The Mitogenic, but Not Differentiative, Response of Crop Tissue to Prolactin Is Circadian Phase Dependent*

Abstract

The circadian time of PRL [prolactin] administration is an important determinant of its stimulatory activity in pigeon crop tissue. Based on previously published experiments 2 phases of the entrained circadian cycle (0 and 9 h after light onset) which represent minimum and maximum crop sensitivity were chosen; several specific biochemical markers of mitogenesis and differentiation were examined. These included DNA synthesis, ornithine decarboxylase activity, total RNA concentration, polyadenylated RNA concentration and a specific PRL-induced mRNA. In confirmation of previous studies, crop weight was increased twice as much by ovine PRL (0.5 .mu.g/g body wt .times. 3 days) injected 9 h after light onset compared with the 0 h time of injection. A single local injection of 10 .mu.g 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. 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.