Inflammation plays an important role in the pathogenesis of depression; however, the underlying mechanisms remain unclear. Apart from the disordered circadian rhythm in animal models and patients with depression, dysfunction of clock genes has been reported to be involved with the progress of inflammation. This study aimed to investigate the role of circadian clock genes, especially brain and muscle ARNT-like 1 (Bmal1), in the linkage between inflammation and depression. Lipopolysaccharide (LPS)-challenged rats and BV2 cells were used in the present study. Four intraperitoneal LPS injections of 0.5 mg/kg were administered once every other day to the rats, and BV2 cells were challenged with LPS for 24 h at the working concentration of 1 mg/L, with or without the suppression of Bmal1 via small interfering RNA. The results showed that LPS could successfully induce depression-like behaviors and an “inflammatory storm” in rats, as indicated by the increased immobility time in the forced swimming test and the decreased saccharin preference index in the saccharin preference test, together with hyperactivity of the hypothalamic–pituitary–adrenal axis, hyperactivation of astrocyte and microglia, and increased peripheral and central abundance of tumor necrosis factor-α, interleukin 6, and C-reactive protein. Moreover, the protein expression levels of brain-derived neurotrophic factor, triggering receptor expressed on myeloid cells 1, Copine6, and Synaptotagmin1 (Syt-1) decreased in the hippocampus and hypothalamus, whereas the expression of triggering receptor expressed on myeloid cells 2 increased. Interestingly, the fluctuation of temperature and serum concentration of melatonin and corticosterone was significantly different between the groups. Furthermore, protein expression levels of the circadian locomotor output cycles kaput, cryptochrome 2, and period 2 was significantly reduced in the hippocampus of LPS-challenged rats, whereas Bmal1 expression was significantly increased in the hippocampus but decreased in the hypothalamus, where it was co-located with neurons, microglia, and astrocytes. Consistently, apart from the reduced cell viability and increased phagocytic ability, LPS-challenged BV2 cells presented a similar trend with the changed protein expression in the hippocampus of the LPS model rats. However, the pathological changes in BV2 cells induced by LPS were reversed after the suppression of Bmal1. These results indicated that LPS could induce depression-like pathological changes, and the underlying mechanism might be partly associated with the imbalanced expression of Bmal1 and its regulated dysfunction of the circadian rhythm.

Graphical Abstract

Conclusions In conclusion, our results indicated that LPS could induce depression-like behaviors and an “inflammatory storm” in vivo, activate microglia in vitro, together with a disruption of synaptic plasticity and circadian rhythm. This mechanism may be involved in the imbalanced expression of Bmal1. Our results can help uncover a novel clock-immunological mechanism of neuroinflammation-induced depression and shed light on the development of new effective pharmacotherapies for depression

中文翻译：

---

Bmal1 在脂多糖诱导的抑郁样行为及其相关中的潜在作用

炎症在抑郁症的发病机制中起着重要作用。然而，其根本机制仍不清楚。据报道，除了动物模型和抑郁症患者的昼夜节律紊乱外，生物钟基因的功能障碍也与炎症的进展有关。本研究旨在探讨生物钟基因，尤其是大脑和肌肉 ARNT 样 1 (Bmal1) 在炎症和抑郁症之间的联系中的作用。本研究使用脂多糖 (LPS) 攻击的大鼠和 BV2 细胞。每隔一天给大鼠腹腔注射4次0.5 mg/kg LPS，并用1 mg/L工作浓度的LPS攻击BV2细胞24 h，有或没有通过小干扰RNA抑制Bmal1 。结果表明，LPS能够成功诱导大鼠抑郁样行为和“炎症风暴”，表现为强迫游泳试验中不动时间增加、糖精偏好试验中糖精偏好指数下降，以及大鼠的过度活跃。下丘脑-垂体-肾上腺轴，星形胶质细胞和小胶质细胞过度活化，以及肿瘤坏死因子-α、白细胞介素 6 和 C 反应蛋白的外周和中枢丰度增加。此外，海马和下丘脑中脑源性神经营养因子、髓系细胞1表达的触发受体、Copine6和突触结合蛋白1（Syt-1）的蛋白表达水平降低，而髓系细胞2表达的触发受体的蛋白表达水平增加。有趣的是，各组之间的温度波动以及褪黑激素和皮质酮的血清浓度存在显着差异。此外，在 LPS 攻击的大鼠海马中，昼夜节律运动输出周期 kaput、隐花色素 2 和 period 2 的蛋白表达水平显着降低，而 Bmal1 表达在海马中显着增加，但在下丘脑中降低，其中 Bmal1 表达水平在下丘脑中显着降低。 -位于神经元、小胶质细胞和星形胶质细胞处。一致地，除了细胞活力降低和吞噬能力增加之外，LPS攻击的BV2细胞呈现出与LPS模型大鼠海马中蛋白质表达变化类似的趋势。然而，抑制Bmal1后，LPS诱导的BV2细胞的病理变化得到逆转。这些结果表明，LPS可引起抑郁样病理变化，其潜在机制可能部分与Bmal1表达失衡及其昼夜节律调节功能障碍有关。

图形概要

结论 总之，我们的结果表明，LPS 可以在体内诱发抑郁样行为和“炎症风暴”，在体外激活小胶质细胞，同时破坏突触可塑性和昼夜节律。该机制可能与Bmal1的表达失衡有关。我们的结果可以帮助揭示神经炎症诱发抑郁症的新型时钟免疫机制，并为开发新的有效抑郁症药物疗法提供线索