The GalR2 galanin receptor mediates galanin-induced jejunal contraction, but not feeding behavior, in the rat: differentiation of central and peripheral effects of receptor subtype activation.

The neuropeptide galanin mediates a diverse array of physiological functions through activation of specific receptors. Roles of the three recently cloned galanin receptors (GalRs) in rat intestinal contraction and food intake were examined using GalR-selective ligands and the results were compared with the pharmacological profiles of defined GalRs. The action profile of these ligands in jejunal contraction resembled only that of GalR2 and only a high level of GalR2 mRNA was detected in the tissue, supporting GalR2 as the receptor mediating jejunal contraction. The action profile for food intake in rats excluded GalR2, GalR3 and the putative pituitary galanin receptor as the 'feeding receptor', suggesting that either GalR1 or an unidentified GalR is responsible for mediating this function.

Differential effects of intracerebroventricular glucagon-like peptide-1 on feeding and energy expenditure regulation.

Intracerebroventricular (i.c.v.) administration of glucagon-like peptide-1-(7-37) amide (GLP-1) has been shown to modulate food and water intake. The present studies further characterize the effects of i.c.v. GLP-1 in the regulation of energy balance in lean and obese animals. In both obese and lean Zucker rats, a single i.c.v. infusion of GLP-1 (1-30 microg) resulted in a dose-dependent reduction of food intake and decrease in respiratory quotient relative to the saline control during the first 2 h of the nocturnal cycle. In obese Zucker rats, the food intake was reduced by 73 +/- 11% of the control at the 30 microg dose, whereas a modest 45 +/- 18% reduction was observed in lean rats. Despite the large reduction in food intake seen with GLP-1, there was no compensatory decrease in nocturnal oxygen consumption in the obese Zucker rats. Interestingly, low doses of GLP-1 (1 microg) in lean Zucker rats, which had minimal effects on food intake, caused a 19 +/- 7% increase in O2 consumption during the first 2 h of the nocturnal cycle. These data suggest that central GLP-1 may be an important factor controlling negative energy balance in both the lean and obese Zucker rats.

Comparison of acetylcholine-dependent relaxation in large and small arteries of rat mesenteric vascular bed.

The relative contributions of nitric oxide (NO) to in vitro relaxation responses elicited by acetylcholine (ACh) were compared in vessels of different sizes from the rat mesenteric vascular bed. ACh elicited an endothelium-dependent relaxation in phenylephrine-contracted superior mesenteric arteries (SMA, unstretched luminal diam 650 microns), which was blocked by compounds that inhibited NO, such as hemoglobin (10 microM), methylene blue (10 microM), and NG-monomethyl-L-arginine (1 mM). In contrast, the endothelium-dependent relaxation induced by ACh in phenylephrine-contracted mesenteric resistance arteries (MRA, unstretched luminal diam 200 microns) was not blocked by hemoglobin, methylene blue, or NG-monomethyl-L-arginine. KCl (25 mM) partially inhibited the ACh-dependent relaxation in MRA. Furthermore, the ACh-dependent relaxation in MRA was selectively inhibited by the Ca(2+)-activated K+ channel blocker charybdotoxin (0.1 microM). In contrast, the ATP-sensitive K+ channel blocker glibenclamide (50 microM) did not block the ACh-dependent relaxation in MRA. We conclude that 1) NO is a major component of the ACh-dependent relaxation in SMA and 2) the ACh-dependent relaxation of MRA is resistant to NO inhibitors but sensitive to a Ca(2+)-activated K+ channel blocker. This suggests that an endothelium-derived hyperpolarization factor may be involved in the relaxation of MRA.

Central melanocortin system modulates energy intake and expenditure of obese and lean Zucker rats.

Melanocortins play a critical role in appetite and body weight regulation, because manipulations of this pathway can lead to the development of obesity in several animal models. The purpose of this study was to use a melanocortin receptor agonist and antagonist to evaluate the involvement of melanocortins in feeding, energy metabolism, and body weight regulation in lean and obese Zucker rats. Central administration of a melanocortin receptor antagonist (SHU9119) elevated food intake and body weight of lean Zucker rats but had little effect in obese Zucker rats. In contrast, the melanocortin receptor agonist MTII reduced food intake in both lean and obese rats but was more potent in the obese Zucker rats. These data indicate the existence of functional melanocortin receptors in both lean and obese Zucker rats but suggest that obese Zucker rats have reduced endogenous melanocortin tone. In addition to its effects on food intake, MTII infusion elevated oxygen consumption and decreased respiratory quotient dose dependently during the light cycle. Our data suggest that a melanocortin receptor agonist can induce weight loss by increasing energy expenditure and promoting body fat utilization in addition to its inhibitory effects on food intake in both obese and lean Zucker rats.

Intracerebroventricular injection of leptin increases thermogenesis and mobilizes fat metabolism in ob/ob mice.

Genetically obese (ob/ob) mice display a number of metabolic alterations, including decreased thermogenesis, hyperphagia, hyperglycemia and increased body fat. A single intracerebroventricular (i.c.v.) injection of these mice with leptin (0.01 to 1 microg) lowered food intake and body weight within 24 h. In addition, i.c.v. administration of leptin increased 22 h energy expenditure while reducing the respiratory quotient (RQ) in a dose-dependent manner. The leptin-induced decrease in RQ suggests a reduction in the fraction of total energy derived from carbohydrate oxidation and a corresponding increase in energy derived from fat oxidation. Our data suggest that leptin controls energy homeostasis through activation of receptor(s) in the central nervous system (CNS) that regulate both food intake and energy metabolism.

Evidence for the presence of a proteinase-activated receptor distinct from the thrombin receptor in vascular endothelial cells.

The thrombin receptor was the first cloned G protein-coupled receptor reported to be activated by proteolytic cleavage of its extracellular amino terminus. A second proteinase-activated receptor (PAR-2) was cloned recently and expressed in Xenopus laevis oocytes. PAR-2 was activated by trypsin and by a peptide (SLIGRL) derived from the new amino terminus. Since PAR-2 mRNA was detected in highly vascularized organs, we compared the physiological functions of the thrombin receptor and PAR-2 in vascular endothelium. Thrombin and trypsin both elicited endothelium-dependent relaxations in prostaglandin F2alpha (PGF2alpha)-contracted strips of porcine coronary artery. Whereas high doses of both thrombin or trypsin (10 U/mL) caused homologous desensitization, trypsin caused further relaxation of thrombin-desensitized tissues. Thrombin and PAR-2-derived peptides (SFLLRN and SLIGRL) both induced endothelium-dependent relaxations in PGF2alpha-contracted porcine coronary arteries. SFLLRN or SLIGRL (30 micronmol/L) also showed homologous desensitization but not cross desensitization. In the presence of the NO synthase inhibitor NG-monomethyl-L-arginine (1 mmol/L), both SFLLRN- and SLIGRL-induced relaxations were partially inhibited. SFLLRN elicited weak contraction in coronary arteries without endothelium, whereas SLIGRL had no effect. Intravenous injection of SFLLRN (1 mg/kg, bolus) into anesthetized rats elicited a transient depressor response followed by pronounced pressor response. In contrast, intravenous administration of SLIGRL (1 mg/kg, bolus) produced only a marked depressor response. Consistent with the in vivo data, SFLLRN contracted the endothelium-rubbed rat aortic rings and aggregated human platelets in vitro, whereas SLIGRL had no effect. The finding that both trypsin and SLIGRL induced endothelium-dependent relaxations indicates the presence of PAR-2 on endothelial cells. In addition, both trypsin and SLIGRL elicited relaxations in thrombin- or SFLLRN-desensitized tissue, suggesting that PAR-2 is distinct from thrombin receptor in vascular endothelium. The lack of PAR-2-mediated platelet aggregation or smooth muscle contraction suggested it might not share the pathogenic properties associated with the thrombin receptor in the vasculature.

Leptin increases energy expenditure and selectively promotes fat metabolism in ob/ob mice.

Obesity occurs whenever energy intake exceeds energy expenditure. The ob gene product leptin is a potent anorectic agent when administered to ob/ob mice, but its effects on energy expenditure have not been investigated in detail. The present study was designed to analyze the acute metabolic effects of leptin in vivo. Analysis of oxygen consumption in ob/ob mice demonstrated a reduction in energy expenditure compared with lean controls; this reduction showed a diurnal fluctuation and was most evident during the light cycle. A single intraperitoneal dose of leptin increased oxygen consumption during the light cycle in ob/ob mice, ablating the circadian fluctuation in this parameter. In addition, leptin had a profound effect on fuel selection: the respiratory quotient was markedly reduced, indicating a reduction in carbohydrate oxidation and an increase in fat oxidation. These acute effects of leptin on metabolic parameters are consistent with the selective loss of body fat observed on chronic leptin treatment and suggest that increased energy utilization plays an important role in the anti-obese actions of leptin.

Activation of the NPY Y5 receptor regulates both feeding and energy expenditure.

Intracerebroventricular (ICV) administration of neuropeptide Y (NPY) has been shown to decrease energy expenditure, induce hypothermia, and stimulate food intake. Recent evidence has suggested that the Y5 receptor may be a significant mediator of NPY-stimulated feeding. The present study attempts to further characterize the role of NPY Y5-receptor subtypes in feeding and energy expenditure regulation. Satiated Long-Evans rats with temperature transponders implanted in the interscapular brown adipose tissue (BAT) displayed a dose-dependent decrease in BAT temperature and an increase in food intake after ICV infusion of NPY. Similar effects were induced by ICV administration of peptide analogs of NPY that activate the Y5 receptor, but not by analogs that activate Y1, Y2, or Y4 receptors. Furthermore, ICV infusion of the Y5 selective agonist D-[Trp(32)]-NPY significantly reduced oxygen consumption and energy expenditure of rats as measured by indirect calorimetry. These data suggest that the NPY Y5-receptor subtype not only mediates the feeding response of NPY but also contributes to brown fat temperature and energy expenditure regulation.