The objective was to determine the effect of central infusion of insulin and (or) glucose on hypothalamic expression of leptin receptor and pituitary secretion of LH in the ewe. Twenty-two ovariectomized ewes (32 wk of age) were fitted with two lateral cerebroventricular (LCV) cannulae and fed 33% of NRC requirements for 8 wk. Ewes (n> or =5/group) were then infused, via LCV cannulae, with artificial cerebrospinal fluid (aCSF) or aCSF containing physiological concentrations of insulin (INS), glucose (GLU), or INS + GLU; the mass of each increasing linearly from Day 0 (mass = 0 units/h) to Day 8 (mass of INS = 80 mIU/hr and GLU = 10 mg/hr). Jugular serum was collected every 12 min for 4 hr on Days 0, 2, and 4. Ewes treated with INS or INS + GLU had greater (P<0.06) mean concentrations of LH than aCSF treated ewes on Day 2 (13.8+/-1.8 and 12.5+/-1.3 > 8.0+/-3.3 ng/ml). Furthermore, on Day 4, concentrations of LH in INS treated ewes exceeded that (P<0.07) of aCSF treated ewes (14.8+/-2.0 > 7.4+/-3.0 ng/ml). Expression of NPY mRNA did not differ between treatments (P = 0.87). Leptin receptor mRNA expression was dramatically reduced (P<0.0002) in INS+GLU versus aCSF treated ewes. These data provide evidence to suggest that insulin may be an important component of hypothalamic mechanisms regulating secretion of LH and expression of leptin receptors in undernourished ruminants.

The physiological regulation of food intake is a critical factor in both the rate at which an animal grows and its reproductive activity. Recently, progress has been made in elucidating a complex system in which insulin, leptin, and neuropeptide Y function to monitor an animal's energy balance and regulate feed intake and fertility. RNA was extracted from ovine hypothalamic, anterior pituitary, pancreas, and adipose tissue. Using the reverse transcription-polymerase chain reaction. cDNAs were cloned and sequenced for leptin (350 base pairs [bp], GenBank accession number U62123 and 441 bp, GenBank accession number U84247), NPY-Y1 receptor (350 bp, GenBank accession no. U62122) and NPY-Y2 receptor (440 bp, GenBank accession no. U83458). Probes generated from these clones were used to detect mRNA expression within tissues thought to be involved in the coregulation of feed intake and reproduction. Leptin was found to be expressed in sheep adipose tissue. The ovine NPY-Y1 receptor mRNA was detected within the arcuate nucleus and paraventricular nucleus of the hypothalamus, the dentate gyrus of the hippocampus and in pancreatic, anterior pituitary, and adipose tissues. Expression of ovine NPY-Y2 receptor mRNA was detected in the hippocampus and within pancreatic tissue. These observations provide evidence of potential mechanisms that exist for mediating communication between peripheral and central tissues within the insulin-leptin-NPY pathway.

Neuropeptide Y (NPY) is highly expressed in hypothalami of undernourished and genetically obese animals, and is a potent regulator of food intake and reproduction. Leptin, a protein expressed by adipocytes, has been reported to reduce hypothalamic NPY expression. We recently detected (by ribonuclease protection assay [RPA]) expression of the NPY receptor subtype Y1 (but not Y2) mRNA in adipose tissue. Based on these observations we hypothesized that NPY-Y1 receptors in adipose may represent a peripheral mechanism by which NPY can regulate leptin expression in a direct and rapid manner. To test this hypothesis, adipose samples were biopsied from the tailhead region of 48 ± 3 kg wether lambs immediately before and 30 min after a single intravenous injection of 50 μg porcine NPY (“treated” animals, n = 5), or vehicle (“control” animals, n = 4). Injection of NPY resulted in an increase in expression (P = 0.013; as measured by RPA) of both leptin and NPY-Y1 mRNA. In treated animals, negative correlations were found between response in leptin expression and body weight (r = −0.82, P = 0.092), and between leptin response and initial leptin mRNA levels (r = −0.81, P = 0.097). These data provide evidence of a peripheral mechanism by which NPY may regulate adipocyte expression of both leptin and NPY-Y1 receptor mRNA.

Infertility associated with suboptimal nutrition is a major concern among livestock producers. Recently, much effort has been put into understanding the role of the protein leptin in regulating feed intake and reproduction. Leptin, produced by adipocytes, has receptors in the hypothalamus, but more precise locations of leptin receptor-expressing cell bodies have not been reported in a livestock species. The leptin receptor transcript has several splice variants in the mouse and human, but only the “long-form” product (OBRL) is capable of signal transduction. A partial ovine long-form leptin receptor cDNA was cloned and used to evaluate OBRL mRNA expression within hypothalamic, anterior pituitary, and adipose tissues of ovariectomized adult ewes. Expression was detected in reverse transcription-polymerase chain reaction products of all tissues examined. OBRL mRNA was detected by in situ hybridization in the ventromedial and arcuate nuclei of the hypothalamus. In ewes that had been feed restricted for 3 wk before tissue collection, the expression of OBRL mRNA in these areas was greater (P<0.05) than that found in well-fed ewes. These findings provide evidence that the full-length leptin receptor is expressed in hypothalamic, anterior pituitary, and adipose tissue (the latter proffering an autoregulatory mechanism for leptin) and that within the hypothalamus, this receptor form is differentially expressed in well-fed vs. feed-restricted animals.