Abstract
In the pathology-oriented study of depression, inflammation hypothesis has received increasing attention for recent years. To mimic the depressive state caused by inflammation, rodents injected intraperitoneally with lipopolysaccharide (LPS) are usually used to stimulate an immune response. However, the dose of LPS that causes depressive-like behavior varies widely across many literatures. Previous study has uncovered the non-linearity in the dose-effect relationship for the depressive-like behavior induced by LPS administration, while the reason for this is still unclear. The present study aims to investigate the underlying mechanisms of this non-linear dose-dependent relationship. Four groups of mice were injected intraperitoneally with different doses of LPS (0, 0.32, 0.8, and 2 mg/kg). The tail suspension test was conducted to evaluate the depressive-like behavior within 23-25 h after the LPS administration. The neuroplasticity was assessed by the levels of related proteins, TrkB and PSD-95, and by the quantification of neurons using Nissl staining. The levels of the two metabolites of the kynurenine (KYN) pathway, 3-hydroxykynurenine (3-HK) and kynurenic acid (KYNA), in the brain were analyzed by LC-MS/MS. Activation of microglia and astrocytes in the brain were also determined by immunohistochemistry and western blotting, respectively. The results showed that, compared with the control group, the mice in the 0.8 mg/kg LPS-treated group exhibited a remarkable increase of immobility time in the tail suspension test. The neuroplasticity of mice in the 0.8 mg/kg LPS-treated group was also significantly reduced. The neurotoxic metabolite, 3-HK, was accumulated significantly in the hippocampus of the 0.8 mg/kg LPS-treated mice. Surprisingly, the 2 mg/kg LPS-treated mice did not exhibit a remarkable change of 3-HK but expressed increased KYNA significantly, which is neuroprotective. Furthermore, the activation of microglia and astrocytes, which were recognized as the primary source of 3-HK and KYNA, respectively, corresponded to the content of these two metabolites of the KYN pathway in each group. Consequently, it was speculated that the homeostasis of different glial cells could lead to a non-linear dose-dependent behavior by regulating the KYN pathway in the LPS-induced depressive-like mice.