Role of dopamine in adolescent maturation of prefrontal cortical circuits and associated behaviors

青春期，人类的年龄跨度大致为12-18岁，啮齿动物的年龄跨度为4-8周，是人生的关键时期，通常会发生成人决策、认知控制和奖励相关行为的轮廓。在大脑回路中，前额叶皮质(PFC)及其与纹状体的连接对这些行为起着至关重要的支持作用。前额叶和纹状体内的发育过程被认为与持续到青春期后期/成年早期的渐进性变化有关。有趣的是，中脑多巴胺(DA)的输入在PFC和基底节特别突出，在青春期左右达到顶峰。由于DA神经支配的发育与皮质/纹状体的发育是平行的，因此认为青春期DA可能在与PFC和纹状体相关的行为的成熟过程中起作用。然而，目前对黑质/腹侧被盖区(SN/VTA)中脑DA神经元的活动如何协调PFC和纹状体的神经适应机制知之甚少。我们的研究计划的目标是检查DA传递的发育内稳态是否以及如何影响神经回路的完善，从而导致特有的成人行为。我们推测，DA通过影响树突棘的发育、突触的修剪和PFC和纹状体神经元的分化来调节PFC和皮质纹状体回路的成熟。我们将在小鼠身上使用一种创新的药物遗传学系统，在特定的发育时期可逆地沉默或过度刺激中脑DA神经元，以研究其长期后果。该方法涉及AAV病毒介导的依赖Cre的DREADD(由设计者药物激活的Designer受体)在转基因DA转运体-Cre小鼠的中脑DA神经元中的表达。利用这一系统，本提案的目的是解决以下问题：(1)青春期过量的DA神经元活动是否会改变小鼠的运动、认知、社会和奖赏相关行为？(2)过量的DA活动对磁共振成像(MRI)监测的大脑发育轨迹有何影响？我们还将用高尔基-考克斯染色和免疫染色研究神经元的形态和功能发育。(3)调节青少年DA操纵效应的机制是什么？我们将检测在青春期突触修剪中起关键作用的细胞因子和小胶质细胞的水平。我们还将研究抑制性中间神经元的发育，特别是表达GABA的小白蛋白神经元。(4)研究DA D_1和D_2样受体是否介导了青少年DA过度活动引起的长期神经元和行为改变。我们希望我们的研究将填补关于神经调节剂在大脑发育基本过程中的作用的知识的一个重要空白。

Adolescence, an age span roughly ranging from 12-18 years in humans and 4-8 weeks in rodents, is a critical period in life when contours of adult decision-making, cognitive control and reward-related behaviors typically take place. Among brain circuits, the prefrontal cortex (PFC) and its connectivity with the striatum critically support these behaviors. Developmental processes within the PFC and striatum are believed to be linked to progressive changes lasting till late adolescent/early adult ages. Interestingly, midbrain dopamine (DA) inputs, that are particularly prominent in the PFC and the basal ganglia, peak around adolescence. Due to the parallel between the development of DA innervation and cortical/striatal development, it has been suggested that adolescent DA may play a role in the maturation of behaviors linked to the PFC and striatum. However, at present, very little is known of the mechanisms by which activity of the midbrain DA neurons of the substantia nigra/ventral tegmental area (SN/VTA) orchestrate neuroadaptations in the PFC and striatum. The goal of our research programme is to examine whether and how the developmental homeostasis of DA transmission affects the refinement of neural circuits leading to characteristic adult behaviors. We hypothesize that DA regulates the maturation of PFC and cortico-striatal circuit by affecting the development of dendritic spines, synaptic pruning and differentiation of neurons in the PFC and striatum. We will use an innovative pharmacogenetic system in mice to silence or overstimulate midbrain DA neurons reversibly at defined developmental periods in order to study its long-term consequences. The method involves AAV virus-mediated expression of Cre dependent DREADD (designer receptor activated by designer drug) in the midbrain DA neurons of transgenic DA transporter-Cre mice. Using this system, the goal of the present proposal is to address the questions: (1) Will excess DA neuronal activity during adolescence alter motor, cognitive, social and reward-related behaviors of mice? (2) What is the impact of excess DA activity on the developmental trajectory of the brain monitored by magnetic resonance imaging (MRI)? We will also study the morphological and functional development of neurons using Golgi-Cox staining and immunostaining. (3) What are the mechanisms that mediate the effects of adolescent DA manipulation? We will examine the levels of cytokines and microglia cells that play a critical role in synaptic pruning during adolescence. We will also examine the development of inhibitory interneurons, specifically the parvalbumin expressing GABA neurons. (4) We want to investigate whether DA D1-like or D2-like receptors mediate the long-term neuronal and behavioral changes induced by adolescent DA hyperactivity. We expect that our studies will to fill an important gap in knowledge regarding the role of neuromodulators in fundamental processes of brain development.