The effects of melatonin on prolactin and growth hormone secretion in ewes under different photoperiods, during the early post partum period

The effects of melatonin on the secretion of prolactin (PRL) and growth hormone (GH) were studied in ewes (Polish Longwool, n = 20) nursing lambs, under different day length (photoperiod) regimes. The animals were divided into four groups: (i) long day control group (LDC, n = 5); (ii) long day melatonin-treated (LDM, n = 5) group, which had lambed in March (spring); (iii) short day control group (SDC, n = 5) and (iv) short day melatonintreated (SDM, n = 5) group, which had lambed in November (autumn). Blood samples were collected from all ewes 5 days after lambing. Four blood collections were performed at 10- day intervals, over a 40-day time period. Sampling started at sunset and continued for 6 h at 20 min intervals. Subcutaneous melatonin implants were inserted in ewes of the LDM and SDM groups after first blood collection. The plasma concentrations of PRL and GH were assayed in samples using RIA. In ewes from the LDC group, the mean plasma PRL concentration increased gradually, reaching a significantly (P < 0.001) higher level, after 3 weeks. In contrast, in the LDM group, the PRL concentration decreased significantly (P < 0.001) after 10 days. The mean plasma GH concentration was significantly (P < 0.001) higher in the LDC than in the LDM group, for the entire experimental period. However, a gradual decrease in the concentration was recorded in both groups. In the SDC and SDM groups, the plasma PRL concentrations did not decrease significantly 2 weeks after the onset of the trial, and did not differ significantly in these groups. The mean plasma GH concentration increased significantly (P < 0.001) in the SDM group, compared to the SDC group, after the third week. The mean plasma GH concentration in the SDM and the SDC group reached a similar level by the end of the trial. It would appear that melatonin may effectively inhibit PRL secretion in ewes during the early post partum period under long photoperiod, and stimulate GH release in such ewes under short photoperiod. The inhibition of PRL secretion in nursing ewes during increasing photoperiod (long days) occurred, despite the strong stimulation of suckling. It can therefore be assumed that melatonin via decreasing PRL concentrations, may be used to shorten the nursing period in early-lactating ewes.