Psychophysical, computational, and neurochemical analysis of gustatory reward

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To succeed in their struggle for survival and reproductive success, animals must make good choices about which goals to pursue and how much effort to invest in attaining them. Benefits and costs must be weighed appropriately in the quest for crucial resources, such as food and water. The taste system, an important input to such decisions, provides information about the nutrient content of foods and warnings about potential toxins. Much has been learned about the cells in the mouth and on the tongue that give rise to the sense of taste and about the pathways that relay signals from these cells to the brain and process them. Much less is known about the brain circuits that translate taste information into decisions about food-seeking behaviour. These circuits are the subject of the proposed experiments, which concern the transformation of sensory information (taste and "mouth feel") into a subjective assessment of value. Given the animal's current physiological state and the work and risk involved in harvesting a particular food, how much is that food worth? I have developed a mathematical model that describes how the value of the food is computed. Using behavioural tests derived from the model, we can determine which of the many influences on food-seeking behaviour are affected by changes in need states, drugs, and brain circuits. The proposed experiments assess the effects of both short- and long-term food shortages on the perceived value of sugar and fat and on the effort rats are willing to invest to obtain foods rich in these substances. We will test the hypothesis that reward-related gustatory information is carried by a particular set of brain pathways, the projections from the pontine parabrachial nucleus to the basal forebrain. We will also study the role of dopamine-containing brain cells in the valuation of foods and of the effort required to obtain them. Working out the brain circuity involved in the evaluation and pursuit of food rewards is a necessary step towards understanding the mechanisms underlying foraging behaviour and should provide information useful to understanding how our contemporary environment, in which food is abundant, highly available, rich, and relatively inexpensive, is so conducive to the development of obesity.