Ontogeny of prolactin mRNA in the rat pituitary gland as evaluated by in situ hybridization.

We have studied the ontogeny of prolactin (PRL) messenger RNA (mRNA) in male and female rats. Quantitative in situ hybridization was performed on sections of fixed pituitaries using a 32S-labeled PRL cDNA probe. With this technique, hybridization signal was first detected on day 19 of gestation. The PRL mRNA levels were very low in foetuses and newborn animals. Higher PRL mRNA levels were found in 5-day-old animals. Thereafter, mRNA concentrations regularly increased to reach a plateau at 60 and 90 days of age in males and females, respectively. Sexual dimorphism was first observed in 20-day-old animals, the PRL mRNA levels being higher in the female than in the male. This difference in PRL mRNA became more marked after puberty such that in 90-day-old animals the amounts of PRL mRNA in females were 2.7 times those observed in males. Since sexual dimorphism in PRL mRNA levels occurs well before sexual dimorphism in PRL secretion, which takes place first during puberty, it is suggested that during sexual maturation PRL secretion is regulated translationally as well as transcriptionally.