Circuit-level control of forelimb motor behaviour and plasticity following injury

中枢神经系统（CNS）的主要输出是通过从大脑到脊髓的下行运动通路来控制运动，以协调运动神经元的活动。在脊髓损伤的情况下，它们的重要性非常明显，因为它们的破坏会导致严重的瘫痪。然而，它们在损伤后运动行为和调节运动系统恢复能力中的具体作用尚不清楚。在这里，我们正在着手一项新的研究计划，旨在利用化学发生和个体发生技术了解脊髓损伤后的神经可塑性和功能代偿。具体来说，我们提出了一系列初始实验，使用最近开发的一种技术，该技术允许非侵入性和短暂的体内特定神经回路沉默，称为设计药物特异性激活的设计受体（DREADDs） （PMID:17360345）。dreadd是一种生物工程g蛋白偶联受体，它可以激活或沉默神经元，以响应不激活体内其他受体的配体。这些配体可以通过腹腔注射（短效）或通过饮用水（长效）无创给药。DREADD受体可以通过两种方式之一靶向特定的神经元群。首先，编码DREADDs的病毒可以被施用，这样通常只有病毒注射区域内的细胞被感染。其次，通过使用cree依赖性构建体，可以通过采用双病毒方法靶向特定的解剖投影，其中一种cree依赖性病毒在神经元细胞体上施用，另一种cree编码病毒在神经元轴突/突触末端逆行感染（PMID: 26889809）。我们已经成功地使用了这种方法（PMID: 27053214）。在目前的项目中，我们将使用沉默的DREADD hM4Di来特异性地沉默与前肢运动控制有关的下行运动通路。本研究的第一个目的是阐明皮质脊髓、结节脊髓和髓系网状脊髓腹侧（MdV）通路在未受伤成年小鼠中枢神经系统运动行为功能中的作用。第二个目的是阐明在金字塔水平上介导皮质脊髓束损伤后恢复的rubrospinal和MdV通路的可塑性。我们将评估小鼠在给予CNO以沉默这些特定神经元群之前和之后的单个食物颗粒到达，梯子穿越，旋转杆和握力的恢复情况，并在同一动物中进行比较，以便每只动物都作为自己的对照。这将揭示特定神经元投射系统在执行特定任务时的功能参与，无论是在正常的未损伤状态还是在不完全损伤后。

The major output of the central nervous system (CNS) is to control movement via descending motor pathways that project from the brain to the spinal cord to orchestrate motoneuron activities. Their importance is strikingly obvious in cases of spinal cord injury, when severe paralysis can ensue because of their disruption. However, their specific roles in motor behaviour and ability to mediate motor system recovery following injury remain unclear. Here, we are embarking on a new research program aiming at an understanding of neural plasticity and functional compensation after spinal cord injury using chemogenetic and ontogenetic technologies. Specifically, we propose a series of initial experiments using a recently developed technology that allows non-invasive and transient silencing of specific neural circuits in vivo called Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) (PMID:17360345). DREADDs are bioengineered G-protein coupled receptors that can activate or silence neurons in response to ligands that do not activate other receptors within the body. These ligands can be administered non-invasively via intraperitoneal injection (short acting) or through drinking water (long acting). DREADD receptors can be targeted to specific neuronal populations in one of two ways. First, viruses encoding DREADDs can be administered such that only cells within the area of viral injection is generally infected. Second, by using cre-dependent constructs, specific anatomical projections can be targeted by employing a dual virus approach, wherein one cre-dependent virus is administered at the neuron's cell body and the second, cre-encoding virus is administered for retrograde infection at the neuron's axon/synaptic terminal (PMID: 26889809). We have used this approach successfully (PMID: 27053214).In the present project we will use the silencing DREADD hM4Di to specifically silence descending motor pathways implicated in forelimb motor control. The first objective of this grant is to elucidate the role of corticospinal, rubrospinal, and medullary reticulospinal ventral (MdV) pathways in motor behavioural function in the uninjured adult mouse CNS. The second objective is to elucidate the plasticity of rubrospinal and MdV pathways in mediating recovery following corticospinal tract injury at the level of the pyramid. We will assess the recovery of single food pellet reaching, ladder crossing, rotarod and grip strength in mice before and following the administration of CNO to silence these specific neuronal populations and make comparisons within the same animal such that each animal acts as its own control. This will reveal the functional involvement of specific neuronal projection systems in the execution of specific tasks, either in the normal uninjured state or after incomplete injury.