Circannual rhythms in steroid receptor concentration and nuclear binding in the chick oviduct

For 3 yr, 1-yr synchronized circannual rhythms were found for the chick oviduct weights, amounts of soluble protein, concentrations of the progesterone receptor in this organ, and capacity of this steroid-receptor complex in cytosol in bind in vitro to deoxyribonucleoprotein acceptor sites. By fitting the data with 1-yr cosine curves, statistically significant circannual rhythms were described for all of these rhythms, with high values occurring in the late summer and low ones in the late winter. Interestingly, no statistically significant rhythms were described for the binding of the progesterone-receptor complex to pure DNA. By analyzing both receptor and nuclear protein preparations isolated at different times of the year, changes in the receptor were found to underlie the rhythms in nuclear binding in vitro. Studies in vivo revealed similar rhythms for the nuclear translocation and binding of both progesterone and estrogen and for the progesterone-induced changes in RNA polymerase II activity. In each instance, marked reductions in nuclear binding and steroid-induced changes in the polymerase activity occurred in the late winter. Blood levels of the 3H-labeled steroids did not vary throughout the year, so differences in whole body metabolism and disposition of the steroids were ruled out as factors in the in vivo rhythms. These results support the validity of the cell-free binding assays and the hypothesis that certain protein-DNA complexes, and not pure DNA, represent the natural acceptor sites for the progesterone receptor. Furthermore, these rhythms emphasize the fact that steroid receptors are not constant entities but change in both amount and function. Although the exact cause of these rhythms in the progesterone receptor is not understood, a rhythm in estrogen action on the oviduct is strongly suggested. These rhythms in the steroid receptors may play a role in seasonal reproduction and molting in animals.