Prolactin messenger ribonucleic acid levels, prolactin synthesis, and radioimmunoassayable prolactin during the estrous cycle in the golden Syrian hamster.

The purpose of this study was to observe the molecular dynamics of pituitary prolactin (PRL) gene expression during the estrous cycle of the Golden Syrian hamster. PRL messenger ribonucleic acid (mRNA) levels, PRL synthesis (3H-PRL in the incubation media or incubated pituitary after a 3 hr incubation with 3H-leucine), and radioimmunoassayable (RIA) PRL (in the incubation media or incubated pituitary after the 3 hr incubation) were measured in the morning (0930-1100 hr) on each day of the cycle. We observed that all of these PRL indices declined or did not change from Day 2 to Day 3 of the cycle. From Day 3 to Day 4 (proestrus), however, PRL mRNA levels increased 33-38% and media 3H-PRL increased 32-42%, while there were no significant changes in pituitary 3H-PRL, or RIA-PRL in the media or pituitary. From Day 4 to Day 1 (estrus) there was a reciprocal change in the levels of 3H-PRL in the pituitary vs. the media, with the former increasing 37-50% and the latter decreasing 25-32%. Pituitary RIA-PRL also increased 45-64% from Day 4 to Day 1 while media RIA-PRL did not change. These data are consistent with the following hypothesis: On the morning of proestrus (Day 4) in the hamster, PRL mRNA levels are elevated compared to those on Day 3, signaling an increase in PRL synthesis. This newly synthesized PRL is shunted into a "readily releasable" pool on the morning of Day 4 (contributing to the afternoon surge of serum PRL), and into a "preferentially stored" pool by the morning of Day 1 (for release in response to cervical stimulation and use as a luteotrophin to maintain early pregnancy should fertilization occur).