Differential Responses of Male and Female Microglia to LPS and Estrogen May Contribute to Sexually Dimorphic Effects of Inflammation on Phrenic Long‐term Facilitation

Chronic intermittent hypoxia (CIH) is a hallmark of obstructive sleep apnea (OSA) that causes neuroinflammation, and acutely (1 night) impairs one form of respiratory motor plasticity, phrenic long‐term facilitation (pLTF). The prevalence of OSA is sexually dimorphic (higher in males) although this difference greatly decreases after menopause, suggesting that estrogen may play a role in protecting the respiratory system in females. Preliminary data suggest that impairment of pLTF by neuroinflammation induced by challenge with gram‐negative bacterial lipopolysaccharide (LPS; 0.1 mg/kg) may also be sexually dimorphic; LPS treatment abolished pLTF in gonadally intact males but not in females treated with estrogen. Although estrogen is well‐known to be anti‐inflammatory, little is known about its ability to control neuroinflammation as it impacts respiratory neuroplasticity. We tested the hypothesis that LPS‐induced neuroinflammation is higher in males, and that estrogen exerts sex‐specific effects on neuroinflammatory gene expression. To begin to test this hypothesis, primary microglia from P4‐P6 male and female rat pups were cultured separately and treated with LPS (100 ng/ml) in the presence and absence of 17β‐estradiol (E2; 1nM). Total RNA was isolated and gene arrays (SA Biosciences) were performed; individual gene changes were confirmed by qRT‐PCR. Whereas LPS increased the expression of multiple pro‐inflammatory cytokines and chemokines in microglia from both sexes, the effects were stronger in male microglia. E2 treatment alone strongly reduced basal pro‐inflammatory gene expression in female microglia, but had little effect in male microglia. E2 had generally similar effects on LPS‐induced gene expression in microglia from both sexes. These data suggest that male microglia have stronger pro‐inflammatory responses to LPS, and that reductions in basal levels of inflammatory genes by estrogen are greater in female microglia. Together, these mechanisms may contribute to higher levels of neuroinflammation in the male CNS following LPS, and to the corresponding sex differences in inflammation‐mediated impairment of pLTF. Results from this work will provide significant insight into mechanisms that can be exploited to minimize destabilization of respiratory plasticity by inflammation in both sexes. Support or Funding Information Supported by R01 NS085226 and an OWHR supplement to R01 HL111598