Ontogeny of Prolactin Releasing and Inhibiting Activities in the Posterior Pituitary of Male Rats

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Abstract
Plasma PRL levels in male rats are highest during the peripubertal period. We previously reported that the posterior pituitary (PP) contains a potent PRL-releasing factor (PRF), a trypsin-insensitive small peptide which is distinct from known PRL secretagogues. The objectives were to determine the ontogeny of PRF activity in the PP as well as age-related alterations in anterior pituitary responsiveness to PRF. We also explored if the PP contains a nondopaminergic PRL-inhibiting factor (PIF). PRF/PIF activities were assessed by the ability of PP extracts to alter PRL release from cultured anterior pituitary cells. The PP were extracted with perchloric acid and lyophilized, thus eliminating endogenous dopamine. PRF activity in PP extracts from 10- and 20 day-old (d) rats was very low, increased gradually in 30d and 40d rats, and remained unchanged in adult (90d) rats. In a second experiment, age-related changes in anterior pituitary responsiveness to PP extracts from adult rats and to TRH were determined. The responsiveness of anterior pituitary cells from 10d rats to PRF was low, increased dramatically in cells from 20d rats, and was reduced in cells from 30d and adult rats. The responsiveness to TRH was highest in cells from 10d rats. In a third experiment, anterior pituitary responsiveness to age-matched PP extracts was assessed. Only PIF activity was observed when PP extracts from 10d rats were incubated with anterior pituitary cells from 10d rats. In contrast, PP extracts from 20d, 30d and adult rats exhibited only PRF activity when incubated with age-matched cells. Treatment of PP extracts from 10d rats with trypsin abolished their PIF activity, but did not alter the PRF activity in PP extracts from older rats. These data indicate that PP from 10d male rats contain a trypsin-sensitive nondopaminergic PIF. Peak PRF activity in the neonatal PP is reached between days 30 and 40, whereas anterior pituitary responsiveness to PRF is highest on day 20. These results suggest that a variable input from the PP, including PRF, dopamine and a nondopaminergic PIF, may be important for the maturation of the PRL-releasing mechanism during the postnatal development of the male rat.