Abstract
Within the hypothalamic-pituitary-gonad (HPG) axis, the major hierarchical component is gonadotropin-releasing hormone (GnRH) neurons, which directly or indirectly receive regulatory inputs from a wide array of regulatory signals and pathways, involving numerous circulating hormones, neuropeptides, and neurotransmitters, and which operate as a final output for the brain control of reproduction. In recent years, there has been an increasing interest in neuropeptides that have the potential to stimulate or inhibit GnRH in the hypothalamus of pigs. Among them, Kisspeptin is a key component in the precise regulation of GnRH neuron secretion activity. Besides, other neuropeptides, including neurokinin B (NKB), neuromedin B (NMB), neuromedin S (NMS), α-melanocyte-stimulating hormone (α-MSH), Phoenixin (PNX), show potential for having a stimulating effect on GnRH neurons. On the contrary, RFamide-related peptide-3 (RFRP-3), endogenous opioid peptides (EOP), neuropeptide Y (NPY), and Galanin (GAL) may play an inhibitory role in the regulation of porcine reproductive nerves and may directly or indirectly regulate GnRH neurons. By combining data from suitable model species and pigs, we aim to provide a comprehensive summary of our current understanding of the neuropeptides acting on GnRH neurons, with a particular focus on their central regulatory pathways and underlying molecular basis.
Highlights
The reproductive function of pigs is controlled by complex regulatory networks, which integrate peripheral and internal cues and impinge at the brain centers driving the reproductive axis
Kisspeptin neurons along with neurokinin B in the preoptic area (POA) regulate gonadotropinreleasing hormone (GnRH) cell bodies and in the arcuate nucleus (ARC) act on GnRH terminal axons in the median eminence, which is essential for GnRH neurons to stimulate LH secretion in pigs
It is speculated that RFamide-related peptide (RFRP) act as the essential upstream regulators in the control of GnRH secretion with an inhibitory effect in pigs, as has been proposed for rats and nonhuman primates, but remains unclear
Summary
The reproductive function of pigs is controlled by complex regulatory networks, which integrate peripheral and internal cues and impinge at the brain centers driving the reproductive axis. GnRH is synthesized in a small subset of hypothalamic neurons, which form the final common pathway for the central control of reproduction (Herbison 2016). They integrate steroidal, lactational, hunger, stress, satiety, circadian, odorant, and pheromone signals (Spergel 2019). LH and FSH, which are required for the development and maintenance of the gonads and for fertility, bind to receptors on the Copyright © The Author(s)
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