Acetylcholine and cerebellar dependent motor learning

运动学习是所有新行为的基础，包括通过练习和对感觉经验（条件反射）的反射反应来提高自主运动技能。虽然运动学习涉及大脑区域的网络，但小脑对两种类型的运动学习都至关重要——当小脑受损时，我们学习新的自主运动和适应反射反应的能力就会严重受损。小脑包含了大脑中80%以上的神经元，这一事实进一步强调了小脑对大脑和行为的重要性。课本上对小脑的描述告诉我们，小脑有两种类型的输入：苔藓纤维和攀爬纤维。然而，还有一类额外的输入在很大程度上被忽视了，它们对小脑回路和小脑介导功能具有重要的调节作用。这些纤维包括使用乙酰胆碱（ACh）作为神经递质并在整个小脑中广泛连接的纤维。这些胆碱能纤维的主要来源是一种叫做桥脚核（PPN）的脑干结构。虽然乙酰胆碱对学习和记忆至关重要，但人们对胆碱能投射到小脑的行为意义几乎一无所知。鉴于小脑在运动学习中的关键作用，这是我们理解中的一个重要空白。目前的研究验证了从PPN到小脑的胆碱能投射调节神经元功能以控制运动学习的假设。要理解小脑对运动学习的贡献，一个重要的组织原则是将小脑划分为一系列被称为模块的功能单元。单个模块如何促进运动学习，尤其是那些涉及肢体运动控制的模块，目前还远不清楚。目前的项目使用小脑的模块化组织作为框架来研究胆碱能在两种不同类型的前肢相关运动学习中对小脑回路的影响：前肢到达任务和反射性前肢屈曲条件反射任务。使用这两种不同类型的任务，可以全面研究运动学习期间胆碱能投射到小脑的作用，以及与明确定义的行为输出相关的作用。这个项目是及时的，因为PPN和小脑之间强烈的生理联系直到最近才被确定，并且现在可以通过转基因大鼠选择性地询问胆碱能回路。我们将使用整个动物行为和大脑切片方法的联合力量。在系统水平上，我们将使用多通道电生理记录方法来检查神经元群活动和单个神经元的尖峰序列，并使用干预方法（药理学/光遗传学）来了解PPN和小脑在运动学习过程中如何协调它们的活动。在细胞水平上，我们将使用遗传方法选择性地刺激PPN释放乙酰胆碱，以确定这种神经调节剂如何在细胞水平上控制神经元和突触功能。总的来说，这些方法将为运动学习的细胞机制和电路基础提供新的见解。实验模型的选择：小脑网络结构和连接模式在包括人类在内的哺乳动物物种中高度保守。然而，选择大鼠作为实验动物是因为我们对基本神经解剖学和生理学的理解在这个物种中是最完整的。重要的是，我们的实验将包括在行为情境中神经网络相互作用的研究，这些行为情境已经在大鼠身上得到了很好的表征，并且与人类运动学习相关。总的来说，我们的研究结果旨在为大脑内的神经回路如何提高我们学习新动作的能力提供一种机制理解。

Motor learning is fundamental to all new behaviours and includes the improvement of voluntary motor skills with practice and adapting reflex responses to sensory experience (conditioning). Whilst motor learning involves a network of brain regions, the cerebellum is critically involved in both types of motor learning - when the cerebellum is damaged our capacity to learn new voluntary movements and adapt reflex responses is severely impaired. The importance of the cerebellum to brain and behaviour is further emphasized by the fact that it contains over 80% of all neurons in the brain. Text book descriptions of the cerebellum tell us there are two types of input to the cerebellum: mossy fibres and climbing fibres. However, there is an additional class of inputs that have been largely overlooked, which have important modulatory effects on cerebellar circuits and cerebellar-mediated function. These include fibres that use acetylcholine (ACh) as a neurotransmitter and have widespread connections throughout the cerebellum. The primary source of these cholinergic fibres is a brainstem structure called the pedunculopontine nucleus (PPN). Whilst ACh is vital for learning and memory, almost nothing is known about the behavioural significance of cholinergic projections to the cerebellum. This is an important gap in our understanding given the critical role of the cerebellum in motor learning.The current study tests the hypothesis that cholinergic projections from the PPN to the cerebellum regulate neuronal function to control motor learning. An important organizational principle of the cerebellum for understanding its contributions to motor learning is a division into a series of functional units called modules. How individual modules contribute to motor learning remains far from clear, especially those involved in the control of limb movements. The current project uses the modular organization of the cerebellum as a framework to study cholinergic effects on cerebellar circuits during two different types of forelimb-related motor learning: a forelimb reaching task, and a reflex forelimb-flexion conditioning task. The use of these two distinct types of task allows a comprehensive investigation of the roles of the cholinergic projections to the cerebellum during motor learning, in relation to well-defined behavioural outputs. The project is timely because a strong physiological connection between the PPN and cerebellum has only recently been identified, and genetically modified rats to selectively interrogate cholinergic circuits are now available. We will use the combined power of whole animal behavioural and brain slice approaches. At the systems level we will use multichannel electrophysiological recording methods to examine neuronal population activity and spike trains of individual neurons, and interventionist methods (pharmacological/optogenetic) to understand how PPN and cerebellum orchestrate their activity during motor learning. At the cellular level we will use genetic approaches to selectively stimulate PPN release of ACh to determine how this neuromodulator controls neuronal and synaptic function at the cellular level. Collectively these approaches will provide novel insights into the cellular mechanisms and circuit basis of motor learning. Choice of experimental model: cerebellar network architecture and patterns of connectivity are highly conserved across mammalian species, including human. However, rats are the experimental animal of choice because our understanding of the basic neuroanatomy and physiology is most complete in this species. Importantly, our experiments will include study of neural network interactions during behavioural situations that have been well characterized in rats and that correlate to human motor learning. Overall, the results of our study aim to provide a mechanistic understanding of how neural circuits within the brain give rise to our ability to learn new movements.