Parasite / host interaction and the neurobiology of fear

描述(申请人提供)：行为操纵假说假设寄生虫可以改变宿主的行为，以增加寄生虫的生殖适合度。原生动物寄生虫弓形虫很好地符合这种描述。弓形虫在啮齿动物的无性生命周期和猫肠道的有性生命周期之间穿梭。在啮齿动物中，寄生虫优先定位于大脑。这种寄生虫进化出了一种能力，一旦在大脑中形成了包囊，就能消除啮齿动物对猫气味的天生恐惧，并将这种恐惧转化为吸引力；这增加了啮齿动物被捕食的可能性，从而完成了寄生虫的生命周期。我们先前的工作已经证明了这种行为现象的健壮性，扁桃体中弓形虫包囊的优先形成，以及寄生虫在杏仁基底外侧减少细胞数量和树突长度的能力。为了更好地理解正常恐惧和病理性恐惧，这项资助进一步探索了弓形虫行为影响的神经生物学。特定目标1确定了参与弓形虫行为影响的候选脑区。一项时间进程研究将指出在弓形虫的行为影响首次出现之前，弓形虫包囊形成的大脑区域。此外，我们将构建对照组大鼠对猫气味反应激活的大脑区域图(使用CFOS表达作为标记)，然后确定弓形虫是否阻止这些区域的这种激活。在这个特定目标中确定的候选大脑区域将在随后的目标中进行研究。特定目的2研究弓形虫是否会在局部脑区引起炎症，以及这是否会导致行为综合症。它还将调查这种寄生虫是否会改变不同脑区的细胞(神经元和胶质细胞)的数量，和/或改变神经元树突过程的复杂性。最后，它将检查弓形虫是否能阻止猫气味对这些候选脑区细胞外谷氨酸、GABA和多巴胺浓度的影响。具体目标3将探索弓形虫作用的还原基础。第一部分将分析与寄生虫行为影响有关的大脑区域的蛋白质表达谱，以及寄生虫中的蛋白质表达谱。第二个实验将是条件培养液实验，以确定培养弓形虫的培养液是否能对大脑产生与寄生虫本身相同的影响。这将是确定寄生虫分泌的因子是否调节行为影响的第一步。这项拨款中提出的与公共卫生相关的实验探索了寄生虫学和神经生物学之间不太可能的交叉，即由于弓形虫感染，啮齿动物失去了天生的恐惧。病理性恐惧是几种精神障碍的核心。了解弓形虫是如何消除恐惧的，将有助于我们了解恐惧最初是如何产生的，以及我们如何管理病理性恐惧。

DESCRIPTION (provided by applicant): The behavioral manipulation hypothesis posits that parasites can change the behavior of hosts to increase the reproductive fitness of the parasite. The protozoan parasite Toxoplasma gondii fits this description well. Toxoplasma shuttles between an asexual life-cycle in rodents and a sexual life-cycle in cat intestine. In rodents, parasites preferentially localize to the brain. The parasite has evolved the capacity to, once having formed cysts in the brain, abolish the innate fear that rodents have of the odors of cats, and to convert that fear into an attraction; this increases the likelihood of the rodent being predated, thereby completing the parasite's life cycle. Our prior work has shown the robustness of this behavioral phenomenon, the preferential formation of Toxoplasma cysts in the amygdala, and the capacity of the parasite to reduce cell number and dendrite length in the basolateral amygdala. The present grant further explores the neurobiology of Toxoplasma's behavioral effects, in order to better understand both normal and pathological fear. Specific Aim 1 identifies brain regions that are candidates for mediating the behavioral effects of Toxoplasma. A time course study will indicate brain regions where Toxoplasma cysts form prior to the first emergence of the behavioral effects of the parasite. In addition, we will construct a map of brain regions activated in response to cat odor in control rats (using cFos expression as a marker), and then determine whether Toxoplasma blocks such activation in any of those regions. The candidate brain regions identified in this Specific Aim will be studied in the subsequent Aims. Specific Aim 2 studies whether Toxoplasma causes inflammation in local brain regions and whether that contributes to the behavioral syndrome. It will also investigate whether the parasite changes the number of cells (both neurons and glia) in different brain regions, and/or changes the complexity of dendritic processes in neurons. Finally, it will examine whether Toxoplasma blocks the effects of cat odor on extracellular concentrations of glutamate, GABA and dopamine in those candidate brain regions. Specific Aim 3 will explore the reductive bases of Toxoplasma's actions. The first part will be an analysis of protein expression profiles in brain regions implicated in the parasite's behavioral effects, and in the parasite as well. The second will be a conditioned medium experiment, determining whether the medium in which cultured Toxoplasma grows can have the same effects in the brain as the parasite itself. This will be a first step in determining whether a factor secreted by the parasite mediates the behavioral effects. PUBLIC HEALTH RELEVANCE Experiments proposed in this grant explore an unlikely intersection of parasitology and neurobiology, that is, loss of innate fear in rodents due to Toxoplasma infection. Pathological fear is central to several psychiatric disorders. Understanding how Toxoplasma abolishes fear will shed light on how fear is generated in the first place and how we can manage pathological fear.