Estradiol-mediated increases in the anorexia induced by intraperitoneal injection of bacterial lipopolysaccharide in female rats

Lipopolysaccharide (LPS) derived from the cell walls of gram-negative bacteria causes a robust acute phase response (APR) that includes fever, anorexia, and many other elements. Because immune system function, including some models of illness anorexia, is sexually differentiated, we investigated the sexual differentiation of the anorexia induced by intraperitoneal LPS injections in rats. Cycling female Long-Evans rats tested either during diestrus or estrus ate less following 6.25 microg/kg LPS than did intact males. Following 12.5 microg/kg LPS, females in estrus ate less than either females during diestrus or males. Similarly, a more pronounced anorexia occurred following 12.5, 25, and 50 microg/kg LPS in ovariectomized females that received cyclic estradiol treatment and were tested on the day modeling estrus than in untreated ovariectomized rats. LPS also increased the length of the rats' ovarian cycles, usually by a day, especially when injected during diestrus. As in male rats, when LPS injections were repeated in the same rats, both estradiol-treated and untreated rats failed to display any significant anorexia. The inhibitory effects of LPS on eating in intact and ovariectomized rats were expressed solely as decreases in spontaneous meal frequency, without significant alteration of spontaneous meal size. These data indicate that anorexia following peripheral LPS administration is sexually differentiated and that estradiol is sufficient to produce this response. The mechanism of the pathophysiological effect of estradiol on meal frequency appears to be different from the physiological effect of estradiol on food intake because the latter is expressed solely as a change in meal size.