DOPAMINE D1, D2, AND D3 RECEPTORS AS BIOMARKERS FOR IMAGING NIGROSTRIATAL NEURONS

描述(申请人提供)：帕金森病(PD)是第二种最常见的神经退行性疾病，在北美有100多万人受到影响，目前还没有任何治疗方法被证明可以减缓病情的发展。神经影像生物标记物有可能为帕金森病的进展提供无偏见的测量。与黑质纹状体神经元丢失相关的多巴胺受体的变化可能会导致多巴胺D2和D3受体的“失调”，导致D2：D3比率的变化。D2/D3比值的改变可能在以D2样受体功能改变为特征的中枢神经系统综合征的行为后果中发挥关键作用。然而，D3在病理生理学和治疗中的确切作用仍有待确定。我们以前开发了一种新的方法在体外测量D2和D3受体的密度，并发现纹状体和黑质的D3受体的变化与帕金森病脑内的D1和D2受体的变化不同。我们先前评估了三种不同的突触前PET示踪剂，[11C]DTBZ，[11C]CFT和[18F]FoloDOPA，在给予广泛剂量的选择性多巴胺能神经毒素MPTP的情况下，对猴子的黑质纹状体神经元进行了研究。这些PET研究表明，纹状体摄取这些神经元中的每一个与黑质纹状体神经元的数量(使用黑质酪氨酸羟基酶免疫染色神经元的无偏立体学计数)呈线性相关，但只有当黑质纹状体神经元的损失不超过50%时才如此。相比之下，这些示踪剂中的每一种都与纹状体多巴胺的体外测量呈线性相关。体内PET测量结果与纹状体内VMAT2和DAT的定量放射自显影有很好的相关性。有趣的是，运动评分与黑质细胞计数完全相关，而与纹状体多巴胺或其他终末野测量无关，这表明除了纹状体终末野丧失外，其他因素也会影响运动性帕金森症。我们假设突触后多巴胺受体的变化可能解释了这种差异。在本项目中，我们建议对41只猴子脑组织中突触后多巴胺受体(D1、D2和D3)进行定量放射自显影测量，其中16只猴子接受广泛范围的单一颈内MPTP剂量(0到0.31 mg/kg)治疗；其中10只猴子将在MPTP治疗后的不同时间进行研究(5只在10天，5只在3周)；3组5只猴子将接受一次MPTP治疗，然后用慢性左旋多巴治疗2个月，左旋多巴是一种多巴胺受体激动剂普拉克索或安慰剂。这些额外的措施有望比单独的突触前措施更全面地反映黑质纹状体神经元的状态。这个为期两年的项目的总体目标是确定去多巴胺后突触后标志物的变化，以确定准确测量黑质纹状体损伤严重程度的最有效的神经成像方法。我们还将研究左旋多巴或普拉克索(一种多巴胺D3受体激动剂)长期治疗对这些标记物的影响。这项资助中提出的研究对于扩大我们对多巴胺受体亚型在帕金森病进展中的功能作用的理解具有很大的潜力。

DESCRIPTION (provided by applicant): Parkinson Disease (PD), the second most common neurodegenerative disease, affects more than one million people in North America, and no treatment has been proven to slow progression. Neuroimaging biomarkers have the potential to provide unbiased measurements of PD progression. Changes in dopamine receptors related to loss of nigrostriatal neurons may lead to "dysregulation" of dopamine D2 and D3 receptors with a change in the D2:D3 ratio. A change in the D2:D3 ratio may play a key role in the behavioral consequences of CNS syndromes characterized by altered D2-like receptor function. However, the precise role of D3 in the pathophysiology and treatment remains to be determined. We previously developed a novel procedure to measure the density of D2 and D3 receptors in vitro and found that D3 receptors in striatum and substantia nigra change differently from D1 and D2 receptors in PD brains. We previously evaluated three different presynaptic PET tracers, [11C]DTBZ, [11C]CFT and [18F]fluoroDOPA for nigrostriatal neurons in monkeys given a wide range of doses of the selective dopaminergic neurotoxin MPTP. These PET studies demonstrated that striatal uptake of each of these linearly correlates with the number of nigrostriatal neurons (using unbiased stereologic counts of tyrosine hydroxylase [TH] immunostained neurons in nigra) but only when the loss of nigrostriatal neurons does not exceed 50%. In contrast, each of these tracers linearly correlates with in vitro measures of striatal dopamine. The in vivo PET measures correlated well with each other and with quantitative autoradiography of VMAT2 and DAT in striatum. Interestingly, motor ratings correlated fully with nigral cell counts and not with striatal dopamine or other terminal field measures, suggesting that factors other than just loss of striatal terminal fields influence motor parkinsonism. We hypothesize that changes in postsynaptic dopamine receptors may account for this discrepancy. In this project, we propose to conduct quantitative autoradiography measures of the postsynaptic dopamine receptors (D1, D2 and D3) in the striatum from brain tissues from 41 monkeys, 16 of them are treated with a wide range of single internal carotid MPTP doses (0 to 0.31 mg/kg); 10 of them will be studied at different times after a MPTP (5 at 10 days and 5 at 3 weeks); 3 groups of 5 will have a single dose of MPTP then treated for 2 months with chronic levodopa, a dopamine receptor agonist pramipexole or placebo. These additional measures are expected to provide a more complete reflection of the state of nigrostriatal neurons than presynaptic measures alone. The overall goal of this 2-year project is to determine how postsynaptic markers change after dopamine denervation to determine the most effective neuroimaging measures for accurate measurement of the severity of nigrostriatal injury. We also will investigate the effects chronic treatment with levodopa or pramipexole (a dopamine D3 receptor agonist) on these markers. The studies proposed in this grant have great potential for extending our understanding of the functional roles of dopamine receptor subtypes in PD progression.