REGULATION OF ESTROGEN-RECEPTOR AND PROGESTERONE-RECEPTOR MESSENGER-RIBONUCLEIC-ACID BY ESTROGEN IN THE BRAIN OF THE WHIPTAIL LIZARD (CNEMIDOPHORUS UNIPARENS)

Receptive behavior in females vertebrates is controlled by hormones, p rincipally estrogen, secreted by the ovary. Estrogen influences behavi or by interacting with a specific estrogen binding protein, or recepto r, located in target cells in certain hypothalamic nuclei. To better u nderstand the molecular mechanisms involved in the control of receptiv e behavior in whiptail lizards, we investigated the effects of exogeno us estrogen on the regulation of estrogen receptor and progesterone re ceptor expression in several regions of the brains of Cnemidophorus un iparens. First we determined a dosage of 17 beta-estradiol 3-benzoate (0.5 mu g) which reliably induced receptive behavior in ovariectomized C. uniparens. Then using in situ hybridization, we examined the effec ts of that dosage on the expression of estrogen receptor and progester one receptor mRNA in the brain 24 h after injection. Estrogen treatmen t resulted in a significant up-regulation of estrogen receptor mRNA ex pression in the ventromedial nucleus of the hypothalamus and torus sem icircularis, down-regulation of estrogen receptor mRNA expression in t he lateral septum, and no change in the periventricular nuclei of the hypothalamus, the periventricular nucleus of the preoptic area, and th e dorsal hypothalamus. The same dosage resulted in increased progester one receptor mRNA expression in the ventromedial nucleus of the hypoth alamus and the periventricular nucleus of the preoptic area; no signif icant changes in progesterone receptor mRNA expression were observed i n the periventricular nuclei of the hypothalamus or the torus semicirc ularis, although the differences in progesterone receptor expression i n the torus semicircularis approached statistical significance. The up -regulation of estrogen receptor gene expression by estrogen in the ve ntromedial nucleus of the hypothalamus of lizards is opposite to that reported in female rats in which estrogen down-regulates estrogen rece ptor expression in the ventromedial nucleus of the hypothalamus. We pr opose that this may be related to differences in reproductive physiolo gy between vertebrates which have extended follicular phases and prolo nged periods of estrus and rats, which have an abbreviated follicular phase and brief periods of estrus.