Abstract
Relapse is common in remitted patients with major depressive disorder (MDD). Arketamine, an (R)-enantiomer of ketamine, has persistent prophylactic actions in an inflammatory model of depression. However, the precise mechanisms underlying these prophylactic actions remain unknown. Given the role of the brain-spleen axis in depression, we sought to identify splenic molecular targets that play a role in the prophylactic actions of arketamine. Lipopolysaccharide (LPS) (1.0 mg/kg) was administered 6 days after a single injection of arketamine (10 mg/kg) or saline. RNA-sequencing analysis found altered expression in the heme biosynthesis II pathway. Quantitative RT-PCR revealed that pretreatment with arketamine blocked increased expression of genes involved in the heme biosynthesis II pathway in LPS-treated mice, namely, 5-aminolevulinase synthase 2 (Alas2), ferrochelatase (Fech), hydroxymethylbilane synthase (Hmbs). Interestingly, there were positive correlations between the expression of these genes and spleen weight or plasma levels of pro-inflammatory cytokines. We also found higher expression of ALAS2 and FECH in the spleen from MDD patients. Pretreatment with a key intermediate precursor of heme, 5-aminolaevulinic acid (300 mg/kg/day for 3 days), caused splenomegaly, higher plasma levels of pro-inflammatory cytokines, and depression-like behavior in low-dose LPS (0.1 mg/kg)-treated mice. Interestingly, pretreatment with a heme biosynthesis inhibitor, succinyl acetone (120 mg/kg/day for 3 days), had prophylactic effects in LPS (1.0 mg/kg)-treated mice. These data suggest a novel role for the heme biosynthesis II pathway in the spleen for inflammation-related depression. Therefore, the heme biosynthesis pathway could be a new target for the prevention of relapse in MDD patients.