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Prior incubation of rat anterior pituitary cells with 17beta-estradiol led to an almost complete reversal of the inhibitory effect of two dopamine agonists, dihydroergocornine and RU 24213, on both basal prolactin release and thyrotropin releasing hormone-induced prolactin release. These experiments thus demonstrate a direct interference of dopamine action by a peripheral hormone. Prolactin secretion by pituitary cells in primary culture could possibly serve as an easily accessible model of a system under dopaminergic control.

Although sex steroids were known to play a role in the control of LH, FSH, TSH and prolactin secretion, in vivo experiments could not discriminate between hypothalamic and pituitary sites of action. In this study, the specific action of sex steroids at the anterior pituitary level could be achieved using rat adenohypophyseal cells in primary culture. While estrogens stimulated the sensitivity of the LH and FSH responses to LHRH, androgens had differential effects on the secretion of the two gonadotropins: marked inhibition of LH and stimulation of FSH secretion. Progesterone, on the other hand, while having no effect in the absence of estrogens, could reverse the stimulatory effect of estrogens on LH release while it led to a stimulation of FSH secretion. Estrogens and thyroid hormone exert respective stimulatory and inhibitory effects on TSH secretion by a direct action at the pituitary level. These effects appear to be mediated changes of the level of adenohy-pophyseal TRH receptors. A close correlation was observed between the specificity of binding of the dopamine agonist (3H)dihydroergocryptine and the control of prolactin release in cells in culture, thus supporting the physiological importance of the dopamine receptor in the control of prolactin release. The high degree precision of this system permits assessment of activity of not only dopamine agonists and antagonists, but also of compounds having mixed agonist-antagonistic activity. Preincubation of anterior pituitary cells with 17beta-estradiol not only stimulated basal and TRH-induced prolactin release but, more unexpectedly, led to an almost complete reversal of the inhibitory effect of dopamine agonists on prolactin secretion. Besides its own interest, the adenohypophyseal cell culture system could well be used as a model system for study of the interaction between estrogens and dopaminergic action.