Anorexic effects of interleukin 1 in the rat

Interleukin 1 (IL-1) administration produces anorexia. Among unanswered questions about this effect are 1) whether it plays a role in the cachexia associated with chronic infection and cancer, and 2) whether IL-1 acts directly on food intake or indirectly by first lowering the set point for body weight. To investigate these questions, rats were infused with recombinant IL-1 continuously for 14 days through osmotic minipumps. Tolerance to the anorexic effects of the infusion developed within a few days. Control experiments showed that neither loss of IL-1 potency nor failure in the delivery system were responsible for recovery of food intake. Prior weight reduction completely overrode the anorexic effects of IL-1; previously food-restricted rats were hyperphagic initially despite receiving IL-1. This result is consistent with the view that IL-1 lowers the set point for body weight, but the development of tolerance prevented the full evaluation of this interpretation.