Effect of stress during handling, seawater acclimation, confinement, and induced spawning on plasma ion levels and somatolactin-expressing cells in mature female Liza ramada

Corticotropin-Releasing Hormone: A Novel Stimulator of Somatolactin in Teleost Pituitary Cells

Corticotropin-releasing hormone (CRH) is known for its crucial role in the stress response system, which could induce pituitary adrenocorticotropic hormone (ACTH) secretion to promote glucocorticoid release in the adrenal gland. However, little is known about other pituitary actions of CRH in teleosts. Somatolactin is a fish-specific hormone released from the neurointermediate lobe (NIL) of the posterior pituitary. A previous study has reported that ACTH was also located in the pituitary NIL region. Interestingly, our present study found that CRH could significantly induce two somatolactin isoforms' (SLα and SLβ) secretion and synthesis in primary cultured grass carp pituitary cells. Pharmacological analysis further demonstrated that CRH-induced pituitary somatolactin expression was mediated by the AC/cAMP/PKA, PLC/IP3/PKC, and Ca2+/CaM/CaMK-II pathways. Finally, transcriptomic analysis showed that both SLα and SLβ should play an important role in the regulation of lipid metabolism in primary cultured hepatocytes. These results indicate that CRH is a novel stimulator of somatolactins in teleost pituitary cells, and somatolactins may participate in the stress response by regulating energy metabolism.

Pituitary Actions of EGF on Gonadotropins, Growth Hormone, Prolactin and Somatolactins in Grass Carp

Simple Summary

In mammals, the functions of epidermal growth factor (EGF) have been widely studied. However, little is known about the pituitary actions of EGF in teleost. Using primary cultured grass carp pituitary cells as model, we found that EGF could reduce pituitary luteinizing hormone β (LHβ) mRNA expression, but induce pituitary growth hormone (GH), prolactin (PRL) and somatolactins (SL) mRNA expression. Furthermore, we also found that NKB could suppress EGF-induced PRL mRNA expression in grass carp pituitary cells. These results suggested that EGF could directly regulate pituitary hormones expression in teleost.

Abstract

In mammals, epidermal growth factor (EGF) plays a vital role in both pituitary physiology and pathology. However, the functional role of EGF in the regulation of pituitary hormones has rarely reported in teleost. In our study, using primary cultured grass carp pituitary cells as an in vitro model, we examined the effects of EGF on pituitary hormone secretion and gene expression as well as the post-receptor signaling mechanisms involved. Firstly, we found that EGF significantly reduced luteinizing hormone (LHβ) mRNA expression via ErbB1 coupled to ERK1/2 pathway, but had no effect on LH release in grass carp pituitary cells. Secondly, the results showed that EGF was effective in up-regulating mRNA expression of growth hormone (GH), somatolactin α (SLα) and somatolactin β (SLβ) via ErbB1 and ErbB2 and subsequently coupled to MEK1/2/ERK1/2 and PI3K/Akt/mTOR pathways, respectively. However, EGF was not effective in GH release in pituitary cells. Thirdly, we found that EGF strongly induced pituitary prolactin (PRL) release and mRNA expression, which was mediated by ErbB1 and subsequent stimulation of MEK1/2/ERK1/2 and PI3K/Akt/mTOR pathways. Interestingly, subsequent study further found that neurokinin B (NKB) significantly suppressed EGF-induced PRL mRNA expression, which was mediated by neurokinin receptor (NK2R) and coupled to AC/cAMP/PKA signal pathway. These results suggested that EGF could differently regulate the pituitary hormones expression in grass carp pituitary cells.

NK3R Mediates the EGF-Induced SLα Secretion and mRNA Expression in Grass Carp Pituitary

Epidermal growth factor (EGF) is a potent regulator of cell function in many cell types. In mammals, the EGF/EGFR system played an important role in both pituitary physiology and pathology. However, it is not clear about the pituitary action of EGF in lower vertebrates. In this study, using grass carp as a model, we found that EGF could stimulate NK3R mRNA and protein expression through pituitary ErbB1 and ErbB2 coupled to MEK/ERK and PI3K/Akt/mTOR pathways. In addition, EGF could also induce pituitary somatolactin α (SLα) secretion and mRNA expression in a dose- and time-dependent manner in vivo and in vitro. The stimulatory actions of EGF on SLα mRNA expression were also mediated by PI3K/Akt/mTOR and MEK/ERK pathways coupled to ErbB1 and ErbB2 activation. Our previous study has reported that neurokinin B (NKB) could also induce SLα secretion and mRNA expression in carp pituitary cells. In the present study, interestingly, we found that EGF could significantly enhance NKB-induced SLα mRNA expression. Further studies found that NK3R antagonist SB222200 could block EGF-induced SLα mRNA expression, indicating an NK3R requirement. Furthermore, cAMP/PKA inhibitors and PLC/PKC inhibitors could both abolish EGF- and EGF+NKB-induced SLα mRNA expression, which further supported that EGF-induced SLα mRNA expression is NK3R dependent.

Evolution of the growth hormone, prolactin, prolactin 2 and somatolactin family

Growth hormone (GH), prolactin (PRL), prolactin 2 (PRL2) and somatolactin (SL) belong to the same hormone family and have a wide repertoire of effects including development, osmoregulation, metabolism and stimulation of growth. Both the hormone and the receptor family have been proposed to have expanded by gene duplications in early vertebrate evolution. A key question is how hormone-receptor preferences have arisen among the duplicates. The first step to address this is to determine the time window for these duplications. Specifically, we aimed to see if duplications resulted from the two basal vertebrate tetraploidizations (1R and 2R). GH family genes from a broad range of vertebrate genomes were investigated using a combination of sequence-based phylogenetic analyses and comparisons of synteny. We conclude that the PRL and PRL2 genes arose from a common ancestor in 1R/2R, as shown by neighboring gene families. No other gene duplicates were preserved from these tetraploidization events. The ancestral genes that would give rise to GH and PRL/PRL2 arose from an earlier duplication; most likely a local gene duplication as they are syntenic in several species. Likewise, some evidence suggests that SL arose from a local duplication of an ancestral GH/SL gene in the same time window, explaining the lack of similarity in chromosomal neighbors to GH, PRL or PRL2. Thus, the basic triplet of ancestral GH, PRL/PRL2 and SL genes appear to be unexpectedly ancient. Following 1R/2R, only SL was duplicated in the teleost-specific tetraploidization 3R, resulting in SLa and SLb. These time windows contrast with our recent report that the corresponding receptor genes GHR and PRLR arose through a local duplication in jawed vertebrates and that both receptor genes duplicated further in 3R, which reveals a surprising asynchrony in hormone and receptor gene duplications.

Novel Functional Role of NK3R Expression in the Potentiating Effects on Somatolactin α Autoregulation in grass carp pituitary cells

In our previous study, NKB/NK3R system has been shown to act at the pituitary level to up-regulate SLα synthesis and secretion in grass carp. However, whether NK3R expression can serve as a regulatory target at the pituitary level and contribute to NKB interactions with other SLα regulators is still unclear. In current study, using grass carp pituitary cells as a model, we have a novel finding that co-treatment of SLα/SLβ with carp TAC3 gene products, could induce a noticeable enhancement in SLα mRNA expression and these potentiating effects occurred with a parallel rise in NK3R transcript level after SLα/SLβ treatment. Interestingly, the stimulatory effects of SLα/SLβ on NK3R gene expression could be further potentiated by co-treatment with IGF-I/-II and simultaneous exposure of carp pituitary cells to SLα/SLβ and IGF-I/-II in the presence of TAC3 gene products was found to markedly elevated SLα mRNA expression (20 fold increase) and this synergistic stimulation was mediated by cAMP/PKA-, PLC/PKC- and Ca²⁺ -dependent cascades functionally coupled with NK3R activation. These findings suggest that local release of SLα via functional interactions with IGF-I/-II and TAC3/NK3R system may constitute a potent stimulatory signal for SLα gene expression in the carp pituitary via up-regulation of NK3R expression.

Salinity effects on plasma ion levels, cortisol, and osmolality in Chinook salmon following lethal sampling

Studies on hydromineral balance in fishes frequently employ measurements of electrolytes following euthanasia. We tested the effects of fresh- or salt-water euthanasia baths of tricaine mesylate (MS-222) on plasma magnesium (Mg(2+)) and sodium (Na(+)) ions, cortisol and osmolality in fish exposed to saltwater challenges, and the ion and steroid hormone fluctuations over time following euthanasia in juvenile spring Chinook salmon (Oncorhynchus tshawytscha). Salinity of the euthanasia bath affected plasma Mg(2+) and Na(+) concentrations as well as osmolality, with higher concentrations in fish euthanized in saltwater. Time spent in the bath positively affected plasma Mg(2+) and osmolality, negatively affected cortisol, and had no effect on Na(+) concentrations. The difference of temporal trends in plasma Mg(2+) and Na(+) suggests that Mg(2+) may be more sensitive to physiological changes and responds more rapidly than Na(+). When electrolytes and cortisol are measured as endpoints after euthanasia, care needs to be taken relative to time after death and the salinity of the euthanasia bath.

Stress response in silver catfish (Rhamdia quelen) exposed to the essential oil of Hesperozygis ringens

This study investigated the effects of prolonged exposure of silver catfish (Rhamdia quelen) to the essential oil (EO) of Hesperozygis ringens. Ventilatory rate (VR), stress and metabolic indicators, energy enzyme activities, and mRNA expression of adenohypophyseal hormones were examined in specimens that were exposed for 6 h to 0 (control), 30 or 50 µL L(-1) EO of H. ringens in water. Reduction in VR was observed in response to each treatment, but no differences were found between treatments. Plasma glucose, protein, and osmolality increased in fish exposed to 50 µL L(-1). Moreover, lactate levels increased after exposure to both EO concentrations. Plasma cortisol levels were not changed by EO exposure. Fish exposed to 30 µL L(-1) EO exhibited higher glycerol-3-phosphate dehydrogenase (G3PDH) activity, while exposure to 50 µL L(-1) EO elicited an increase in glucose-6-phosphate dehydrogenase (G6PDH), fructose-biphosphatase (FBP), and 3-hydroxyacyl-CoA-dehydrogenase (HOAD) activities compared with the control group. Expression of growth hormone (GH) only decreased in fish exposed to 50 µL L(-1) EO, while somatolactin (SL) expression decreased in fish exposed to both concentrations of EO. Exposure to EO did not change prolactin expression. The results indicate that GH and SL are associated with energy reorganization in silver catfish. Fish were only slightly affected by 30 µL L(-1) EO of H. ringens, suggesting that it could be used in practices where a reduction in the movement of fish for prolonged periods is beneficial, i.e., such as during fish transportation.

Kisspeptin induction of somatolactin-α release in goldfish pituitary cells: functional role of cAMP/PKA-, PLC/PKC-, and Ca(2+)/calmodulin-dependent cascades

Although the importance of kisspeptin in the pituitary is firmly established, the signaling mechanisms for the pituitary actions of kisspeptin are still largely unknown. Somatolactin (SL), a member of the growth hormone (GH)/prolactin (PRL) family, is a pituitary hormone with pleiotropic functions in fish, but its regulation by kisspeptin has not been examined. To investigate the functional role of kisspeptin in SL regulation, expression of two paralogues of goldfish Kiss1 receptors (Kiss1ra and Kiss1rb) were confirmed in immunoidentified SLα but not SLβ cells isolated by RT-PCR coupled with laser capture microdissection. In goldfish pituitary cells prepared from neurointermediate lobe (NIL), synthetic goldfish Kiss decapeptides (gKiss1-10 and gKiss2-10) could increase SLα release. Consistent with the lack of Kiss1r expression in SLβ cells, SLβ release was not altered by kisspeptin stimulation. In parallel experiments, goldfish gKiss1-10 could elevate cyclic adenosine monophosphate (cAMP) production, upregulate protein kinase A (PKA) and protein kinase C (PKC) activities, and trigger a rapid rise in intracellular Ca(2+) levels in goldfish NIL cells. Using a pharmacological approach, cAMP/PKA and phospholipase C (PLC)/PKC pathways and subsequent activation of Ca(2+)/calmodulin (CaM)-dependent cascades were shown to be involved in SLα release induced by gKiss1-10. Apparently, the Ca(2+)-dependent cascades were triggered by extracellular Ca(2+) entry via voltage-sensitive Ca(2+) channels and mobilization of inositol trisphosphate-sensitive intracellular Ca(2+) stores. Our results demonstrate that gKiss1-10 can act directly at the pituitary level to trigger SLα release via a complex network of post-receptor signaling mechanisms.

Effects of Pro-Tex on zebrafish (Danio rerio) larvae, adult common carp (Cyprinus carpio) and adult yellowtail kingfish (Seriola lalandi)

Aquaculture practices bring several stressful events to fish. Stressors not only activate the hypothalamus-pituitary-interrenal-axis, but also evoke cellular stress responses. Up-regulation of heat shock proteins (HSPs) is among the best studied mechanisms of the cellular stress response. An extract of the prickly pear cactus (Opuntia ficus indica), Pro-Tex, a soluble variant of TEX-OE(®), may induce expression of HSPs and reduce negative effects of cellular stress. Pro-Tex therefore is used to ameliorate conditions during stressful aquaculture-related practices. We tested Pro-Tex in zebrafish (Danio rerio), common carp (Cyprinus carpio L.) and yellowtail kingfish (Seriola lalandi) exposed to aquaculture-relevant stressors (thermal stress, net confinement, transport) and assessed its effects on stress physiology. Heat shock produced a mild increase in hsp70 mRNA expression in 5-day-old zebrafish larvae. Pro-Tex increased basal hsp70 mRNA expression, but decreased heat-shock-induced expression of hsp70 mRNA. In carp, Pro-Tex increased plasma cortisol and glucose levels, while it did not affect the mild stress response (increased plasma cortisol and glucose) to net confinement. In gills, and proximal and distal intestine, stress increased hsp70 mRNA expression; in the distal intestine, an additive enhancement of hsp70 mRNA expression by Pro-Tex was seen under stress. In yellowtail kingfish, Pro-Tex reduced the negative physiological effects of transport more efficiently than when fish were sedated with AQUI-S(®). Overall, our data indicate that Pro-Tex has protective effects under high levels of stress only. As Pro-Tex has potential for use in aquaculture, its functioning and impact on health and welfare of fish should be further studied.

The effects of ammonia and water hardness on the hormonal, osmoregulatory and metabolic responses of the freshwater silver catfish Rhamdia quelen

The aim of the present study was to assess the effects of ammonia and water hardness on endocrine, osmoregulatory and metabolic parameters in silver catfish (Rhamdia quelen). The specimens (60-120g) were subjected to six treatments in triplicate, combining three levels of un-ionized ammonia (NH3) (0.020±0.008mg/L [1.17±0.47μM], 0.180±0.020mg/L [10.57±1.17μM] and 0.500±0.007mg/L [29.36±0.41μM]) and two levels of water hardness (normal: 25mgCaCO3/L and high: 120mgCaCO3/L), and sampled after two exposure times (1 and 5 days post-transfer). Plasma cortisol, metabolites, osmolality and ionic values were determined concomitantly with the mRNA expression levels of different adenohypophyseal hormones (growth hormone, GH; prolactin, PRL; and somatolactin, SL). Previously, full-length PRL and SL as well as β-actin cDNAs from R. quelen were cloned. Exposure to high NH3 levels enhanced plasma cortisol levels in fish held under normal water hardness conditions but not in those kept at the high hardness value. The increase in water hardness did not alter plasma metabolites, whereas it modulated the osmolality and ion changes induced by high NH3 levels. However, this hardness increase did not lead to the decreased GH expression that was observed 5 days after exposure to 0.18mg/L NH3 in fish held at the normal water hardness level, whereas PRL expression was enhanced after one day of exposure under the increased hardness conditions. Additionally, SL expression decreased in specimens exposed for 5 days to 0.18mg/L NH3 and maintained at the high water hardness level. The results showed that increasing water hardness attenuated the hormonal parameters evaluated in R. quelen specimens exposed to high NH3 levels, although plasma metabolism do not appear to suffer major changes.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates release of somatolactin (SL)-α and SL-β from cultured goldfish pituitary cells via the PAC₁ receptor-signaling pathway, and affects the expression of SL-α and SL-β mRNAs

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide that stimulates the release of adenohypophyseal hormone from the pituitary in fish. In the goldfish, PACAP induces the release of somatolactin (SL), in particular, from cultured pituitary cells. SL belongs to the growth hormone and prolactin family, and comprises two molecular variants termed SL-α and SL-β in goldfish. However, there is no information about the involvement of PACAP in the regulation of SL-α and SL-β release and the expression of their mRNAs. Therefore, we examined the effect of PACAP on SL-α and SL-β release from cultured goldfish pituitary cells. Treatment with PACAP (10(-10)-10(-7)M) increased the release of both SL-α and SL-β. The stimulatory action of PACAP (10(-9)M) on SL-α and SL-β release was blocked by treatment with a PACAP-selective receptor (PAC1R) antagonist, PACAP(6-38) (10(-6)M). We also examined whether PACAP affects the expression of SL-α and SL-β mRNAs in cultured pituitary cells. Treatment with PACAP (10(-9) and 10(-8)M) for 6h decreased the expression level of SL-α mRNA but increased that of SL-β mRNA. The action of PACAP (10(-8)M) on SL-β mRNA expression was blocked by treatment with PACAP(6-38) (10(-6)M), whereas PACAP(6-38) elicited no change in the expression of SL-α mRNA. These results indicate that in cultured goldfish pituitary cells, PACAP stimulates the release of SL-α and SL-β, and expression of SL-β mRNA, via the PAC1R-signaling pathway. However, the mechanism whereby PACAP inhibits the expression of SL-α mRNA does not seem to be mediated by PAC1R signaling.

The octadecaneuropeptide stimulates somatolactin release from cultured goldfish pituitary cells

The present study aimed to investigate the distribution of the octadecaneuropeptide (ODN) in the goldfish brain and to look for a possible effect of ODN on somatolactin (SL) release from pituitary cells. A discrete population of ODN-immunoreactive neurones was localised in the lateral part of the nucleus lateralis tuberis. These neurones sent projections through the neurohypophyseal tract towards the neurohypophysis, and nerve fibres were seen in the close vicinity of SL-producing cells in the pars intermedia. Incubation of cultured goldfish pituitary cells with graded concentrations of ODN (10(-9) -10(-5 ) m) induced a dose-dependent stimulation of SL-β, but not SL-α, release. ODN-evoked SL release was blocked by the metabotrophic endozepine receptor antagonist cyclo(1-8) [DLeu(5) ]OP but was not affected by the central-type benzodiazepine receptor antagonist flumazenil. ODN-induced SL release was suppressed by treatment with the phospholipase C (PLC) inhibitor U-73122 but not with the protein kinase A (PKA) inhibitor H-89. These results indicate that, in fish, ODN produced by hypothalamic neurones acts as a hypophysiotrophic neuropeptide stimulating SL release. The effect of ODN is mediated through a metabotrophic endozepine receptor positively coupled to the PLC/inositol 1,4,5-trisphosphate/protein kinase C-signalling pathway.

Signal transduction mechanisms for autocrine/paracrine regulation of somatolactin-α secretion and synthesis in carp pituitary cells by somatolactin-α and -β

Pituitary hormones can act locally via autocrine/paracrine mechanisms to modulate pituitary functions, which represents an interesting aspect of pituitary regulation other than the traditional hypothalamic input and feedback signals from the periphery. Somatolactin, a member of the growth hormone (GH)/prolactin (PL) family, is a pleiotropic hormone with diverse functions, but its pituitary actions are still unknown. Recently, two SL isoforms, SLα and SLβ, have been cloned in grass carp. Based on the sequences obtained, recombinant proteins of carp SLα and SLβ with similar bioactivity in inducing pigment aggregation in carp melanophores were produced. In carp pituitary cells, SLα secretion and cell content were elevated by static incubation with recombinant carp SLα and SLβ, respectively. These stimulatory actions occurred with a parallel rise in SLα mRNA level with no changes in SLβ secretion, cell content, and gene expression. In contrast, SLα mRNA expression could be reduced by removing endogenous SLα and SLβ with immunoneutralization. At the pituitary cell level, SLα release, cell content, and mRNA expression induced by carp SLα and SLβ could be blocked by inhibiting JAK2/STAT5, PI3K/Akt, MEK1/2, and p38 MAPK, respectively. Furthermore, SLα and SLβ induction also triggered rapid phosphorylation of STAT5, Akt, MEK1/2, ERK1/2, MKK3/6, and p38 MAPK. These results suggest that 1) SLα and SLβ produced locally in the carp pituitary can serve as novel autocrine/paracrine stimulators for SLα secretion and synthesis and 2) SLα production induced by local release of SLα and SLβ probably are mediated by the JAK2/STAT5, PI3K/Akt, and MAPK signaling pathways.