Progesterone Inhibits Female-Typical Receptive Behavior and Decreases Hypothalamic Estrogen and Progesterone Receptor Messenger Ribonucleic Acid Levels in Whiptail Lizards (GenusCnemidophorus)

Female-typical sexual behavior in tetrapods is mediated primarily by estrogen and progesterone acting through intracellular receptors at specific sites in the mediobasal hypothalamus. Progesterone exerts both faciliatory and inhibitory actions on female sexual behavior and in well-studied rodent models, the inhibitory actions are exerted through downregulation of progesterone and estrogen receptors. This study examined progesterone effects on both female-typical sexual behavior and hypothalamic estrogen and progesterone receptor mRNA expression (ER- and PR-mRNA) in a sexual and parthenogenetic species of whiptail lizard. Progesterone capsules administered to ovariectomized femaleCnemidophorus inornatusandCnemidophorus uniparensfollowing a receptivity-inducing dosage of estradiol benzoate (EB) strongly inhibited receptive behavior as compared to blank implanted controls. Progesterone capsules administered either before or after an EB injection also strongly downregulated ER- and PR-mRNA abundance in the ventromedial nucleus of the hypothalamus relative to blank implanted controls. The correlated decrease in both EB-induced receptive behavior and ER- and PR-mRNAs following progesterone administration are similar to findings in rats and guinea pigs, suggesting that this is an evolutionarily conserved mechanism in the regulation of female sexual behavior.