Kisspeptin and Phoenixin: Emerging Neuropeptide Tools for the Management of Sheep Breeding

Reproductive efficiency remains the single greatest determinant of profitability in sheep enterprises worldwide. Seasonal anoestrus, variable puberty onset, and the need for tight estrus synchrony for artificial insemination (AI) continue to challenge producers, particularly in breeds with strong photoperiodic reproductive rhythms. Conventional control of the ovine estrous cycle relies heavily on exogenous progestagens (e.g., intravaginal sponges/CIDRs), gonadotropins (PMSG/eCG, hCG), and prostaglandin F2α analogues. While effective, these protocols carry concerns related to hormone residues, animal welfare, cost, and consumer acceptance of hormone-treated food animals. Two hypothalamic neuropeptides-kisspeptin and phoenixin - have emerged over the last two decades as central regulators of the reproductive axis and as promising, more "physiological" alternatives or adjuncts for reproductive management in sheep. This article reviews the neuroendocrine biology of kisspeptin and phoenixin, their interaction with the hypothalamic–pituitary–gonadal (HPG) axis in ewes and rams, and their practical and experimental applications in oestrus induction, synchronization, and treatment of seasonal and lactational anoestrus. 1. Introduction: Sheep are short-day seasonal polyoestrous animals in most temperate breeds, meaning reproductive activity is driven largely by photoperiod acting through the pineal melatonin signal (Malpaux et al., 1999). This seasonality, combined with nutritional and social cues, produces prolonged periods of anovulation that limit the ability to breed out of season, shorten the breeding window, and complicate synchronization for AI or embryo transfer programs. Traditional endocrine control tools - progestagens, PMSG, and prostaglandins—work downstream of the hypothalamus and do not correct the underlying neuroendocrine "gate" that suppresses gonadotropin-releasing hormone (GnRH) pulsatility during anoestrus. The discovery that the KISS1 gene and its product kisspeptin are essential upstream activators of GnRH neurons (de Roux et al., 2003; Seminara et al., 2003) transformed reproductive neuroendocrinology and opened a new avenue for manipulating the reproductive axis at its origin. More recently, phoenixin, a peptide derived from the SMIM20 (small integral membrane protein 20) precursor, has been identified as a modulator that interacts with the kisspeptin–GnRH pathway and independently stimulates gonadotropin release (Yosten et al., 2013; Stein et al., 2016). Together, these peptides represent a shift from managing the symptoms of anoestrus (low circulating gonadotropins) to targeting the central drivers of reproductive quiescence. 2. The Kisspeptin System: 2.1 Molecular Biology and Receptor Kisspeptin is encoded by the KISS1 gene and is proteolytically cleaved into a family of peptides (kisspeptin-54, -14, -13, and -10) that share a common C-terminal decapeptide required for biological activity. Kisspeptin signals through the G protein–coupled receptor GPR54 (also called KISS1R), which is densely expressed on GnRH neurons (Messager et al., 2005; Clarkson & Herbison, 2006). 2.2 Kisspeptin Neuron Populations in the Ovine Hypothalamus Two principal populations of kisspeptin neurons have been characterized in sheep: Arcuate nucleus (ARC) population – co-expresses neurokinin B and dynorphin (the so-called "KNDy" neurons), acts as the pulse generator for GnRH/LH secretion, and is the primary site of negative-feedback regulation by estradiol and progesterone (Goodman et al., 2007; Lehman et al., 2010). Preoptic area (POA) population – mediates the estradiol-induced preovulatory GnRH/LH surge and is sexually dimorphic, being more prominent in ewes than rams (Franceschini et al., 2006; Smith et al., 2007). 2.3 Kisspeptin as the Photoperiodic and Metabolic Integrator Kisspeptin neurons are the point of convergence for the major inhibitory signals of seasonal and lactational anoestrus: Photoperiod/melatonin: Long days suppr…