Long-term orexigenic effects of AgRP-(83---132) involve mechanisms other than melanocortin receptor blockade |
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Authors: | Hagan M M Rushing P A Pritchard L M Schwartz M W Strack A M Van Der Ploeg L H Woods S C Seeley R J |
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Institution: | Department of Psychiatry, University of Cincinnati Medical Center, Cincinnati, Ohio 45267-0559, USA. haganmm@email.uc.edu |
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Abstract: | Overexpression of agouti-related peptide (AgRP), an endogenous melanocortin (MC) 3 and 4 receptor antagonist (MC3/4-R), causes obesity. Exogenous AgRP-(83---132) increases food intake, but its duration and mode of action are unknown. We report herein that doses as low as 10 pmol can have a potent effect on food intake of rats over a 24-h period after intracerebroventricular injection. Additionally, a single third ventricular dose as low as 100 pmol in rats produces a robust increase in food intake that persists for an entire week. AgRP-(83---132) completely blocks the anorectic effect of MTII (MC3/4-R agonist), given simultaneously, consistent with a competitive antagonist action. However, when given 24 h prior to MTII, AgRP-(83---132) is ineffective at reversing the anorectic effects of the agonist. These results support a critical role of MC tone in limiting food intake and indicate that the orexigenic effects of AgRP-(83---132) are initially mediated by competitive antagonism at MC receptors but are sustained by alternate mechanisms. |
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