Investigating the impact of early life corticosteroid exposure on neurocognitive and neuropathological outcomes

Abstract

Synthetic glucocorticoids, such as dexamethasone, are commonly used between 24-34 weeks of gestation to reduce risks associated with preterm. However, evidence is emerging that use in late pregnancy (37–39 weeks) may adversely affect neurodevelopment, leading to long-term cognitive and neuropathological impairments in the offspring. This thesis investigates the impact of early life synthetic glucocorticoid exposure at postnatal day (PND) 7 (corresponding to late pregnancy in humans in relation to brain development) on neurocognitive and neuropathological outcomes in adolescent and adult offspring. We also set out to examine the feasibility of a double-hit model using a pre- (maternal immune activation (mIA)) and a post-natal (neonatal dexamethasone) insult in a rodent model. In chapters 3 & 4 male and female Wistar rats were administered Vehicle (0.9% saline) or dexamethasone (0.2 mg/kg, 0.5 mg/kg or 1 mg/kg, S.C.) or left untreated on PND 7. After 90 minutes, offspring were culled, and brains were collected to measure dexamethasone concentration using Liquid chromatography-tandem mass spectrometry (LC–MS/MS). Behavioural assays, including novel object recognition (NOR), object location task (OLT), open field (OFT), light/dark box (LDB), and social interaction (SI) tests were conducted during adolescent (PND 35-40) and adulthood (PND 70-75). Protein levels of synaptic and neuronal markers were assessed in the dorsal hippocampus and amygdala using both the Simple Western Protein Analyzer (Abby) and immunohistochemistry. In chapter 5 pregnant Wistar dams were injected with a viral mimetic polyinosinic:polycytidylic acid (poly I:C, 10 mg/kg i.p.) or Vehicle (0.9% saline) on gestational day (GD) 15. To confirm an immune response, levels of the proinflammatory cytokine TNFα was measured using enzyme-linked immunosorbent assay (ELISA) 3 hours post administration. In the first study male and female offspring were culled at PND 7 and glucocorticoid receptor (GR) protein levels were measured in the brain (hippocampus) in PND 7 offspring using Simple Western Protein Analyzer (Abby). In the second study male and female mIA offspring were evaluated for early neurodevelopmental behaviours including negative geotaxis (NG) and surface righting reflex (SRR) on PND 6, followed by cognitive (NOR) and anxiety-related (LDB) behaviours during adolescence and adulthood. Data from chapters 3 & 4 show that dexamethasone concentrations were significantly, and dose dependently increased in the brain 90 minutes following administration on PND 7. We also observed sex and dose dependent effects on a range of behavioural outcomes in adolescent and adult animals. These behavioural changes were accompanied by a reduction in synaptic and neuronal (parvalbumin interneurons) markers. Interestingly, most of the significant alterations in behaviours were seen in the lowest dose group (0.2 mg/kg). Taken together, these findings suggest that early-life dexamethasone exposure may induce long-term and behavioural and neurodevelopmental alterations that are not reliably dose-dependent. This non-linear response underscores the need for caution in antenatal glucocorticoid administration, particularly during late gestational weeks or when neonatal benefit is unclear. Findings from chapter 5 demonstrate that administration of poly(I:C) at 10 mg/kg on GD15 leads to increased bodyweight and GR protein levels in the brain in PND 7 offspring. Although an interesting finding in relation to potential susceptibility to a second stressor like dexamethasone administration our subsequent studies were unable to identify reliable early neurodevelopmental behaviours as predictors for identifying resilient and susceptible offspring prior to the second hit. Subsequent behavioural studies identified resilient and susceptible offspring, in relation to behavioural outcomes on PND 35, following prenatal exposure to mIA. Assessment of the same behaviours at PND 70 revealed a potential role for the LDB but not the NOR task as a behavioural readout for assigning groups at PND 35 for future studies aimed at assessing novel interventions. Overall, this work contributes to growing evidence that early-life exposure to glucocorticoids can influence brain development during critical periods. In addition, prenatal poly(I:C) exposure altered body weight and increased GR protein expression in the brains of PND 7 offspring. Together, these findings underscore the need for careful consideration regarding the timing of antenatal glucocorticoid administration and the management of infection during pregnancy. They also highlight the importance of further research to elucidate the underlying mechanisms.