Corticosterone is required for the prolactin receptor gene expression in the late pregnant mouse mammary gland

In order to clarify the prolactin receptor (PRL-R) gene expression al lactogenesis, the levels of the long and short forms of PRL-R mRNA were determined by the competitive RT-PCR in the pregnant, lactating and ovariectomized midpregnant mouse mammary gland. Plasma concentrations of corticosterone and progesterone were determined by RIA. The long form of PRL-R mRNA level was low until 10:00 on day 18, increased 3.3-fold at 22:00 on day 18 of pregnancy and further increased to 4.6-fold at 10:00 on day 0 of lactation. The short form of PRL-R mRNA level remained unchanged during this time period. The corticosterone:progesterone ratio increased 15.5-fold during the last 1.5 days of pregnancy. Corticosterone increased the long form of PRL-R mRNA level when the tissues on day 17 were cultured. On day 12 of pregnancy and following ovariectomy, corticosterone was exceedingly high from 2 h to 8 h and the corticosterone:progesterone ratio changed prior to the increase in the long form of PRL-R mRNA level. We conclude that corticosterone increases the PRL-R gene expression in the mammary gland before the onset of parturition. (C) 1997 Elsevier Science Ireland Ltd.