Elisabeth Jönsson

Growth Hormone Endocrinology of Salmonids: Regulatory Mechanisms and Mode of Action

The focus of this review is on the regulatory mechanisms and the mode of action of GH in salmonids. To stimulate further research, it aims at highlighting areas where numerous important breakthroughs have recently been made, as well as where data are currently lacking. The regulation of GH secretion is under complex hypothalamic control, as well as under negative feedback control by GH and IGF-I. Further, the recently characterized ghrelin is a potent GH secretagogue, and may prove to be a link between feed intake and growth regulation. GH plasma profiles show indications of diurnal changes, but whether salmonids have true pulsatile GH secretion remains to be elucidated. The recent cloning and characterization of the salmon GH receptor (GHR) is a major research break-through which will give new insights into the mechanisms of GH action. It should also stimulate research into circulating GH-binding proteins (GHBPs), as they appear to be a soluble form of the GHR. The salmonid GHR sequences show evolutionary divergence from other fish species, but with a high degree of identity within the salmonid group. Radioreceptorassay studies have found GHR present in all tissues examined, which is in line with the highly pleiotropic action of GH. Data are currently scarce on the plasma dynamics of GH in salmonids, and further studies on GHR and GHBPs dynamics coupled to assessments of GH clearance rates and pathways are needed. The direct versus indirect nature of GH action remains to be clarified, but GH appears to act both locally at the target tissue level to stimulate the autocrine/paracrine action of IGF-I, as well as on the liver to increase plasma IGF-I levels. In addition, GH interacts with other hormones such as cortisol, thyroid hormones, insulin, and reproductive hormones, generating a wide range of physiological effects. GH may act both peripherally and directly at the level of the central nervous system to modify behavior, probably by altering the dopaminergic activity in the brain.

Towards Fish Lipid Nutrigenomics: Current State and Prospects for Fin-Fish Aquaculture

Lipids are the predominant source of energy for fish. The mechanisms by which fish allocate energy from lipids for metabolism, development, growth, and reproduction are critical for understanding key life-history strategies and transitions. Currently, the major lipid component in aquaculture diets is fish oil (FO), derived from wild capture fisheries that are exploited at their maximum sustainable limit. The increasing demand from aquaculture for FO will soon exceed supply and threaten the viability of aquaculture. Thus, it is essential to minimize FO use in aquaculture diets. This might be achieved by a greater understanding of lipid storage and muscle growth, or the identification of alternatives to FO in feeds. This review focuses on recent research applying molecular and genomic techniques to the study of fin-fish lipid metabolism from an aquaculture perspective. Accordingly, particular emphasis will be given to fatty acid metabolism and to highly unsaturated fatty acid (HUFA) biosynthesis and to the transcriptional mechanisms and endocrine factors that regulate these processes in fish. Comparative studies of gene function and distribution are described, which, when integrated with recent fish genome sequence information, provide insights into lipid homeostasis and the outcomes associated with the replacement of FO in fish diets.

Growth Hormone Increases Predation Exposure of Rainbow Trout

The energetic state of an animal strongly influences decisions that balances feeding against predation risk. Growth hormone increases the metabolic demands, which should elevate the feeding motivation of an animal. This, in turn, may increase the willingness to risk exposure to predators during feeding. To test this hypothesis, we studied the effect of growth hormone on the behavioural response of rainbow trout (Oncorhynchus mykiss) to simulated attacks from a model heron. After attacks, growth hormone treated trout foraged closer to the water surface, resumed feeding earlier, and ate more food than did control trout. Such behaviour should increase the susceptibility to aerial predation. Thus, predation may select against high endogenous growth hormone secretion in wild fish. Furthermore, genetic manipulations to increase growth hormone levels, intended to improve growth performance in aquaculture, may result in individuals with substantially altered behavioural patterns. In light of the increasing potential for interactions between farmed and wild fish, growth hormone transgenic fish may pose a threat to wild fish populations.

Aerobic scope fails to explain the detrimental effects on growth resulting from warming and elevated CO2 in Atlantic halibut

As a consequence of increasing atmospheric CO2, the worlds oceans are becoming warmer and more acidic. Whilst the ecological effects of these changes are poorly understood, it has been suggested that fish performance including growth will be reduced mainly as a result of limitations in oxygen transport capacity. Contrary to the predictions given by the oxygen- and capacity-limited thermal tolerance hypothesis, we show that aerobic scope and cardiac performance of Atlantic halibut (Hippoglossus hippoglossus) increase following 14–16 weeks exposure to elevated temperatures and even more so in combination with CO2-acidified seawater. However, the increase does not translate into improved growth, demonstrating that oxygen uptake is not the limiting factor for growth performance at high temperatures. Instead, long-term exposure to CO2-acidified seawater reduces growth at temperatures that are frequently encountered by this species in nature, indicating that elevated atmospheric CO2 levels may have serious implications on fish populations in the future.

Dominance, Nutritional State, and Growth Hormone Levels in Rainbow Trout (Oncorhynchus mykiss)

This study addressed three questions concerning interactions between physiology and dominance in juvenile rainbow trout: (1) the validity of a model predicting a time-dependent effect of fasting on competitive ability (i.e., the ability to obtain contested food items) was tested in a series of dominance trials between fed and progressively more fasted trout, as was (2) the association between fasting and plasma growth hormone levels. (3) The relationship between plasma growth hormone levels and the competitive ability of individual trout was also studied. The main results were as follows: (1) The predictions of the time-dependent model were supported by the fasting-dominance experiment. After 3 days, fasted fish were dominant over fed fish, whereas after 6 and 9 days, the competitive ability of fed and fasted fish was similar. After 12 days, there was a tendency for fed fish to be dominant over their fasted competitors. (2) Sampling of plasma from fed and fasted trout, after 3, 6, 9, and 12 days, demonstrated that plasma growth hormone levels increases in food-deprived rainbow trout after more than 6 days of fasting, which is consistent with previous work. (3) No difference in plasma growth hormone levels was found between paired dominant and subordinate trout. Possible interactions between nutritional state, growth hormone levels, and dominance, and their implications are discussed.

Ghrelin decreases food intake in juvenile rainbow trout (Oncorhynchus mykiss) through the central anorexigenic corticotropin-releasing factor system

Ghrelin stimulates pituitary growth hormone (GH) release, and has a key role in the regulation of food intake and adiposity in vertebrates. To investigate the central effect of native rainbow trout ghrelin (rtghrelin) on food intake in rainbow trout, as well as its possible mode of action, four groups of fish received a single injection into the third brain ventricle (i.c.v. injection): (1) control group (physiological saline) (2) ghrelin-treated group (2.0 ng rtghrelin g bwt(-1)), (3) group given the corticotropin-releasing hormone receptor antagonist alpha-helical CRF 9-41 (ahCRF) (4.0 ng g bwt(-1)) and (4) group receiving the same dose of both ghrelin and ahCRF. Food intake was assessed 1h after treatment. In addition, the presence of the GHS-R (the ghrelin receptor) in the rainbow trout CNS was examined with Western blot. To investigate peripheral effects of ghrelin, rainbow trout received an intraperitoneal cholesterol-based implant with or without rtghrelin, and daily food intake was measured during 14 days. Weight and length were measured at the start and termination of the experiment and specific growth rates were calculated. Mesenteric fat stores, muscle and liver lipid content were analysed after the treatment period. Central ghrelin injections decreased food intake compared with controls, and treatment with ahCRF abolished the ghrelin-effect. Western blot analysis of the GHS-R revealed a single band at around 60 kDa in pituitary, hypothalamus, brain and stomach. Long-term peripheral ghrelin treatment decreased daily food intake compared with controls. This was reflected in a ghrelin-induced decrease in weight growth rate (p<0.06). There was no effect of ghrelin on plasma GH levels or tissue fat stores. The conclusion from this study is that the GHS-R is indicated in the CNS in rainbow trout and that ghrelin may act there as an anorexigenic hormone, through a CRF-mediated pathway. Elevated peripheral ghrelin levels also seem to lead to decreased feed intake in the longer term.

Growth Hormone Increases Aggressive Behavior in Juvenile Rainbow Trout

The aim of the present study was to clarify the role of growth hormone in social interactions in juvenile salmonids. Growth hormone increases the metabolic demands and feeding motivation in teleost fish. As a consequence, growth hormone may increase aggression levels and/or fighting ability. To test these hypotheses we observed agonistic behavior in pairs of juvenile rainbow trout (Oncorhynchus mykiss) consisting of two control fish (C/C pairs), two growth hormone-treated fish (GH/GH pairs), or one growth hormone-treated and one control fish (C/GH pairs). The initiator and the winner of each act of aggression were registered. Aggression was lowest in the C/C pairs, intermediate in the C/GH pairs, and highest in the GH/GH pairs, with the difference between the C/C pairs and the GH/GH pairs being significant. This supports the hypothesis that GH increases aggression levels. However, in the C/GH pairs, the number of conflicts won by GH-treated and control fish did not differ significantly. Thus, because social status was not increased, GH did not appear to affect fighting ability. We suggest that growth hormone affects aggression indirectly by increasing the swimming activity, and/or by inducing defense of a larger territory, thereby increasing the encounter rate between opponents. Since increased aggression can incur energetic and mortality costs, there may be selection against high GH levels in natural populations.

The role of ghrelin in energy balance regulation in fish.

Knowledge about the endocrine regulation of energy balance in fish is of interest for basic as well as aquaculture research. Ghrelin is a peptide hormone that was first identified in fish 10 years ago and has important roles in the control of food intake and metabolism. Both ghrelin and its receptor, the growth hormone secretagogue receptor (GHS-R), have been found in numerous fish species. Their tissue distributions support the idea that ghrelin has an integrative role in the regulation of energy balance at both the central nervous system level and systemic level. In tilapia and goldfish, ghrelin treatment appears to increase food intake and to stimulate lipogenesis and tissue fat deposition to promote a more positive energy status. In rainbow trout, on the other hand, ghrelin decreases food intake. Goldfish and rainbow trout are the fish species in which the mode of action of ghrelin on food intake has been most thoroughly investigated. The results from these studies indicate that ghrelin alters food intake by acting on well-known appetite signals, such as CRH, NPY and orexin, in the hypothalamus in a species-specific manner. In goldfish, sensory fibres of the vagus nerve convey the signal from gut-derived ghrelin to modulate appetite. The data also indicate that ghrelin may modulate foraging/swimming activity and the perception of food in fish. Results related to the effects of energy status, temperature, and stressors on plasma ghrelin/tissue ghrelin mRNA levels are occasionally inconsistent between short- and long-term studies, between the protein and mRNA, and between species. Recent data also imply a role of ghrelin in carbohydrate metabolism. More functional studies are required to understand the role of ghrelin and its mechanisms of action in the regulation of energy balance among fish.

Central nervous system actions of growth hormone on brain monoamine levels and behavior of juvenile rainbow trout

Growth hormone (GH) has been demonstrated to alter the behavior of juvenile salmonids. However, the mechanisms behind this action are not yet understood. In mammals and birds, peripheral GH treatment has been shown to affect monoaminergic activity in the central nervous system, which may be a mechanism whereby GH alters behavior. To investigate if GH may influence behavior directly at the central nervous system, juvenile rainbow trout were injected with GH into the third ventricle of the brain, whereupon physical activity and food intake were observed during 2 h. Thereafter, brains were sampled and the content of serotonin, dopamine, and noradrenaline and their metabolites were measured in hypothalamus, telencephalon, optic tectum, and brainstem. The GH-treated fish increased their swimming activity relative to sham-injected controls, while appetite remained unchanged, compared with sham-injected controls. Analysis of brain content of monoamines revealed that the GH treatment caused a decrease in the dopamine metabolite homovanillic acid in the hypothalamus, indicating a lowered dopaminergic activity. It is concluded that GH may alter behavior by acting directly on the central nervous system in juvenile rainbow trout. Furthermore, GH seems to alter the dopaminergic activity in the hypothalamus. Whether this is a mechanism whereby GH affects swimming activity remains to be clarified.

The impact of temperature on the metabolome and endocrine metabolic signals in Atlantic salmon (Salmo salar)

The aim was to elucidate the effects of elevated temperature on growth performance, growth- and appetite-regulating hormones and metabolism in Atlantic salmon, Salmo salar. Post-smolts in seawater (average mass 175g) that had been reared at 12°C were kept at three temperatures (8, 12 and 18°C) and sampled after one and three months. After three months, the fish kept in 18°C had decreased growth rate and condition factor, and elevated plasma levels of growth hormone (GH) and leptin, compared with fish kept at the lower temperatures. Food conversion efficiency was also decreased at 18°C, while at the same time protein uptake was improved and thus was not a limiting mechanism for growth. Redistribution of energy stores in fish at the highest temperature is evident as a preference of maintaining length growth during times of limited energy availability. NMR-based metabolomics analyses of plasma revealed that several metabolites involved in energy metabolism were negatively affected by temperature in the upper temperature range of Atlantic salmon. Specifically, the high temperature induced a decline of several amino acids (glutamine, tyrosine and phenylalanine) and a shift in lipid metabolism. It appears likely that the decreased food intake at the highest temperature is linked to an anorexigenic function of leptin, but also that the decreased food intake, feed conversion efficiency and condition factor can be linked to changes in GH endocrinology.