The role of bacterial endotoxin in the immature brain

Abstract

Aim: Epidemiological evidence suggests that infections may increase the vulnerability of the immature brain to further insults. The aim of this thesis was to investigate the role of bacterial endotoxin (lipopolysaccharide, LPS) on hypoxic-ischemic (HI) injury in the immature rat brain, and to evaluate possible underlying mechanisms of increased sensitivity.Methods: A low dose of LPS was administered (intraperitoneally) to neonatal rats and animals were thereafter subjected to HI, induced by ligation of the left carotid artery and subjected to 7.7% O2. Both the role of different periods of HI and the time interval between LPS and HI were studied. Temperature, cerebral blood flow (CBF) and blood glucose was measured in separate animals. Neuropathological examination was performed to evaluate brain injury. The global gene expression after LPS exposure was examined using microarray technique (Affymetrix GeneChip® Rat Expression Set 230A) and the statistical program, Significance analysis of microarray (SAM, FDR <5% and fold change 1.7).Results: A low dose of LPS or 20 minutes of HI did not cause brain damage but the combination induced severe brain injury. The time interval between LPS and HI played a role in neuropathological outcome as an interval of 4h, 6h and 72h induced increased sensitivity to HI, whereas a 24h interval had a preconditioning effect on the brain injury. The increased brain injury after LPS/20minHI, could not be explained by changes in body temperature or cerebral blood flow. LPS/20minHI resulted in prolonged hypoglycaemia, lasting up to 24h after LPS administration. Glucose supplementation after LPS administration had ameliorating effects on the brain injury in the cerebral cortex and hippocampus, but the damage in striatum was not affected. Microarray analysis demonstrated a total of 234 genes that were regulated in the brain, of which 96% had not previously been described in the immature brain after LPS. LPS initiated a biphasic inflammatory response in the brain and several genes related to the NF-êB, JNK and caspase pathways were differentially expressed. Conclusions: LPS induced a primary and secondary phase of sensitization of the immature brain to a subsequent HI insult, which was associated with a biphasic inflammatory response in the brain. The sensitizing effects of LPS were not related to changes in body temperature or CBF and could only partially be explained by blood glucose levels. LPS conferred tolerance in an intermediate phase, demonstrating that the temporal relationship between LPS and HI in the immature brain is critical for neuropathological outcome.