Progestins affect reproductive behavior and androgen receptor dynamics in male guinea pig brain.

The present study extends previous studies of ours by comparing the anti-androgenic effect of a progestin agonist (R5020) with progesterone (P). Intact male guinea pigs treated with P (1 and 10 mg/day) and R5020 (100 micrograms/day) had greater latency to mount and lower numbers of mounts and intromissions compared to controls. Ejaculation and plasma testosterone concentration were not affected. Specific brain regions were analyzed for androgen receptor (AR) content. Progestins produced fewer (P less than 0.01) nuclear AR in hypothalamus-preoptic area and pituitary without associated changes in cytosolic AR. These data are best interpreted by postulating an effect of P on AR dynamics mediated through the P receptor and not by competition for androgen binding to its receptor.