Acetylcholine and cerebellar dependent motor learning

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.

Motor learning is fundamental to all new behaviours and includes the improvement of motor skills with practice (procedural learning), and adapting reflex responses to sensory experience (associative conditioning). Whilst motor learning involves a network of brain regions, the cerebellum (cbm) is a crucial element for the acquisition of conditioned reflexes and procedural learning. An extensive network of cholinergic fibres courses throughout the cbm. Acetylcholine (ACh) is vital for learning and memory, but almost nothing is known about the behavioural significance of cholinergic projections to the cbm and the role they play in motor learning.

Our overarching hypothesis is that cholinergic projections to the cbm regulate neuronal function to control the encoding and consolidation of motor learning. We will use three mutually reinforcing lines of study to test our hypothesis:

1.In vivo studies will define the neural interactions between the primary source of cholinergic fibres to the cbm, the pedunculopontine nucleus (PPN), and the cbm. This will be performed during two motor learning paradigms - a skilled forelimb reaching task and a classical conditioning forelimb reflex paradigm. These experiments will identify the times during different phases of motor learning when PPN-cbm interactions occur.

2.In vitro experiments will use optogenetic approaches to activate cholinergic PPN projections. These experiments will identify the cellular and synaptic mechanisms by which endogenous cholinergic receptor activation regulates cbm neurons.

3.Behavioural intervention studies will manipulate cholinergic receptor function in the cbm during motor learning. These experiments will determine the causal effect of cholinergic neuromodulation on encoding and consolidation of motor learning.

Overall, the results of this project will provide fundamental new insights into how cholinergic projections to the cbm regulate neuronal function to control motor learning.

Learning new motor skills is impaired in healthy ageing and in neurodegenerative diseases such as spinocerebellar ataxias, and therefore contributes substantially to a wide burden of symptoms that remain largely refractory to conventional drug-based therapies.

The research will be of benefit to:
(i) The academic research team involved in the project;
(ii) The wider academic community;
(iii) Members of the general public with an interest in movement disorders;
(iv) The pharmaceutical industry;
(v) Patients suffering from cerebellar-related movement disorders, their families, charities and organizations seeking to support patients.

How will they benefit from this research?
(i)The named researcher (JP) recently completed her PhD in our lab and is a talented neuroscientist with both in vitro and in vivo skills that will transfer admirably to the proposed plan of work. The project will allow her to develop her expertise further in highly novel and state-of-the-art research techniques (such as optogenetics) that will aid her future career. There is a worldwide skills shortage of researchers with experimental animal in vivo research expertise. By taking a lead role in the research programme JP will also develop her management, communication, team working and other transferable skills.
(ii) International academia in the fields of preclinical and clinical movement disorders, as well as basic scientists in the fields of sensorimotor, learning & memory and behavioural neuroscience are likely to benefit from the scientific progress made by this research.
(iii) Members of the general public. The findings from this project are applicable to understanding human mobility and ageing. Such knowledge is of wide interest to the general public. The findings will therefore be appropriate to disseminate through public engagement activities.
(iv) Pharmaceutical industry. Worldwide clinical and preclinical drug research will benefit from advances in understanding of basic brain biology and its relevance to areas of major human disease that cause a substantial burden on society.
(v) Patients suffering from cerebellar-related movement disorders. At present there are no satisfactory treatments. In large part this is because the underlying neurobiology of these disorders is unknown. By providing insights into normal brain circuit function associated with motor learning, the research will enable charities to realise their mission of providing education and help to patients and their carers. Progress in understanding network dysfunction that leads to movement disorders requires a global perspective on brain function i.e. it is not sufficient to study one brain region in isolation. The network analysis we seek to provide will offer a more accurate picture of the neurobiology involved. The impact of the research on patient groups and their families will be in terms of identifying potential new targets for therapies; and being able to provide a better understanding of the brain circuitry that underpins movement disorders of cerebellar origin.