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      Effect of Estrogen on Melanocortin-3 Receptor mRNA Expression in Mouse Pituitary Glands in vivo and in vitro

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          Abstract

          Alpha-melanocyte-stimulating hormone (α-MSH) stimulates prolactin (PRL) release and mammotrope proliferation in mouse anterior pituitary glands. The present study investigated the regulation of melanocortin-3 receptor (MC3-R) mRNA levels in mice using semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Relative MC3-R mRNA levels in the anterior lobes of female mice increased between 20 and 45 days of age, and a significant difference in MC3-R mRNA expression between sexes was seen at 45 days. Ovariectomy decreased MC3-R mRNA expression in the female anterior lobes, and estradiol-17β (E<sub>2</sub>) treatment increased MC3-R mRNA levels in ovariectomized mouse anterior lobes and cultured anterior pituitary cells. E<sub>2</sub> treatment increased proopiomelanocortin (POMC) mRNA levels in ovariectomized mouse neurointermediate lobes and cultured neurointermediate pituitary cells. On the other hand, E<sub>2</sub> treatment did not affect POMC mRNA expression in mouse anterior lobes or cultured anterior pituitary cells. These results suggest that α-MSH directly stimulates PRL release and mammotrope proliferation through MC3-Rs expressed in mammotropes, while estrogen stimulates MC3-R gene transcription in the anterior pituitary cells and POMC gene transcription in the intermediate lobes. In lactating mice, POMC mRNA levels in the neurointermediate lobes were elevated compared with in non-lactating mice. The present study suggests that α-MSH is involved in augmented PRL secretion by estrogen and during lactation.

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          Most cited references 20

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          Single-step method of RNA isolation by acid guanidinium thiocyanate-phenol-chloroform extraction.

          A new method of total RNA isolation by a single extraction with an acid guanidinium thiocyanate-phenol-chloroform mixture is described. The method provides a pure preparation of undegraded RNA in high yield and can be completed within 4 h. It is particularly useful for processing large numbers of samples and for isolation of RNA from minute quantities of cells or tissue samples.
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            Ptx1, a bicoid-related homeo box transcription factor involved in transcription of the pro-opiomelanocortin gene.

            The pituitary gland contains six distinct hormone-producing cell types that arise sequentially during organogenesis. The first cells to differentiate are those that express the pro-opiomelanocortin (POMC) gene in the anterior pituitary lobe. The other lineages, which appear later, include cells that are dependent on the POU factor Pit-1 and another POMC-expressing lineage in the intermediate pituitary lobe. Using AtT-20 cells as a model for early expression of POMC in the anterior pituitary, we have defined a regulatory element conferring cell specificity of transcription and cloned a cognate transcription factor. This factor, Ptx1 (pituitary homeo box 1), contains a homeo box related to those of the anterior-specific genes bicoid and orthodenticle in Drosophila, and Otx-1 and Otx-2 in mammals. Ptx1 activates transcription upon binding a sequence related to the Drosophila bicoid target sites. Ptx1 is the only nuclear factor of this DNA-binding specificity that is detected in AtT-20 cells, and it is expressed at high levels in a subset of adult anterior pituitary cells that express POMC. However, Ptx1 is expressed in most cells of Rathke's pouch at an early time during pituitary development and before final differentiation of hormone-producing cells. Thus, Ptx1 may have a role in differentiation of pituitary cells, and its early expression pattern suggests that it may have a role in pituitary formation. In the adult pituitary gland, Ptx1 appears to be recruited for cell-specific transcription of the POMC gene.
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              Evidence That Hypothalamic Periventricular Dopamine Neurons Innervate the Intermediate Lobe of the Rat Pituitary

              The purpose of the present study was to provide neurochemical and endocrinological evidence that dopamine (DA) neurons terminating in the intermediate lobe of the rat pituitary originate in the periventricular nucleus of the hypothalamus. One week following surgical separation of the periventricular nucleus from the mediobasal hypothalamus, DA and 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations in the intermediate lobe were reduced by 50%, and this was accompanied by an increase in plasma α-melanocyte-stimulating hormone (α-MSH) concentrations. In contrast, this procedure had no effect on concentrations of prolactin in the plasma, or DA or DOPAC in the median eminence, the region of the mediobasal hypothalamus containing terminals of tuberoinfundibular DA neurons. Electrical stimulation of the periventricular nucleus increased the ratio of DOPAC/DA in the intermediate lobe and reduced the concentrations of α-MSH in the plasma, whereas in these same animals the DOPAC/DA ratio in the median eminence and concentrations of prolactin in the plasma were unaltered. These results indicate that approximately 50% of all the DA neurons terminating in the intermediate lobe of the rat pituitary originate in or project through the periventricular nucleus of the hypothalamus, and that these DA neurons regulate the secretion of α-MSH from intermediate lobe melanotrophs.
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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
                18 January 2005
                : 80
                : 3
                : 143-151
                Affiliations
                Department of Biology, Faculty of Science, Okayama University, Okayama, Japan
                Article
                82355 Neuroendocrinology 2004;80:143–151
                10.1159/000082355
                15583471
                © 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: 10, References: 34, Pages: 9
                Categories
                Gonadal Steroid Feedback and Gonadotropins

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