Comparing the influence of immune activation and toxin administration on various aspects involved in feeding

One of the most common symptoms of immune activation in mammals is a reduction in food intake. The present thesis compared the associative and non-associative properties of the immune stimulant, lipopolysaccharide (LPS), and the toxin, lithium chloride (LiCl), on both the appetitive and consummatory components involved in feeding in male Long Evans rats. Appetitive behaviors are food finding behaviors displayed by an animal prior to the consumption of the food, whereas consummatory behaviors are the behaviors involved in the actual consumption of food. The results showed that LiCl treatment resulted in an unconditioned decrease in sucrose intake whether sucrose was infused intraorally or whether it was voluntarily consumed from a bottle. In contrast, LPS treatment produced decreases in sucrose intake only when assessed via bottle intake, but not intraorally, suggesting that LPS produces unconditioned adverse effects only on the appetitive responses. Furthermore, when sucrose was voluntarily consumed from a bottle during conditioning, LPS treatment resulted in conditioned avoidance of the taste but not aversion to the taste. In contrast, LiCl treatment produced both conditioned avoidance as well as aversion. However, when conditioning involved intraoral infusions of sucrose, LPS treatment did produce evidence of a conditioned aversion. Investigation of the effects of these agents on the consumption and palatability of a mildly aversive sucrose-quinine solution produced different results. Both LPS and LiCl produced a typical conditioned avoidance of the bitter-sweet taste in the consumption measure. LPS treatment produced an unconditioned increase in oral rejection responses relative to saline controls or LiCl-treated rats, suggesting that immune activation can have effects on consummatory responding when unpalatable tastes are used. The findings of this thesis underscore the importance of the method in which the taste stimulus is delivered in the induction of conditioning, suggesting that activation of the immune system produces differential effects on the appetitive and consummatory components of feeding dependent on the conditioning route. As well, the findings of this thesis are consistent with evidence for separate underlying mechanisms responsible for conditioned avoidance and conditioned aversion.