It has previously been shown that administration of the indoleamine melatonin to advance the breeding season of ewes is also associated with an increase in ovulation rate and subsequent litter size.
Experiment 1 assessed whether, in ewes receiving melatonin to advance the breeding season, the indoleamine acts directly on the corpus luteum to enhance progesterone secretion or indirectly through increased activity of the hypothalamic pulse generator. Six ewes received 3 mg melatonin orally at 15.00 h daily from 22 March onwards, six were induced to ovulate during mid-anoestrus following withdrawal of a progestagen pessary and injection of exogenous gonadotrophin and six acted as naturally ovulating controls. First overt oestrus occurred between 17 May and 8 July in melatonin-treated ewes, between 21 October and 3 January in control ewes and on 8 July in all induced ewes. On days 2 and 10 after the first overt oestrus, melatonin-treated ewes had pulsatile LH activity characteristic of that measured in control ewes ovulating naturally during the breeding season. There was an absence of any pulsatile LH activity in the induced ewes. Progesterone concentrations between days 7 and 12 following oestrus were significantly higher in melatonin-treated than in control and induced ewes, suggesting a luteotrophic role for melatonin.
Experiment 2 was carried out to determine whether administration of melatonin commencing after induced ovulation and insemination would alter the endocrine status of the ewe and thereby influence the establishment of pregnancy and embryo survival. Thirty-two anoestrous ewes were induced to ovulate on 29 June. Starting 24 h after intra-uterine insemination, 16 ewes were given melatonin daily for 60 days and 16 acted as controls. Daily LH concentrations were higher in melatonin-treated than in control ewes from days 2 to 22 after oestrus, while prolactin concentrations declined in melatonin-treated ewes over the same period. Plasma progesterone concentrations were enhanced in melatonin-treated ewes between days 4 and 9 following oestrus, yet ovulation rates were the same as for controls. Successful pregnancies occurred in 0·56 control (9 of 16) and 0·69 melatonin-treated (11 of 16) ewes. For these ewes the number of fetuses surviving to term as a proportion of ovulation rate was 0·43 and 0·51 for the control and melatonin treatment respectively.
J. Endocr. (1988) 119, 523–530
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