Progesterone-induced decrease of pineal protein synthesis in rats. Possible participation in estrous-related changes of pineal function.

Pineal protein synthesis in female rats, estimated from the incorporation of labeled amino acids into proteins in vitro, exhibited significant changes as a function of the stage of the estrous cycle. These changes were restricted to the proestrous and estrous days; pineal protein synthesis attained its maximum on the morning of proestrus declining abruptly by 53% during the evening, at the time of the expected gonadotrophin and prolactin release. Pineal serotonin-N-acetyltransferase activity increased by 10 to 15 times during night-time on every day of cycle; no appreciable modification of its daily rhythm was detected along the estrous cycle. Spayed rats treated for 2 days with progesterone showed a dose-dependent decrease of amino acid incorporation into pineal proteins, regardless of whether estradiol was simultaneously administered or not. Pineal protein synthesis in spayed rats administered with estradiol for 2 days and killed at 11 a.m. and 5 p.m. on the third day, did not show differences as a function of time of sacrifice. When progesterone was injected on the morning of the third day a significant decline in protein synthesis was observed at 5 p.m. Only in the latter group serum LH levels showed significantly greater values at 5 p.m. Pineal serotonin content of estradiol-treated rats increased significantly at evening, an effect which was obliterated by the administration of progesterone; progesterone alone did not affect pineal serotonin content. Radioactivity uptake by pineal glands incubated with labeled progesterone did not show changes along the estrous cycle. These data argue in favour of the involvement of progesterone in the changes of pineal protein synthesis observed during the "critical period" for gonadotrophin and prolactin release.