Acute Neonatal Inflammation Differentially Activates Adult Microglia In Respiratory Neural Control Regions

Abstract

Adult respiratory motor plasticity is abolished by neonatal inflammation, but is restored with an adult anti-inflammatory treatment, suggesting lasting inflammatory signaling underlies deficiencies in adult respiratory motor plasticity. While neonatal inflammation impairs adult hippocampal plasticity by increasing adult microglial number (resident immune cells of the brain) and inflammatory gene expression, it is unclear if adult microglia in respiratory control regions are similarly affected by neonatal inflammation. Thus, we hypothesized that acute neonatal inflammation would increase the number of and inflammatory transcriptomic profiles in adult microglia in respiratory control regions (medulla and cervical spinal cord). Using flow cytometry, a single neonatal inflammatory challenge (1 mg/kg LPS, i.p., postnatal day 4) significantly increased adult medullary microglial frequency in males (neonatal LPS: 25 ± 3% microglia, n=11; neonatal saline: 12 ± 2% microglia, n=11, p<0.0001), although the effect was not statistically significant in females (neonatal LPS: 21 ± 4% microglia, n=10; neonatal saline: 12 ± 3% microglia, n=11, p=0.0626). Interestingly, these differences were not observed in spinal microglia in either sex (male neonatal LPS: 13 ± 5% microglia, n=12; male neonatal saline: 11 ± 3% microglia, n=10, p=0.9; female neonatal LPS: 16 ± 6% microglia, n=12; female neonatal saline: 13 ± 5% microglia, n=12, p=0.9), suggesting a neonatal LPS challenge region-specifically increases adult microglia. Similar regional differences were also observed in preliminary transcriptomic data from adult females challenged acutely as neonates; more microglial genes were differentially increased in isolated medullary microglia (271 genes, n=3) than in spinal microglia (23 genes, n=6). In addition, preliminary data suggest potential sex differences in adult microglial inflammatory gene responses in respiratory control regions following an acute neonatal LPS challenge. More inflammatory genes were increased in adult female microglia (18 genes, n=9) than in male microglia (7 genes, n=7). In conclusion, neonatal inflammation region-dependently increased medullary microglial frequency in adult males, while medullary microglia have increased inflammatory transcriptomic profiles in females. These microglial changes may contribute to the mechanisms underlying abolition of adult respiratory motor plasticity following an early life neonatal inflammatory challenge.