Lipopolysaccharide and interleukin-1β decrease sucrose intake but do not affect expression of place preference in rats

Immune system activation has been shown to induce decreased interest in pleasurable stimuli. Studies of this phenomenon have assessed the effect of cytokines or lipopolysaccharide (LPS) on behavior maintained by primary reinforcers, stimuli, such as palatable solutions, that effectively reinforce behavior without prior training. The studies reported in this paper replicated findings of immune system activation decreasing intake of a palatable solution and assessed the effects of immune activation on behavior maintained by a conditioned reinforcer, a stimulus paired with a the palatable solution. Using a conditioned place preference procedure, the effects of LPS and interleukin-1β (IL-1β) on sucrose intake (primary reinforcer) and preference for a sucrose-paired environment (conditioned reinforcer) were tested. LPS and IL-1β decreased sucrose intake but had no effect on the expression of a sucrose-induced place preference, indicating a differential effect of immune system activation on appetitive behaviors maintained by primary and conditioned reinforcers. Finally, it was shown that a sucrose-induced place preference is sensitive to the motivational state of the subjects at the time of testing; a sucrose-induced place preference was demonstrated if rats were tested when water deprived but not if tested after free access to water.