Barnes Maze. Mitochondrial function and morphology were assessed in synaptosomes post-mortem. In addition, expression of TNF-α, IL-1ß, and ROMO1 were examined in the hippocampus and prefrontal cortex. Circulating pro- and anti-inflammatory cytokines in the periphery were assessed following the first and last LPS injection as well as at the time of tissue collection. Circulating ROMO1 was assessed in terminal samples.
Results: Exposure to repeated predatory stress increased time spent in the corners of the open field, suggestive of anxiety-like behavior, in both males and females. There were no significant group differences in the social interaction test and minimal effects were evident in the Barnes maze. A history of chronic stress interacted with chronic LPS in male mice to lead to a deficit in synaptosomal respiration. Female mice were more sensitive to both chronic stress and chronic LPS such that either a history of chronic stress or a history of chronic LPS was sufficient to disrupt synaptosomal respiration in females. Both stress and chronic LPS were sufficient to increase inflammation and reactive oxygen in males in the periphery and centrally. Females had increased markers of peripheral inflammation following acute LPS but no evidence of peripheral or central increases in inflammatory factors or reactive oxygen following chronic exposures.
Conclusion: Collectively, these data suggest that while metrics of inflammation and reactive oxygen are disrupted in males following chronic stress and chronic LPS, only the combined condition is sufficient to alter synaptosomal respiration. Conversely, although evidence of chronic inflammation or chronic elevation in reactive oxygen is absent, females demonstrate profound shifts in synaptosomal mitochondrial function with either a history of chronic stress or a history of chronic inflammation. These data highlight that differential mechanisms are likely in play between the sexes and suggest that female sensitivity to neurogenerative conditions may be precipitated by influence of life experiences on mitochondrial function in the synapses.