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Effects of Neonatal Exposure to Estradiol on Prolactin Secretion and Activity of the Tubero‐lnfundibular Dopamine System in Young Adulthood: Comparison with Neonatal Prolactin Deficiency *

Previous results from this laboratory indicate that female rats who consume milk deficient in prolactin (PRL) during the neonatal period subsequently display hyperprolactinemia, associated with decreased activity in the tubero‐infundibular dopamine (DA) system and decreased lactotrope responsiveness to DA receptor stimulation. The present studies tested whether these neuroendocrine consequences of neonatal PRL deficiency can be mimicked by exposure of neonatal rats to estradiol. Female rats were injected sc with 1 mUg estradiol benzoate or oil vehicle on postpartum Days one to 3, while in other experiments, females were made neonatally deficient in PRL through treatment of their mothers with the DA agonist bromocriptine, a treatment that reduces the levels of PRL in milk. Females treated neonatally with estradiol benzoate, as well as offspring of the bromocriptine‐treated mothers, displayed hyperprolactinemia as young adults, as compared to their respective vehicle‐matched controls, and in both cases, this was abolished by ovariectomy, indicating dependence upon ovarian secretions. As reported previously in neonatal PRL‐deficient females, neonatal estradiol benzoate‐treated animals also exhibited reduced steady state levels and decreased turnover rates of DA in the median eminence when 35 days of age. DA levels and turnover rates in this region were still significantly reduced on postpartum Day 60. The DA agonist bromocriptine suppressed PRL release to a similar extent in cultured anterior pituitary cells from neonatal estrogen‐treated and control rats, suggesting normal responsiveness of DA receptors on lactotrope cells in both groups. The present results confirm the ability of estradiol treatment or induction of a PRL deficiency during the early neonatal period to induce subsequent hyperprolactinemia in female rats, and further indicate that the hyperprolactinemic conditions resulting from either neonatal manipulation are dependent on the ovary and are associated with decreased levels and turnover of DA in the median eminence during the prepubertal period. Although these findings suggest that increased exposure to estradiol during the neonatal period may underlie the similar effects of neonatal PRL deficiency, the further observations in neonatal estrogen‐treated rats that 1) decreased DA turnover in the median eminence persists at Day 60, and 2) lactotrope responsiveness to DA is normal, differ from results obtained previously in PRL‐deficient rats. Thus, enhanced exposure to estrogen during the neonatal period does not appear to account for all of the neuroendocrine consequences of neonatal PRL deficiency.