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      Plasticity in the Melanotrope Neuroendocrine Interface of Xenopus laevis

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          Abstract

          Melanotrope cells of the amphibian pituitary pars intermedia produce α-melanophore-stimulating hormone (α-MSH), a peptide which causes skin darkening during adaptation to a dark background. The secretory activity of the melanotrope of the South African clawed toad Xenopus laevis is regulated by multiple factors, both classical neurotransmitters and neuropeptides from the brain. This review concerns the plasticity displayed in this intermediate lobe neuroendocrine interface during physiological adaptation to the environment. The plasticity includes dramatic morphological plasticity in both pre- and post-synaptic elements of the interface. Inhibitory neurons in the suprachiasmatic nucleus, designated suprachiasmatic melanotrope-inhibiting neurons (SMINs), possess more and larger synapses on the melanotrope cells in white than in black-background adapted animals; in the latter animals the melanotropes are larger and produce more proopiomelanocortin (POMC), the precursor of α-MSH. On a white background, pre-synaptic SMIN plasticity is reflected by a higher expression of inhibitory neuropeptide Y (NPY) and is closely associated with postsynaptic melanotrope plasticity, namely a higher expression of the NPY Y1 receptor. Interestingly, melanotrope cells in such animals also display higher expression of the receptors for thyrotropin-releasing hormone (TRH) and urocortin 1, two neuropeptides that stimulate α-MSH secretion. Possibly, in white-adapted animals melanotropes are sensitized to neuropeptide stimulation so that, when the toad moves to a black background, they can immediately initiate α-MSH secretion to achieve rapid adaptation to the new background condition. The melanotrope cell also produces brain-derived neurotrophic factor (BDNF), which is co-sequestered with α-MSH in secretory granules within the cells. The neurotrophin seems to control melanotrope cell plasticity in an autocrine way and we speculate that it may also control presynaptic SMIN plasticity.

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          Most cited references 58

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          BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis.

          Interest in BDNF as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF-TrkB to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and plasticity. Despite individual breakthroughs, an integrated understanding of BDNF function in synaptic plasticity is lacking. Here, we attempt to distill current knowledge of the molecular mechanisms and function of BDNF in LTP. BDNF activates distinct mechanisms to regulate the induction, early maintenance, and late maintenance phases of LTP. Evidence from genetic and pharmacological approaches is reviewed and tabulated. The specific contribution of BDNF depends on the stimulus pattern used to induce LTP, which impacts the duration and perhaps the subcellular site of BDNF release. Particular attention is given to the role of BDNF as a trigger for protein synthesis-dependent late phase LTP--a process referred to as synaptic consolidation. Recent experiments suggest that BDNF activates synaptic consolidation through transcription and rapid dendritic trafficking of mRNA encoded by the immediate early gene, Arc. A model is proposed in which BDNF signaling at glutamate synapses drives the translation of newly transported (Arc) and locally stored (i.e., alphaCaMKII) mRNA in dendrites. In this model BDNF tags synapses for mRNA capture, while Arc translation defines a critical window for synaptic consolidation. The biochemical mechanisms by which BDNF regulates local translation are also discussed. Elucidation of these mechanisms should shed light on a range of adaptive brain responses including memory and mood resilience.
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            7B2 is a neuroendocrine chaperone that transiently interacts with prohormone convertase PC2 in the secretory pathway.

            The neuroendocrine polypeptide 7B2 is a highly conserved secretory protein selectively present in prohormone-producing cells equipped with a regulated secretory pathway. We find that the amino-terminal half of 7B2 is distantly related to chaperonins, a subclass of molecular chaperones. When incubated in vitro with newly synthesized pituitary proteins, recombinant 7B2 specifically associates with prohormone convertase PC2. Metabolic cell labeling combined with coimmunoprecipitation studies showed that, in vivo, the precursor form of 7B2 interacts with the proform of PC2. Pulse-chase analysis revealed that this association is transient in that it commences early in the secretory pathway, while dissociation in the later stages appears to coincide with the cleavages of 7B2, proPC2, and prohormone. Our results suggest that 7B2 is a novel type of molecular chaperone preventing premature activation of proPC2 in the regulated secretory pathway.
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              Overview of the P2 receptors.

              The release of nucleotides in extracellular fluids can result from cell necrosis, exocytosis of secretory granules (such as platelet dense granules), or efflux through membrane channels. In addition, recent evidence suggests that vesicular trafficking is an important pathway of nucleotide release. Once in the extracellular fluids, they are rapidly degraded by ectonucleotidases, such as CD39, that play a key role in neutralizing the platelet aggregatory action of adenosine diphosphate (ADP), and act on two families of receptors: the ionotropic P2X receptors and the G-protein-coupled P2Y receptors. The family of P2X receptors encompasses seven genes. Currently, there are eight genuine P2Y receptors that can be subdivided into two structurally distinct subfamilies. Whereas P2X receptors are receptors of ATP, the different P2Y receptors are activated by distinct nucleotides, diphosphates or triphosphates, or purines or pyrimidines, some of them being conjugated to sugars. The study of knockout mice has demonstrated that P2X receptors play important roles in the neurogenic control of smooth muscle contraction, in pain and visceral perception, and in macrophage functions. The phenotype of P2Y null mice so far is more restricted: inhibition of platelet aggregation to ADP and increased bleeding time in P2Y (1)(-/-) and P2Y (12)(-/-) mice and lack of epithelial responsiveness to nucleotides in airways (P2Y (2)(-/-)) and intestine (P2Y (4)(0/-)).
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                Author and article information

                Journal
                NEN
                Neuroendocrinology
                10.1159/issn.0028-3835
                Neuroendocrinology
                S. Karger AG
                0028-3835
                1423-0194
                2007
                June 2007
                27 March 2007
                : 85
                : 3
                : 177-185
                Affiliations
                Department of Cellular Animal Physiology, Radboud University Nijmegen, Nijmegen, The Netherlands
                Article
                101434 Neuroendocrinology 2007;85:177–185
                10.1159/000101434
                17389778
                © 2007 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: 3, References: 72, Pages: 9
                Categories
                Signaling Mechanisms Underlying Neuroendocrine Function

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