Melatonin modulation of the daily prolactin secretion in intact and ovariectomized ewes. Relation to a phase of the estrous cycle and to the presence of estradiol.
The aim of this study was to investigate whether melatonin might modulate the daily prolactin secretion in the ewe during a period of ovarian activity and, if so, whether this modulatory action of melatonin was related to the presence of estradiol in the organism. Ewes in the late follicular and luteal phase, as well as overiectomized ewes without (OVX) and after 7 days of estradiol injections (OVX+E2) were examined. Melatonin was infused into the third brain ventricle (100 microgram/100 microliter/h) from 14.00 to 18.00 h. The concentration of prolactin increased significantly during the infusion of melatonin in late follicular-phase ewes, but not in luteal-phase ewes, as compared to the concentration before the infusion: range from 204.0 +/- 31.7 to 272.2 +/- 50.1 ng/ml vs. range from 68.2 +/- 31.8 to 94.7 +/- 33.1 ng/ml (mean +/- SEM, n = 4, p < 0.01) and to the concentration noted during control infusions: range from 130.0 +/- 58.0 to 179.3 +/- 55.6 ng/ml (mean +/- SEM, n = 4, p < 0.05). In ovariectomized ewes, the concentration of prolactin during infusion of melatonin increased significantly, unrelated to the presence of estradiol, as compared to the concentration before infusion: range from 136.7 +/- 20.3 to 260.0 +/- 11.6 ng/ml vs. range from 41.6 +/- 2.6 to 152.3 +/- 14.6 ng/ml in OVX ewes (mean +/- SEM, n = 4, p < 0.01) and range from 161.5 +/- 66.5 to 250.2 +/- 24.3 ng/ml vs. range from 61.2 +/- 1.7 to 159.2 +/- 43.3 ng/ml in OVX+E2 ewes (mean +/- SEM, n = 4, p < 0.01). Concentrations during infusion of melatonin in OVX and OVX+E2 ewes were also significantly higher than during the control infusions: range from 7.2 +/- 1.7 to 22.2 +/- 4.1 ng/ml (mean +/- SEM, n = 4, p < 0.001). These results indicate that melatonin may affect the daily secretion of prolactin in ewes during the breeding season, and suggest that the variable response of prolactin to the melatonin signal in intact and ovariectomized ewes relates to the interaction between both ovarian steroids - estradiol and progesterone - and the prolactin-releasing factor.
Misztal
T
T
The Kielanowski Institute of Animal Physiology and Nutrition, Polish Academy of Science, Jablonna, Poland. infizys@atos.warman.com.pl
Romanowicz
K
K
Barcikowski
B
B
D000818:Animals; D001921:Brain / Q000187:drug effects; D002940:Circadian Rhythm; D004958:Estradiol / Q000494:pharmacology; D005260:Female; D005498:Follicular Phase; D007700:Kinetics; D008183:Luteal Phase; D008550:Melatonin / Q000008:administration & dosage / Q000494:pharmacology*; D010052:Ovariectomy; D011388:Prolactin / Q000378:metabolism*; D012621:Seasons; D012756:Sheep / Q000502:physiology*