Physiological significance of short-loop feedback of prolactin in female mice

Twice daily subcutaneous injections of 1 mg human placental lactogen for 1 week to female mice significantly suppressed the pituitary prolactin synthesis, which was measured by the in vitro incorporation of [14C]leucine into prolactin. On the other hand, 2 isologous pituitary grafting affected neither synthesis nor release of prolactin in the in situ pituitary even after 8 weeks, while the grafting resulted in the constant elevation of circulating prolactin to the proestrous level, the highest under normal physiological conditions, and induced marked mammary growth. Based on these observations, the significance of short-loop feedback of prolactin is discussed.