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      Role of Glucocorticoids in the Regulation of Pituitary Somatostatin Receptor Subtype (sst1–sst5) mRNA Levels: Evidence for Direct and Somatostatin-Mediated Effects

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          Glucocorticoids can differentially regulate somatostatin (SRIH) receptor subtype expression depending on the duration of treatment, dose used and tissue type examined. In order to determine if glucocorticoids are critical regulators of pituitary SRIH receptor synthesis in vivo, we examined the effect of adrenalectomy (ADX), with and without dexamethasone (DEX; 200 µg/day for 8 days) treatment, on the relative expression levels of the SRIH receptor subtypes, sst1–sst5, by multiplex RT-PCR. ADX increased pituitary sst2 mRNA levels, but did not significantly alter mRNA levels of the other SRIH receptor subtypes. These findings indicate that pituitary sst2 synthesis is normally under inhibitory control of endogenous glucocorticoids. High-dose DEX resulted in a decrease in sst1–sst4 mRNA and an increase in sst5 mRNA, independent of adrenal status. DEX also decreased sst2, sst3 and sst4 mRNA levels and increased sst5 mRNA levels by short-term in vitro application (10 n M, 4 h) in primary rat pituitary cell cultures, indicating DEX regulation of sst2–sst5 in vivo is at least in part due to a direct action at the level of the pituitary. However, the inhibitory actions of DEX on sst1 mRNA levels observed in vivo were not consistently replicated in vitro. In order to determine if the somatotrope population of the pituitary would display a similar response to DEX, fluorescent-activated cell sorting was used to obtain somatotrope-enriched cultures (>95% growth hormone immunopositive cells). DEX treatment (10 n M, 4 h) of somatotropes decreased sst2 and sst3, but did not alter sst5 mRNA levels. These results indicate that the effects of DEX on sst5 mRNA levels observed in unsorted pituitary cell cultures might be due to changes in sst5 expression in pituitary cell types other than somatotropes. Since excess glucocorticoids are thought to enhance SRIH tone, we also tested if ligand activation of SRIH receptor subtypes in vitro could mimic any of the actions of DEX on SRIH receptor mRNA levels observed in vivo. To this end, unsorted pituitary cell cultures and somatotrope-enriched cultures were treated with SRIH (1 and 100 n M) for 4 h. SRIH increased sst3 and sst5 mRNA levels, in both culture systems. These results suggest that the DEX-induced increase in endogenous SRIH tone may contribute to enhanced sst5 mRNA levels observed in vivo. However, the stimulatory actions of SRIH on sst3 mRNA levels observed in vitro might be overridden by direct inhibitory actions of DEX.

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          Most cited references 16

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          Molecular mechanisms of glucocorticoid action: what is important?

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            Rapid identification of subtype-selective agonists of the somatostatin receptor through combinatorial chemistry.

            Nonpeptide agonists of each of the five somatostatin receptors were identified in combinatorial libraries constructed on the basis of molecular modeling of known peptide agonists. In vitro experiments using these selective compounds demonstrated the role of the somatostatin subtype-2 receptor in inhibition of glucagon release from mouse pancreatic alpha cells and the somatostatin subtype-5 receptor as a mediator of insulin secretion from pancreatic beta cells. Both receptors regulated growth hormone release from the rat anterior pituitary gland. The availability of high-affinity, subtype-selective agonists for each of the somatostatin receptors provides a direct approach to defining their physiological functions.
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              In vivo and in vitro Effects of Ghrelin/Motilin-Related Peptide on Growth Hormone Secretion in the Rat

              Ghrelin (Ghr), a 28 amino acid gastric peptide with an n-octanoylation on Ser 3, has recently been identified as an endogenous ligand of the growth hormone secretagogue (GHS) receptor. A cDNA was also isolated from a mouse stomach library encoding a protein named prepromotilin-related peptide (ppMTLRP) which shares sequence similarities with prepromotilin. Mouse and rat ppMTLRP sequences (rGhr) are identical and show 89% identity with human ghrelin (hGhr). By analogy with promotilin, cleavage of proMTLRP into an 18 amino acid endogenous processed peptide can be assumed on the basis of a conserved dibasic motif in position 9–10 of its sequence. In the present work, we compared the GH-releasing activity of rGhr28/MTLRP and of hGhr28/MTRLP with that of a shorter form of the peptide, hGhr18. A short peptide devoid of Ser-3 n-octanoylation hGhr18[–] was also tested. Addition of rGhr28, hGhr28 and hGhr18 stimulated GH release to the same extent from superfused pituitaries. The effect was dose dependent in a 10 –8 to 10 –6 M concentration range. In contrast, hGhr 18[–] was inactive. In freely moving animals, both rGhr28 and hGhr28 (10 µg, i.v.) stimulated GH release, whereas the same dose of hGhr18 or of hGhr18[–] was ineffective. After rGhr28, GH plasma levels increased as early as 5 min after injection and returned to basal values within 40–60 min. Expressed as percent stimulation, administration of rGhr28 was equally effective when injected during troughs or peaks of GH. Plasma concentrations of prolactin, adrenocorticotropin and leptin were not modified. Spontaneous GH secretory episodes were no longer observed within 3 h of rGhr28 treatment, but repeated administration of the secretagogue at 3- to 4-hour intervals resulted in a similar GH response. Activation of somatostatin (SRIH) release by ether stress did not blunt the GH response to rGhr28. This suggests that the secretagogue acts in part by inhibiting endogenous SRIH, as further substantiated by the ability of rGhr28 (10 –6 M ), to decrease the amplitude of 25 m M K + -induced SRIH release from perifused hypothalami. In conclusion, (1) n-octanoylation of Ghrs and the shorter form hGhr18 is essential for the direct pituitary GH-releasing effect of this new family of endogenous GHSs; (2) only the longer forms are active in vivo and (3) inhibition of SRIH release appears involved in the mechanism of Ghr action.

                Author and article information

                S. Karger AG
                September 2003
                02 October 2003
                : 78
                : 3
                : 163-175
                aDepartment of Pharmacology, Kyunghee University School of Medicine, Seoul, Korea; bDepartment of Medicine, Nippon Medical School, Tokyo, Japan, and cDepartment of Medicine, Section of Endocrinology and Metabolism, University of lllinois at Chicago, Chicago, Ill., USA
                72798 Neuroendocrinology 2003;78:163–175
                © 2003 S. Karger AG, Basel

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                Page count
                Figures: 7, Tables: 1, References: 54, Pages: 13
                Growth Hormone and Leptin


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