Comparison of the effects of bacterial lipopolysaccharide and muramyl dipeptide on food intake.
For further characterization of the mechanism involved in the anorexia during bacterial infection, we investigated whether muramyl dipeptide (MDP), the minimal immunologically active structure of gram-positive bacterial cell walls, affects rats' food intake in the same way as lipopolysaccharide (LPS) from E. coli. MDP (1.6 mg/kg body weight = b.wt.) injected intraperitoneally (IP) reduced food intake by decreasing meal frequency without affecting meal size. Indomethacin (2.5 mg/kg b.wt., IP) but not verapamil (5 mg/kg b.wt., IP) attenuated the hypophagic effect of MDP. In further experiments, MDP and LPS (100 micrograms/kg b.wt., IP) both inhibited gastric emptying and indomethacin failed to block this effect of LPS. Hepatic vagotomy did not attenuate the hypophagic effects of MDP or LPS. LPS reduced water intake only when food was available, but reduced food intake also during water deprivation. MDP did not affect water intake. MDP and LPS both had an aversive effect, but LiCl, which was also aversive, failed to reduce feeding under the conditions tested. This questions the role of a conditioned taste aversion in the hypophagia induced by MDP or LPS. The results suggest that a stimulation of eicosanoid synthesis contributes to MDP-induced hypophagia and may therefore also contribute to the anorexia during infection. In contrast, an inhibition of gastric emptying, an activation of hepatic satiety signals or a reduction of water intake, does not seem to be crucial for the hypophagic effects of MDP or LPS.