Melatonin Modulation of the Daily Prolactin Secretion in Intact and Ovariectomized Ewes

Abstract

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 µg/100 µl/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.

Circadian Determinations of Cortisol, Prolactin and Melatonin in Chronic Methyl-Phenyl-Tetrahydropyridine-Treated Monkeys

“…Hormonal and neurovegetative alterations in PD are probably the consequence of hypothalamic alterations, although histological studies of the hypothalamus reveal no neuronal loss. Reflecting the findings of the present work, several authors have confirmed the preservation of hypothalamic dopaminergic neurons in PD [77, 78, 79]and MPTP-treated monkeys [70]. However, specific changes in prolactin and melatonin regulation [19, 20, 21, 22]suggest alterations in the pituitary-hypothalamic system, findings which deserve more attention.…”

“…Moreover, there are melatonin receptors in the pars tuberalis of the pituitary [69], and melatonin may have a direct action on dopamine transmission. Melatonin may increase prolactin release [70], causing a secondary inhibition of dopamine release [71]. Besides, the suprachiasmatic nucleus of primates contains dopamine receptors, an observation which may explain the inhibitory effect of dopamine on melatonin regulation [72].…”

The Effect of the Photoperiod and Exogenous Melatonin on the Protein Content in Sheep Milk

“…In natural conditions, the maximum concentration of prolactin in the blood is identified in the long-day period, and at this time, the level of melatonin decreases. The lowest concentration of prolactin is observed during the short-day period, which is when the level of melatonin is the highest [32,33]. Experiments performed by Molik et al [34,35] demonstrated that day length likely determines the milk yield of sheep.…”

The effect of the diversified signal of melatonin on milk yields in seasonally breeding sheep

“…Increased secretion of MLT in the period of shorter days contributed to lower milk secretion in sheep. The studies by Lincoln et al (1992Lincoln et al ( , 2000 and Misztal et al (1999) carried out on barren sheep and rams demonstrated similar trends of the MLT profile. The milk-performance parameters in seasonally breeding sheep are undoubtedly affected by the profile of MLT which modulates secretion of lactothroph hormones.…”