Alpha 1- and alpha 2-noradrenergic receptor binding in guinea pig brain: sex differences and effects of ovarian steroids.

Noradrenergic transmission mediated by alpha 1-receptors is involved in the regulation of ovarian steroid-dependent reproductive processes in female guinea pigs. In addition, estradiol treatment is known to affect alpha 2-receptor binding in some steroid-sensitive regions of guinea pig brain. In these experiments, we further assessed the effects of ovarian hormones on alpha-noradrenergic receptors with quantitative autoradiographic methods. Binding of alpha 1- and alpha 2-noradrenergic receptors was measured in male and female guinea pigs treated with either estradiol or oil to determine if sex differences in steroid sensitivity might be related to differences in the distribution or modulation of alpha-receptors. In a second experiment, the possibility was tested that progesterone might alter the sensitivity to norepinephrine by modulating alpha 1- or alpha 2-receptor binding in estradiol-primed female guinea pigs. Results of these studies showed that estradiol treatment reduced alpha 1- and alpha 2-receptor binding of the ventromedial hypothalamic nucleus only in females and increased alpha 2-receptor binding in several preoptic nuclei of both sexes. Progesterone administration, however, had no effect on either alpha-receptor subtype. These studies indicate that estradiol-induced changes in alpha-receptor binding might be involved in steroid-dependent reproductive processes and in the differential responsiveness of males and females to ovarian hormones. Changes in these receptors do not appear to be involved in the stimulatory effects of progesterone on neuroendocrine processes.