Endocrine responses of ovariectomized ewes to i.c.v. infusion of urocortin.

Urocortin is a novel corticotropin-releasing factor-like peptide, first isolated from the rat midbrain, which has anorexigenic properties, possibly associated with its involvement in the stress axis. Urocortin has been implicated in blood pressure regulation, ACTH release and feed intake, but its role as an integral component of the reproduction-nutrition axis has not been examined. The present experiment was designed to determine the effects of i.c.v. infusion of urocortin on feed intake and endocrine profiles of LH, GH, IGF-I, cortisol and leptin in ovariectomized ewes. Ewes were fitted with two laterocerebroventricular cannulae and urocortin was continuously infused in a linearly increasing manner from 0.001 microg/h on day 0, to a maximum of 31.6 microg/h on day 5. Blood samples were collected via jugular catheters at 10 min intervals for 4 h on day 1, 3 and 5, and assayed by RIA for LH, GH, IGF-I, cortisol and leptin. All ewes were allowed free access to feed and water, and feed intake was recorded daily. Urocortin-infused ewes responded with a significant decrease in feed intake beginning on day 1 (P<0.02) and were aphagic for the remainder of the experiment. Serum concentrations of LH were elevated in individual samples from urocortin-treated compared with saline-treated ewes on day 3 (treatment x day x sample, P=0.05), but were not different on day 1 or 5. Mean serum concentrations of GH increased (P<0.04) over days with urocortin treatment, although concentrations of IGF-I were not influenced by treatment (P>0.5). Serum concentrations of cortisol were markedly increased by urocortin treatment (P<0.001). Leptin tended to be influenced by treatment and day (P=0.08), with leptin levels tending to be elevated in urocortin-treated vs saline-treated ewes on day 5 (P=0.08). The ability of urocortin to decrease feed intake while increasing LH, GH, cortisol and leptin provides evidence that urocortin is not only an integral component of the stress axis, but possibly of the nutrition-reproduction axis in sheep.

Central infusion of leptin into well-fed and undernourished ewe lambs: effects on feed intake and serum concentrations of growth hormone and luteinizing hormone.

Leptin has been implicated in the regulation of feed intake, growth, and reproduction. The objective of this study was to determine if centrally administered leptin would affect feed intake and the secretion of growth hormone (GH) and luteinizing hormone (LH) in ewe lambs. Eighteen ewe lambs were ovariectomized and fitted with intracerebroventricular (i.c.v.) cannulae. Lambs were randomly assigned to receive either a maintenance diet (fed), or a diet that provided 38% of maintenance requirements (diet-restricted) for 14 weeks. Subsequently, recombinant ovine leptin or vehicle was continuously infused, via i.c.v. cannulae, in a linearly increasing dose for 8 days, reaching a maximum of 1.25 microg/kg per h. Feed intake was recorded on days -1 to 7. Blood was collected via jugular cannulae every 10 min for 4 h on days 0, 2, 4, 6 and 8 for the determination of serum leptin, insulin, LH and GH. Leptin suppressed feed intake in fed lambs on days 4 to 7 (P<0.001), but had no effect on feed intake in diet-restricted lambs (P>0.25). Fed lambs had greater serum concentrations of leptin than diet-restricted lambs (P=0.007). Also, although not different on day 0 (pretreatment), on day 8 serum leptin concentrations were greater in leptin-treated lambs than in saline-treated lambs (P=0.003). Insulin was lower in diet-restricted than in fed lambs (P=0.003), but was not affected by leptin treatment (P=0.82). LH pulse frequencies were lower in diet-restricted lambs than in fed lambs (P=0.038), but were not affected by leptin treatment (P=0.85). Mean serum GH was greater in diet-restricted than in fed lambs (P<0.01). In diet-restricted lambs treated with leptin or saline, mean GH did not differ on day 0, but increased in response to leptin treatment (P<0.006). Treatment of fed lambs with leptin did not affect serum GH (P>0.32). From this work, we propose that leptin represents an important functional link between adipose stores and hypothalamic function in ruminants. We demonstrate that leptin concentrations change in response to reduced nutritional status, and that leptin has the ability to regulate multiple physiological processes in lambs, including both feed intake and secretion of GH.

Neural levels of cholecystokinin peptide and mRNA during dietary stimulation of growth and LH secretion in growth-retarded lambs.

Chronic feed restriction of prepubertal male lambs adversely affects reproductive development by inhibiting the pulsatile release of luteinizing hormone (LH). Because this effect can be reversed by ad libitum feeding, the associations between diet-induced increases in LH release and concurrent changes in body weight gain, serum glucose. CCK peptide, and CCK mRNA were examined. Neonatally castrated male lambs received a restricted ration to maintain their respective weaning weights beginning at 8 wk of age. At 23 wk of age, lambs were assigned randomly to receive additional feed equivalent to 0%, 50%, or 100% of their previous daily intake. Serum profiles of LH and glucose were determined 2 and 4 wk after onset of the increased intakes. At 27 wk of age, lambs were euthanized and both hypothalamic and cerebral cortical tissues were collected for analysis of CCK peptide and CCK mRNA. With additional intakes, body weight gain increased (P < 0.001) proportional to the graded increases in feed intake. Mean serum LH concentrations, LH peak frequencies, and serum glucose concentrations also increased (P < 0.05) progressively among the 0%, 50%, and 100% dietary intake groups. Neither CCK peptide nor CCK mRNA differed (P > 0.05) among dietary groups suggesting that endogenous CCK in the whole hypothalamus did not change with the feeding-induced increase in LH release. Concentrations of CCK peptide in cerebral cortex were greater (P < 0.05) than hypothalamic concentrations, but there were no differences between hypothalamus and cerebral cortex in the relative abundance of CCK mRNA. In summary, dietary stimulation of growth-retarded male lambs resulted in progressive increases in body weight gain, mean serum LH, serum glucose, and LH peak frequencies. Because hypothalamic levels of CCK peptide and CCK mRNA did not change during feeding-induced secretion of LH, endogenous CCK in the hypothalamus seems unlikely as a chronic mediator of nutrition-sensitive LH release.