Developmental expression of the prolactin gene in the chicken.

Steady-state levels of prolactin (PRL) mRNA in the pituitary gland during embryonic development were determined by dot blot analyses to relate the changes with those of pituitary and plasma levels of PRL. Steady-state levels of the 1.38-kb mRNA encoding the PRL prohormone remained low until Day 18 of incubation, increased on Day 19 of incubation, and reached maximum levels on the day of hatch but decreased 1 day after hatch. Changes in both pituitary and plasma concentrations of PRL closely mimicked those changes in PRL mRNA levels. Subsequently, both the levels of the pituitary PRL mRNA and PRL remained unchanged whereas those of plasma PRL increased 7 days after hatch. The results indicate that a progressive expression of PRL gene in the pituitary gland occurs 1-2 days before the hatch and concomitant increases in plasma concentrations of prolactin may be associated with physiologic changes in pulmonary respiration and hatching.