The effects of lipopolysaccharide and lithium chloride on the ingestion of a bitter-sweet taste: Comparing intake and palatability

Activation of the immune system with lipopolysaccharide (LPS) has been shown to result in decreased consumption of normally preferred substances while at the same time not affecting palatability. The present study examined the effects LPS administration on both intake and palatability of a relatively unpalatable bitter-sweet taste. Bitter is thought to signal a danger cue to an animal representing a potential toxin-containing food. Using a one-bottle consumption test, voluntary intake of a sucrose-quinine (0.15 M sucrose + 0.00015 M quinine; S-Q) solution was assessed in rats on two conditioning days (days 1 and 4) after a systemic injection with LPS, LiCl, or NaCl. On the test day (day 7), rats were given 1 h access to the same solution in the absence of any injection. In a separate experiment, rats fitted with intraoral cannulae received similar testing schedules, however, the solution was delivered intraorally, activating only the consummatory responses of the animal. During conditioning, rats received 5 brief (1 min) intraoral infusions of the taste across a 1 h period following injections of LPS, LiCl or NaCl. Individual taste reactivity responses were recorded and analyzed. Both LPS and LiCl resulted in decreased consumption of the unpalatable taste relative to controls on the test day, suggesting typical conditioned taste avoidance. When the consummatory responses were examined, LPS-treatment produced an increase in active oral rejection relative to NaCl- and LiCl-treated groups on both conditioning days. The present study demonstrates that although both LPS- and LiCl-treatment result in similar conditioned avoidance using an intake measure, they do not elicit similar patterns of taste reactivity responding to intraoral infusions of the bitter-sweet taste. Furthermore, the present results suggest that immune activation with LPS-treatment results in increased rejection of a mildly aversive stimulus and supports the hypothesis that reorganization of behavioral priorities occurs during bacteria-induced sickness.