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  • Author: SE Wendelaar Bonga x
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PP Pepels, H Van Helvoort, SE Wendelaar Bonga and PH Balm

High concentrations (up to 600 pg/ml) of corticotropin-releasing hormone (CRH) were detected in plasma of the teleost fish Oreochromis mossambicus (tilapia) when screening peripheral tissues of tilapia exposed to stress. Notably, the plasma CRH response to stressors in tilapia is much more pronounced than that in higher vertebrates, such as rats. After characterisation by RIA, by spiking plasma with synthetic tilapia CRH and by methanol-acid extraction, it is concluded that the immunoreactive (ir) material in plasma represents tilapia CRH(1-41). Results indicate that a CRH-binding protein is absent in tilapia plasma. Unstressed fish had plasma CRH levels under the limit of detection (<2 pg/ml), but following capture stress plasma CRH levels (170-300 pg/ml) as well as plasma cortisol levels (120 ng/ml) increased rapidly to plateau levels, which were reached after approximately 5 min. Tilapia CRH(1-41) tested at concentrations between 10(-11) and 10(-7) M in vitro did not stimulate the cortisol release from interrenal tissue. Also pretreatment of interrenal tissue with 10(-9) M CRH did not sensitise the cortisol-producing cells to a subsequent ACTH challenge. Forty-eight hours of net confinement or 48 h of cortisol treatment abolished the plasma CRH response and cortisol response to capture stress. The rapidity of the plasma CRH response and its inhibition after 48 h of stress or cortisol treatment point to release by central nervous tissue. Therefore the distribution of glucocorticoid receptors (GRs) in the brain and pituitary of tilapia was investigated. Main GR-ir cell clusters were found in the medial part (Dm) and posterior part of the dorsal telencephalon, in the preoptic region, in the inferior lobe of the hypothalamus and in the cerebellum. We conclude from comparison of CRH brain contents of unstressed and stressed fish that plasma CRH was released by CRH-ir cells located in the lateral part of the ventral telencephalon (Vl), and suggest that the cortisol feedback on CRH release by Vl is mainly exerted via the forebrain Dm region. We propose that CRH is mobilised during stress to fulfil peripheral functions, such as the regulation of circulating leukocytes or of cardiac output, as CRH receptors have been reported in these organs for fish species.

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RJ Arends, R van der Gaag, GJ Martens, SE Wendelaar Bonga and G Flik

Pro-opiomelanocortin (POMC) is the precursor of a number of biologically active peptides, including adrenocorticotropic hormone, alpha-melanocyte-stimulating hormone and beta-endorphin, which are released by the pituitary glands of fish as well as mammals. To quantify the levels of expression of the two POMC mRNAs relative to one another during the response of the common carp to temperature-induced stress, we used reverse transcriptase PCR combined with capillary electrophoresis and laser-induced fluorescence detection. The ratio of POMC-I mRNA to POMC-II mRNA determined in wild-type and four isogenic carp strains was found to be strain-dependent and influenced by temperature. In strain E20xR8, the ratio had altered in favour of POMC-I from 1:3.2 (POMC-I:POMC-II) in fish adapted to 24 degreesC to 1:1.2 in fish adapted to a decrease of 9 degreesC in ambient temperature. A rapid drop in temperature from 24 to 15 degreesC decreased the POMC mRNA ratio at the expense of POMC-I from 1:1.9 in the control fish (strain E4xR3R8) to 1:4.2 3 h after the temperature drop of 9 degreesC. We conclude that both POMC genes are expressed in the common carp and that their expression ratio is strain-dependent and changes in response to ambient temperature.

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S Varsamos, SE Wendelaar Bonga, G Flik, R Quere and T Commes

Proopiomelanocortin (POMC) cDNA was cloned from sea bass (Dicentrarchus labrax) pituitary gland. A 743 nucleotide sequence was obtained coding for the following sequences flanked by sets of proteolytic cleavage sites: ACTH (Ser(88)-Met(127)), alpha-MSH (Ser(88)-Gly(102)), CLIP (Pro(106)-Met(127)), beta-LPH (Glu(131)-Gln(208)), gamma-LPH (Glu(131)-Ser(175)), beta-MSH (Asp(159)-Ser(175)), and beta-endorphin (Tyr(178)-Gln(208)). No region homologous to gamma-MSH/joining peptide (a tetrapod POMC feature) was found. Amino acid sequence identity was high with other teleostean species considered (tilapia: 73%) and lower with elasmobranchs (dogfish: 42%). However, the presumed biologically active peptides were highly conserved within all species considered: alpha-MSH (93-100%), ACTH (80-95%) and beta-endorphin (54-90%). Real-time PCR allowed us to quantify the expression of the POMC in different tIssues of the sea bass: pituitary gland, liver, gonad and head kidney. No significant POMC expression was found in the integument. In pituitary gland, gonads, head kidney and liver, POMC expression was respectively, 1.26x10(10), 2.67x10(5), 2.06x10(4) and 1.67x10(4) copies/ micro g mRNA.

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RJ Arends, J Rotllant, Metz JR, JM Mancera, SE Wendelaar Bonga and G Flik

MSH is a pituitary hormone derived by post-translational processing from POMC and involved in stress and background adaptation. N-terminal acetylation of MSH to monoacetyl alpha-MSH or diacetyl alpha-MSH increases the bioactivity of the peptide. The aim of this study was to characterize alpha-MSH acetylation in the sea bream (Sparus aurata L.) pituitary gland in response to the stressors air exposure and confinement, as well as in fish adapted for 15 days to a white, gray or black background. Pituitary homogenates were purified by reversed-phase HPLC (RP-HPLC). The alpha-MSH content of fractions was measured by RIA. Immunoreactive RP-HPLC fractions were further analyzed by electrospray mass spectrometry and the peptide sequence determined as SYSMEHFRWGKPV-NH2. In the pituitary gland of sea bream, des-, mono- and diacetyl alpha-MSH were identified. Then plasma alpha-MSH levels were measured in sea bream adapted to different backgrounds. Surprisingly, we found the highest plasma alpha-MSH levels in white-adapted as compared with black-adapted sea bream with intermediate values for gray-adapted fish. This observation is in contrast with results that have been obtained in eel, trout or terrestrial vertebrates. Next, des-, mono- and diacetyl alpha-MSH forms were measured in homogenates of the pituitary gland and in plasma of sea bream exposed to air, to confinement, or to different backgrounds. Monoacetyl alpha-MSH was the predominant form in all control and experimental groups. The lowest content of monoacetyl alpha-MSH relative to des- and diacetyl alpha-MSH was found in white-adapted fish. Levels of des- and diacetyl alpha-MSH forms were similar under all conditions. We observed that monoacetyl alpha-MSH is the most abundant isoform in the pituitary gland after background adaptation, confinement and air exposure, in sea bream. These data indicate that the physiologically most potent isoform of alpha-MSH may vary from species to species.

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Metz JR, MO Huising, J Meek, AJ Taverne-Thiele, SE Wendelaar Bonga and G Flik

Adrenocorticotropic hormone (ACTH) takes a central role in the hypothalamo-pituitary-interrenal axis (HPI axis), which is activated during stress. ACTH is produced by the corticotrope cells of the pituitary pars distalis (PD) and is under control of factors from the nucleus preopticus (NPO). The distribution of ACTH in the hypothalamo-pituitary system in common carp (Cyprinus carpio L.) was assessed by immunohistochemistry. ACTH and beta-endorphin immunoreactivity was observed in the ACTH cells in the PD and in the NPO. Nerve fibers, originating from the NPO and projecting to the pituitary gland, contain beta-endorphin, but not ACTH, and these fibers either control the pituitary pars intermedia (PI) through beta-endorphin or release it to the blood. The release of pituitary ACTH (studied in a superfusion setup) must in vivo be under predominant inhibitory control of dopamine. Release of ACTH is stimulated by corticotropin-releasing hormone, but only when ACTH cells experience dopaminergic inhibition. The expression of the precursor pro-opiomelanocortin in (POMC) NPO, PD and PI was studied in an acute restraint stress paradigm by real-time quantitative polymerase chain reaction (RQ-PCR). POMC gene expression is upregulated in these three key tissues of the hypothalamo-pituitary complex, revealing a hitherto unforeseen complex role for POMC-derived peptides in the regulation of responses to stress.

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RJ Arends, JM Mancera, JL Munoz, SE Wendelaar Bonga and G Flik

We investigated short-term effects (up to 24 h) of air exposure and confinement, and long-term effects (up to 11 days) of confinement, to elucidate signalling pathways in the stress response of gilthead sea bream Sparus aurata L. Plasma glucose and lactate were taken as indicators of sympathetic activation, and alpha-melanocyte stimulating hormone (alpha-MSH), adrenocorticotrophic hormone (ACTH) and cortisol as indicators of activation of the brain-pituitary-interrenal (BPI) axis. Air exposure for 3 min resulted, within 30 min, in an increase in plasma concentrations of cortisol, alpha-MSH, glucose, lactate, osmolality and plasma Na, Cl and Mg. Plasma ACTH and beta-endorphin and plasma K, Ca and P did not change. We conclude that air exposure mainly activates the brain-sympathetic-chromaffin cell (BSC) axis. In fish confined at a density of 70 kg/m(3) (compared with 4 kg/m(3) in controls), cortisol, ACTH and alpha-MSH increased within 1 h, indicating activation of the BPI axis. Plasma glucose, Na, Cl and Mg increased with an 8 h delay compared with the response to air exposure. No changes in plasma lactate, osmolality, K, Ca and P were observed. Long-term confinement induced a biphasic cortisol response with peaks at 1 h and at 2 and 3 days. A gradual increase in plasma beta-endorphin concentrations peaked at 7 days; the concentration of alpha-MSH increased rapidly within 1 h and then declined to control values 4 days after the onset of confinement. No changes in ACTH were detected. Our data provide evidence that a stressor-specific activation of the BSC and BPI axes may occur in Sparus aurata.

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AJ Stouthart, EC Lucassen, FJ van Strien, PH Balm, RA Lock and SE Wendelaar Bonga

Whole-body levels of ACTH, alpha-MSH and cortisol in eggs and larvae of the common carp (Cyprinus carpio) were determined periodically up until 168 h after fertilisation. ACTH, alpha-MSH and cortisol immunoreactivity was detected in unfertilised eggs, and endogenous production of ACTH and alpha-MSH was observed 24 h after fertilisation and that of cortisol 36 h after fertilisation. ACTH immunoreactivity reached peak levels before hatching (56-72 h after fertilisation) and remained relatively stable thereafter, while alpha-MSH immunoreactivity started to increase after hatching. At 36 h after fertilisation, whole-body cortisol levels increased rapidly reaching peak levels at the end of hatching (72 h after fertilisation), remaining stable until the end of the experiment. From 50 h after fertilisation onwards, embryos and larvae increased their whole-body cortisol levels when subjected to handling (mechanical pressure during egg stage or netting during the larval stage). It is concluded that the pituitary-interrenal axis in carp is fully functional at the time of hatching. No indications of a stress non-responsive period after hatching were observed. To characterise ACTH and alpha-MSH immunoreactivities in carp larvae, whole-body homogenates were analysed by HPLC, with pituitary homogenates of adult carp serving as a reference. ACTH and alpha-MSH immunoreactivity in carp larvae homogenates consisted of three and two products respectively. HPLC of adult carp pituitaries revealed the presence of two ACTH immunoreactive products, which may represent a phosphorylated and a non-phosphorylated ACTH variant, while the three alpha-MSH peaks most likely represent des-acetylated, mono-acetylated and di-acetylated alpha-MSH, the latter being the predominant form. In carp larvae, however, one of the ACTH immunoreactive products co-eluted with the non-phosphorylated ACTH, while the two alpha-MSH products identified co-eluted with des-acetylated and mono-acetylated alpha-MSH, indicating that POMC processing at this stage of development is different from prohormone processing in adult fish.

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EH van Den Burg, Metz JR, RJ Arends, B Devreese, I Vandenberghe, J Van Beeumen, SE Wendelaar Bonga and G Flik

Carp beta-endorphin is posttranslationally modified by N-terminal acetylation and C-terminal cleavage. These processes determine the biological activity of the beta-endorphins. Forms of beta-endorphin were identified in the pars intermedia and the pars distalis of the pituitary gland of the common carp (Cyprinus carpio), as well as the forms released in vitro and into the blood. After separation and quantitation by high performance liquid chromatography (HPLC) coupled with radioimmunoassay, the beta-endorphin immunoreactive products were identified by electrospray ionisation mass spectrometry and peptide sequencing. The release of beta-endorphins by the pituitary gland was studied after stimulation with corticotrophin-releasing factor (CRF) in vitro. In the pars intermedia, eight N-acetylated truncated forms were identified. Full length N-acetyl beta-endorphin(1-33) coeluted with N-acetyl beta-endorphin(1-29) and these forms together amounted to over 50% of total immunoreactivity. These products were partially processed to N-acetyl betaendorphin(1-15) (30.8% of total immunoreactivity) and N-acetyl beta-endorphin(1-10) (3.1%) via two different cleavage pathways. The acetylated carp homologues of mammalian alpha- and gamma-endorphin were also found. N-acetyl beta-endorphin(1-15) and (1-29) and/or (1-33) were the major products to be released in vitro, and were the only acetylated beta-endorphins found in blood plasma, although never together. CRF stimulated the release of opioid beta-endorphin from the pars distalis. This non-acetylated beta-endorphin represents the full length peptide and is the most abundant form in plasma.