Effect of Early Calf-Hood Nutrition on the Hypothalamic-Pituitary Axis in Holstein-Friesian Bulls

Improved early life nutrition can advance age at onset of puberty in bull calves. While the precise biochemical mechanisms have not been fully elucidated the effect is most likely mediated through a direct influence of metabolic status on hypothalamic gonadotrophin releasing hormone (GnRH) pulsatility, ultimately leading to enhanced luteinizing hormone (LH) pulsatility and testosterone synthesis and release. The objective of this study was to examine the effect of nutrition during the first 18 weeks of life on selected genes in the arcuate nucleus of the hypothalamus and anterior pituitary in Holstein-Friesian bull calves. Holstein-Friesian bull calves with a mean (±S.D.) age and bodyweight of 19 (±8.2) days and 47.5 (±5.3) kg, respectively, were assigned to either a high (n=10) or low (n=10) plane of nutrition, with target growth rates of 1.2 and 0.5 kg per day, respectively. At 126±3 days of age, calves were euthanized following intravenous administration of sodium pentobarbitone and the arcuate nucleus region of the hypothalamus together with the anterior pituitary were excised. RNA was isolated and cDNA was synthesised. Quantitative PCR was performed to detect mRNA expression of the selected genes. The following genes were selected for analysis in the arcuate nucleus: Kiss1R, GPR54, GnRH, AgRP, NPY, POMC, MC4R, IGF1R, IGF1, OBR and GHSR while: LHβ, FSHβ, GHSR, GH1, GnRHR, IGF1 and IGF1R were selected for the anterior pituitary. UBQ and RSP9 were selected as stable reference genes. Data were analysed using mixed models methodology (PROC MIXED; SAS). For variables that were not normally distributed, λ and log transformations were used. Only one gene, GHSR was upregulated in the anterior pituitary (P<0.05) and tended towards greater transcript abundance in the arcuate nucleus (P<0.1) in the low compared with the high plane of nutrition calves. This is the first study to investigate the effect of early calfhood nutrition on aspects of the molecular regulation of the hypothalamic-pituitary axis of the bull calf. The inverse relationship between nutritional status and transcript abundance for the ghrelin receptor (GHSR) in both arcuate nucleus and anterior pituitary tissue is consistent with the well-defined role of this gene in energy homeostasis and regulation of body weight growth and most likely reflects poorer metabolic status of the calves on the low plane of nutrition. Ghrelin is a known inhibitor of GnRH pulsatility, the magnitude of its receptor is related to precocious puberty in cattle.