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      Relation between the Hypothalamic-Pituitary-Thyroid (HPT) Axis and the Hypothalamic-Pituitary-Adrenal (HPA) Axis during Repeated Stress

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          Abstract

          Previous work has indicated that acute and repeated stress can alter thyroid hormone secretion. Corticosterone, the end product of hypothalamic-pituitary-adrenal (HPA) axis activation and strongly regulated by stress, has been suggested to play a role in hypothalamic-pituitary-thyroid (HPT) axis regulation. In the current study, we sought to further characterize HPT axis activity after repeated exposure to inescapable foot-shock stress (FS), and to examine changes in proposed regulators of the HPT axis, including plasma corticosterone and hypothalamic arcuate nucleus agouti-related protein (AGRP) mRNA levels. Adult male Sprague-Dawley rats were subjected to one daily session of inescapable FS for 14 days. Plasma corticosterone levels were determined during and after the stress on days 1 and 14. Animals were killed on day 15, and trunk blood and brains were collected for measurement of hormone and mRNA levels. Repeated exposure to FS led to a significant decrease in serum levels of 3,5,3′-triiodothyronine (T<sub>3</sub>) and 3,5,3′,5′-tetraiodothyronine (T<sub>4</sub>). Stress-induced plasma corticosterone levels were not altered by repeated exposure to the stress. Despite the decrease in peripheral hormone levels, thyrotropin-releasing hormone (TRH) mRNA levels within the paraventricular nucleus of the hypothalamus were not altered by the stress paradigm. Arcuate nucleus AGRP mRNA levels were significantly increased in the animals exposed to repeated FS. Additionally, we noted significant correlations between stress-induced plasma corticosterone levels and components of the HPT axis, including TRH mRNA levels and free T<sub>4</sub> levels. Additionally, there was a significant correlation between AGRP mRNA levels and total T<sub>3</sub> levels. Changes in body weight were also correlated with peripheral corticosterone and TRH mRNA levels. These results suggest that repeated exposure to mild-electric foot-shock causes a decrease in peripheral thyroid hormone levels, and that components of the HPA axis and hypothalamic AGRP may be involved in stress regulation of the HPT.

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          Most cited references 21

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          Corticosteroid inhibition of ACTH secretion.

          Corticosteroid feedback inhibits the brain-hypothalamo-pituitary units of the adrenocortical system. Naturally occurring corticosteroids may have their primary actions in vivo at brain and hypothalamic sites of feedback, whereas synthetic glucocorticoids that do not bind to transcortin may act primarily on corticotropes and regions of brain outside the blood-brain barrier. There appear to be three major time frames of corticosteroid action: fast, intermediate and slow. These time frames probably are the consequence of three separate mechanisms of corticosteroid action at feedback-sensitive sites. The rapidity of occurrence of fast feedback is not compatible with a nuclear site of corticosteroid action, and protein synthesis is not required. The action of CRF on ACTH release may be inhibited by a rapid effect of corticosteroids at the cell membrane. Since stimulated, but not basal, ACTH and CRF release are inhibited in vitro, the corticosteroids may inhibit some event in stimulus-secretion coupling (e.g., cAMP production). Intermediate feedback also decreases ACTH release in response to stimulation of the corticotrope, but does not affect ACTH synthesis; CRF synthesis and release both appear to be affected by the intermediate corticosteroid action. The mechanism of intermediate feedback requires the presence of a protein whose synthesis is corticosteroid-dependent; however, the role of this protein is unknown. Intermediate feedback, like fast feedback, apparently does not involve inhibition of total ACTH stores or the releasable pool of ACTH since basal secretion of ACTH is also not inhibited in vitro within this time domain. On the other hand, slow feedback apparently involves the classical genomic steroid mechanism of action; slow feedback reduces pituitary ACTH content by decreasing levels of mRNA encoding for POMC, the ACTH precursor molecule. Slow feedback, therefore, inhibits basal as well as stimulus induced ACTH secretion. Corticosteroid-induced inhibition of basal ACTH secretion has been shown to occur within 2 h in vivo but not in vitro. The time course and sensitivity of this feedback effect is different than that demonstrated for stimulus induced secretion. This difference suggests that basal secretion is activated by different pathways to (CRF and) ACTH secretion. There is some evidence that suggests that whereas comparator elements are not reset during stress, a comparator element is reset during the course of the circadian rhythm so that different basal levels of steroid are achieved.(ABSTRACT TRUNCATED AT 400 WORDS)
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            Thyroid hormone receptors: multiple forms, multiple possibilities

             M A Lazar (1993)
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              Habituation to repeated stress is stressor specific.

              Rats were exposed to 15 min of restraint or footshock or forced running in an activity wheel once a day for 10 days. Control groups were handled only. On the 11th day, rats from each stressor group and controls were exposed to 15 min of one stressor in a crossed design such that all combinations of one chronic stressor and one acute stressor were performed. Rats were sacrificed immediately following removal from their home cage or after 15 min stressor exposure on the 11th day and plasma corticosterone and prolactin and pituitary cyclic AMP levels were determined. There were no measured differences in these stress indices among groups of rats sacrificed immediately upon removal from their home cage on day 11 regardless of previous history on days 1 through 10. Plasma corticosterone and plasma prolactin and pituitary cyclic AMP levels were elevated in all rats exposed to any of the three stressors immediately prior to sacrifice as compared to all rats not exposed to stress immediately before sacrifice. However, plasma prolactin and pituitary cyclic AMP responses to each of the 3 stressors were attenuated in rats which had previous exposure to that specific stressor as compared to rats which had previous experience with a different or no stressor. We conclude that habituation results from behavioral experience with a particular stressor rather than biochemical adaptation resulting from repeated challenge to hormonal and neurochemical systems responsive to stress.
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                Author and article information

                Journal
                NEN
                Neuroendocrinology
                10.1159/issn.0028-3835
                Neuroendocrinology
                S. Karger AG
                0028-3835
                1423-0194
                2005
                July 2005
                29 July 2005
                : 81
                : 3
                : 183-192
                Affiliations
                aDepartment of Biology and Neuroscience Program, Middlebury College, Middlebury, Vt. and bUniversity of Michigan Medical School, Mental Health Research Institute, Ann Arbor, Mich., USA
                Article
                87001 Neuroendocrinology 2005;81:183–192
                10.1159/000087001
                16020927
                © 2005 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: 38, Pages: 10
                Categories
                Original Paper

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