Neonatal lipopolysaccharide has long-term effects on steroids and gene expression in the adult mouse brain

Early-life infections can exert profound and lasting effects on brain development and function, potentially through sustained alterations in glucocorticoid (GC) signaling. Here, we investigated whether a neonatal immune challenge alters brain steroids and gene expression in adulthood. Male and female C57BL/6J mice were injected intraperitoneally with saline vehicle or 50 μg/kg lipopolysaccharide (LPS) at postnatal day (PND) 4 and PND6, and then with either vehicle or LPS in adulthood (2 × 2 design). The brain and serum were collected 4 h after adult treatment. Using liquid chromatography-tandem mass spectrometry, we measured steroids in serum (total and free) and in the prefrontal cortex (PFC), dorsal hippocampus, ventral hippocampus, amygdala, and hypothalamus. In both groups of adults, neonatal LPS increased progesterone levels in the PFC. In LPS-treated adults, neonatal LPS increased corticosterone levels (total and free) in serum, as well as in all regions. Moreover, in LPS-treated adults, neonatal LPS increased 11-dehydrocorticosterone (DHC) levels in the dorsal hippocampus, ventral hippocampus, and hypothalamus, as well as aldosterone levels in the dorsal hippocampus and amygdala. Using bulk RNA sequencing, we examined gene expression in the dorsal hippocampus. Weighted gene co-expression network analysis identified modules showing impacts of neonatal LPS, adult LPS, and sex. Further, correlations between steroid levels and module eigengenes suggest that neonatal LPS modulates the effects of adult LPS on gene expression. These findings clarify how neonatal LPS exposure shapes brain steroid profiles and gene expression, providing new insights into the enduring impacts of early-life infection on brain function and behavior.