Understanding the role of dopamine in vertebrate motor control

Nerve cells (neurons) communicate by secreting neurotransmitters, small chemical messengers, at tiny junctions between cells called synapses. Dopamine is a specialised messenger molecule that alters the way other neurons processes information. In the normal brain, dopamine controls information processing in neurons that affect movement, attention and motivation. However, disturbances in dopamine release can have devastating effects on brain function. For example, disorders such as schizophrenia, Parkinson's disease and restless leg syndrome are linked to imbalances in dopamine secretion. For this reason, dopamine secreting neurons are a major focus of bioscience research.

Our understanding of dopamine function comes mainly from the study of neurons in the midbrain, a brain region that coordinates many of our sensory and motor functions. However, dopamine releasing neurons are also found in the diencephalon, a region responsible for processing sensory information, regulating motivation and controlling other body functions. A specific group of these cells extend long outgrowths (axons) into the spinal cord where they communicate with networks of neurons responsible for generation of movements such as walking, running and swimming. These dopamine neurons are thought to facilitate and stabilise locomotor behaviours, but their specific roles have not been properly analysed, and little is known about the specific mechanisms by which they might exert their effects in the spinal cord. One key reason why the roles of these cells are not well understood is because most studies have been conducted on isolated pieces of spinal cord tissue that cannot generate naturally-occurring movements.

We are using zebrafish larvae to determine how dopamine neurons affect naturally-occurring forms of locomotion. These tiny larval fish contain a full complement of dopamine neurons, yet lack bone tissue and skin pigments. These features mean that we can observe, record from and experimentally manipulate neurons in intact, living fish that can produce natural swimming patterns. Our preliminary work with this model has provided some exciting new insights into the possible role for diencephalic dopamine neurons in controlling the frequency and intensity of swimming: different groups of spinal cord neurons are activated at different speeds of swimming, with cells in one region used to produce slow, weak movements and those in another region used for fast, intense movements. We have found that laser removal of spinally-projecting dopamine neurons reduces the activity in spinal cells used for fast, intense movements. Thus, we suspect that dopamine precisely regulates the production of more intense forms of locomotor behaviour.

We will test this suggestion by using a laser to selectively remove the dopamine neurons of interest. We will then record the electrical activity patterns of spinal cord neurons involved in the generation of swimming. This will allow us to study loss of dopamine signals in the spine affects nerve cells used for low and high speed movements. Once we have identified the cell types that dopamine affects, we will make recordings of the electrical activity of these individual neurons to examine the mechanisms underlying these changes. Finally, we will use high speed video motion capture to understand how removal of dopamine neurons affects the movements of freely-behaving zebrafish.

In completing this work we aim to shed important new light on the fundamental processes underpinning vertebrate motor control. Moreover, as spinal cord-projecting dopamine neurons have been implicated in disorders such as Parkinson's disease and restless leg syndrome, our finding may help us to better understand dopamine-related diseases that can have debilitating effects on locomotor behaviour.

Dopamine (DA) controls a vast array of behavioural processes and defective DA signalling has been implicated in a number of neurological disorders. Current understanding of DA's relevance to behaviour is derived from the study of midbrain DA neurons that project to the forebrain. However, the diencephalon also contains a population of DA neurons that project posteriorly to innervate spinal cord locomotor circuits. These cells, termed 'diencephalospinal DA neurons' (DDNs), are believed to facilitate expression of flexible motor behaviours. However, the true function of these cells is unknown, principally because few in vivo studies have been undertaken.

In this proposal we will use larval zebrafish as an in vivo model for delineating the functional role of DDNs. Preliminary data obtained by our laboratories suggests that DDNs modulate speed-related shifts in spinal neuron recruitment: in zebrafish, small, ventral neurons are recruited at low locomotor speeds whilst more dorsal neurons are recruited as speed increases. DDNs appear to selectively modulate dorsal neuron recruitment, suggesting that these neurons control expression of high frequency locomotion.

In this project we will test the hypothesis that DDNs modulate speed-related recruitment of spinal neurons. Using patch clamp electrophysiology and calcium imaging, we will determine how DDNs influence recruitment patterns across a range of swimming speeds. Subsequently, we will pinpoint the cellular and synaptic targets of DA's effects with patch clamp methods. Finally, the impact of DDN ablation on swimming behaviour will be determined with motion capture. This work will provide a detailed understanding of how DDNs control of vertebrate motor output and may also shed new light on the role these cells play in DA-related motor disorders such as Parkinson's disease and restless leg syndrome.

Our project will provide a detailed understanding of the role dopamine plays in vertebrate motor control. This is a topic of high current interest to neuroscientists studying fundamental aspects of dopamine signalling and vertebrate motor control. It is relevant to important clinical ventures seeking to understand how defective dopamine signalling affects human wellbeing. Beneficiaries of our project will therefore include the scientists undertaking the work, our collaborators, the wider neuroscience community, clinicians and commercial biomedical organizations. Moreover, members of the public will benefit from knowledge of how our motor activity is regulated.

The project offers a broad-based multidisciplinary training opportunity and scientists working on the project will develop expertise in much-needed, highly specialized research techniques. These include in vivo patch clamp electrophysiology, in vivo calcium imaging and high speed kinematics. The McDearmid group is the only UK-based group that employs in vivo patch clamp methods to study zebrafish spinal cord physiology. Thus, our work will help build national expertise in techniques required to maintain the UKs international competitiveness. Scientists working on the project will also develop transferable skills including analytical methods, problem solving, oral presentation and scientific writing; skills that will be essential for their future career prospects. Our project will thus help generate researchers with the broad skill set needed for both academic and industrial research and development sectors.

In the long-term, we anticipate our findings may lead to a better understanding of disorders such as restless legs syndrome and Parkinson's disease, which may help to instruct the development of new treatments for these neurological conditions.

Impact will be achieved through publication of our findings in high-impact scientific journals, presentations at international conferences and invited University talks. Our findings will also be presented in a lay format at public open days and in media communications. The ideas behind our research are comprehensible and interesting to the general public, and we will capitalize on this through production of non-technical articles, press releases, open meetings and public lectures. Dissemination and public engagement will be facilitated by the University of Leicester Press Office.