Hypercapnia‐evoked Chronic Stress Alters IL‐1β Levels in Brain Respiratory Chemoreceptor Regions

Abstract

Chronic stress often elicits neuronal, hormonal, and immune responses. Long term hypercapnia is a respiratory stressor linked with sleep apnea, sudden infant death syndrome (SIDS) and chronic obstructive pulmonary disease (COPD), however, mechanisms of central respiratory dysfunction in these conditions are unclear. We aimed to explore the neuroinflammatory responses to hypercapnia in key brain respiratory regions. We tested the hypothesis that long-term exposure to 8% CO2 activates the releaseof brain stress hormone corticosterone (CORT) in the circulation which may contribute to inflammation in brainstem circuits that regulate CO2-stimulated breathing. Age-matched adult male C57BL/6J mice (n=10 mice/group) were exposed to hypercapnia (8% CO2; balance room air) in the short-term (6 hours) or long-term (10 or 14 days), or room air (control) conditions.Following anesthesia, plasma samples were collected for corticosterone (CORT) assay and brain tissues including cerebellum, hypothalamus, the nucleus of the solitary tract (NTS), rostral and caudal retrotrapezoid nucleus (rRTN, cRTN) were micro-dissected and processed for cytokine assays. Plasma CORT levels were not different between hypercapnia and room air groups in the short-term, but a significantly higher CORT level (p < 0.01, 2 way ANOVA) was observed following 10 days of exposure to hypercapnia compared to control conditions. CORT level returned to baseline following 14 days of hypercapnia compared to control (14 days room air) conditions. In the brain, the pro-inflammatory cytokine interleukin-1β (IL-1β) was raised in the cerebellum (p < 0.01, 2 way ANOVA) following 6 hours but not 10 days of hypercapnia exposure, compared to control conditions. In the hypothalamus and cRTN, IL-1β was unaltered at the 6 hours time point, significantly upregulated (p < 0.05, p < 0.01, 2 way ANOVA) at 10 days of hypercapnia exposure, compared to controls. In these brain regions, IL-1β levels returned to baseline after 14 days hypercapnia exposure time point. There was no change in IL-1β in the rRTN and NTS following 6 hours, 10 days, and 14 days of hypercapnia, compared to controls. Taken together, the level of plasma CORT positively correlates to brain IL-1β during long-term hypercapnia. These results suggest that hypercapnia activates a physiological stress response, and the associated brain inflammation depends on the duration of hypercapnia exposure. We propose that the stress and inflammatory adaptive responses observed are part of compensatory mechanisms following long-term hypercapnia. The cellular source and underlying mechanisms of interactions between plasma CORT and brain IL-1β are under investigation.