α1- and α2-noradrenergic receptors in steroid-sensitive brain areas: development and response to estradiol-17β benzoate in neonatal guinea pigs

Abstract

Immature female guinea pigs are relatively insensitive to the effects of ovarian steroids on certain reproductive processes including the facilitation of sexual receptivity and the induction of hypothalamic progestin receptors by estrogen. In adult guinea pigs, α-receptor-mediated noradrenergic transmission is known to affect both of these processes. In the experiments reported here, we investigated the possibility that age-related differences in α 1- and α 2-noradrenergic receptors might underlie the insensitivity of neonates to ovarian steroids. With tritium-sensitive film autoradiography, the distribution of α 1- and α 2-receptors in neonatal and adult female guinea pigs was compared and the effects of exogenous estradiol-17β on α-receptor binding was examined in neonatal guinea pigs. The results of these experiments showed that in animals not treated with estrogen the binding of both α-receptor subtypes differed between adults and neonates in several brain regions including areas known to be involved in the regulation of reproduction. In all regions where differences occurred, α-receptor levels were higher in neonatal females than in adult females. In addition, in contrast to previously reported results in adults, estrogen did not affect α-receptor binding in any region of neonatal guinea pig brain.