Dopaminergic innervation of the rainbow trout pituitary and stimulatory effect of dopamine on growth hormone secretion in vitro

The effect of environmental stressors on growth in fish and its endocrine control

Fish body growth is a trait of major importance for individual survival and reproduction. It has implications in population, ecology, and evolution. Somatic growth is controlled by the GH/IGF endocrine axis and is influenced by nutrition, feeding, and reproductive-regulating hormones as well as abiotic factors such as temperature, oxygen levels, and salinity. Global climate change and anthropogenic pollutants will modify environmental conditions affecting directly or indirectly fish growth performance. In the present review, we offer an overview of somatic growth and its interplay with the feeding regulatory axis and summarize the effects of global warming and the main anthropogenic pollutants on these endocrine axes.

Nutrient regulation of somatic growth in teleost fish. The interaction between somatic growth, feeding and metabolism

This review covers the current knowledge on the regulation of the somatic growth axis and its interaction with metabolism and feeding regulation. The main endocrine and neuroendocrine factors regulating both the growth axis and feeding behavior will be briefly summarized. Recently discovered neuropeptides and peptide hormones will be mentioned in relation to feeding control as well as growth hormone regulation. In addition, the influence of nutrient and nutrient sensing mechanisms on growth axis will be highlighted. We expect that in this process gaps of knowledge will be exposed, stimulating future research in those areas.

Grass Carp Follisatin: Molecular Cloning, Functional Characterization, Dopamine D1 Regulation at Pituitary Level, and Implication in Growth Hormone Regulation

Activin is involved in pituitary hormone regulation and its pituitary actions can be nullified by local production of its binding protein follistatin. In our recent study with grass carp, local release of growth hormone (GH) was shown to induce activin expression at pituitary level, which in turn could exert an intrapituitary feedback to inhibit GH synthesis and secretion. To further examine the activin/follistatin system in the carp pituitary, grass carp follistatin was cloned and confirmed to be single-copy gene widely expressed at tissue level. At the pituitary level, follistatin signals could be located in carp somatotrophs, gonadotrophs, and lactotrophs. Functional expression also revealed that carp follistatin was effective in neutralizing activin's action in stimulating target promoter with activin-responsive elements. In grass carp pituitary cells, follistatin co-treatment was found to revert activin inhibition on GH mRNA expression. Meanwhile, follistatin mRNA levels could be up-regulated by local production of activin but the opposite was true for dopaminergic activation with dopamine (DA) or its agonist apomorphine. Since GH stimulation by DA via pituitary D1 receptor is well-documented in fish models, the receptor specificity for follistatin regulation by DA was also investigated. Using a pharmacological approach, the inhibitory effect of DA on follistatin gene expression was confirmed to be mediated by pituitary D1 but not D2 receptor. Furthermore, activation of D1 receptor by the D1-specific agonist SKF77434 was also effective in blocking follistatin mRNA expression induced by activin and GH treatment both in carp pituitary cells as well as in carp somatotrophs enriched by density gradient centrifugation. These results, as a whole, suggest that activin can interact with dopaminergic input from the hypothalamus to regulate follistatin expression in carp pituitary, which may contribute to GH regulation by activin/follistatin system via autocrine/paracrine mechanisms.

Physiology during smoltification in Atlantic salmon: effect of melatonin implants

Melatonin implants were used to override natural melatonin rhythm in groups of juvenile Atlantic salmon, Salmo salar, raised at simulated natural photoperiod (SNP) and constant light (LL) from mid-March until end of August. The experiment contained also both sham control (with non-melatonin implants) and control (no implants). No differences were found in the experimental variables between these two control groups. Growth and food intake were negatively affected by melatonin implantation. Overall, higher GH levels were observed in the SNP melatonin-implanted group, whereas no differences in GH levels were seen between the SNP control, LL control, or the LL melatonin-implanted groups. Highest food intake was seen in the LL control group. No differences in food intake were recorded between the LL melatonin-implanted and SNP control groups. Gill Na(+), K(+), ATPase (NKA) activity was influenced by time as well as the interaction between photoperiod and time. No differences in gill NKA activity or plasma chloride levels following transfer to seawater were seen between the groups with melatonin implants and their controls. Based on the present results, it seems apparent that melatonin does play a role in regulating food intake and growth in Atlantic salmon smolts.

Isolation and characterization of melanopsin photoreceptors of Atlantic salmon (Salmo salar)

Melanopsins constitute a recently described group of vertebrate opsin photoreceptors that are involved in nonvisual photoreception. Here we describe the identification of six melanopsin genes of Atlantic salmon (Salmo salar), a valuable teleost model for studying nonvisual photoreception and the basis of photoperiodism. The results show that genes belonging to two different groups, the mammalian-like (Opn4m) and the Xenopus-like (Opn4x) melanopsins have been duplicated in teleosts. In addition, two pairs of salmon duplicates were identified, presumably originating from the salmon lineage whole genome duplication event. The expression pattern of melanopsins was studied by in situ hybridization. The results show that Opn4m and Opn4x melanopsins are differentially expressed in the brain and retina, indicating a functional divergence. In the retina, Opn4m and Opn4x melanopsin are differentially expressed in ganglion, amacrine, and horizontal cells. In the brain, Opn4m is expressed in the dorsal thalamus and in the nucleus lateralis tuberis of the hypothalamus, which is closely connected to and involved in the regulation of pituitary function. Opn4x melanopsins are expressed in the dopaminergic, hypophysiotrophic cell population of the suporaoptic/chiasmatic nucleus and in the serotonergic cell population of the left habenula. The results suggest that melanopsin photoreceptors can be involved in signaling of photoperiodic information through multiple pathways, involving both the retina and possibly as deep-brain photoreceptors directly transmitting photoperiodic information to the hypothalamus-pituitary axis.

Neuroendocrine control of growth hormone in fish

The biological actions of growth hormone (GH) are pleiotropic, including growth promotion, energy mobilization, gonadal development, appetite, and social behavior. Accordingly, the regulatory network for GH is complex and includes many endocrine and environmental factors. In fish, the neuroendocrine control of GH is multifactorial with multiple inhibitors and stimulators of pituitary GH secretion. In fish, GH release is under a tonic negative control exerted mainly by somatostatin. Sex steroid hormones and nutritional status influence the level of brain expression and effectiveness of some of these GH neuroendocrine regulatory factors, suggesting that their relative importance differs under different physiological conditions. At the pituitary level, some, if not all, somatotropes can respond to multiple regulators. Therefore, ligand- and function-specificity, as well as the integrative responses to multiple signals must be achieved at the level of signal transduction mechanisms. Results from investigations on a limited number of stimulatory and inhibitory GH-release regulators indicate that activation of different but convergent intracellular pathways and the utilization of specific intracellular Ca(2+) stores are some of the strategies utilized. However, more work remains to be done in order to better understand the integrative mechanisms of signal transduction at the somatotrope level and the relevance of various GH regulators in different physiological circumstances.

Regulation of somatostatins and their receptors in fish

The multifunctional nature of the somatostatin (SS) family of peptides results from a multifaceted signaling system consisting of many forms of SS peptides that bind to a variety of receptor (SSTR) subtypes. Research in fish has contributed important information about the components, function, evolution, and regulation of this system. Somatostatins or mRNAs encoding SSs have been isolated from over 20 species of fish. Peptides and deduced peptides differ in their amino acid chain length and/or composition, and most species of fish possess more than one form of SS. The structural heterogeneity of SSs results from differential processing of the hormone precursor, preprosomatostatin (PPSS), and from the existence of multiple genes that give rise to multiple PPSSs. The PPSS genes appear to have arisen through a series of gene duplication events over the course of vertebrate evolution. The numerous PPSSs of fish are differentially expressed, both in terms of the distribution among tissues and in terms of the relative abundance within a tissue. Accumulated evidence suggests that nutritional state, season/stage of sexual maturation, and many hormones [insulin (INS), glucagon, growth hormone (GH), insulin-like growth factor-I (IGF-I), and 17beta-estradiol (E2)] regulate the synthesis and release of particular SSs. Fish and mammals possess multiple SSTRs; four different SSTRs have been described in fish and several of these occur as isoforms. SSTRs are also wide spread and are differentially expressed, both in terms of distribution of tissues as well as in terms of relative abundance within tissues. The pattern of distribution of SSTRs may underlie tissue-specific responses of SSs. The synthesis of SSTR mRNA and SS-binding capacity are regulated by nutritional state and numerous hormones (INS, GH, IGF-I, and E2). Accumulated evidence suggests the possibility of both tissue- and subtype-specific mechanisms of regulation. In many instances, there appears to be coordinate regulation of PPSS and of SSTR; such regulation may prove important for many processes, including nutrient homeostasis and growth control.

Peripherally administered growth hormone increases brain dopaminergic activity and swimming in rainbow trout

There is increasing evidence that growth hormone (GH) has important behavioral effects in fish, but the underlying mechanisms are not well understood. To investigate if peripherally administered GH influences the monoaminergic activity of the brain, and how this is correlated to behavior, juvenile rainbow trout were implanted intraperitoneally with ovine GH. Fish were either kept isolated or in groups of five. The physical activity and food intake of the isolated fish were observed after 1 and 7 days, when brains were also sampled. The content of serotonin, dopamine, and noradrenaline and their metabolites in hypothalamus, telencephalon, optic tectum, and brain stem was then analyzed. For fish kept isolated for 7 days following implant, GH increased swimming activity and the levels of the dopamine metabolite 3, 4-hydroxy-phenylacetic acid (DOPAC) were higher in all brain parts examined. In the optic tectum, the levels of the dopamine metabolite homovanillic acid (HVA) were lowered by the GH treatment. One-day GH implant did not affect behavior or monoamine levels of isolated fish. In the fish kept in groups, a 7-day GH implant increased the hypothalamic levels of DOPAC, but not in the other brain parts examined, which may indicate an effect on the brain dopaminergic system from social interactions. It can be concluded that peripherally administered GH may function as a neuromodulator, affecting the dopaminergic activity of the rainbow trout brain, and this is associated with increased swimming activity.

Distribution of dopamine D2 receptor mRNAs in the brain and the pituitary of female rainbow trout: an in situ hybridization study

The distribution of D(2)R (dopamine D(2) receptor) mRNAs was studied in the forebrain of maturing female rainbow trout by means of in situ hybridization using a (35)S-labeled riboprobe (810 bp) spanning the third intracytoplasmic loop. A hybridization signal was consistently obtained in the olfactory epithelium, the internal cell layer of the olfactory bulbs, the ventral and dorsal subdivisions of the ventral telencephalon, and most preoptic subdivisions, with the notable exception of the magnocellular preoptic nucleus, and the periventricular regions of the mediobasal hypothalamus, including the posterior tuberculum. In the pituitary, the signal was higher in the pars intermedia than in the proximal and the rostral pars distalis, but no obvious correspondence with a given cell type could be assigned. Labeled cells were also located in the thalamic region, some pretectal nuclei, the optic tectum, and the torus semicircularis. These results provide a morphologic basis for a better understanding on the functions and evolution of the dopaminergic systems in lower vertebrates.

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.