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      Combined Hypervolemia and Hypoosmolality Alter Hypothalamic-Pituitary-Adrenal Axis Response to Endotoxin Stimulation

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          Abstract

          Changes in corticotropin (ACTH) and glucocorticoid secretion have been described during disturbances of body fluid homeostasis and attributed to alterations in arginine vasopressin (AVP) secretion from magnocellular hypothalamic neurons. In order to further characterize the mechanisms involved in the interactions between body fluid alterations and pituitary adrenal function, we manipulated osmolality and volemia in sheep under stimulation of the pituitary-adrenal axis by acute injection of endotoxin. We have recently shown that endotoxin injection induces a long-lasting release of both corticotropin releasing hormone (CRH) and AVP into hypophysial portal blood, and an early stimulation of AVP secretion into peripheral vessels, thus suggesting a joint activation of magnocellular and parvocellular neurons of the PVN. We used the same experimental model to investigate the effect of combined volume loading and plasma dilution (achieved by 1-deamino-8-D-arginine (dDAVP) administration together with infusion of 2 liters of 2.5% glucose solution) on CRH, AVP, ACTH and cortisol responses to endotoxin stimulation. In volume-loaded animals, ACTH and cortisol responses to endotoxin were significantly blunted and we observed a parallel decrease in portal CRH and jugular and portal AVP levels. These data show that hypoosmolality and/or hypervolemia reduce(s) ACTH and cortisol response to stress in sheep as in other species. They strongly suggest that this reduction in ACTH and cortisol responses to endotoxin involve not only magnocellular hypothalamic neurons secreting AVP, as usually assumed, but also PVN parvocellular neurons secreting both CRH and AVP.

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          Most cited references 5

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          Vasopressinergic control of pituitary adrenocorticotropin secretion comes of age.

           F. Antoni (1993)
          This article summarizes the importance of arginine vasopressin (AVP) in the control of adrenocorticotropin (ACTH) secretion, with special reference to interactions with corticotropin releasing factor (CRF-41), glucocorticoids, and the purported corticotropin release inhibiting peptide atriopeptin. AVP that participates in the regulation of ACTH release at the pituitary level is produced in two main groups of neurons in the hypothalamus: parvicellular cells in the paraventricular nucleus, which also produce CRF-41, and magnocellular neurons in the supraoptic and paraventricular nuclei. The role of the latter in anterior pituitary hormone release has been debated for many years. Evidence generated in the last 5 years shows quite convincingly that AVP released by magnocellular neurons is, in fact, also involved in the control of ACTH. Nevertheless, it is clear that corticotrope cells require CRF-41 to maintain their capacity to secrete ACTH. This is at least due partly to the fact that AVP does not increase proopiomelanocortin mRNA transcription, while CRF-41 is a potent inducer of this gene. New developments in the area of corticotrope cell physiology are discussed, highlighting evidence for dual ACTH secreting pathways in anterior pituitary cells, which may be controlled separately by AVP and CRF-41. Evidence for interactions between ACTH secretagogues and peptidergic as well as glucocorticoid inhibitors of ACTH secretion is reviewed to demonstrate that an important aspect of AVP/CRF-41 dualism may be associated with the ability of the secretagogues to selectively modulate the efficacy of inhibitory factors. Finally, by citing examples from physiological studies on the regulation of ACTH secretion, it is shown how the multicomponent hypothalamic regulatory system operates, emphasizing the considerable signal integrating role of the adenohypophysial corticotrope cell.
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            Corticotropin-releasing factor mRNA in the hypothalamus is affected differently by drinking saline and by dehydration

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              Connections of the hypothalamic paraventricular nucleus with the neurohypophysis, median eminence, amygdala, lateral septum and midbrain periaqueductal gray: an electrophysiological study in the rat.

              Extracellular recordings were obtained from 555 paraventricular (PVN) nucleus neurons in pentobarbital-anesthetized male rats. Cells were examined for their spontaneous activity patterns and response to single 1-Hz electrical stimulation of the neurohypophysis, median eminence, amygdala, lateral septum (LS) and midbrain periaqueductal gray (PAG). Neurohypophyseal stimulation evoked antidromic activation from 109 neurons. Among spontaneously active neurohypophyseal neurons, evidence of a recurrent inhibitory pathway usually required pituitary stimulus intensities twice threshold for antidromic activation. Orthodromic excitatory or inhibitory responses followed amygdala and LS stimulation, but not PAG stimulation. The amygdala influence was predominantly inhibitory to 'phasic' (putative vasopressin-secreting) PVN neurohypophyseal neurons. Neurohypophyseal stimulation evoked orthodromic responses from 124 PVN cells; some of these neurons were also responsive to stimulation in other sites. Median eminence stimulation evoked antidromic responses from 37 PVN neurons; some of these cells also displayed phasic activity but no evidence for recurrent inhibition. Twelve cells in this group were also activated antidromically from both the median eminence and the neurohypophysis; collision tests suggest that the median eminence innervation may be an axon collateral of a neurohypophyseal pathway. Amygdala stimulation was inhibitory to some cells in this category. Amygdala, LS and PAG stimulation evoked antidromic activation from a small number of PVN cells, but none of these cells appeared to innervate more than one area, including the neurohypophysis, and none displayed phasic activity. Orthodromic responses were recorded among other PVN neurons after stimulation in these sites; however, PAG stimulation was the least effective stimulation area. These observations provide additional electrophysiological data that confirm efferent PVN connections to all areas tested, afferent connections from amygdala and LS but not PAG, and the possibility for coordinated activity among PVN neurons through local recurrent or common afferent connections.
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                Author and article information

                Journal
                NEN
                Neuroendocrinology
                10.1159/issn.0028-3835
                Neuroendocrinology
                S. Karger AG
                0028-3835
                1423-0194
                1999
                May 1999
                20 May 1999
                : 69
                : 5
                : 352-359
                Affiliations
                aLaboratoire de Neuroendocrinologie Expérimentale, INSERM U 501, Institut Fédératif Jean-Roche, Faculté de Médecine, et bService d’Endocrinologie, Maladies Métaboliques et de la Nutrition, Hôpital Nord, Marseille, France; cDivision d’Endocrinologie et du Métabolisme, CHU Vaudois, Lausanne, Suisse
                Article
                54437 Neuroendocrinology 1999;69:352–359
                10.1159/000054437
                10343176
                © 1999 S. Karger AG, Basel

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                Page count
                Figures: 4, References: 36, Pages: 8
                Categories
                Regulation of Corticotropin and Adrenal Steroid Feed-Back

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