The mitogenic, but not differentiative, response of crop tissue to prolactin is circadian phase dependent.

The circadian time of PRL administration is an important determinant of its stimulatory activity in pigeon crop tissue. Based on previously published experiments we chose two phases of the entrained circadian cycle (0 and 9 h after light onset) which represent minimum and maximum crop sensitivity and examined several specific biochemical markers of mitogenesis and differentiation. These included DNA synthesis, ornithine decarboxylase activity, total RNA concentration, polyadenylated RNA concentration, and a specific PRL-induced messenger RNA. In confirmation of previous studies, crop weight was increased twice as much by ovine PRL (0.5 micrograms/g BW X 3 days) injected 9 h after light onset compared with the 0 h time of injection. A single local injection of 10 micrograms of ovine-PRL increased DNA synthesis by 4-fold when injection was made at 9 h but had no effect when injection was made at 0 h after light onset. In contrast, PRL stimulation of gene expression, including total RNA, polyadenylated RNA, and a specific PRL-induced messenger RNA, were quantitatively identical at each phase of the circadian cycle. Corollary with its central role in cell proliferation, ornithine decarboxylase activity was induced by PRL injected at 9 h after light onset. The mitogenic and differentiational PRL effects in crop are therefore partially dissociable and may depend on distinct mechanisms.