Evidence of prolactin short loop feedback in the postpartum lactating rat

Serum prolactin was studied in lactating and nonlactating rats after restraint stress, chronic estradiol benzoate treatment, and acute L-tryptophan treatment. As expected, these treatments evoked increases in serum prolactin in nonlactating rats. In lactating rats these treatments did not increase basal serum prolactin when litters were removed for 4 hr. Daily estradiol benzoate treatment increased pituitary gland prolactin content after 14 days, but there was no effect on basal serum prolactin. Daily estradiol treatment did not augment prolactin release in response to 30 min of suckling. It is suggested that the short loop feedback by prolactin (i.e., prolactin inhibiting its own secretion) makes the lactating rat refractory to these stimuli of prolactin secretion.