Understanding the dopamine signals triggered directly by striatal acetylcholine: The circuits, their timing and the actions of addictive drugs

Context of the research: The chemical transmitter in the brain dopamine is critically involved in our ability to select our everyday actions and movements. Drugs that are addictive, like nicotine and cocaine, release too much dopamine, promoting the actions that underlie more drug seeking, and hence the cycle of addiction. Conversely, loss of dopamine in Parkinson's disease (PD) causes problems selecting and starting movements. But, we still do not fully understand how the dopamine release is controlled. We need to grasp the underlying neural mechanisms if we are to understand the functions of dopamine better and how to treat dopamine disorders.
It has long been assumed that the only trigger for dopamine release is the electrical impulses generated in the cell soma, and that these travel along the long cables, or axons, to axon endings where dopamine is released. However, we have new evidence that radically revises this thinking. Using state-of-the-art methods to drive specific neural circuits in the brains of mice engineered to respond to flashes of light, we have identified that a different set of neurons can trigger dopamine to be released. These neurons release the chemical acetylcholine directly onto dopaminergic axons which triggers dopamine release, equally as powerfully as impulses in dopamine cells themselves. These findings cause a paradigm shift in our current thinking and place cholinergic neurons central to the functions of dopamine. Here in this research grant we propose to advance our understanding of how acetylcholine neurons and their inputs evoke DA signals. Excitingly, new technology allows us for the first time to address this highly timely question.
Aims and objectives of the research: The broad aim of this research is to promote our understanding of how acetylcholine-producing neurons drive brain dopamine signals, and to understand whether these mechanisms contribute to the neural actions of drugs of addiction. We will address this through three specific aims.
Aim 1. Acetylcholine neurons in striatum powerfully trigger the release of dopamine from axons, but when does this happen? Our data suggest that this might happen when cholinergic neurons are excited by their inputs. If we could identify which inputs were important, we would understand which circuits we could target in dopamine dysfunction, and begin to understand what kinds of functions we might expect this dopamine signal to have. Therefore, in Aim 1, we will address - what are the inputs to cholinergic neurons that can drive dopamine release?
Aim 2. We already know that electrical impulses generated in the dopamine cell bodies themselves can trigger dopamine release. So which of these two triggers is the most important one and how do they interact? We will address this in Aim 2. The answers to this question will tell us a huge amount about the best way to drive dopamine release.
Aim 3. Addictive drugs like nicotine and cocaine are able to boost dopamine signals in the brain. But is acetylcholine-evoked dopamine release a mechanism targeted by these drugs? Our pilot data suggest it might be. We will address this in Aim 3. The answer to this question would have significant impact on our understanding of drug addiction.
Potential applications and benefits of the research: Through this work, we will build a more complete picture of the neural circuits that drive dopamine transmission. In turn, we will shed light on fundamental mechanisms of normal brain function that might be disrupted, and then targeted for therapeutic benefit, in disorders of dopamine neuron function, including addiction disorders and PD. In addition, the technical infrastructure that we will establish will be cutting-edge and an excellent investment for future studies to investigate dopamine function further e.g. in disease. Therefore, whatever the outcome, our work will provide a fundamental advance in knowledge and be of direct benefit to future work.

Striatal dopamine (DA) critically participates in the regulation of motivation and action selection. Dysregulation of DA signaling underlies a variety of psychomotor disorders, including addiction and Parkinson's disease. Surprisingly however, our grasp of the neuronal mechanisms that govern DA transmission remains incomplete. Activity in midbrain DA neurons has long been assumed to be its principal driver, but we have obtained new evidence for a radical shift to this paradigm, requiring us to rethink the mechanisms that govern DA release, and in turn, DA function and dysfunction.
Using an optogenetic strategy to light-activate striatal cholinergic interneurons (ChIs) that express channelrhodpsin2 (ChR2), and by detecting DA release at microelectrodes with subsecond resolution, we have identified that single action potentials in a population of ChIs triggers striatal DA release directly from DA axons by activating presynaptic nicotinic receptors (Threlfell et al, Neuron, in press). In addition, DA release was triggered by inputs to striatum from thalamus which coordinate ChI firing in vivo. Furthermore, ChI-evoked DA release overrode ascending activity in DA axons, suggesting that ChIs and their inputs might dominate as a trigger. These findings place ChIs central to our understanding of DA function and, potentially, dysfunction. It is imperative now to address key questions about how ChIs evoke DA release.
We will exploit optogenetic technologies to incorporate established and new light-activated channelrhodopsins (ChR2 and C1V1) in DA neurons, ChIs and/or their inputs to identify: (1) which inputs recruit ChIs to drive DA transmission?; (2) how do dynamic repertoires of activity in ChIs and DA neurons interact to govern DA transmission?; and (3) are these mechanisms targeted by addictive drugs? These studies are critical if we are to improve our knowledge of DA system function, and gain new insights about strategies for future therapies in DA dysfunction.

The wider impact of this work to non-academic beneficiaries could be far reaching in the longer term. This work is likely to impact upon a variety of non-academic groups including clinicians, sufferers and carers of a diverse group of DA-centred psychomotor diseases, the general public and pharmaceutical companies seeking new targets for drug development. The timescales over which this work is likely to impact these different groups of people will vary. Short-term benefits will be felt by all these groups as a result of increased understanding of neurotransmission in the brain, and progression of research into psychomotor disorders resulting from our findings. Longer-term societal and economic benefits may be felt by disease sufferers and pharmaceutical companies should new therapeutic targets emerge as a result of our research.