FIGURE 15 Control of prolactin secretion. Dashed line indicates inhibition. A physiological role for TRH (thyrotropin-releasing hormone) and other postulated releasing hormones has not been established. Estradiol stimulates secretion and may interfere with the inhibitory action of dopamine.

To date, there is no known product of prolactin action that feeds back to regulate prolactin secretion. The effects of suckling and estrogen on prolactin secretion are open loops. Experiments in animals suggest that prolactin itself may act as its own "short-loop" feedback inhibitor by stimulating dopaminergic neurons in the arcuate nucleus. It is not certain that such an effect is applicable to humans. If prolactin is a negative effector of its own secretion, it is not clear what mechanisms override feedback inhibition to allow prolactin to rise to high levels during pregnancy.

Cellular Regulation of Prolactin Secretion

As in most other endocrine cells, the secretory activity of lactotropes is enhanced by increased cytosolic concentrations of calcium and cyclic adenosine monophosphate (cAMP) (Fig. 16). Dopamine acting through G-protein-coupled receptors, inhibits prolactin secretion

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