Neuroendocrine signaling pathways and the nutritional control of puberty in heifers.
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Puberty is a complex physiological process in females that requires maturation of the reproductive neuroendocrine system and subsequent initiation of high- frequency, episodic release of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH). Genetics and nutrition are two major factors controlling the timing of puberty in heifers. While nutrient restriction during the juvenile period delays puberty, accelerated rates of body weight gain during this period have been shown to facilitate pubertal development by programming hypothalamic centers that underlie the pubertal process. Among the different metabolic factors, leptin plays a critical role in conveying nutritional information to the neuroendocrine axis and controlling pubertal progression. Because GnRH neurons are devoid of the leptin receptor, leptin's effects on GnRH neurons must be relayed via an afferent neuronal network. Two neuronal populations located in the arcuate nucleus (ARC) that express the orexigenic peptide neuropeptide Y (NPY), and the anorexigenic peptide alpha melanocyte-stimulating hormone (MSH), are key components of afferent pathways that convey inhibitory (NPY) and excitatory (MSH) inputs to GnRH neurons. In addition, ARC neurons expressing kisspeptin, a potent stimulator of GnRH release, are also involved in the nutritional regulation of puberty. Our studies have demonstrated that increased planes of nutrition during juvenile development result in morphological and functional changes in hypothalamic pathways comprising NPY, proopiomelanocortin (POMC), and kisspeptin neurons. Changes included differential expression of NPY, POMC, and Kiss1 in the ARC, and plasticity in the axonal projections to GnRH and kisspeptin neurons. Additionally, increased rates of body weight gain also promoted changes in the pattern of DNA methylation, a key epigenetic mechanism for regulation of gene expression. Finally, our most recent findings suggest that maternal nutrition during gestation can also induce structural and functional changes in hypothalamic neurocircuitries that are likely to persist long after pubertal maturation and influence reproductive performance throughout adulthood in cattle.