Optical activation of a C. elegans neural circuit underpinning feeding behaviour

Communication between neurones is a fundamental aspect of brain function. This is mediated by chemical neurotransmitters that signal from one nerve cell to the next at specific sites on the nerve cell called synapses. There are trillions of these synapses in the human brain and their correct function is essential for good mental health. Dysfunction in this synaptic communication is the basis for the majority of psychiatric and neurodegenerative disorders including depression, anxiety, schizophrenia, Parkinson's disease, Alzheimer's disease and many other prevalent and distressing conditions. Indeed, many of the drugs used to treat these disorders act to improve synaptic communication to ameliorate the symptoms of the disease. However in many cases these drugs are fairly 'blunt instruments'. A major problem is that chemical neurotransmission is complex and also very plastic. There is still a relatively superficial understanding of its subtleties. Specifically, any given synapse will store a multitude of neurotransmitters which can be released in different proportions depending on the activity of the nerve cell giving rise to a synaptic 'cocktail'. Different 'cocktails' have different consequences for signalling at the synapse. It is clear that the ability of the synapse to 'mix a cocktail' and alter its signalling properties is related to the ability of the neuronal circuits, and ultimately the brain, to alter its output and change behaviour. Therefore a fundamental goal of neuroscience is to understand the role of distinct classes of neurotransmitters within neuronal circuits and how altered signalling leads to a change in behaviour. This is a complex problem and many neuroscientists, including ourselves, have chosen to work on simple invertebrate nervous systems to address it. The advantage of the model that we have chosen, the nematode worm C. elegans, is that although it has a very simple nervous system with only 5000 synapses rather than trillions, it has the same complex mechanism for signalling between nerve cells as humans. Furthermore, the animal exhibits simple behaviours and will adapt in an appropriate way according to its environment. This indicates that it has the capacity to re-configure its neural circuits in much the same way as higher animals. The attraction of this as a model system is that it is easy to study a specific problem from the level of the gene through to the molecule, neurone, circuit and the intact behaving animal. The behaviour we are investigating is the response of the animal to food deprivation. Here we are taking advantage of a new technical advance that will enable us to activate specific elements of the neuronal circuit that regulates this adaptive response and record synaptic signalling. By performing this analysis in different genetic mutants we will be able to define the properties of the signal in a circuit and describe changes in the signal that parallel the change in the behaviour of the animal. This will provide a unique insight in to the fundamental problem of how altered synaptic signalling leads to a change in behaviour and pave the way for further genetic analysis of this problem.

We are addressing the problem of neural substrates of behaviour. This problem is relatively intractable in higher animals due to the need to correlate changes in behaviour in the intact animal with changes that occur at the level of circuits and synapses. C. elegans has defined neural circuits that underpin specific behaviours and thus is a more tractable system in which to relate neural substrates to behaviour. We focus on a microcircuit of 20 neurones that comprises the pharyngeal nervous system and regulates feeding behaviour. We will employ 'optogenetics', a new tool that utilises light-sensitive channels to allow optical modulation of cells. In the context of neuronal circuits this has the potential to enable precise activation of discrete components for the first time. The attraction is that it enables a genetic and molecular dissection of signalling pathways. Furthermore, it is now feasible to define specific components of a neural circuit whilst the animal is in different behavioural states. We will develop this for use in a defined microcircuit, the pharyngeal nervous system, in C. elegans. This will involve the generation of transgenic lines expressing the light-sensitive channel CHOP in subsets of neurones within the pharyngeal circuit. In pilot experiments we have demonstrated light-activated responses by the expression of CHOP within the muscle. We have also shown expression of CHOP in subsets of pharyngeal neurones. We will optimise the technique so that we can use it to interrogate signalling in the pharyngeal circuit. Specifically we are interested in the role of neuropeptide signalling in behavioural plasticity. This has grown out of work in the Holden-Dye lab which studies NPY-like receptors in context-dependent behaviour and is reinforced by genetic evidence that neuropeptides have a key role in the adaptive response. Thus we wish to apply optogenetics to the problem of defining the signalling properties of discrete elements within the circuit.