Resolution of Lipopolysaccharide-Induced Inflammation Followed by DNA Hypomethylation and Increased Tetrahydrobiopterin Biosynthesis in Mouse Hippocampus

<b>Background</b>: Robust evidence supports the role of tetrahydrobiopterin (BH4) metabolism in sustaining inflammation; however, the mechanisms underlying the persistent upregulation of the BH4 pathway remain incompletely understood. This study investigated the epigenetic regulation of BH4 metabolism following a single injection of lipopolysaccharide (LPS) in the mouse hippocampus. <b>Methods</b>: Male C57BL/6J mice received either saline or LPS (0.33 mg/kg, i.p.) and were sacrificed at 4 h or 24 h post injection. Behavioral assessments and analyses of hippocampal neurotransmitter metabolism, DNA methylation profile, oxidative stress, and inflammasome activation were performed. Neopterin levels, a marker of immune system activation, were measured in both the plasma and hippocampus. <b>Results</b>: LPS-treated mice exhibited sickness behavior, including reduced locomotor and exploratory activity at both 4 and 24 h. While exploratory behavior showed partial recovery by 24 h, locomotor activity remained impaired. Neopterin levels increased in both the plasma and hippocampus following LPS administration but returned to baseline in the hippocampus by 24 h. Despite the normalization of neopterin, a persistent pro-inflammatory state in the hippocampus was evident at 24 h, as shown by increased expression of Ikbkb and components of the NLRP3 inflammasome, along with elevated oxidative stress markers. Upregulation of <i>Nrf-2</i> and <i>Hmox1</i> suggested activation of a protective antioxidant response. Dopaminergic metabolism was disrupted, indicating impaired BH4-dependent dopamine turnover. Epigenetic analysis revealed increased expression of DNA methyltransferases (<i>Dnmt1</i>, <i>Dnmt3a</i>, <i>Dnmt3b</i>) and <i>Tet2</i>, along with reduced expression of <i>Tet1</i> and <i>Tet3</i>. Promoter hypomethylation of <i>Gch1</i> and <i>Ptps</i> was observed, correlating with increased hippocampal expression and potentially elevated BH4 levels. <b>Conclusions</b>: Together, these findings show that a single LPS challenge was sufficient to induce the activation of the BH4 synthesis pathway during the late acute inflammatory phase, both systemically and in the hippocampus, potentially driven by epigenetic modifications such as promoter hypomethylation. This may contribute to the perpetuation of neuroinflammation.