Physiology and Endocrinology: Feed Intake, Metabolism and Maternal Nutrition

Leptin has been proposed to act via NPY in the hypothalamus to modulate the effect of nutrition on sexual maturation. The objective was to evaluate the effect of oLeptin infusion and of high or low energy intake on the expression of neuropeptide Y (NPY), and 2 NPY receptors, NPY-Y1 and NPY-Y2. Thirty 6 prepubertal Nelore heifers, 18 to 20 mo-old, 275.8 ± 17.2kg BW and BCS of 5 ± 0.5 (1 to 9 scale) were randomly assigned to each of 3 treatments (n = 12): H (high energy diet), L (low energy diet), and LL (low energy diet + oLeptin). Diets were formulated to promote weight gain of 0.4 kg/day (groups L and LL) or 1.2 kg/day (H group). Heifers were fed ad libitum once a day, they were weighed and had their BCS evaluated twice weekly. After 21 d of adjustment, heifers in LL group received subcutaneous injections of oLeptin at 4.8μg/kg BW twice a day, for 56 d. Groups H and L received similar injections of 2mL saline solution. Age at puberty was considered to be the age on first detection of a corpus luteum by twice weekly transrectal ultrasonography, confirmed by plasma concentrations of progesterone of >1ng/ml. Twenty 4 heifers were slaughtered at the time of puberty for harvesting of the hypothalamus. Total RNA was extracted with Trizol, treated with DNaseI and reverse transcribed to cDNA. Expression of transcripts of NPY, NPY-Y1 and NPY-Y4 was quantified by real-time PCR. Changes in gene expression were calculated by relative quantification with the ΔΔCt method, using the gene ribosomal protein L-19 as the reference gene. There was no effect of leptin administration on expression of NPY (P = 0.70), NPY-Y1 (P = 0.72) and NPY-Y4 (P = 0.96) at the time of puberty. However, high energy intake reduced expression of NPY-Y1 (P = 0.02), without affecting expression of the other 2 genes. Downregulation of NPY-Y1 could indicate lower sensibility of the hypothalamus to NPY action. NPY is a well known inhibitor of puberty therefore this result indicates that a reduction in NPY-Y1 expression could permit attainment of puberty at a younger age in Zebu heifers.