Immune, Hormonal and Neural Mechanisms of Ingestive 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 plant (and some 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 foods, post ingestional effects provide important information about the nutritional and toxic characteristics of the ingested foods. We have been characterizing the effects of high doses of toxin (e.g., LiCl) on these defence mechanisms by examining learning mechanisms involved in aversion to foods which contain toxins. We have shown that large doses of toxin will not only condition strong disgust responding (e.g., gaping and head shaking) in rats to the taste of the foods containing the toxin, but that a novel context will also acquire the ability to elicit conditioned disgust responses when paired with the toxin. However, most foods in the environment tend to contain low levels of toxin and we have modeled this process by mixing low levels of LiCl into a food and examining both food intake and changes in behavior indicative of taste palatability. We have found that rats will reduce their food intake to keep the toxin level below a certain threshold value, indicating that rats are able to track ingested toxin levels. We have also found that female rats show a stronger conditioned response to high levels of toxin than males and we plan to examine this sex difference in more detail as well as examine the role of the female gonadal hormones, estradiol and progesterone, in conditioned disgust to both taste and context with high levels of toxin. We also plan to examine sex differences, and the role of gonadal hormones, in the ability of rats to regulate their intake of low levels of toxin in food. Regulation of the toxin threshold will also be examined in pregnant rats. The overall thrust of this program of research is to determine the role of the toxin defence system in feeding and to determine if the defence mechanism(s) may be stronger in females in order to prevent adverse reproductive outcomes.