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Suckling Unmasks the Stimulatory Effect of Dopamine on Prolactin Release: Possible Role for -Melanocyte- Stimulating Hormone as a Mammotrope Responsiveness Factor^*

Division of Molecular and Cellular Endocrinology, Department of Anatomy and Cell Biology, Medical University of South Carolina Charleston, South Carolina 29425

Address all correspondence and requests for reprints to: Dr. L. Stephen Frawley, Division of Molecular and Cellular Endocrinology, Department of Anatomy and Cell Biology, Medical University of South Carolina, 171 Ashley Avenue, Charleston, South Carolina 29425.

Abstract

Mounting evidence indicates that dopamine (DA) can stimulate as well as inhibit PRL release when given in appropriately low doses. In the present study, we investigated whether the suckling stimulus could influence this response. Pituitary cultures from suckled or nonsuckled rats were exposed to DA (10^–16–10^–6 M) during a reverse hemolytic plaque assay for PRL. Pituitary cells from nonsuckled rats exhibited only the inhibitory response to DA; exposure to high-dose DA (10^–6 M) reduced plaque area to 42.3 ± 7.2% (mean ± SEM) of control. A low dose of DA (10^–12 M) had no effect on PRL secretion (79.3 ± 13.3% of control). In striking contrast, a brief suckling stimulus (10 min) rendered the mammotropes responsive to stimulation by low-dose DA (to 152.7 ± 12.5% of control). Thus, suckling appears to be a requirement for expression of the stimulatory effect of DA in lactators. In a subsequent series of experiments we explored the possibility that a hypophysial factor, released during nursing, might mimic the effects of suckling on mammotrope responsiveness. Accordingly, we tested the ef- fects of -melanocyte-stimulating hormone (10^–7 M) and lowdose DA, alone or in combination, on pituitary cells from non-suckled rats. Although neither agent alone had a dramatic effect on PRL secretion, concurrent administration of both of these significantly stimulated PRL release to 130.0 ± 4.2% of control. Taken together, these results demonstrate that suckling renders mammotropes responsive to the stimulatory effects of DA. Moreover, our data indicate that -melanocyte-stimulating hormone could function as a responsiveness factor in this phenomenon.

Footnotes

^* This work was supported by NIH Grant DK-38215 (to L.S.F.)

Received February 4, 1991.

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