Cytokines in the rat brain during hypoxemic hypoxia and their influence on cell death

The underlying mechanisms of secondary acute brain injury in type-1 respiratory failure could be either hypoxia-mediated or inflammation-mediated cell death in the brain. This study was aimed to explore changes in expression and concentrations of hypoxia- and inflammation-related cytokines in the brain interstitial fluid (ISF) and plasma. Conscious Sprague-Dawley rats were maintained at low PiO2 for 48 h and interstitial fluid (ISF) samples from the CC and plasma samples were collected. Concentrations of 32 cytokines in those samples were estimated, and expression of >80 cytokines at the transcript level in the CC samples determined. The number of dead cells was determined using the TUNEL assay. The concentrations of cytokines in the ISF and plasma samples were tested with Repeated-Measures ANOVA and Mixed-Measures ANOVA, respectively. The number of dead cells was compared with one-way ANOVA with Multiple comparisons. Fold changes in the amounts of mRNAs were compared by Student’s t-test. Statistical significance was set at p<0.05. We could estimate concentrations of 14 cytokines in ISF samples, with no significant effect of the duration of hypoxia on their concentrations. There was a >2-fold increase in concentrations of proinflammatory cytokines MIP-1α and TNFα and of anti-inflammatory cytokine IL-10. In plasma, we could determine concentrations of 10 cytokines in all samples and the duration of hypoxia exerted significant effects on the concentrations of 5 cytokines. Manifold increases in erythropoietin (EPO) and FGF21 concentration were observed. Hypoxia caused downregulation in the expression of genes for six pro-inflammatory, three biphasic and two anti-inflammatory cytokines in the CC, while the expression of vascular-endothelial growth factor (VEGF) and Osteopontin was upregulated. Thus, a complex pattern of cytokine signaling, rather than a clear pro- or anti-inflammatory response was revealed. In addition, inhibition of (EPO) and (VEGF) signaling in the brain significantly reduced the number of dead cells during hypoxia, while inhibition of peripheral TNFα was detrimental for the hypoxic brain.