Food intake is not only regulated by homeostatic processes as is evidenced by eating beyond homeostatic needs if presented with highly palatable food. The palatability of taste, i.e. hedonic tone, is unquestionably a key factor in guiding food intake. A positive hedonic tone stimulates feeding and is one of many dietary factors associated with excess energy intake and, in the long term, may lead to obesity, diabetes, and hypertension. The object of this project is to understand how neural activity in the cortex and limbic system control taste processing in subcortical gustatory nuclei. The long-term objective is to elucidate how the brain assigns hedonic value to taste stimuli that reinforce or weaken their consumption. There must be neural circuitry by which gustatory hedonic value can be modified. For example, an unpleasant experience with food-learned taste aversion-can result in the rejection of a previously accepted taste, while a mineral imbalance--Na+ appetite-can transform a normally rejected Na+ solution into a highly preferred one. In normal animals, changes in the motivational qualities of taste stimuli are coincident with selective alterations in the responses of brainstem taste cells. Although the isolated brainstem contains the sensory and motor apparatus necessary to produce feeding behavior, ventral forebrain mechanisms are necessary for motivational states to modify these behaviors. One interpretation is that feedback from rostral structures normally modulates the afferent neural activity that conveys the hedonic value of a taste stimulus. Using electrophysiological, neuroanatomical, neurochemical, and lesion analysis, this project will test hypotheses about the centrifugal control of taste processing in the brainstem. Specific experiments examine (1) differences between the influence of forebrain activity on NST and PBN taste neurons, (2) the route through which descending cortical and limbic projections modulate PBN taste neurons, and (3) the neurochemicals released in the PBN by activation of cortical and limbic system nuclei.
