Contrasting effects of different levels of food intake and adiposity on LH secretion and hypothalamic gene expression in sheep

Abstract

Body reserves (long-term) and food intake (short-term) both contribute nutritional feedback to the hypothalamus. Reproductive neuroendocrine output (GnRH/LH) is stimulated by increased food intake and not by high adiposity in sheep, but it is unknown whether appetiteregulating hypothalamic neurons show this differential response. Castrated male sheep (Scottish Blackface) with oestradiol implants were studied in two 4 week experiments. In Experiment 1, sheep were fed to maintain the initial body condition (BC) score of 2·0 0·00 (lower BC (LBC), n=7) or 2·9 0·09 (higher BC (HBC), n=9), and liveweight of 43 1·1 and 59 1·6 kg respectively. LBC and HBC sheep had similar mean plasma LH concentration, pulse frequency and amplitude, but HBC animals had higher mean plasma concentrations of insulin (P<0·01), leptin (P<0·01) and glucose (P<0·01). Gene expression (measured by in situ hybridisation) in the hypothalamic arcuate nucleus (ARC) was higher in LBC than HBC sheep for neuropeptide Y (NPY; 486% of HBC, P<0·01), agouti-related peptide (AGRP; 467%, P<0·05) and leptin receptor (OB-Rb; 141%, P<0·05), but lower for cocaine-and amphetamine-regulated transcript (CART; 92%, P<0·05) and similar between groups for pro-opiomelanocortin (POMC). In Experiment 2, sheep with initial mean BC score 2·4 0·03 and liveweight 55 0·8 kg were fed a liveweight-maintenance ration (low intake, LI, n=7) while sheep with initial mean BC score 2·0 0·03 and liveweight 43 1·4 kg were fed freely so that BC score increased to 2·5 0·00 and liveweight increased to 54 1·4 kg (high intake, HI, n=9). Compared with LI, HI sheep had higher mean plasma LH (P<0·05), baseline LH (P<0·01) and pulse amplitude (P<0·01) and showed a trend towards higher pulse frequency. Although there were no differences in final mean plasma concentrations, there were significant increases over time in mean concentrations of insulin (P<0·001), leptin (P<0·05) and glucose (P<0·001) in HI sheep. Gene expression for AGRP in the ARC was higher in HI than LI animals (453% of LI; P<0·05), but expression levels were similar for NPY, OB-Rb, CART and POMC. Thus, the hypothalamus shows differential responses to steady-state adiposity as opposed to an increase in food intake, in terms of both reproductive neuroendocrine activity and hypothalamic appetiteregulating pathways. Differences in hypothalamic gene expression were largely consistent with contemporary levels of systemic leptin and insulin feedback; however, increased nutritional feedback was stimulatory to GnRH/LH whereas constant high feedback was not. The hypothalamus therefore has the ability to retain a nutritional memory that can influence subsequent responses.

Selected hormonal and neurotransmitter mechanisms regulating feed intake in sheep

“…Indeed, feeding a proteinrestricted energy balanced diet (8% v. 18% protein) resulted in an increase in immunohistochemical staining for NPY in the PeVN and ARC (Polkowska and Gladysz, 2001). In other studies, negative energy balance created by a 4-day fast resulted in an increase in NPY gene expression in the ARC with similar data obtained in castrate male sheep in low body condition (Archer et al, 2002a). In another study comparing lean ewes to fat ewes, the number of neurons positive for NPY was increased by 225% as well as an increase in NPY-positive cells co-expressing leptin receptors (Kurose et al, 2005).…”

“…Agouti-related protein AgRP mRNA Adam et al, 2002;Archer et al, 2002a) and AgRP protein (Wagner et al, 2004) were localized to the ARC nucleus in sheep. Cloning of a partial sequence of AgRP mRNA indicated a high homology with bovine AgRP but the comparison of the human message indicated only about 56% homology for the portion cloned (Wagner et al, 2004).…”

“…In one study, POMC gene expression in the ARC was found to be unaffected by a short-term (4-day) fast . In a study of feed intake and adiposity, castrate male sheep with estradiol implants were fed (4 weeks) to maintain body condition or to produce low or high body condition (Archer et al, 2002a). The expression of POMC was similar across all groups.…”

“…More importantly, feed intake remained at normal levels even after the removal of NPY. Therefore, NPY has been characterized as a neurotransmitter of the appetite system that regulated increased feed intake and is in turn regulated by nutritional as well as photoperiod manipulations in sheep.Agouti-related protein AgRP mRNA Adam et al, 2002;Archer et al, 2002a) and AgRP protein (Wagner et al, 2004) were localized to the ARC nucleus in sheep. Cloning of a partial sequence of AgRP mRNA indicated a high homology with bovine AgRP but the comparison of the human message indicated only about 56% homology for the portion cloned (Wagner et al, 2004).…”

The effect of nutrition on the seasonality of reproduction in ewes

“…By using OVX+E2 Rasa Aragonesa ewes at the beginning and the end of the seasonal anoestrus following a twoweek period of overfeeding, we observed the same effect on plasma LH concentrations, although the effects of nutrition on LH pulse frequency were not significant; possibly, because of the short time over which the nutritional treatments were administered [32]. Although high food intake appears to have a stronger effect on LH secretions than does a constant level of body fat [33,34], the initial increase in LH secretion induced by a high food intake is maintained for only a few weeks, despite consistent increases in body mass caused by high food intake. That effect occurred in intact [2,35] and in castrated/oestradiolimplanted animals [34], which probably reflects long-term adaptations that allow animals to reach a new equilibrium in energy balance after their nutritional state has changed.…”