Abstract
Timing of puberty has a great influence on animal productivity. For example, reproduction in sheep can be affected by seasonality, leading to fluctuations in availability of animal products. Therefore, optimization of birth dates would improve reproductive success in sheep. Since the discovery of the major role of kisspeptin and Kiss1R, its cognate receptor, in reproductive function, there are new opportunities for interventions. Repeated or continuous administration of native kisspeptin are able to hasten puberty and induce ovulation during breeding and non-breeding seasons of sheep. However, due to the short half-life of kisspeptin, protocols involving native kisspeptin are usually proof of concept, but not practical under field conditions. Consequently, there are efforts to develop kisspeptin analogues capable of replicating effects of repeated/continuous administration of native kisspeptin. In this review, we intended to provide a comprehensive summary of the neuroendocrine requirements for puberty onset and ovulation in adult ewes, focusing on kisspeptin, its physiological effects and responses to its analogues on reproductive function in ewes.
Highlights
In sheep, the onset of puberty occurs when there are metabolic cues that sufficient growth has occurred and when photoperiod becomes permissive
The hypothalamus become less sensitive to the negative feedback of estradiol (E2), stimulating increased pulse frequency for both gonadotropin releasing hormone (GnRH) and luteinizing hormone (LH)
During early stages of juvenile development, the number of Kiss1-expressing cells increase in both POA and arcuate nucleus (ARC)
Summary
The onset of puberty occurs when there are metabolic cues that sufficient growth has occurred and when photoperiod becomes permissive. These data supported the assertion that NKB acts in an autocrine/paracrine manner, indirectly influencing GnRH secretion Dyn, another co-expressed neuropeptide in the arcuate KNDy neurons, is an endogenous opioid peptide that selectively binds the k-opioid receptor (KOR). Corroborating the hypothesis of opposing effects of Dyn vs Kp/NKB, Dyn expression is higher during the early follicular phase, whereas Kp/ NKB expression peak during the surge (Fergani et al, 2017) Based on these data, it has been suggested that KNDy neurons of the ARC nucleus could be the GnRH pulse generator. It has been suggested that KNDy neurons of the ARC nucleus could be the GnRH pulse generator Another recently discovered neuropeptide, GnIH (Gonadotropin-inhibitory hormone), may have a role in physiological control of reproduction, due to its inhibitory effect on GnRH release in quails (Tsutsui et al, 2000). Further work will be necessary to establish the role, if any, of RFRP-3 in controlling sheep reproduction
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