Lipopolysaccharide-Induced Systemic Inflammation in the Neonatal Period Increases Microglial Density and Oxidative Stress in the Cerebellum of Adult Rats.

Inflammatory processes occurring in the perinatal period may affect different brain regions, resulting in neurologic sequelae. Injection of lipopolysaccharide (LPS) at different neurodevelopmental stages produces long-term consequences in several brain structures, but there is scarce evidence regarding alterations in the cerebellum. The aim of this study was to evaluate the long-term consequences on the cerebellum of a systemic inflammatory process induced by neonatal LPS injection. For this, neonatal rats were randomly assigned to three different groups: naïve, sham, and LPS. Saline (sham group) or LPS solution (1 mg/kg) was intraperitoneally injected on alternate postnatal days (PN) PN1, PN3, PN5, and PN7. Spontaneous activity was evaluated with the open field test in adulthood. The cerebellum was evaluated for different parameters: microglial and Purkinje cell densities, oxidative stress levels, and tumor necrosis factor alpha (TNF-α) mRNA expression. Our results show that administration of LPS did not result in altered spontaneous activity in adult animals. Our data also indicate increased oxidative stress in the cerebellum, as evidenced by an increase in superoxide fluorescence by dihydroethidium (DHE) indicator. Stereological analyses indicated increased microglial density in the cerebellum that was not accompanied by Purkinje cell loss or altered TNF-α expression in adult animals. Interestingly, Purkinje cells ectopically positioned in the granular and molecular layers of the cerebellum were observed in animals of the LPS group. Our data suggest that neonatal LPS exposure causes persistent cellular and molecular changes to the cerebellum, indicating the susceptibility of this region to systemic inflammatory insults in infancy. Further investigation of the consequences of these changes and the development of strategies to avoid those should be subject of future studies.

Neonatal inflammatory pain increases hippocampal neurogenesis in rat pups.

Preterm infants undergo several painful procedures during their stay in neonatal intensive care units. Previous studies suggest that early painful experiences may have an impact on brain development. Here, we used an animal model to investigate the effect of neonatal pain on the generation of new neurons in the dentate gyrus region of the hippocampus. Rat pups received intraplantar injections of complete Freund's adjuvant (CFA), a painful inflammatory agent, on either P1 or P8 and were sacrificed on P22. We found that rat pups injected with CFA on P8 had more BrdU-labeled cells and a higher density of cells expressing doublecortin (DCX) in the subgranular zone of the dentate gyrus. No change in BrdU-labeling or DCX expression was observed in pups injected with CFA on P1. These findings indicate that neonatal pain can increase hippocampal neurogenesis, suggesting that early painful experiences may shape brain development and thereby influence behavioral outcome.

Repetitive nociceptive stimuli in newborn rats do not alter the hippocampal neurogenesis.

The aim of this study was to evaluate the effects of acute repetitive nociceptive stimuli to newborn rats over neurogenesis rate in the dentate gyrus of hippocampus. Newborn rats were randomly distributed in three groups, according to the type of stimulus received from d 1 to d 7 of life four times per day: acute nociceptive stimuli, tactile stimuli, and none (controls). At d 21 of life, rats received BrdU (5-bromo-2'-deoxyuridine, a marker of cell proliferation), and on d 28, they were killed. After brain sectioning, the tissue was processed for immunohistochemical identification of proliferating cells. BrdU-positive cell counts were performed in the granule cell layer of the dentate gyrus. Acute nociceptive stimulation during the first week of life was not associated to alterations in the weight gain among experimental groups. On examination 1 wk after BrdU injection, the number of BrdU-labeled cells in the dentate gyrus of the hippocampus was not significantly different between stimulated (painful and tactile stimuli) and nonstimulated animals. These results show that both types of stimuli used here, when applied very early in the development, do not affect the neurogenesis rate in the rat dentate gyrus.