Plasma prolactin as determined by radioimmunoassay was barely detectable in 21-day-old fetuses and on the day of birth (day 0). It had declined by days 3–6 and then rose gradually to attain adult levels at day 35. To further evaluate (he development of the prolactin control system, the ability of estrogen to elicit prolactin release was studied at various ages by injecting different doses of estradiol benzoate (Eb) SC once daily lor 2 days and measuring plasma prolactin 24 hr after the last injection. At 11 days of age, there was no significant increase in prolactin with any of the doses of Eb used (0.05–0.4 μg/100 g body wt). In 27-day-old rats, the 0.4 μg dose induced a dramatic rise in plasma prolactin and the 0.05, 0.10 and 0.2 μg dose induced much smaller but significant increases in prolactin. By 37 days, the 0.2 μg dose became highly effective and the 0.4 μg dose induced an even larger increase. The response of adult females to this dose was significantly less than that observed in 37-day-old animals. In a subsequent experiment, the time of first response to estrogen was determined more exactly t o be 24 days of age. The maturation of the dopaminergic inhibitory control of prolactin release was studied by injecting the dopamine (DA) receptor blocker, Pimozide (0.63 mg/kg, SC), at different ages. The drug significantly elevated plasma prolactin as early as the third day of life and the magnitude of this response then increased progressively up to 35 days of age. It is suggested that plasma prolactin levels during development in the female rat depend on the balance between the stimulatory effect of estrogen on prolactin release and the inhibitory control exerted by the hypothalamic dopaminergic system.
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