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      A Role for Agouti-Related Protein in Appetite Regulation in a Species with Continuous Nutrient Delivery

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          Abstract

          Knowledge of specific neurotransmitters as well as the pathways and mechanisms regulating appetite in ruminants that continually graze, such as sheep, is incomplete. Although fundamentally agouti-related protein (AGRP) has a similar function across species to increase food intake, the regulation of AGRP may vary across grazing and intermittent feeders. To investigate the role of orexigenic peptides in the regulation of feed intake, we first extracted messenger RNA from sheep that were fasted for 3 days, which was then used for PCR followed by cloning and sequencing to demonstrate the presence of hypothalamic AGRP expression. Ovine AGRP was closely related to the bovine, but contained sequence differences with human and mouse AGRP. Analysis of genomic DNA also revealed a similar gene structure to other published species. Secondly, using dual-labeled immunohistochemistry, we determined that there was both increased AGRP immunoreactivity and increased abundance of c-Fos immunoreactivity in AGRP neurons in the arcuate nucleus of fasted sheep. Because AGRP neurons are activated by fasting, we hypothesized that AGRP would stimulate feeding in this ruminant species. Sheep fed ad libitum were injected intracerebroventricularly with concentrations of AGRP at 0.2 and 2.0 nmol/kg. AGRP at 2.0 nmol/kg significantly increased food intake at 4, 6 and 12 h (p < 0.05). A 4th study was done to investigate the interactions of AGRP and neuropeptide Y (NPY) on food intake over a 24-hour period. Intracerebroventricular injections of either AGRP or NPY significantly increased cumulative food intake over saline controls. When AGRP and NPY were injected in combination, food intake was increased over saline controls; however, AGRP did not potentiate the effects of NPY. These results demonstrate that AGRP stimulates food intake in sheep and highlights the important differences between this species and rodent models.

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          Most cited references 24

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          Neuropeptides, food intake and body weight regulation: a hypothalamic focus.

          Energy homeostasis is controlled by a complex neuroendocrine system consisting of peripheral signals like leptin and central signals, in particular, neuropeptides. Several neuropeptides with anorexigenic (POMC, CART, and CRH) as well as orexigenic (NPY, AgRP, and MCH) actions are involved in this complex (partly redundant) controlling system. Starvation as well as overfeeding lead to changes in expression levels of these neuropeptides, which act downstream of leptin, resulting in a physiological response. In this review the role of several anorexigenic and orexigenic (hypothalamic) neuropeptides on food intake and body weight regulation is summarized.
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            Hypothalamic localization of the feeding effect of agouti-related peptide and alpha-melanocyte-stimulating hormone.

            The melanocortin-4 receptor (MC4R) in the hypothalamus is thought to be important in physiological regulation of food intake. We investigated which hypothalamic areas known to express MC4R are involved in the regulation of feeding by using alpha-melanocyte-stimulating hormone (alpha-MSH), an endogenous MC4R agonist, and agouti-related peptide (Agrp), an endogenous MC4R antagonist. Cannulae were inserted into the rat hypothalamic paraventricular (PVN), arcuate (Arc), dorsomedial (DMN), and ventromedial (VMN) nuclei; the medial preoptic (MPO), anterior hypothalamic (AHA), and lateral hypothalamic (LHA) areas; and the extrahypothalamic central nucleus of the amygdala (CeA). Agrp (83-132) (0.1 nmol) and [Nle4, D-Phe7]alpha(-MSH (NDP-MSH) (0.1 nmol), a stable alpha-MSH analog, were administered to fed and fasted rats, respectively. The PVN, DMN, and MPO were the areas with the greatest response to Agrp and NDP-MSH. At 8 h postinjection, Agrp increased feeding in the PVN by 218 +/- 23% (P < 0.005), in the DMN by 268 +/- 42% (P < 0.005), and in the MPO by 236 +/- 31% (P < 0.01) compared with a saline control group for each nucleus. NDP-MSH decreased food intake in the PVN by 52 +/- 6% (P < 0.005), in the DMN by 44 +/- 6% (P < 0.0001), and in the MPO by 55 +/- 6% (P < 0.0001) at 1 h postinjection. Injection into the AHA and CeA resulted in smaller alterations in food intake. No changes in feeding were seen after the administration of Agrp into the Arc, LHA, or VMN, but NDP-MSH suppressed food intake in the Arc and LHA. This study indicates that the hypothalamic nuclei expressing MC4R vary in their sensitivity to Agrp and alpha-MSH with regard to their effect on feeding.
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              Use of cryoprotectant to maintain long-term peptide immunoreactivity and tissue morphology

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                Author and article information

                Journal
                NEN
                Neuroendocrinology
                10.1159/issn.0028-3835
                Neuroendocrinology
                S. Karger AG
                0028-3835
                1423-0194
                2004
                January 2005
                25 January 2005
                : 80
                : 4
                : 210-218
                Affiliations
                aDepartment of Anatomy, Physiology and Pharmacology, College of Veterinary Medicine, Auburn University, Auburn, Ala., USA; bAgResearch Ltd., Hamilton, New Zealand; cPediatric Endocrinology, Oregon Health and Science University, Portland, Oreg., and dAnimal and Range Science, South Dakota State University, Brookings, S. Dak., USA
                Article
                82735 Neuroendocrinology 2004;80:210–218
                10.1159/000082735
                15604600
                © 2004 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                Page count
                Figures: 4, References: 32, Pages: 9
                Categories
                Original Paper

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