Effects of interleukin-1 on sexual attractivity in a model of sickness behavior.

Interleukin-1 (IL-1), a cytokine secreted by activated macrophages, inhibits sexual behavior in female but not male rats. The present study examined the effects of IL-1 on sexual attractiveness of the injected animal and on the sexual responses exhibited by its mating partner. In Experiment 1, a male rat was placed with an estrous female, injected with either IL-1 beta (2 or 10 micrograms/kg) or saline. Males exhibited more mounts and intromissions per ejaculation and longer ejaculation latencies with IL-1- than with saline-injected females. In a second experiment, a male was placed with two estrous females, one injected with IL-1 beta (5 micrograms/kg) and the other with saline. Males performed less sexual behavior and spent less time with the IL-1-injected female. In a third experiment, an estrous female was placed with two males, one injected with IL-1 beta (5 or 20 micrograms/kg) and the other with saline. IL-1 had no effect on the time spent by the female with each male, and only the high dose reduced proceptive (courtship) behavior. In conclusion, IL-1 administration to females reduces the quality of the sexual act, thus reducing the chances for conception during infection, which is associated with spontaneous abortion and abnormal development of the fetus. In males, the chances for reproduction are less affected by IL-1, possibly because reproduction during infection is not as risky in males as in females.