Effects of estrogens on the characteristics of [3H]spiroperidol and [3H]RU24213 binding in rat anterior pituitary gland and brain.

Since estrogens have been found to exert a marked inhibitory effect on dopaminergic action at the anterior pituitary and striatal levels in the rat, the effect of estrogen treatment has been studied on the binding characteristics of the dopamine (DA) antagonist [3H]spiroperidol and of the new DA agonist [3H]N-n-propyl-N-phenylethyl-beta-(3-hydroxyphenyl)ethylamine hydrochloride ([3H]RU24213) in rat striatum, nucleus accumbens + olfactory tubercle, frontal cortex and anterior pituitary gland. Specificity of binding was carefully examined in order to investigate possible changes of the agonist and antagonist states of the DA receptor. Estrogen treatment led to a small increase (approx. 20%) of [3H]spiroperidol and [3H]RU24213 binding in rat striatum, nucleus accumbens + olfactory tubercle and frontal cortex while no significant effect was found in the anterior pituitary gland. That the increased binding is due to a corresponding increased number of binding sites and not to higher affinity is indicated by the absence of effect of estrogen treatment on the IC50 values for displacement of the two labeled ligands by a variety of unlabeled compounds. Specificity of binding of DA agonists and antagonists remained unchanged after estrogen treatment. The present data suggest that the potent desensitizing effect of estrogen on DA action at the striatal and pituitary levels is exerted at a step subsequent to binding of DA to its receptor.