Persistence of prolactin (PRL) surges induced by progesterone in ovariectomized rats.
The continuity of PRL surges induced by progesterone was examined. Cycling rats were ovariectomized and simultaneously implanted with Silastic tubes containing progesterone. Administration of progesterone was continued by renewing these tubes every 7 days. Five days after the onset of progesterone treatment, they exhibited high plasma PRL concentration at 3:00 h, which revealed the appearance of nocturnal PRL surge. This was confirmed by the observation at 2 h intervals on Days 6 and 14. On both days, diurnal and nocturnal PRL surges were detected. Contrarily, there was no significant PRL release on Day 21. So, PRL surges might terminate between 10 and 15 days after their onset. Because there was apparent nocturnal surge on Day 21 in rats cervically stimulated one day before, disappearance of PRL surges was not the result of attenuation in responsiveness of pituitary gland to hypothalamic control.These data suggest the existence of a specific central mechanism responsible for the termination of PRL surges even under high progesterone milieu.