Neuroimmune Mechanisms of Depressive-Like Behavior During Aging

描述（由申请人提供）：炎症现在被认为是导致人口老龄化的主要健康问题，导致诸如肥胖、代谢综合征、心脏病和2型糖尿病中的胰岛素抵抗等代价高昂的疾病。即使是健康衰老的慢性过程也与低度炎症的发展有关。然而，年龄相关的炎症变化主要被认为与全身性疾病有关。我们和其他人现在已经收集了大量证据，表明这种炎症状态，由促炎细胞因子的过度表达所定义，不仅限于外周，而且在大脑中也有发现。脑部炎症引起的疾病症状通常与微生物感染有关，这可能是导致老年人共病行为和心理障碍的重要原因。事实上，65岁以上的人中有五分之一患有抑郁症，这是普通人群患病率的两倍多。随着年龄的增长，抑郁症患病率增加的一个可能机制是血清素合成的减少，血清素是调节情绪的关键神经递质，是由大脑中的促炎细胞因子引起的。这种作用是由免疫诱导的色氨酸降解酶吲哚胺2,3双加氧酶（IDO）的激活介导的。这一过程降低了合成血清素所需色氨酸的生物利用度。我们的初步数据表明，外周免疫激活激活IDO并诱导抑郁样行为改变，与成年小鼠相比，这些影响在老年小鼠中加剧。基于这些证据，我们认为外周免疫激活促进了老年人情绪障碍的发生。我们建议在暴露于急性和慢性外周免疫激活的老年小鼠中测试这一假设，我们已经证明这两种事件可以增加大脑IDO活性。在第一个目标中，我们将确定老年小鼠对急性和慢性外周免疫激活的反应是否会加剧抑郁样的行为改变。在第二个目标中，我们将评估外周和大脑IDO以及脑色氨酸和血清素的增加在这些行为变化中的作用，而第三个目标将确定脑胶质细胞对年龄相关的脑IDO增加的贡献。然后，我们将使用新的药理学方法来确定靶向脑炎症（目标4）或脑IDO（目标5）是否会减弱衰老对抑郁样行为发展的功能后果。这些令人兴奋的实验将首次使用综合神经免疫方法来评估IDO在年龄相关的外周和大脑炎症增加以及老年人情绪障碍患病率增加之间的关键中介作用。

DESCRIPTION (provided by applicant): Inflammation is now recognized to be responsible for major health problems of the aging population, contributing to costly diseases such as obesity, the metabolic syndrome, heart disease and insulin resistance in type-2 diabetes. The chronic process of even healthy aging is also associated with development of low grade inflammation. However, age-related changes in inflammation have mainly been considered in relation to systemic disorders. We and others have now collected substantial evidence to show that this inflammation status, as defined by the overexpression of proinflammatory cytokines, is not restricted to the periphery but is also found in the brain. Brain inflammation causes symptoms of sickness that are usually associated with microbial infections, which is likely to make an important contribution to the comorbid behavioral and psychological disturbances that occur in the elderly. Indeed, one in five individuals over the age of 65 suffer from depressive disorders, which is more than twice the prevalence found in the general population. A likely mechanism for the increased prevalence of depressive disorders during aging is a reduction in the synthesis of serotonin, a key neurotransmitter in the regulation of mood, caused by proinflammatory cytokines acting in the brain. This action is mediated by immune-induced activation of the tryptophan-degrading enzyme, indoleamine 2,3 dioxygenase (IDO). This process decreases the bioavailability of tryptophan for the synthesis of serotonin. Our preliminary data indicate that peripheral immune activation activates IDO and induces depressive-like behavioral alterations, and these effects are exacerbated in aged compared to adult mice. Based on this evidence, we propose that peripheral immune activation precipitates the occurrence of mood disorders in aged individuals. We propose to test this hypothesis in aged mice exposed to both acute and chronic peripheral immune activation, two events that we have already shown to increase brain IDO activity. In the first objective, we will determine whether the depressive-like behavioral alterations that develop in response to both acute and chronic peripheral immune activation are exacerbated in aged mice. In the second objective, we will assess the role of increases in peripheral and brain IDO, as well as brain tryptophan and serotonin, in these behavioral changes, whereas the third objective will determine the contribution of brain glial cells to the age-associated increase in brain IDO. We will then use novel pharmacological approaches to determine if targeting brain inflammation (Objective 4) or brain IDO (Objective 5) attenuates the functional consequences of aging on development of depressive like behavior. These exciting experiments will be the first to use integrative neuroimmune approaches to evaluate IDO as the critical mediator between the age-related increase in peripheral and brain inflammation and the increased prevalence of mood disorders in aged individuals.