Oestrogen receptors in cell nuclei of the hypothalamus-preoptic area-amygdala following an injection of oestradiol or the antioestrogen CI-628

Oestrogen receptors were measured by an exchange assay in cell nuclei of the hypothalamus-preoptic area-amygdala (HPA) of ovariectomized-adrenalectomized (OVX-ADX) rats following an intravenous injection of oestradiol or an antioestrogen, CI-628. Receptors were translocated to the nucleus by both compounds. Receptors translocated by the antioestrogen were specific for oestrogens; testosterone, corticosterone and progesterone were not bound by these receptors. Several properties of antioestrogen- and oestradiol-receptor complexes were compared. The time-course in cell nuclei in vivo showed that receptors were still present in HPA nuclei 24 h after administration of CI-628 but not 24 h after oestradiol. This prolonged increase of nuclear receptors after the antioestrogen treatment was attributed to the continued presence of CI-628 and its metabolites in plasma. The maximum level of receptors produced in the HPA cell/nuclei following CI-628 treatment was lower than the peak level of nuclear receptors following oestradiol. The dissociation rate in vitro of nuclear antioestrogen-receptor complexes formed in vivo was more rapid at 0°C than that of nuclear oestradiol-receptor complexes. This property may be related to the lower peak level of binding after CI-628 treatment and to the inhibitory actions of antioestrogens.