Fonken Laura K, Frank Matthew G, Kitt Meagan M, Barrientos Ruth M, Watkins Linda R, Maier Steven F
Department of Psychology and Neuroscience, University of Colorado, Boulder, CO 80309, USA.
Department of Psychology and Neuroscience, University of Colorado, Boulder, CO 80309, USA.
Brain Behav Immun. 2015 Mar;45:171-9. doi: 10.1016/j.bbi.2014.11.009. Epub 2014 Nov 26.
The circadian system regulates many physiological functions including inflammatory responses. For example, mortality caused by lipopolysaccharide (LPS) injection varies depending on the time of immunostimulation in mammals. The effects of more subtle challenges on the immune system and cellular mechanisms underlying circadian differences in neuroinflammatory responses are not well understood. Here we show that adult male Sprague-Dawley rats injected with a sub-septic dose of LPS during the light phase displayed elevated sickness behaviors and hippocampal cytokine production compared to rats injected during the dark phase. Microglia are the primary central nervous system (CNS) immune cell type and may mediate diurnal differences in sickness response, thus we explored whether microglia demonstrate temporal variations in inflammatory factors. Hippocampal microglia isolated from adult rats rhythmically expressed inflammatory factors and circadian clock genes. Microglia displayed robust rhythms of TNFα, IL1β and IL6 mRNA, with peak cytokine gene expression occurring during the middle of the light phase. Microglia isolated during the light phase were also more reactive to immune stimulation; such that, ex vivo LPS treatment induced an exaggerated cytokine response in light phase-isolated microglia. Treating microglia with corticosterone ex vivo induced expression of the circadian clock gene Per1. However, microglia isolated from adrenalectomized rats maintained temporal differences in clock and inflammatory gene expression. This suggests circadian clock gene expression in microglia is entrained by, but oscillates in the absence of, glucocorticoids. Taken together, these findings demonstrate that microglia possess a circadian clock that influences inflammatory responses. These results indicate time-of-day is an important factor to consider when planning inflammatory interventions such as surgeries or immunotherapies.
昼夜节律系统调节包括炎症反应在内的许多生理功能。例如，在哺乳动物中，脂多糖（LPS）注射所致的死亡率因免疫刺激时间而异。对于免疫系统更细微的挑战以及神经炎症反应中昼夜差异的潜在细胞机制，人们尚未完全了解。在此，我们表明，与在黑暗阶段注射LPS的大鼠相比，在光照阶段注射亚致死剂量LPS的成年雄性Sprague-Dawley大鼠表现出更高的疾病行为和海马细胞因子产生。小胶质细胞是中枢神经系统（CNS）主要的免疫细胞类型，可能介导疾病反应中的昼夜差异，因此我们探究了小胶质细胞是否在炎症因子方面表现出时间变化。从成年大鼠分离出的海马小胶质细胞有节律地表达炎症因子和昼夜节律时钟基因。小胶质细胞呈现出强大的TNFα、IL1β和IL6 mRNA节律，细胞因子基因表达峰值出现在光照阶段中期。在光照阶段分离出的小胶质细胞对免疫刺激也更具反应性；因此，体外LPS处理在光照阶段分离的小胶质细胞中诱导出夸张的细胞因子反应。体外使用皮质酮处理小胶质细胞可诱导昼夜节律时钟基因Per1的表达。然而，从肾上腺切除大鼠分离出的小胶质细胞在时钟和炎症基因表达方面仍保持时间差异。这表明小胶质细胞中昼夜节律时钟基因的表达受糖皮质激素的影响，但在没有糖皮质激素的情况下仍会振荡。综上所述，这些发现表明小胶质细胞拥有一个影响炎症反应的昼夜节律时钟。这些结果表明，在规划诸如手术或免疫疗法等炎症干预措施时，一天中的时间是一个需要考虑的重要因素。
