Resolution of estrogen binding species in hypothalamus and pituitary

In addition to the classic Type I estrogen receptor, a second estrogen binding species has been reported in rat uterine nuclear material. We have examined nuclear material from hypothalamus and pituitary for the presence of Type II estrogen receptor by the sucrose pad/exchange assay. Extensive (0.05-40.0 nM) saturation analyses were performed on crude chromatin isolated from hypothalami and pituitaries of hyperestrogenized ovariectomized rats. Analysis of data by an adaptation of the graphical method of Rosenthal suggests that there is only a single class of estrogen binding sites in these tissues with a Kd (0.2 nM) close to that reported for Type I receptors in other systems. However, a definitive resolution of the binding component(s) was not possible due to noise in the upper regions of the saturation plot. Therefore, we pooled 7 hypothalamic and 4 pituitary saturation experiments and analyzed the data with LIGAND, a nonlinear curve fitting program. Computer generated curves indicate that the data from both target tissues are approximated most closely by a model describing a single high affinity binding species (Type I) (Kd = 0.17 - 0.38 nM) and a linear or very low affinity nonspecific binding component. Thus, we conclude that there is no evidence for the presence of a second or lower affinity estrogen binding component in hypothalamus or pituitary. The absence of this binding species in these two estrogen target tissues is consistent with the concept that Type II sites may be involved in estrogen's control of tissue hypertrophy and hyperplasia--estrogen responses not found in hypothalamus or pituitary.