Pre-obese and obese agouti mice are sensitive to the anorectic effects of peptide YY(3-36) but resistant to ghrelin.
OBJECTIVE
The role of the melanocortin system in the feeding effects of peripheral peptide YY(3-36) (PYY(3-36)) and ghrelin was investigated using the agouti (A(y)/a) mouse as a model of abnormal melanocortin signalling. Furthermore, we examined whether the ectopic expression of agouti protein in A(y)/a mice results in complete MC4-R inhibition, by studying the effects of peripheral alpha-melanocyte-stimulating hormone (alpha-MSH) and leptin on food intake.


METHODS
Adult A(y)/a mice were studied in the pre-obese state (7-8 weeks) and obese state (14-15 weeks). Animals received PYY(3-36) (0.02 micromol/kg), NDP-alpha-MSH (0.2 micromol/kg), leptin (2 micromol/kg) (all 24 h fasted state) and ghrelin (0.2 micromol/kg) (fed state) by intraperitoneal (i.p.) injection. Age-matched A(y)/a controls received i.p. saline. A separate cohort of wild-type (WT), age-matched controls received the same peptide dose or saline. Food intake was measured at 1, 2, 4, 8 and 24 h post-injection and compared in all four groups. Plasma leptin-, ghrelin- and PYY-like immunoreactivity (IR) were measured using radioimmunoassay (RIA).


RESULTS
At 2 h post-injection, PYY(3-36) reduced food intake in pre-obese and obese A(y)/a mice, whereas ghrelin had no effect. Plasma ghrelin levels were significantly reduced in pre-obese and obese A(y)/a mice compared to WT controls. Peripheral administration of NDP-alpha-MSH and leptin acutely suppressed feeding (0-2 h) in pre-obese and obese A(y)/a mice.


CONCLUSIONS
Responsiveness of pre-obese and obese A(y)/a mice to PYY(3-36) suggests that the melanocortin system may not be essential for the anorectic effects of this peptide. Melanocortinergic antagonism by agouti protein in A(y)/a mice may be sufficient to block the effects of endogenous, but not exogenous PYY(3-36), alpha-MSH and leptin. The mechanism underlying ghrelin resistance in A(y)/a mice may result from antagonism of hypothalamic melanocortin receptors-4 by agouti protein, supporting a role for the melanocortin system in mediating ghrelin's actions.
Martin
N M
NM
Department of Metabolic Medicine, Imperial College, Hammersmith Campus, London, UK.
Small
C J
CJ
Sajedi
A
A
Patterson
M
M
Ghatei
M A
MA
Bloom
S R
SR
D054366:Agouti Signaling Protein; D000818:Animals; D001066:Appetite / Q000187:drug effects; D004351:Drug Resistance; D004435:Eating / Q000187:drug effects*; D005215:Fasting; D054439:Ghrelin; D036341:Intercellular Signaling Peptides and Proteins / Q000378:metabolism; D020738:Leptin / Q000097:blood / Q000494:pharmacology; D009074:Melanocyte-Stimulating Hormones / Q000037:antagonists & inhibitors; D051379:Mice; D008810:Mice, Inbred C57BL; D008820:Mice, Obese; D008822:Mice, Transgenic; D009765:Obesity / Q000097:blood / Q000503:physiopathology*; D010446:Peptide Fragments; D036361:Peptide Hormones / Q000097:blood / Q000494:pharmacology*; D019894:Peptide YY / Q000097:blood / Q000494:pharmacology*; D044105:Receptor, Melanocortin, Type 4 / Q000235:genetics / Q000502:physiology; D000521:alpha-MSH / Q000494:pharmacology
D054366:Agouti Signaling Protein; D054439:Ghrelin; D036341:Intercellular Signaling Peptides and Proteins; D020738:Leptin; D010446:Peptide Fragments; D036361:Peptide Hormones; D044105:Receptor, Melanocortin, Type 4; C515183:a protein, mouse; D019894:Peptide YY; C061785:peptide YY (3-36); D000521:alpha-MSH; D009074:Melanocyte-Stimulating Hormones