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      Regulation of Expression of Somatostatin Genes by Sex Steroid Hormones in Goldfish Forebrain

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          Recently, our laboratory has identified three distinct pre-pro-somatostatin (PSS) genes in goldfish brain: PSS-I encodes for somatostatin (SRIH)-14, PSS-II encodes SRIH-28, which contains [Glu<sup>1</sup>, Tyr<sup>7</sup>, Gly<sup>10</sup>] SRIH-14 at its C-terminus, and PSS-III encodes [Pro<sup>2</sup>] SRIH-14. In goldfish, increasing levels of the sex steroid estradiol increase the plasma levels of growth hormone (GH). However, whether sex steroids act at the level of the brain to regulate GH release is unclear. In the present study, the effects of sex steroids on the expression of the three PSS genes in goldfish forebrain were examined. The results demonstrate that treatment with estradiol significantly increases the expression of PSS-I and PSS-III genes in both male and female fish. The effects of estradiol were evident after only 2.5 days of treatment. Testosterone treatment increased the expression of PSS-I and PSS-III genes in female but not male fish, and only at the highest dose used. In addition, the effects of testosterone were evident only after treatment for 5 or 10 days and were blocked by an aromatase inhibitor, suggesting that testosterone must be converted to estradiol to exhibit the effect. Neither estradiol nor testosterone treatment had effects on the expression of the PSS-II gene. These results suggest that sex steroids can act either directly or indirectly on the brain to regulate PSS-I and PSS-III gene expression, influencing in turn the regulation of GH secretion.

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          Most cited references 6

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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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            The interrelation between photoperiod, growth hormone, and sexual maturation of adult Atlantic salmon (Salmo salar).

            The plasma profiles of growth hormone (GH) in adult male and female Atlantic salmon were determined in relation to manipulation of the photoperiod and to the development and timing of sexual maturation. Fish were exposed to natural light (NL) or NL + 24L:0D additional light over the netpens from January (ALJ) or March (ALM) to July. Thereafter, these groups were brought indoors, subdivided, and subjected to simulated natural photoperiod (SNP), continuous light (24L), or short day (8L). Assay of salmon GH by RIA in monthly plasma samples revealed that GH levels were generally < 1 ng ml-1 during January to June and were only slightly affected by additional light in January or March. ALJ-24L treatment, and to a lesser extent, ALM-24L treatment, was effective in preventing sexual maturation, and GH levels of immature fish continued to be < or = 1.5 ng ml-1. On the other hand, in sexually maturing fish, GH levels increased to 2-5 ng ml-1 months prior to ovulation. Short-day photoperiod (8L) from July advanced ovulation and spermiation, whereas continuous light from July delayed these processes. The timing of the increase of GH levels was shifted in a parallel manner, indicating a functional relationship between plasma GH levels and the process of sexual maturation.
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              Hypothalamic and thyroidal regulation of growth hormone in tilapia.

              A radioimmunoassay (RIA) for recombinant tilapia growth hormone (GH) was established and validated. The ability of various hypothalamic factors to regulate GH secretion in the tilapia hybrid (Oreochromis niloticus x Oreochromis aureus) was studied. Somatostatin1-14 (SRIF1-14; 10-100 micrograms/kg) was found to reduce circulating GH levels in a dose-dependent manner. SRIF1-14 (0.1-1000 nM) inhibited GH release from perifused pituitary fragments (ED50 0.83 nM). Human growth hormone-releasing hormone fragment 1-29 (hGHRH1-29; 100 micrograms/kg) doubled circulating GH levels and modestly stimulated GH secretion in vitro. Carp growth hormone-releasing hormone (cGHRH) stimulated GH secretion in vitro to a similar degree at the same dose (1 microM). Injection of salmon gonadotropin-releasing hormone (sGnRH) superactive analog (10-100 micrograms/kg) increased plasma GH levels sixfold. sGnRH also stimulated GH release in vitro (ED50 142.56 nM). Dopamine (0.1-10 microM) and the D1 DA receptor agonist SKF 38393 increased GH secretion from perifused pituitary fragments dose-relatedly. Thyrotropin-releasing hormone (TRH) had no effect on GH secretion from perifused pituitary fragments, but increased plasma GH levels, as did bovine thyroid stimulating hormone (bTSH). The increased plasma GH in the bTSH-treated fish coincided with a dramatic increase in T4; however, TRH increased GH without changing T4 levels. T3 increased the synthesis of GH by isolated pituitaries (incorporation of [3H]leucine). SRIF1-14 seems to be a most potent hypothalamic regulator of GH secretion in tilapia; sGnRH and DA both increased GH secretion, although sGnRH elicited considerably greater responses at lower doses. Two forms of GHRH increased GH levels, although the unavailability of the homologous peptide prevented an accurate evaluation of its importance in regulating GH secretion. The thyroid axis (TRH, TSH, and T3) stimulates both synthesis and release of GH, although TRH did not appear to have a direct effect on the level of the pituitary.

                Author and article information

                S. Karger AG
                July 2002
                01 July 2002
                : 76
                : 1
                : 8-17
                Biological Sciences Department, University of Alberta, Edmonton, Alta., Canada
                63679 Neuroendocrinology 2002;76:8–17
                © 2002 S. Karger AG, Basel

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                Page count
                Figures: 7, Tables: 2, References: 38, Pages: 10
                Gonadal Steroids: Central Effects and Neurotransmitter Regulation


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