Modifications of the High and Low Affinity Pituitary Domperidone-Binding Sites in Chronic Estrogenized Rats*

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Abstract
The effect of chronic estrogen treatment on the anterior pituitary domperidone-binding sites was studied in female rats. The rats were implanted from 1-6 mo. with a Silastic capsule containing 17.beta.-estradiol. The Feldman analysis of [3H]domperidone binding to anterior pituitary membranes of control or estrogenized rats revealed the presence of 2 sites. The binding characteristics of the higher affinity site were identical for both groups (Kd of the high affinity site, 0.30-0.45 nM; maximum number of binding sites of the high affinity site, 74-95 fmol/mg protein); however, those of the lower affinity site were affected by the estrogen treatment: the Kd of the low affinity site increased from 17.4 .+-. 3.2 to 41.5 .+-. 9 (.+-. SE) nM, and the maximum number of binding sites of the low affinity site increased from 214 .+-. 22 to 343 .+-. 35 fmol/mg protein. In chronic estrogenized rats, the total number of binding sites was increased by 54%. These changes, induced by chronic estrogenization, were reversible, since 2 wk after removal of the 17.beta.-estradiol pellet, the binding characteristics were no longer different from those observed in control rats. In contrast to chronic estrogen treatment, ovariectomy reduced markedly the total number of [3H]domperidone-binding sites in anterior pituitary membranes (-70%). Feldman analysis revealed that this reduction resulted from the complete disappearance of the low affinity sites in those membranes. No significant change in the binding characteristics of the high affinity site was detected in ovariectomized rats. Since estradiol induces a decrease in the anterior pituitary content of dopamine, a denervation supersensitivity-like mechanism might be responsible for the increase in pituitary domperidone-binding sites in estrogenized rats. Conversely, a hyposensitivity mechanism could be implicated in the decrease in the total number of the pituitary domperidone-binding sites in ovariectomized rats, since pituitary dopamine levels are increased in those animals. Whether the antidopaminergic properties of estrogen are also involved in this modulation after chronic estradiol treatment requires further investigation.