Effects of progestins on sexual behaviour in castrated lizards (Cnemidophorus inornatus).

In many male vertebrates, androgens stimulate, while progesterone inhibits, sexual behaviour. Testicular androgens also control sexual behaviour in males of the lizard Cnemidophorus inornatus. However, administration of progesterone will reinstate sexual behaviour in castrated animals. Increased doses of progesterone did not result in increased numbers of responders; at all doses tested approximately 36% (n = 45) of animals treated with progesterone responded as reliable courters. A high percentage (60-68%) of progesterone-sensitive individuals maintained their sensitivity over a period of months. A synthetic progestin 17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione with low rates of metabolism and conversion, also stimulated sexual behaviour in castrated C. inornatus. Finally, the synthetic antiprogestin 11 beta-4-dimethylaminophenyl-17 beta-hydroxy-17 alpha-1-propinyl- 4,9-oestradiene-3-one inhibited the stimulatory effects of progesterone on sexual behaviour but failed to inhibit the stimulatory effects of androgens. These findings indicate an unusual departure from the characteristic pattern of steroid sensitivities of neural centres controlling sexual behaviour.