Abstract
ABSTRACTThe seasonal breeding in sheep poses a challenge in improving productivity, owing to the incomplete knowledge of the mechanics of seasonal breeding, effective strategies for increasing the lamb crop are restricted. A major mechanism responsible for seasonal reproduction is a striking increase in the responsiveness of gonadotropin-releasing hormone (GnRH) neurons to the negative feedback effects of estradiol that peaks during anestrous and decreases in the breeding season. Melatonin does not directly drive seasonal changes in reproduction; rather it appears to synchronize an endogenous circannual rhythm. The Pars Tuberalis (PT) is a single common region across species that shows strongest MT1 expression. Melatonin acts through high affinity, G-protein coupled receptors located in thyrotroph cells in PT of the pituitary, controlling the production of the hormone, thyrotropin (TSH). Probably, the thyroid hormones serve as a connecting link between environmental cues and reproductive activity. Two very important peptides viz. kisspeptins and RFamide relative peptide appear to be key targets of thyroid hormones. Kisspeptin has been demonstrated as a potent GnRH stimulator and RFRP as an inhibitor which gets modulated with the season and controls reproductive activity in sheep.
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