Pinealectomy of melatonin implantation does not affect prolactin surge or luteal function in pseudopregnant rats

The effects of melatonin on twice daily surges in PRL and luteal function were studied in pseudopregnant (PSP) rats. Cyclic rats received pinealectomy or sham operation under pentobarbital anesthesia on diestrus 1. Pinealectomized rats immediately received a melatonin capsule (the PINX+Mel group) or a blank capsule (the PINX group), and sham group of rats received a blank capsule (the control group). After operation, all rats were maintained under the same photoperiod conditions (14L: 10D), and estrous cyclicity was examined. At the 8th estrous cycle after operation, PSP was induced by mechanical stimulation of the uterine cervix. The last day on which the rat exhibited the estrous smear was designated as day 1 of PSP. Blood samples were obtained by jugular venipuncture under light ether anesthesia between 1200 and 1300 h on days 1, 3, 5, 7, 9 and 11, and between 0000 and 0100 h on day 13 of PSP. All rats were decapitated between 1200 and 1300 h on day 13 of PSP, and trunk blood was collected. A silicon catheter was inserted into the right jugular vein under ether anesthesia on day 4 of PSP. Blood samplings were performed every 2 h between 1000 h on day 5 and 1000 h on day 6 of PSP. Serum concentrations of melatonin, PRL and progesterone were measured by radioimmunoassay. Pinealectomy or melatonin installation did not disturb the estrous cyclicity or the induction of PSP. There were no differences among the three groups of rats during PSP in the pattern or serum PRL concentrations, and there were no significant differences among these three groups in the serum progesterone concentration or weight of the corpus luteum. The present results indicated that the continuously high or low melatonin in the physiological range did not affect the rhythm or strength of PRL surges or luteal function in PSP rats.