ANDROGEN TREATMENT DECREASES ESTROGEN-RECEPTOR BINDING IN THE VENTROMEDIAL NUCLEUS OF THE RAT-BRAIN - A QUANTITATIVE IN-VITRO AUTORADIOGRAPHIC ANALYSIS

Androgens oppose the actions of estrogen on a number neuroendocrine functions in the rat including prolactin and gonadotropin secretion and the activation of the female pattern of sex behavior. Although in nonneural tissues antiestrogenic actions of androgens have been related to actions at the level of the estrogen receptor, previous attempts to demonstrate effects of nonaromatizable androgens on estrogen receptor levels in the brain have been unsuccessful, possibly because of the poor anatomical resolution of the methods used. We have used a new in vitro autoradiographic assay combined with an I-125-labeled estrogen receptor ligand to test the hypothesis that the nonaromatizable androgen, 5 alpha-dihydrotestosterone (5 alpha-DHT), may act to reduce estrogen binding in specific regions of the brain involved in reproductive neuroendocrine and behavioral responses. This in vitro autoradiographic method allows selective measurement of occupied estrogen receptors in tissue sections. Gonadectomized/adrenalectomized rats were divided into two groups per sex. All animals received daily injections of estradiol benzoate (EB: 40 mu g/kg body wt) for 4 days. Animals in the 5 alpha-DHT treatment group received 5 alpha-DHT (10 mg/kg body wt) every 12 h for 4 days, while animals in the control group received vehicle injections. Animals were killed 4 h after the final EB/5 alpha-DHT injection and their brains processed for in vitro autoradiography. As previously reported, higher levels of estrogen binding were observed in the ventrolateral aspect of the ventromedial nucleus (vIVMN) and the periventricular and medial preoptic area of the female compared to the male. 5 alpha-DHT treatment resulted in an overall decrease in estrogen binding which attained statistical significance only in the vIVMN of the female. This effect of 5 alpha-DHT was inhibited by flutamide, a nonsteroidal androgen mediated through the androgen receptor system. When similarly treated female rats were injected (iv) with a saturating dose of unconjugated estradiol 1 h before sacrifice to occupy and thus detect all available receptor, a similar 5 alpha-DHT-induced decrease in estrogen-binding capacity in the vIVMN was observed. Thus, it is unlikely that 5 alpha-DHT treatment resulted in downregulation of estrogen receptor occupation through a local effect on estrogen metabolism. These data demonstrate for the first time that nonaromatizable androgens can act in the brain to downregulate estrogen receptor. As the vIVMN is the principal site for regulation of female sex behavior in the female rat, these findings support the hypothesis that androgens may act to inhibit estrogen-induced female sex behavior by decreasing estrogen receptor binding. (C) 1994 Academic Press, Inc.