Role of Capsaicin-Sensitive Peripheral Sensory Neurons in Anorexic Responses to Intravenous Infusions of Cholecystokinin, Peptide YY-(3–36), and Glucagon-Like Peptide-1 in Rats

Cholecystokinin (CCK)-induced suppression of feeding is mediated by vagal sensory neurons that are destroyed by the neurotoxin capsaicin (CAP). Here we determined whether CAP-sensitive neurons mediate anorexic responses to intravenous infusions of gut hormones peptide YY-(3–36) [PYY-(3–36)] and glucagon-like peptide-1 (GLP-1). Rats received three intraperitoneal injections of CAP or vehicle (VEH) in 24 h. After recovery, non-food-deprived rats received at dark onset a 3-h intravenous infusion of CCK-8 (5, 17 pmol·kg−1·min−1), PYY-(3–36) (5, 17, 50 pmol·kg−1·min−1), or GLP-1 (17, 50 pmol·kg−1·min−1). CCK-8 was much less effective in reducing food intake in CAP vs. VEH rats. CCK-8 at 5 and 17 pmol·kg−1·min−1 reduced food intake during the 3-h infusion period by 39 and 71% in VEH rats and 7 and 18% in CAP rats. In contrast, PYY-(3–36) and GLP-1 were similarly effective in reducing food intake in VEH and CAP rats. PYY-(3–36) at 5, 17, and 50 pmol·kg−1·min−1 reduced food intake during the 3-h infusion period by 15, 33, and 70% in VEH rats and 13, 30, and 33% in CAP rats. GLP-1 at 17 and 50 pmol·kg−1·min−1 reduced food intake during the 3-h infusion period by 48 and 60% in VEH rats and 30 and 52% in CAP rats. These results suggest that anorexic responses to PYY-(3–36) and GLP-1 are not primarily mediated by the CAP-sensitive peripheral sensory neurons (presumably vagal) that mediate CCK-8-induced anorexia

Effects of Solid‐Phase Extraction of Plasma in Measuring Gut Metabolic Hormones in Fasted and Fed Blood of Lean and Diet‐Induced Obese Rats

Glucagon‐like peptide‐1 (GLP‐1), peptide YY (3‐36) [PYY(3‐36)], amylin, ghrelin, insulin, and leptin are thought to act as hormonal signals from periphery to brain to control food intake. Here, we determined the effects of solid‐phase extraction of plasma in measuring these hormones in blood of lean and diet‐induced obese rats. Individual enzyme‐linked immunoassays and a multiplex assay were used to measure active GLP‐1, total PYY, active amylin, active ghrelin, insulin, leptin, and total GIP in response to (1) addition of known amounts of the peptides to lean and obese plasma, (2) a large meal in lean and obese rats, and (3) intravenous infusions of anorexigenic doses of GLP‐1, PYY(3‐36), amylin, and leptin in lean rats. Extraction of lean and obese plasma prior to assays produced consistent recoveries across assays for GLP‐1, PYY, amylin, ghrelin, and insulin, reflecting losses inherent to the extraction procedure. Plasma extraction prior to assays generally revealed larger meal‐induced changes in plasma GLP‐1, PYY, amylin, ghrelin, and insulin in lean and obese rats. Plasma extraction and the multiplex assay were used to compare plasma levels of GLP‐1, PYY, and amylin after a large meal with plasma levels produced by IV infusions of anorexigenic doses of GLP‐1, PYY(3‐36), and amylin. Infusions produced dose‐dependent increases in plasma peptide levels, which were well above their postprandial levels. These results do not support the hypothesis that postprandial plasma levels of GLP‐1, PYY(3‐36), and amylin are sufficient to decrease food intake by an endocrine mechanism

Effects of leptin replacement alone and with exendin-4 on food intake and weight regain in weight-reduced diet-induced obese rats

Reidelberger R, Haver A, Chelikani PK, Apenteng B, Perriotte-Olson C, Anders K, Steenson S, Blevins JE. Effects of leptin replacement alone and with exendin-4 on food intake and weight regain in weight-reduced diet-induced obese rats. Am J Physiol Endocrinol Metab 302: E1576–E1585, 2012. First published April 17, 2012; doi:10.1152/ajpendo.00058.2012.—Weight loss in obese humans produces a relative leptin deficiency, which is postulated to activate potent orexigenic and energy conservation mechanisms to restrict weight loss and promote weight regain. Here we determined whether leptin replacement alone or with GLP-1 receptor agonist exendin-4 attenuates weight regain or promotes greater weight loss in weight-reduced diet-induced obese (DIO) rats. Forty percent restriction in daily intake of a high-fat diet in DIO rats for 4 wk reduced body weight by 12%, body fat by 29%, and plasma leptin by 67% and normalized leptin sensitivity. When food restriction ended, body weight, body fat, and plasma leptin increased rapidly. Daily administration of leptin [3-h intraperitoneal (ip) infusions (4 nmol·kg-1·h-1)] at onset and end of dark period for 3 wk did not attenuate hyperphagia and weight regain, nor did it affect mean daily meal sizes or meal numbers. Exendin-4 (50 pmol·kg-1·h-1) infusions during the same intervals prevented postrestriction hyperphagia and weight regain by normalizing meal size. Coadministration of leptin and exendin-4 did not reduce body weight more than exendin-4 alone. Instead, leptin began to attenuate the inhibitory effects of exendin-4 on food intake, meal size, and weight regain by the end of the second week of administration. Plasma leptin in rats receiving leptin was sevenfold greater than in rats receiving vehicle and 17-fold greater than in rats receiving exendin-4. Together, these results do not support the hypothesis that leptin replacement alone or with exendin-4 attenuates weight regain or promotes greater weight loss in weight-reduced DIO rats. OBESITY IS A CHRONIC, stigmatized, and costly disease that is rarely curable and is increasing in prevalence in most of the world (4). Current therapies for producing weight loss in obese individuals, i.e., dieting, exercise, and medications, are woefully ineffective in producing long-term weight loss. This is likely due to redundancy and plasticity in the complex physiological system that controls food intake and regulates energy reserves. Many experts believe that multidrug therapy aimed at different components of this regulatory system will be required to produce a significant reduction in adiposity (4, 13, 21, 22). An important early step in the development of antiobesity drugs is determining whether chronic administration of anorexigenic substances, alone or in combination, can produce a prolonged decrease in daily food intake and adiposity in experimental animals. Methods of administration usually include daily injections or insertion of an osmotic minipump beneath the skin or into the peritoneal cavity to deliver substances continuously for 1 wk or more. Treatments typically produce only transient decreases in food intake, resulting in relatively small or no decreases in body weight and adiposity (e.g., see Refs. 16, 28, 38, 43, and 44). Likely reasons include development of a compensatory increase in food intake between injections, desensitization and downregulation of receptors in response to continuous or frequent administration of high doses, and weight loss-induced activation of counteracting orexigenic and energy conservation mechanisms to restore energy reserves (14, 24, 37, 39)