Ovarian Steroids Modulate the Release of Dopamine into Hypophysial Portal Blood and the Density of Anterior Pituitary [3H]Spiperone-Binding Sites in Ovariectomized Rats*

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Abstract
The concentration of dopamine (DA) in pituitary stalk plasma and the number of DA D-2 receptor sites in the anterior pituitary gland were evaluated in ovariectomized (OVX) rats treated for 48-78 h with estradiol (E2) or vehicle and for 2-30 h with progesterone (P) or vehicle. Only the combined administration of E2 for 72-78 h and P for 24-30 h altered the release of DA into pituitary stalk blood and the number of dopaminergic binding sites in the anterior pituitary gland. The DA concentrations of pituitary stalk plasma 72 h after initiation of treatment were 1.9 .+-. 0.3 (mean .+-. standard error of the mean), 2.6 .+-. 0.2, and 2.4 .+-. 0.3 ng/ml in OVX rats treated with vehicle, E2, and P alone, respectively, and were increased significantly to 4.3 .+-. 0.4 ng/ml in E2- and P-treated rats. The binding of [3H]spiperone to DA D-2 receptors was measured by a single point assay in homogenates of the individual anterior pituitary glands obtained from the same rats. Anterior pituitary tissue was incubated with 0.4 nM [3H]spiperone in the presence or absence of 10-6 M (+)-butaclamol. Binding was similar in all animals except those treated with E2 and P, in which a significant increase occurred. Three days after initiation of treatment, binding was equivalent to 49.8 .+-. 5.8, 51.3 .+-. 5.1, and 55.4 .+-. 4.9 fmol/mg protein in OVX rats treated with vehicle, E2, and P, respectively. In contrast, a significant increase in binding to 83.7 .+-. 4.6 fmol/mg protein was observed in rats given E2 and P. No differences in either parameter were apparent 48 h after starting the steroid/vehicle treatment, nor were there any differences between morning and afternoon values in any single group. Scatchard analyses were made on the binding kinetics of DA receptors in anterior pituitary glands obtained from additional rats, and 1.5- to 2-fold increases in the density of sites were observed in animals that had received both E2 and P, while the apparent affinity of the receptors was unchanged. Treatment with the combination of E2 and P is required to increase the release of DA into hypophyseal portal blood and it also increases the density of DA receptors within the anterior pituitary gland.