Short- and long-term effects of manipulation of the pineal/melatonin axis in ewes.

The experiments which have provided insight into the role of the pineal gland and melatonin in sheep reproduction are reviewed. There is now strong evidence that timed daily melatonin administration and continuous administration via implants are equally effective in promoting the same physiological consequences as short daylength. Thus melatonin treatments lasting six weeks can result in an advance of the breeding season and an advance in the seasonal peak in ovulation rate. Much longer periods of exposure to melatonin or long-term pinealectomy result in a different response. In the second part of this review the long-term effects of pineal manipulation in juvenile and pubertal ewes are compared and contrasted. Five years after pinealectomy (at 7.5 months) ewes displayed normal timed breeding seasons without exhibiting a normal annual pattern of LH sensitivity to oestradiol. Pinealectomy at 2.5 months of age resulted in delayed puberty and breeding seasons out of synchrony with normal ewes two years later. These animals maintained an LH sensitivity to oestradiol consistent with their own breeding season. Melatonin implants which delivered melatonin for over a year caused similar effects as pinealectomy at 2.5 months but had essentially opposite effects to pinealectomy in adults. These results generate many questions about the different perception of the melatonin signal in prepubertal versus adult ewes and the factors involved in the onset and offset of the breeding season.