Reward networks and appetitive behaviour

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Animals and humans must eat to fulfil the energy requirements of their bodies, and they have evolved powerful mechanisms to increase appetite when required. An excellent example of one such mechanism is the production of a hormone, called ghrelin, by the gut in between meals. Ghrelin acts on the brain to increase food seeking, but also to make food seem more rewarding. It is easy to see why hunger has evolved to make an animal go and search for and procure food. But why make eating food so rewarding? It is believed that this evolved so that animals will maximise eating if food is normally scarce. Food sources that are rich in energy, for example which contain a lot of sugar or fat, are preferred, especially if an animal has limited opportunities to eat or is under threat of predation while it is out in the open. The downside of food being rewarding is that, in a modern environment, where high-energy food is abundant and easily available, we are still motivated to over consume. It is a widely held view that the current epidemic in human obesity is due to the fact that over-weight people, even though they have adequate energy stored in their bodies, are still driven to over eat because they do not disengage reward circuits. Simply put, even after eating a full meal, we will still find space for some pudding!In order to understand why obese humans over eat, it will help to understand how the brain responds to hunger, and what may change as we put weight on. Importantly, we wish to understand the different brain pathways which control different aspects of the hunger response. For example, how eating food turns off the negative, unpleasant feelings of hunger, versus how eating food turns on the positive, pleasant feelings of eating a nice meal. These behaviours are controlled by an extremely complex network of brain cells (neurones), which we are only just beginning to understand. In this project, we want to determine how different types of these neurones connect with each other. We are able to do this because, for the first time, we can see the different neurones in mice because they have been made to shine with fluorescent light. We can record the activity of these neurones while stimulating the other cells that connect with them - all in a petri dish! However, what is also very new is that we can stimulate specific types of neurone in mice while they are behaving perfectly normally. This can be done by either giving the mice an injection of a new drug or by shining light into the mouse's brain using an optic fibre. We can also inhibit the activity of the same neurones and see if the mice still respond to hunger or to the hormone, ghrelin. By doing this, we can see whether the mice eat normal food, or if they prefer to put in a bit more effort to receive a more rewarding sugar pellet. By stimulating different pathways in the brain, we will be able to build up a complete picture of the complex network of neurones that control eating behaviour. Only when we have done this, will we then be able to ask what changes when a mouse or a human becomes obese.