Immune, Hormonal and Neural Mechanisms of Feeding Behavior

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All organisms need to solve the problem of obtaining proteins, carbohydrates, fats and other nutrients from their environment without absorbing significant amounts of toxins, as many plants (and animals) produce toxins to protect against herbivory (or predation). Mammals have evolved a number of mechanisms, both physiological and behavioral, that act as defences against the ingestion and absorption of toxins in food or water. In addition to orosensory assessment of food, postingestional effects provide important information about the nutritional and toxic characteristics of the ingested food. We have characterized the effects of high doses of the toxin LiCl on learning mechanisms involved in the toxin defence system and low doses of this toxin, presented in food, on feeding behaviors. We have shown that large doses of the toxin will not only condition strong disgust responses (gaping) in rats when a novel taste is paired with the effects of the toxin, but a novel context also acquires the ability to elicit disgust responses when paired with the toxin. Of interest to us are the neural systems that are mediating the conditioning of disgust reactions to a context and we will use a lesion approach to examine the roles of the vagus nerve, the brainstem chemosensor (area postrema) and the gustatory insular cortex. We have also demonstrated that rats made sick by activating their immune system with a bacterial mimetic, will not show this type of conditioned disgust, either to a taste cue or a context. Of interest here is to determine how activation of the immune system blocks memory formation. When small doses of LiCl are mixed into a food rats will reduce their food intake to keep the toxin level below a certain threshold value, indicating that the rats are able to track ingested toxin levels. We will again be examining the roles of the same neural structures (vagus, area postrema, insular cortex) in this important phenomenon by using a lesion approach. The role of immune system stimulation in this process will also be examined as will putative age (adolescence and adult) and sex differences in this toxin tracking phenomenon. The overall thrust of this project is to characterise the negative side of incentive motivation (nausea and disgust) related to the toxin defence system.