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To clarify the roles of prolactin (PRL) and GH in the control of the immune system, the effects of environmental salinity, hypophysectomy, and PRL and GH administration on several immune functions were examined in tilapia (Oreochromis mossambicus). Transfer from fresh water (FW) to seawater (SW) did not alter plasma levels of immunoglobulin M (IgM) and lysozyme. The superoxide anion (O(2)(-)) production in head kidney leucocytes accompanied by phagocytosis was elevated in SW-acclimated fish over the levels observed in FW fish. Hypophysectomy of the fish in FW resulted in a reduction in O(2)(-) production in leucocytes isolated from the head kidney, whereas there was no significant change in plasma levels of IgM or lysozyme. Treatment with tilapia GH and PRLs (PRL(177) and PRL(188)) enhanced O(2)(-) production in vitro in head kidney leucocytes in a dose-related manner. Extrapituitary expression of two PRLs, GH and IGF-I mRNA was detected in lymphoid tissues and cells such as head kidney, spleen, intestine and leucocytes from peripheral blood and head kidney. PRL-receptor mRNA was detected in head kidney leucocytes, and the level of expression was higher in SW-acclimated fish than that in FW fish. Treatment with PRL(177) caused higher production of O(2)(-) in the head kidney leucocytes isolated from SW tilapia than that from FW fish. In view of the fact that PRL acts antagonistically to osmoregulation in SW, its immunomodulatory actions in this euryhaline fish would appear to be independent of its osmoregulatory action.
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Abstract Studies in mammals have shown that synthetic Met-enkephalin derivatives, called growth hormone-releasing peptides (GHRPs), stimulate growth hormone (GH) release. The present study was conducted to determine whether the GHRP, KP-102, specifically stimulates GH release in a teleost. Tilapia (Oreochromis mossambicus) were given a single intraperitoneal injection of KP-102 (D-Ala-D-beta;-Nal-Ala-Trp-D-Phe-Lys-NH(2)) or bovine GHRH(1-29)-amide or vehicle and blood was sampled at 1, 6 and 12 h after injection. KP-102 was administered at two doses of 1 ng/g and 10 ng/g body weight, whereas GHRH (positive control) was administered at a single dose of 10 ng/g body weight. Plasma levels of tilapia GH and prolactins (tPRL(177) and tPRL(188)) were determined by radioimmunoassay. As expected, GHRH injection significantly (P<0.001) elevated plasma GH levels (ng/ml) in tilapia at 6 h post-injection. KP-102 also significantly elevated GH levels (at the low dose) at 6 (P<0.05) and 12 (P<0.01) hours post-injection. There were no significant effects on plasma PRL(s) levels, although mean levels of both PRLs were elevated at 6 h post-injection. These results show for the first time that GHRPs stimulate GH release in teleosts and suggest that the GHRP receptor and possibly a "Ghrelin-like" ligand are also present in lower vertebrates.
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We examined the effects of environmental salinity on circulating levels of the two prolactins (tPRL177 and tPRL188) and levels of pituitary tPRL177 and tPRL188 mRNA in the euryhaline tilapia, Oreochromis mossambicus. Fish were sham-operated or hypophysectomized and the rostral pars distalis (RPD) autotransplanted onto the optic nerve. Following post-operative recovery in (1/4) seawater, tilapia were transferred to fresh water (FW), (1/4) seawater (SW) or SW. Serum tPRL177 and tPRL188 levels in sham-operated and RPD-autotransplanted fish were highest in FW and decreased as salinity was increased. tPRL177 and tPRL188 mRNA levels in RPD implants as well as in pituitaries from the sham-operated fish were also highest in FW and decreased with increasing salinity. Serum osmolality increased with salinity, with the highest levels occurring in the seawater groups. We conclude that some plasma factor (probably plasma osmolality), in the absence of hypothalamic innervation, exerts a direct regulatory action on prolactin release and gene expression in the pituitary of O. mossambicus. This regulation is in accord with the actions of the two prolactins in the freshwater osmoregulation of the tilapia.
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We purified ghrelin from stomach extracts of a teleost fish, the Japanese eel (Anguilla japonica) and found that it contained an amide structure at the C-terminal end. Two molecular forms of ghrelin with 21 amino acids were identified by cDNA and mass spectrometric analyses: eel ghrelin-21, GSS(O-n-octanoyl)FLSPSQRPQGKDKKPP RV-amide and eel ghrelin-21-C10, GSS(O-n-decanoyl) FLSPSQRPQGKDKKPPRV-amide. Northern blot and RT-PCR analyses revealed high gene expression in the stomach. Low levels of expression were found only in the brain, intestines, kidney and head kidney by RT-PCR analysis. Eel ghrelin-21 increased plasma growth hormone (GH) concentrations in rats after intravenous injection; the potency was similar to that of rat ghrelin. We also examined the effect of eel ghrelin on the secretion of GH and prolactin (PRL) from organ-cultured tilapia pituitary. Eel ghrelin-21 at a dose of 0.1 nM stimulated the release of GH and PRL, indicating that ghrelin acts directly on the pituitary. The present study revealed that ghrelin is present in fish stomach and has the ability to stimulate the secretion of GH from fish pituitary. A novel regulatory pathway of GH secretion by gastric ghrelin seems to be conserved from fish to human.
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Glucocorticoids are known to impede somatic growth in a wide range of vertebrates. In order to clarify the mechanisms through which they may act in an advanced teleost fish, we examined the effects of cortisol administration on the growth hormone (GH)/insulin-like growth factor-I (IGF-I)/IGF-binding protein (IGFBP) system in the tilapia (Oreochromis mossambicus). In a short-term experiment, fish were injected intraperitoneally with cortisol (2 or 10 microg/g), and killed at 2, 4, 8 and 24 h after the injection. In a longer-term experiment, fish were killed 24 and 48 h after cortisol injection (2, 10 and 50 microg/g). Cortisol at doses of 2 and 10 microg/g significantly increased IGFBPs of four different sizes (24, 28, 30, and 32 kDa) in the plasma within 2 h without altering plasma levels of IGF-I or GH. On the other hand, cortisol at doses of 10 and 50 microg/g significantly reduced plasma IGF-I levels after 24 and 48 h. IGF-I mRNA levels in the liver were also significantly reduced by cortisol at doses of 10 and 50 microg/g after 48 h, suggesting that a decrease in plasma IGF-I levels is mediated through the attenuation of IGF-I gene expression in the liver. In contrast, no significant change was observed in plasma or pituitary contents of GH at any time point examined, which would appear to indicate that cortisol reduces IGF sensitivity to GH (GH-resistance). These results clearly indicate that cortisol induces a rapid increase in plasma IGFBPs and a more delayed decrease in IGF-I production. The dual mode of cortisol action may contribute to the inhibitory influence of cortisol on somatic growth in teleosts.
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There is considerable evidence that the GH/IGF-I axis plays an important role in female reproduction. We report the isolation and characterization of the GH receptor (GH-R) and its gene expression profile during oogenesis in the tilapia, Oreochromis mossambicus. cDNA encoding GH-R was cloned and sequenced from the tilapia liver. The predicted GH-R preprotein consisted of 635 amino acids and contained a putative signal peptide, an extracellular region with a characteristic motif, a single transmembrane region, and a cytoplasmic region with conserved box 1 and 2 domains. The tilapia GH-R shared 34-74% identities with known GH-Rs in vertebrates. A binding assay using COS-7 cells showed that the cloned GH-R bound specifically to tilapia GH. Northern blot analysis showed a single mRNA transcript in the liver and ovary. In situ hybridization revealed intense signals of GH-R in the cytoplasm and nucleus of immature oocytes. The granulosa and theca cells surrounding vitellogenic oocytes also contained the GH-R mRNA signals. About a tenfold greater level of GH-R mRNA was found in the immature oocytes versus the mature oocytes, along with high levels of IGF-I mRNA. There were no significant changes in mRNA levels of GH-R and IGF-I in the liver or in plasma IGF-I levels during oocyte development. No correlation was found between hepatic GH-R mRNA and ovarian GH-R mRNA. These results suggest that the GH/IGF-I axis in the ovary may be involved in the early phases of oogenesis, under a different regulatory mechanism of GH-R gene expression from that of the liver.
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Three forms of gonadotropin-releasing hormone (GnRH) are isolated and identified here by chemical sequence analysis for one species of tilapia, Oreochromis niloticus, and by HPLC elution position for a second species of tilapia, O. mossambicus. Of the three GnRH forms in O. mossambicus, chicken GnRH-II (cGnRH-II) and sea bream GnRH (sbGnRH) are present in greater abundance in the brain and pituitary than salmon GnRH (sGnRH). These three native forms of GnRH are shown to stimulate the release of prolactin (PRL) from the rostral pars distalis (RPD) of the pituitary of O. mossambicus in vitro with the following order of potency: cGnRH-II > sGnRH > sbGnRH. In addition, a mammalian GnRH analog stimulated the release of PRL from the pituitary RPD incubated in either iso-osmotic (320 mosmol/l) or hyperosmotic (355 mosmol/l) medium, the latter normally inhibiting PRL release. The response of the pituitary RPD to GnRH was augmented by co-incubation with testosterone or 17 beta-estradiol. The effects of GnRH on PRL release appear to be direct effects on PRL cells because the RPD of tilapia contains a nearly homogeneous mass of PRL cells without intermixing of gonadotrophs. Our data suggest that GnRH plays a broad role in fish, depending on the species, by affecting not only gonadotropins and growth hormone, but also PRL.
