Chapter 1 Neuroendocrine Systems of the Fish Brain

Identification of C1q/TNF-related protein 4 as a novel appetite-regulating peptide that reduces food intake in Siberian sturgeon (Acipenser baerii)

Emerging findings point to a role for C1q/TNF-related protein 4 (CTRP4) in feeding in mammals. However, it remains unknown whether CTRP4 regulates feeding in fish. This study aimed to determine the feeding regulation function of CTRP4 in Siberian sturgeon (Acipenser baerii). In this study, the Siberian sturgeon ctrp4 (Abctrp4) gene was cloned, and Abctrp4 mRNA was shown to be highly expressed in the hypothalamus. In the hypothalamus, Abctrp4 mRNA decreased during fasting and reversed after refeeding. Subsequently, we obtained the AbCTRP4 recombinant protein by prokaryotic expression and optimized the expression and purification conditions. Siberian sturgeon (81.28 ± 14.75 g) were injected intraperitoneally using 30, 100, and 300 ng/g Body weight (BW) AbCTRP4 to investigate its effect on feeding. The results showed that 30, 100, and 300 ng/g BW of the AbCTRP4 significantly reduced the cumulative food intake of Siberian sturgeon at 1, 3, and 6 h. Finally, to investigate the potential mechanism of CTRP4 feeding inhibition, 300 ng/g BW AbCTRP4 was injected intraperitoneally. The findings demonstrated that AbCTRP4 treatment for 1 h significantly promoted the mRNA levels of anorexigenic peptides (pomc, cart, and leptin) while suppressing the mRNA abundances of orexigenic peptides (npy and agrp).In addition, the jak2/stat3 pathway in the hypothalamus was significantly activated after 1 h of AbCTRP4 treatment. In conclusion., this study confirms the anorexigenic effect of CTRP4 in Siberian sturgeon.

First neurotranscriptome of adults Tambaquis (Colossoma macropomum) with characterization and differential expression between males and females

The Tambaqui is one of the most representative Amazon fish species, being highly exploited in fisheries, aquaculture and as a research model. Nonetheless, data about functional genome are still required to evaluate reproductive and nutrition parameters as well as resistance to pathogens. The of next-generation sequencing has allows assessing the transcriptional processes in non-model species by providing comprehensive gene collections to be used as a database in further genomic applications and increased performance of captive populations. In this study, we relied on RNAseq approach to generate the first transcriptome of the telencephalon from adult males and females of Colossoma macropomum, resulting in a reference dataset for future functional studies. We retrieved 896,238 transcripts, including the identification of 267,785 contigs and 203,790 genes. From this total, 91 transcripts were differentially expressed, being 63 and 28 of them positively regulated for females and males, respectively. The functional annotation resulted in a library of 40 candidate genes for females and 20 for males. The functional enrichment classes comprised reproductive processes (GO:0,048,609; GO:0,003,006; GO:0,044,703; GO:0,032,504; GO:0,019,953) being related to sex differentiation (e.g., SAFB) and immune response (e.g., SLC2A6, AHNAK, NLRC3, NLRP3 and IgC MHC I alpha3), thus indicating that the genes in the neurotranscriptome of Tambaqui participate in sex differentiation and homeostasis of captive specimens. These data are useful to design the selection of genes related to sex determination and animal welfare in raising systems of Tambaqui.

Agouti-Signalling Protein Overexpression Reduces Aggressiveness in Zebrafish

Feeding motivation plays a crucial role in food intake and growth. It closely depends on hunger and satiation, which are controlled by the melanocortin system. Overexpression of the inverse agonist agouti-signalling protein (ASIP) and agouti-related protein (AGRP) leads to enhanced food intake, linear growth, and weight. In zebrafish, overexpression of Agrp leads to the development of obesity, in contrast to the phenotype observed in transgenic zebrafish that overexpress asip1 under the control of a constitutive promoter (asip1-Tg). Previous studies have demonstrated that asip1-Tg zebrafish exhibit larger sizes but do not become obese. These fish display increased feeding motivation, resulting in a higher feeding rate, yet a higher food ration is not essential in order to grow larger than wild-type (WT) fish. This is most likely attributed to their improved intestinal permeability to amino acids and enhanced locomotor activity. A relationship between high feeding motivation and aggression has been previously reported in some other transgenic species showing enhanced growth. This study aims to elucidate whether the hunger observed in asip1-Tg is linked to aggressive behaviour. Dominance and aggressiveness were quantified using dyadic fights and mirror-stimulus tests, in addition to the analysis of basal cortisol levels. The results indicate that asip1-Tg are less aggressive than WT zebrafish in both dyadic fights and mirror-stimulus tests.

Effects of temperature on food intake and the expression of appetite regulators in three Characidae fish: The black-skirted tetra (Gymnocorymbus ternetzi), neon tetra (Paracheirodon innesi) and Mexican cavefish (Astyanax mexicanus)

The Characidae family of fish is composed of commercially important species for which little is known about the regulation of feeding. Fish are ectotherms so that their body temperature fluctuates with the temperature of the surrounding water. Changes in water temperature can thus have major effects on the physiology of fish, in particular their feeding. The mechanisms by which appetite is influenced by changes in temperatures in fish remain unclear. In this study, we examined the effects of temperature on feeding behavior, food intake and the expression of appetite regulators in three characid fish (black tetra, neon tetra and cavefish) by submitting them to four different temperatures for 2 weeks (20°C, 24°C, 28°C, 32°C). In all species, food intake increased with increasing temperature. In neon and black tetras, increasing temperatures decreased expressions of orexin and leptin and increased that of cocaine and amphetamine regulated transcript (CART). In cavefish, temperature had no effect on brain orexin, leptin or CART. In all three species, higher temperatures induced increases in intestine expression of cholecystokinin (CCK), but no effects were seen for intestine ghrelin and peptide YY expressions. Our results show that temperature affects feeding in Characidae fish and induces species-specific changes in the expression of appetite regulators.

Neurosecretory protein GL in GIFT tilapia (Oreochromis niloticus): cDNA cloning, tissue distribution and effects of feeding on its expression

Neurosecretory protein GL (NPGL), a novel neuropeptide, has been identified in the hypothalamus of chicks and rodents. NPGL plays a crucial role in monitoring energetic status via the regulation of feeding and metabolism. However, no study on NPGL has been reported in fish thus far. In the present study, the full-length cDNA of NPGL was identified from the hypothalamus of GIFT tilapia (Oreochromis niloticus). The ORF of tilapia NPGL is 471 bp and encodes a precursor peptide with a size of 156 a.a, consisting of a 26 a.a signal peptide and an 82 a.a mature peptide. Tissue distribution profiles of npgl in tilapia were acquired using semiquantitative PCR and in situ hybridization (ISH). The results showed that the highest npgl mRNA is expressed in the telencephalic-preoptic complex, which comprises both the telencephalon and the anterior preoptic area (POA) of male tilapia, and in the ovary of female tilapia. In addition, in male tilapia, the ISH results showed that the cells containing npgl mRNA were distributed exclusively in the anterior periventricular pretectal nucleus (Ppa) of the POA. FISH results demonstrated that npgl mRNA is also expressed in the lateral tuberal nucleus of the hypothalamus (NLT). Real-time PCR showed that npgl mRNA significantly increased in the telencephalic-preoptic complex of male tilapia that were fasted for 24 h and then fed a full diet for 20 min compared with the unfed group. Results of the FISH study showed that parvocellular cells containing npgl mRNA in the Ppa of fed fish were apparently more abundant than those of the unfed group. Few npgl positive signals also appeared in the NLT after full feeding, where pomc mRNA is highly expressed. These results indicate that NPGL may be a short-term satiety factor in fish and that the coexpression of NPGL and POMC may be present in the hypothalamus of male tilapia.

Recurrent oxidant treatment induces dysregulation in the brain transcriptome of Atlantic salmon (Salmo salar) smolts

Peracetic acid (PAA) is an organic peroxide that produces free radicals, which contribute to its potent disinfection power. At therapeutic doses, PAA is considered a mild stressor that can trigger transient local and systemic oxidative stress in fish, but the resulting consequences in the brain have yet to be identified. Therefore, we report the brain transcriptome of Atlantic salmon (Salmo salar) smolts that have been periodically exposed to PAA. Fish were treated three times (every 15 days) with PAA with either short (15 min) or long (30 min) exposure periods. After the third treatment, the whole brain was collected and subjected to biochemical and transcriptomic analyses. The level of reactive oxygen species in the brain was not significantly affected by recurrent PAA treatments. Microarray analysis was performed on the whole brain and revealed 205 differentially expressed genes (DEGs), regardless of the duration of the treatment. The short exposure duration had a more considerable impact on the brain transcriptome, correlating with 70% more DEGs than the long exposure. Strikingly, the brain transcriptome was characterised by the downregulation of gene expression, especially in the short exposure group, and around 82% of the identified DEGs were downregulated. Some of the highly affected genes were key molecules of the vasotocinergic and isotocinergic systems and the corticotropin-releasing factor signalling system, indicating interference of the stress axis but could also suggest an anxiolytic effect. In addition, there were alterations in genes involved in cellular metabolism and processing, signalling and trafficking, and innate immunity, which underscores the physiological changes in the brain following recurrent PAA treatment. Overall, the transcriptomic data reveal that recurrent oxidant treatment could influence brain functions, and although the magnitude was marginal, the alterations suggested neurological adaptations of fish to PAA as a potential chemical stressor. The results identify the risks of PAA, which would be valuable in drafting a framework for its empirically driven use in fish farming.

Possible role of transcription factors (BSX, NKX2.1, IRX3 and SIRT1) in the regulation of appetite in goldfish (Carassius auratus)

The homeobox genes play important roles in the embryonic development of animals. Recent evidence suggests they might also regulate feeding and act as transcription factors of appetite regulators. Examples of these genes are a brain-specific homeobox transcription factor (BSX), NK2 homeobox 1 (NKX2.1) and the Iroquois homeobox 3 (IRX3). Sirtuin1 (SIRT1) acts as a transcription factor for nutrient (e.g. lipid, glucose) homeostasis and responds to stress and nutrient availability, and has been shown to interact with appetite regulators. Very little is known about the role of these genes in the regulation of feeding and nutrient homeostasis in fish. In this study, we assessed the roles of BSX, NKX2.1, IRX3 and SIRT1 in the central regulation of feeding in goldfish by examining their mRNA brain distribution, assessing the effects of fasting on their brain expression and assessing the effects of peripheral injections of cholecystokinin (CCK, a brain-gut peptide), on their brain expression. All genes showed a widespread distribution in the brain, with high levels in the hypothalamus. In both hypothalamus and telencephalon, fasting induced increases in BSX, IRX3 and NKX2.1 expressions but had no effect on SIRT1 expression levels. CCK injections increased hypothalamic expression levels of IRX3 and SIRT1, and telencephalic expression levels of NKX2.1 and SIRT1, with no effect on either hypothalamic BSX or NKX2.1 expression levels or telencephalon BSX or IRX3 expression levels. Our results suggest that, in goldfish as in mammals, central BSX, NKX2.1, IRX3 and SIRT1 are present in regions of the brain regulating feeding, are sensitive to nutrient status and interact with appetite-regulating peptides.

Feed intake and gene expression of appetite-regulating hormones in Salminus brasiliensis fed diets containing soy protein concentrate

Dourado (Salminus brasiliensis) is a large carnivorous fish with high commercial value for which sustainable aquaculture relies on the substitution of expensive dietary animal protein sources in aquafeeds, in particular fish meal (FM), by cheaper plant protein, such as soy protein concentrate (SPC). This study aimed at evaluating feed intake and gene expression of appetite- regulating hormones [orexin, cocaine and amphetamine regulated transcript (CART), leptin, cholecystokinin (CCK) and peptide YY (PYY)] in the intestine, pyloric caeca and hypothalamus of juvenile dourado fed diets containing graded levels of SPC and FM as dietary protein sources for a period of three weeks. Increasing dietary plant protein contents reduced daily feed consumption and the expressions of the anorexigenic hormone CCK in the anterior intestine and in pyloric caeca and PYY in pyloric caeca. No changes were detected in the hypothalamic expression of appetite-regulating hormones, suggesting that gastrointestinal hormones are more involved in the decrease in feeding induced by plant protein diets than central appetite-regulating systems.

Gene expression and latitudinal variation in the stress response in Fundulus heteroclitus

Atlantic killifish, Fundulus heteroclitus, are intertidal marsh fish found along the east coast of North America. Associated with the thermal gradient along this coast, northern and southern killifish populations are known to differ in morphology, behavior, and physiology, including in their cortisol stress response. Our goal was to explore population differences in the stress response and identify underlying molecular mechanisms. We measured responses to both acute and repeated stress in plasma cortisol, stress axis mRNA expression, and body condition in northern and southern killifish. Following an acute stressor, the southern population had higher cortisol levels than the northern population but there was no difference between populations following repeated stress. In the brain, both corticotropin releasing factor and its binding protein had higher expression in the southern than the northern population, but the northern population showed more changes in mRNA levels following a stressor. In the head kidney, Melanocortin 2 Receptor and steroidogenic acute regulatory protein mRNA levels were higher in the southern population suggesting a larger capacity for cortisol synthesis than in the northern fish. Lastly, the glucocorticoid receptor GR1 mRNA levels were greater in the liver of southern fish, suggesting a greater capacity to respond to cortisol, and GR2 had differential expression in the head kidney, suggesting an interpopulation difference in stress axis negative feedback loops. Southern, but not northern, fish were able to maintain body condition following stress, suggesting that these differences in the stress response may be important for adaptation across latitudes.

Proximate causes and ultimate effects of common antidepressants, fluoxetine and venlafaxine, on fish behavior

Antidepressant (AD) drugs are widely prescribed for the treatment of psychiatric disorders, including depression and anxiety disorders. The continuous use of ADs causes significant quantities of these bioactive chemicals to enter the aquatic ecosystems mainly through wastewater effluent discharge. This may result in many aquatic organisms being inadvertently affected by these drugs. Fluoxetine (FLX) and venlafaxine (VEN) are currently among the most widely detected ADs in aquatic systems. A growing body of experimental evidence demonstrates that FLX and VEN have a substantial capacity to induce neurotoxicity and cause behavioral dysfunctions in a wide range of teleost species. At the same time, these studies often report seemingly contradictory results that are confounding in nature. Hence, we clearly require comprehensive reviews that attempt to find overarching patterns and establish possible causes for these variable results. This review aims to explore the current state of knowledge regarding the neurobehavioral effects of FLX and VEN on fishes. This study also discusses the potential mechanistic linkage between the neurotoxicity of ADs and behavioral dysfunction and identifies key knowledge gaps and areas for future research.

Comparative insights into the integration mechanism of neuropeptides to starvation and temperature stress

Stress is known as the process of biological responses evoked by internal or external stimuli. The ability to sense, integrate and respond to stress signals is a requisite for life. Temperature and photoperiod are very important environmental factors for animals. In addition, stress signals can also be inputted from peripheral tissue, such as starvation and inflammation. Through afferent pathways, stress signals input to the central nervous system (CNS), where various signals will integrate, and the integrated information will transmit to the peripheral effectors. As the regulators of neural activity, neuropeptides play important roles in these processes. The present review summarizes recent findings about the integration mechanism of stress signals in the CNS, emphasizing on the role of neuropeptides.

Neuroendocrine Regulation of Plasma Cortisol Levels During Smoltification and Seawater Acclimation of Atlantic Salmon

Diadromous fishes undergo dramatic changes in osmoregulatory capacity in preparation for migration between freshwater and seawater. One of the primary hormones involved in coordinating these changes is the glucocorticoid hormone, cortisol. In Atlantic salmon (Salmo salar), cortisol levels increase during the spring smoltification period prior to seawater migration; however, the neuroendocrine factors responsible for regulating the hypothalamic-pituitary-interrenal (HPI) axis and plasma cortisol levels during smoltification remain unclear. Therefore, we evaluated seasonal changes in circulating levels of cortisol and its primary secretagogue-adrenocorticotropic hormone (ACTH)-as well as transcript abundance of the major regulators of HPI axis activity in the preoptic area, hypothalamus, and pituitary between migratory smolts and pre-migratory parr. Smolts exhibited higher plasma cortisol levels compared to parr across all timepoints but circulating ACTH levels were only elevated in May. Transcript abundance of preoptic area corticotropin-releasing factor b1 and arginine vasotocin were ~2-fold higher in smolts compared to parr in February through May. Smolts also had ~7-fold greater hypothalamic transcript abundance of urotensin 1 (uts-1a) compared to parr in May through July. When transferred to seawater during peak smolting in May smolts rapidly upregulated hypothalamic uts-1a transcript levels within 24 h, while parr only transiently upregulated uts-1a 96 h post-transfer. In situ hybridization revealed that uts-1a is highly abundant in the lateral tuberal nucleus (NLT) of the hypothalamus, consistent with a role in regulating the HPI axis. Overall, our results highlight the complex, multifactorial regulation of cortisol and provide novel insight into the neuroendocrine mechanisms controlling osmoregulation in teleosts.

Application of transcriptome analysis to understand the adverse effects of hydrogen peroxide exposure on brain function in common carp (Cyprinus carpio)

Hydrogen peroxide (H₂O₂), as a common disinfectant, has been extensively used in aquaculture. The toxicity of high ambient H₂O₂ for gills and liver of fish has received attention from many researchers. However, whether H₂O₂ exposure induced brain injury and neurotoxicity has not been reported in fish. Therefore, this study aimed to explore the potential mechanism of H₂O₂ toxicity in brain of common carp via transcriptome analysis and biochemical parameter detection. The fish were exposed to 0 (control) and 1 mM of H₂O₂ for 1 h per day lasting 14 days. The results showed that H₂O₂ exposure caused oxidative damage in brain evidenced by decreased glutathione (GSH), total antioxidant capacity (T-AOC) and nicotinamide adenine dinucleotide (NAD⁺) levels, and increased formation of malondialdehyde (MDA) and 8-hydroxy-2'-deoxyguanosine (8-OHdG). Meanwhile, H₂O₂ exposure reduced 5-hydroxytryptamine (5-HT) level, and down-regulated tryptophan hydroxylase 1 (tph1a), tph2, 5-hydroxytryptamine receptor 1A-beta (htr1ab) and htr2b expression in brain. Transcriptome analysis showed that H₂O₂ exposure up-regulated 604 genes and down-regulated 1209 genes in brain. Go enrichment displayed that the differently expressed genes (DEGs) were enriched mainly in cellular process, single-organism process, metabolic process, and biological regulation in the biological process category. Further, KEGG enrichment indicated that H₂O₂ exposure led to dysregulation of neurotransmitter signals including depression of glutamatergic synapse, GABAergic synapse and endocannabinoid signaling. Also, we found the alteration of three key pathways including calcium, cAMP and HIF-1 in brain after H₂O₂ exposure. In conclusion, our data indicated that H₂O₂ exposure induced oxidative damage and neurotoxicity, possibly related to dysregulation of neurotransmitters and calcium, cAMP and HIF-1 signaling pathways, which may adversely affect learning, memory and social responses of common carp. This study provided novel insight into biological effects and underlying mechanism of H₂O₂ toxicity in aquatic animal, and contributed to proper application of H₂O₂ in aquaculture.

Ancient fishes and the functional evolution of the corticosteroid stress response in vertebrates

The neuroendocrine mechanism underlying stress responses in vertebrates is hypothesized to be highly conserved and evolutionarily ancient. Indeed, elements of this mechanism, from the brain to steroidogenic tissue, are present in all vertebrate groups; yet, evidence of the function and even identity of some elements of the hypothalamus-pituitary-adrenal/interrenal (HPA/I) axis is equivocal among the most basal vertebrates. The purpose of this review is to discuss the functional evolution of the HPA/I axis in vertebrates with a focus on our understanding of this neuroendocrine mechanism in the most ancient vertebrates: the agnathan (i.e., hagfish and lamprey) and chondrichthyan fishes (i.e., sharks, rays, and chimeras). A review of the current literature presents evidence of a conserved HPA/I axis in jawed vertebrates (i.e., gnathostomes); yet, available data in jawless (i.e., agnathan) and chondrichthyan fishes are limited. Neuroendocrine regulation of corticosteroidogenesis in agnathans and chondrichthyans appears to function through similar pathways as in bony fishes and tetrapods; however, key elements have yet to be identified and the involvement of melanotropins and gonadotropin-releasing hormone in the stress axis in these ancient fishes warrants further investigation. Further, the identities of physiological glucocorticoids are uncertain in hagfishes, chondrichthyans, and even coelacanths. Resolving these and other knowledge gaps in the stress response of ancient fishes will be significant for advancing knowledge of the evolutionary origins of the vertebrate stress response.

Effects of Short-Term Fasting on mRNA Expression of Ghrelin and the Peptide Transporters PepT1 and 2 in Atlantic Salmon (Salmo salar)

Food intake is a vital process that supplies necessary energy and essential nutrients to the body. Information regarding luminal composition in the gastrointestinal tract (GIT) collected through mechanical and nutrient sensing mechanisms are generally conveyed, in both mammals and fish, to the hypothalamic neurocircuits. In this context, ghrelin, the only known hormone with an orexigenic action, and the intestinal peptide transporters 1 and 2, involved in absorption of dietary di- and tripeptides, exert important and also integrated roles for the nutrient uptake. Together, both are potentially involved in signaling pathways that control food intake originating from different segments of the GIT. However, little is known about the role of different paralogs and their response to fasting. Therefore, after 3 weeks of acclimatization, 12 Atlantic salmon (Salmo salar) post-smolt were fasted for 4 days to explore the gastrointestinal response in comparison with fed control (n = 12). The analysis covered morphometric (weight, length, condition factor, and wet content/weight fish %), molecular (gene expression variations), and correlation analyses. Such short-term fasting is a common and recommended practice used prior to any handling in commercial culture of the species. There were no statistical differences in length and weight but a significant lower condition factor in the fasted group. Transcriptional analysis along the gastrointestinal segments revealed a tendency of downregulation for both paralogous genes slc15a1a and slc15a1b and with significant lowered levels in the pyloric ceca for slc15a1a and in the pyloric ceca and midgut for slc15a1b. No differences were found for slc15a2a and slc15a2b (except a higher expression of the fasted group in the anterior midgut), supporting different roles for slc15 paralogs. This represents the first report on the effects of fasting on slc15a2 expressed in GIT in teleosts. Transcriptional analysis of ghrelin splicing variants (ghrl-1 and ghrl-2) showed no difference between treatments. However, correlation analysis showed that the mRNA expression for all genes (restricted to segment with the highest levels) were affected by the residual luminal content. Overall, the results show minimal effects of 4 days of induced fasting in Atlantic salmon, suggesting that more time is needed to initiate a large GIT response.

Gonadotropin-inhibitory hormone (GnIH) distribution in the brain of the ancient fish Atractosteus tropicus (Holostei, Lepisosteiformes)

The Holostei group occupies a critical phylogenetic position as the sister group of the Teleostei. However, little is known about holostean pituitary anatomy or brain distribution of important reproductive neuropeptides, such as the gonadotropin-inhibitory hormone (GnIH). Thus, the present study set out to characterize the structure of the pituitary and to localize GnIH-immunoreactive cells in the brain of Atractosteus tropicus from the viewpoint of comparative neuroanatomy. Juveniles of both sexes were processed for general histology and immunohistochemistry. Based on the differences in cell organization, morphology, and staining properties, the neurohypophysis and three regions in the adenohypophysis were identified: the rostral and proximal pars distalis (PPD) and the pars intermedia. This last region was found to be innervated by the neurohypophysis. This organization, together with the presence of a saccus vasculosus, resembles the general teleost pituitary organization. A vast number of blood vessels were also recognized between the infundibulum floor of the hypothalamus and the PPD, evidencing the characteristic presence of a median eminence and a portal system. However, this well-developed pituitary portal system resembles that of tetrapods. As regards the immunohistochemical localization of GnIH, we found four GnIH-immunoreactive (GnIH-ir) populations in three hypothalamic nuclei (suprachiasmatic, retrotuberal, and tuberal nuclei) and one in the diencephalon (prethalamic nucleus), as well as a few scattered neurons throughout the olfactory bulbs, the telencephalon, and the intersection between them. GnIH-ir fibers showed a widespread distribution over almost all brain regions, suggesting that GnIH function is not restricted to reproduction only. In conclusion, the present study describes, for the first time, the pituitary of A. tropicus and the neuroanatomical localization of GnIH in a holostean fish that exhibits a similar distribution pattern to that of teleosts and other vertebrates, suggesting a high degree of phylogenetic conservation of this system.

Multimodal hypothalamo-hypophysial communication in the vertebrates

The vertebrate pituitary is arguably one of the most complex endocrine glands from the evolutionary, anatomical and functional perspectives. The pituitary plays a master role in endocrine physiology for the control of growth, metabolism, reproduction, water balance, and the stress response, among many other key processes. The synthesis and secretion of pituitary hormones are under the control of neurohormones produced by the hypothalamus. Under this conceptual framework, the communication between the hypophysiotropic brain and the pituitary gland is at the foundation of our understanding of endocrinology. The anatomy of the connections between the hypothalamus and the pituitary gland has been described in different vertebrate classes, revealing diverse modes of communication together with varying degrees of complexity. In this context, the evolution and variation in the neuronal, neurohemal, endocrine and paracrine modes will be reviewed in light of recent discoveries, and a re-evaluation of earlier observations. There appears to be three main hypothalamo-pituitary communication systems: 1. Diffusion, best exemplified by the agnathans; 2. Direct innervation of the adenohypophysis, which is most developed in teleost fish, and 3. The median eminence/portal blood vessel system, most conspicuously developed in tetrapods, showing also considerable variation between classes. Upon this basic classification, there exists various combinations possible, giving rise to taxon and species-specific, multimodal control over major physiological processes. Intrapituitary paracrine regulation and communication between folliculostellate cells and endocrine cells are additional processes of major importance. Thus, a more complex evolutionary picture of hypothalamo-hypophysial communication is emerging. There is currently little direct evidence to suggest which neuroendocrine genes may control the evolution of one communication system versus another. However, studies at the developmental and intergenerational timescales implicate several genes in the angiogenesis and axonal guidance pathways that may be important.

Changes in corticotropin releasing factor system transcript levels in relation to feeding condition in Acipenser dabryanus

CRF system, structural conservation, has an association with feeding regulation in mammals. However, mammals and fish have different physiological mechanisms, the potential role of CRF system for feeding regulation in teleost fish are most unknown. To better explore possible feeding mechanisms of CRF system in Acipenser dabryanus, the gene expression patterns of CRF system have been investigated after different energy status. CRF and two receptors have been studied in Acipenser dabryanus in previous study, thus, four components of CRF system (UI, UCN2, UCN3 and CRF-BP) have been studied in this study. Results showed post-prandial increased UCNs mRNA expressions, and 10 days fasting decreased UCNs mRNA expressions, and the mRNA abundance of CRF-BP has no significant differences. Above, this study confirmed the CRF system has potential role for feeding regulation in Acipenser dabryanus.

Structure and function analysis of various brain subregions and pituitary in grass carp (Ctenopharyngodon idellus)

It has been generally acknowledged that environment could alter the morphology and functional differentiation of vertebrate brain. However, as the largest group of all vertebrates, studies about the structures and functions of various brain subregions in teleost are still scarce. In this study, using grass carp as a model, histology method and RNA-sequencing were recruited to examine the microstructure and transcript levels among different brain subregions and pituitary. Histological results showed that the grass carp brain was composed of six parts, including olfactory bulb, telencephalon, hypothalamus, optic tectum, cerebellum, and medulla oblongata. In addition, compared to elasmobranchs and non-teleost bony ray-finned fishes, grass carp lost the hypothalamo-hypophyseal portal system, instead the hypophysiotropic neurons were directly terminated in the pituitary cells. At the transcriptomic level, our results suggested that the olfactory bulb might be related to reproduction and immune function. The telencephalon was deemed to be involved in the regulation of appetite and reproduction. The optic tectum might play important roles in the vision system and feeding. The hypothalamus could regulate feeding, and reproduction process. The medulla oblongata was related with the auditory system. The pituitary seemed to play pivotal roles in energy metabolism, organ development and reproduction. Finally, the correlation analysis suggested that the hypothalamus and the telencephalon were highly related, and close anatomical connection and overlapping functions suggested that the telencephalon and hypothalamus might be the regulation center of feeding and reproduction among teleost brain. This study provided a global view of the microstructures and specific functions of various brain subregions and pituitary in teleost. These results will be very helpful for further study in the neuroendocrinology regulation of growth and reproduction in teleost brain-pituitary axis.

Morphological and immunohistochemical comparison of the pituitary gland between a tropical Paracheirodon axelrodi and a subtropical Aphyocharax anisitsi characids (Characiformes: Characidae)

ABSTRACT Cardinal tetra Paracheirodon axelrodi and bloodfin tetra Aphyocharax anisitsi are two species of characids with high trade value as ornamental fish in South America. Although both species inhabit middle water layers, cardinal neon exhibits a tropical distribution and bloodfin tetra a subtropical one. Generally, these species are difficult to grow, so it becomes essential to know some key components of the neuroendocrine system to achieve their reproduction in captivity. Considering the importance of deepening the knowledge of the reproductive physiology through functional morphology, for the first time in this work we performed an anatomical, morphological and immunohistochemical analysis of the pituitary gland of these two species. In both species, a leptobasic type pituitary is found in the ventral zone of the hypothalamus and it is characterized by a neurohypophysis which has a well-developed pituitary stalk and a globular adenohypophysis. The pituitary components, characterized by histochemistry and immunohistochemistry, shows a distribution pattern of cells types similar to other teleost species, with only slight differences in the distribution of βFSH and βLH for P. axelrodi.

Transcriptional study of appetite regulating genes in the brain of zebrafish (Danio rerio) with impaired leptin signalling

The hormone leptin is a key regulator of body weight, food intake and metabolism. In mammals, leptin acts as an anorexigen and inhibits food intake centrally by affecting the appetite centres in the hypothalamus. In teleost fish, the regulatory connections between leptin and other appetite-regulating genes are largely unknown. In the present study, we used a zebrafish mutant with a loss of function leptin receptor to investigate brain expression patterns of 12 orexigenic and 24 anorexigenic genes under different feeding conditions (normal feeding, 7-day fasting, 2 and 6-hours refeeding). Expression patterns were compared to wild-type zebrafish, in order to identify leptin-dependent differentially expressed genes under different feeding conditions. We provide evidence that the transcription of certain orexigenic and anorexigenic genes is influenced by leptin signalling in the zebrafish brain. We found that the expression of orexigenic genes was not affected by impaired leptin signalling under normal feeding conditions; however, several orexigenic genes showed increased transcription during fasting and refeeding, including agrp, apln, galr1a and cnr1. This suggests an inhibitory effect of leptin signal on the transcription of these orexigenic genes during short-term fasting and refeeding in functional zebrafish. Most pronounced effects were observed in the group of anorexigenic genes, where the impairment of leptin signalling resulted in reduced gene expression in several genes, including cart family, crhb, gnrh2, mc4r, pomc and spx, in the control group. This suggests a stimulatory effect of leptin signal on the transcription of these anorexigenic genes under normal feeding condition. In addition, we found multiple gain and loss in expression correlations between the appetite-regulating genes, in zebrafish with impaired leptin signal, suggesting the presence of gene regulatory networks downstream of leptin signal in zebrafish brain. The results provide the first evidence for the effects of leptin signal on the transcription of various appetite-regulating genes in zebrafish brain, under different feeding conditions. Altogether, these transcriptional changes suggest an anorexigenic role for leptin signal, which is likely to be mediated through distinct set of appetite-regulating genes under different feeding conditions.

NPY and NPY receptors in the central control of feeding and interactions with CART and MC4R in Siberian sturgeon

Neuropeptide Y (NPY) is the most powerful central neuropeptide implicated in feeding regulation via its receptors. Understanding the role of NPY system is critical to elucidate animal feeding regulation. Unlike mammal, the possible mechanisms of NPY system in the food intake of teleost fish are mostly unknown. Therefore, we investigated the regulatory mechanism of NPY and NPY receptors in Siberian sturgeon. In this study, we cloned the cDNA encoding NPY, and assessed the effects of different energy status on npy mRNAs abundance. The expression of npy was decreased in the brain after feeding 1 and 3 h. Besides, the expression of npy was increased after fasting within 15 days, while exhibiting significant decrease after refeeding. In order to further characterize the role of NPY receptor in fish, we performed acute intraperitoneal (i.p.) injection of NPY Y1 and Y2 receptor agonists, which is [Leu 31, Pro 34] NPY and NPY13-36 respectively. The results showed that the food intake of Siberian sturgeon was increased within 30 mins after injection of both Y1 and Y2 receptor agonist. To explore the relationship between NPY, NPY receptors and another appetite peptides, we examined the level of npy, cocaine- and amphetamine-regulated transcript (cart) and melanocortin-4 receptor (mc4r) by injected Y1 and Y2 receptor agonist. The results suggested that cart expression was regulated by NPY which acts on Y1 receptor or Y2 receptor. While mc4r expression just was mediated by NPY and Y1 receptor.

Fish as models for understanding the vertebrate endocrine regulation of feeding and weight

The frequencies of eating disorders and obesity have increased worldwide in recent years. Their pathophysiologies are still unclear, but recent evidence suggests that they might be related to changes in endocrine and neural factors that regulate feeding and energy homeostasis. In order to develop efficient therapeutic drugs, a more thorough knowledge of the neuronal circuits and mechanisms involved is needed. Although to date, rodents have mostly been used models in the area of neuroscience and neuroendocrinology, an increasing number of studies use non-mammalian vertebrates, in particular fish, as model systems. Fish present several advantages over mammalian models and they share genetic and physiological homology to mammals with close similarities in the mechanisms involved in the neural and endocrine regulation of appetite. This review briefly describes the regulation of feeding in two model species, goldfish and zebrafish, how this regulation compares to that in mammals, and how these fish could be used for studies on endocrine regulation of eating and weight and its dysregulations.

Brain and Pituitary Response to Vaccination in Gilthead Seabream (Sparus aurata L.)

Vaccination is a widely used therapeutical strategy in aquaculture, but whether vaccination elicits stress responses in the central neuroendocrine system and enhances the crosstalk between the immune and endocrine systems in the brain or pituitary after vaccination is unclear. To answer this question two experiments using two different vaccine exposure routes, i.e., bath or intraperitoneal (i.p.) injection, were carried out on gilthead seabream (Sparus aurata L.). In the first one, the stress responses of fish subjected to waterborne Vibrio anguillarum bacterin were compared with responses after air exposure or their combination. In the second experiment, fish were subjected to an intraperitoneal injection of Lactococcus garvieae bacterin and we assessed the central stress response and also whether or not a significant immune response was induced in brain and pituitary. In both experiments, blood, brain and pituitary tissues were collected at 1, 6, and 24 h post stress for plasma hormone determination and gene expression analysis, respectively. Results indicated that bath vaccination induced a decreased central stress response compared to air exposure which stimulated both brain and pituitary stress genes. In the second experiment, injection vaccination kept unchanged plasma stress hormones except cortisol that raised at 6 and 24 h. In agreement, non-significant or slight changes on the transcription of stress-related genes were recorded, including the hormone genes of the hypothalamic pituitary interrenal (HPI) axis and other stress markers such as hsp70, hsp90, and mt genes in either brain or pituitary. Significant changes were observed, however, in crhbp and gr. In this second experiment the immune genes il1β, cox2, and lys, showed a strong expression in both brain and pituitary after vaccination, notably il1β which showed more than 10 fold raise. Overall, vaccination procedures, although showing a cortisol response, did not induce other major stress response in brain or pituitary, regardless the administration route. Other than main changes, the alteration of crhbp and gr suggests that these genes could play a relevant role in the feedback regulation of HPI axis after vaccination. In addition, from the results obtained in this work, it is also demonstrated that the immune system maintains a high activity in both brain and pituitary after vaccine injection.

Age-related central regulation of orexin and NPY in the short-lived African killifish Nothobranchius furzeri

Orexin A (OXA) and neuropeptide Y (NPY) are two hypothalamic neuropeptides involved in the regulation of feeding behavior and food intake in all vertebrates. Accumulating evidences document that they undergo age-related modifications, with consequences on metabolism, sleep/wake disorders and progression of neurodegenerations. The present study addressed the age related changes in expression and distribution of orexin A (its precursor is also known as hypocretin-HCRT) and NPY, and their regulation by food intake in the short-lived vertebrate model Nothobranchius furzeri. Our experiments, conducted on male specimens, show that: (a) HCRT and OXA and NPY mRNA and protein are localized in neurons of diencephalon and optic tectum, as well as in numerous fibers projecting through the entire neuroaxis, and are colocalized in specific nuclei; (b) in course of aging, HCRT and NPY expressing neurons are localized also in telencephalon and rhombencephalon; (c) HCRT expressing neurons increased slightly in the diencephalic area of old animals and in fasted animals, whereas NPY increased sharply; (d) central HCRT levels are not regulated neither in course of aging nor by food intake; and (e) central NPY levels are augmented in course of aging, and regulated by food intake only in young. These findings represent a great novelty in the study of central orexinergic and NPY-ergic systems in vertebrates', demonstrating an uncommon and unprecedented described regulation of these two orexigenic neuropeptides.

Glyphosate commercial formulation effects on preoptic area and hypothalamus of Cardinal Neon Paracheirodon axelrodi (Characiformes: Characidae)

ABSTRACT In Colombia the use of glyphosate commercial formulations (Roundup™) for spraying have left deleterious effects on animals and humans. Much of this spraying takes place at the Orinoco basin, habitat of one of the most exported ornamental fish in Colombia, Cardinal neon. To evaluate the effect of Roundup Activo™ four experimental treatments were carried out with 0 mg/L (T1), 0.1 mg/L (T2), 1 mg/L (T3) and 5 mg/L (T4) during 30 days of exposure. The fishes were processed for high-resolution optical microscopy. The main finding of Roundup Activo™ exposure was an increase in mast cells number in brain blood vessels and some neuronal nuclei of the preoptic and posterior diencephalic areas, including hypothalamus. A correlation between concentrations and mast cells number was observed, with the largest mast cells number in T4 treatment. Mast cells presence is a stress benchmark, suggesting the beginning of allergic, inflammatory and apoptotic events. Presence of mast cells in these brain areas may lead to alterations on reproduction, visual and olfactory information integration among other processes. These alterations may result in diminished survival, affecting the conservation of this species in its natural habitat.

The Gonadotropin-Inhibitory Hormone: What We Know and What We Still Have to Learn From Fish

Gonadotropin-inhibitory hormone, GnIH, is named because of its function in birds and mammals; however, in other vertebrates this function is not yet clearly established. More than half of the vertebrate species are teleosts. This group is characterized by the 3R whole genome duplication, a fact that could have been responsible for the great phenotypic complexity and great variability in reproductive strategies and sexual behavior. In this context, we revise GnIH cell bodies and fibers distribution in adult brains of teleosts, discuss its relationship with GnRH variants and summarize the few reports available about the ontogeny of the GnIH system. Considering all the information presented in this review, we propose that in teleosts, GnIH could have other functions beyond reproduction or act as an integrative signal in the reproductive process. However, further studies are required in order to clarify the role of GnIH in this group including its involvement in development, a key stage that strongly impacts on adult life.

Nutrient Regulation of Endocrine Factors Influencing Feeding and Growth in Fish

Endocrine factors regulate food intake and growth, two interlinked physiological processes critical for the proper development of organisms. Somatic growth is mainly regulated by growth hormone (GH) and insulin-like growth factors I and II (IGF-I and IGF-II) that act on target tissues, including muscle, and bones. Peptidyl hormones produced from the brain and peripheral tissues regulate feeding to meet metabolic demands. The GH-IGF system and hormones regulating appetite are regulated by both internal (indicating the metabolic status of the organism) and external (environmental) signals. Among the external signals, the most notable are diet availability and diet composition. Macronutrients and micronutrients act on several hormone-producing tissues to regulate the synthesis and secretion of appetite-regulating hormones and hormones of the GH-IGF system, eventually modulating growth and food intake. A comprehensive understanding of how nutrients regulate hormones is essential to design diet formulations that better modulate endogenous factors for the benefit of aquaculture to increase yield. This review will discuss the current knowledge on nutritional regulation of hormones modulating growth and food intake in fish.

Kiss1 hexadecapeptide directly regulates gonadotropin-releasing hormone 1 in the scombroid fish, chub mackerel

Here we report that the Kiss1 hexadecapeptide (Kiss1-16) directly regulates the functional form of gonadotropin-releasing hormone (GnRH) in the preoptic area (POA) of a scombroid fish model. In this study, we analyzed the localization of two kisspeptin (kiss1 and kiss2) neurons and two kisspeptin receptors (kissr1 and kissr2) in the brain of adult chub mackerel using in situ hybridization to determine whether the kisspeptin receptors co-localize with GnRH1 neurons. The kiss1- and kiss2-expressing neurons were mainly localized in the nucleus recessus lateralis (NRL) and the nucleus of the posterior recess (NRP) in the hypothalamus. Kissr1 was present in the anterior POA and the habenular nucleus. Kissr2 was widely distributed, including in the POA, lateral tuberal nucleus, NRL, and NRP. Notably, GnRH1 was expressed in neurons in the POA, and these neurons co-expressed kissr1. In contrast, kissr2 was expressed abundantly in the vicinity of GnRH1 neurons, but their co-expression did not seem to occur. We also characterized the endogenous mature form of the Kiss1 peptide. An in vitro reporter gene assay clearly showed that Kiss1-16 (HQDMSSYNFNSFGLRY-NH2) was more potent at receptor activation than Kiss1 pentadecapeptide (Kiss1-15), which is the form of Kiss1 found in other fish species. This study strongly suggests that kisspeptin signaling, especially Kiss1 signaling, is important for regulating reproduction in scombroid fish.

Impact of Air Exposure on Vasotocinergic and Isotocinergic Systems in Gilthead Sea Bream (Sparus aurata): New Insights on Fish Stress Response

The hypothalamus-pituitary-interrenal (HPI) and hypothalamus-sympathetic-chromaffin cell (HSC) axes are involved in the regulation of the stress response in teleost. In this regard, the activation of a complex network of endocrine players is needed, including corticotrophin-releasing hormone (Crh), Crh binding protein (Crhbp), proopiomelanocortin (Pomc), thyrotropin-releasing hormone (Trh), arginine vasotocin (Avt), and isotocin (It) to finally produce pleiotropic functions. We aimed to investigate, using the gilthead sea bream (Sparus aurata) as a biological model, the transcriptomic response of different endocrine factors (crh, crhbp, pomcs, trh), neuropeptides (avt and it), and their specific receptors (avtrv1a, avtrv2, and itr) in four important target tissues (hypothalamus, pituitary, kidney and liver), after an acute stress situation. We also investigated several stress hormones (catecholamines and cortisol). The stress condition was induced by air exposure for 3 min, and hormonal, metabolic and transcriptomic parameters were analyzed in a time course response (15 and 30 min, and 1, 2, 4, and 8 h post-stress) in a total of 64 fish (n = 8 fish per experimental group; p = 0.05; statistical power = 95%). Our results showed that plasma noradrenaline, adrenaline and cortisol values increased few minutes after stress exposure. At hypothalamic and hypophyseal levels, acute stress affected mRNA expression of all measured precursors and hormonal factors, as well as their receptors (avtrs and itr), showing the activation, at central level, of HPI, HSC, and Avt/It axes in the acute stress response. In addition, stress response also affected mRNA levels of avtrs and itr in the head kidney, as well as the steroidogenic acute regulatory protein (star) and tyrosine hydroxylase (th) expression, suggesting their participation in the HPI and HSC axes activation. Moreover, the pattern of changes in hepatic avtrs and itr gene expression also highlights an important role of vasotocinergic and isotocinergic pathways in liver metabolic organization after acute stress events. Our results demonstrate, both at transcriptional and circulating levels of several hormones, the existence of a complex activation of different endocrine pathways in S. aurata related to the stress pathways, where vasotocinergic and isotocinergic systems can also be considered key players of the acute stress response orchestration.

Effects of acute handling stress on short-term central expression of orexigenic/anorexigenic genes in zebrafish

Physiological mechanisms driving stress response in vertebrates are evolutionarily conserved. These mechanisms involve the activation of both the hypothalamic-sympathetic-chromaffin cell (HSC) and the hypothalamic-pituitary-adrenal (HPA) axes. In fish, the reduction of food intake levels is a common feature of the behavioral response to stress but the central mechanisms coordinating the energetic response are not well understood yet. In this work, we explore the effects of acute stress on key central systems regulating food intake in fish as well as on total body cortisol and glucose levels. We show that acute stress induced a rapid increase in total body cortisol with no changes in body glucose, at the same time promoting a prompt central response by activating neuronal pathways. All three orexigenic peptides examined, i.e., neuropeptide y (npy), agouti-related protein (agrp), and ghrelin, increased their central expression level suggesting that these neuronal systems are not involved in the short-term feeding inhibitory effects of acute stress. By contrast, the anorexigenic precursors tested, i.e., cart peptides and pomc, exhibited increased expression after acute stress, suggesting their involvement in the anorexigenic effects.

Dynamics of ovarian maturation throughout the reproductive cycle of the Neotropical cichlid fishCichlasomadimerus(Teleostei, Cichliformes)

In this study, we analyzed gene expression profiles, plasma steroids concentrations, and gonadal morphology throughout the reproductive cycle of female Cichlasoma dimerus (Heckel, 1840), a monogamous cichlid fish exhibiting social hierarchies. Fish were analyzed at six phases encompassing their annual cycle, namely resting (during the nonreproductive period), prespawning, 30 h post spawning, 4 days post spawning, 10 days post spawning, and subordinate (during the reproductive period). The histological and histomorphometric analysis showed that C. dimerus exhibits asynchronous ovarian development. Similar to resting females, subordinate females showed low gonadosomatic index, reduced expression levels of vitellogenin (vtgAb), zona pellucida (zpB), gonadal aromatase (cyp19a1A), and low concentrations of plasma sex steroids, thus indicating that social intimidation by dominant conspecifics elicited reproductive arrest. In reproductively active females, a direct positive correlation between plasma estradiol, vtgAb expression, percentage of late vitellogenic oocytes, and gonadosomatic index was observed. These parameters were maximal at the prespawning phase, decreased at 30 h post spawning and 4 days post spawning, and then reached a peak at 10 days post spawning. Our results indicate that female C. dimerus become spawning capable after 10 days post spawning, coincidently with the shortest time interval between successive spawns recorded in captivity.

Hypothalamic Integration of Metabolic, Endocrine, and Circadian Signals in Fish: Involvement in the Control of Food Intake

The regulation of food intake in fish is a complex process carried out through several different mechanisms in the central nervous system (CNS) with hypothalamus being the main regulatory center. As in mammals, a complex hypothalamic circuit including two populations of neurons: one co-expressing neuropeptide Y (NPY) and Agouti-related peptide (AgRP) and the second one population co-expressing pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) is involved in the integration of information relating to food intake control. The production and release of these peptides control food intake, and the production results from the integration of information of different nature such as levels of nutrients and hormones as well as circadian signals. The present review summarizes the knowledge and recent findings about the presence and functioning of these mechanisms in fish and their differences vs. the known mammalian model.

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

Evidence obtained in recent years in a few species, especially rainbow trout, supports the presence in fish of nutrient sensing mechanisms. Glucosensing capacity is present in central (hypothalamus and hindbrain) and peripheral [liver, Brockmann bodies (BB, main accumulation of pancreatic endocrine cells in several fish species), and intestine] locations whereas fatty acid sensors seem to be present in hypothalamus, liver and BB. Glucose and fatty acid sensing capacities relate to food intake regulation and metabolism in fish. Hypothalamus is as a signaling integratory center in a way that detection of increased levels of nutrients result in food intake inhibition through changes in the expression of anorexigenic and orexigenic neuropeptides. Moreover, central nutrient sensing modulates functions in the periphery since they elicit changes in hepatic metabolism as well as in hormone secretion to counter-regulate changes in nutrient levels detected in the CNS. At peripheral level, the direct nutrient detection in liver has a crucial role in homeostatic control of glucose and fatty acid whereas in BB and intestine nutrient sensing is probably involved in regulation of hormone secretion from endocrine cells.

Topological and histological description of preoptic area and hypothalamus in cardinal tetra Paracheirodon axelrodi (Characiformes: Characidae)

ABSTRACT Topological and histological descriptions of the preoptic area and hypothalamus of the cardinal tetra Paracheirodon axelrodi were performed. Standard histological paraffin sections were used and stained with Nissl technique, and plastic sections for high-resolution optic microscopy (HROM). The preoptic area showed some differences related to the location of the magnocellular preoptic nucleus (PM) and the size of the neurons in this region, as they were the biggest in all the preoptic area. Additionally, within the preoptic area, the different structures that comprise the organum vasculosum of the lamina terminalis (OVLT) were identified and characterized. The hypothalamus could be subdivided in three regions - the ventral, the dorsal and the caudal hypothalamic regions - neuron morphology, size and staining pattern were similar in all of them.

Neuroendocrine-immune interaction: Evolutionarily conserved mechanisms that maintain allostasis in an ever-changing environment

It has now become accepted that the immune system and neuroendocrine system form an integrated part of our physiology. Immunological defense mechanisms act in concert with physiological processes like growth and reproduction, energy intake and metabolism, as well as neuronal development. Not only are psychological and environmental stressors communicated to the immune system, but also, vice versa, the immune response and adaptation to a current pathogen challenge are communicated to the entire body, including the brain, to evoke adaptive responses (e.g., fever, sickness behavior) that ensure allocation of energy to fight the pathogen. This phenomenon is evolutionarily conserved. Hence it is both interesting and important to consider the evolutionary history of this bi-directional neuroendocrine-immune communication to reveal phylogenetically ancient or relatively recently acquired mechanisms. Indeed, such considerations have already disclosed an extensive "common vocabulary" of information pathways as well as molecules and their receptors used by both the neuroendocrine and immune systems. This review focuses on the principal mechanisms of bi-directional communication and the evidence for evolutionary conservation of the important physiological pathways involved.

The antidepressant venlafaxine disrupts brain monoamine levels and neuroendocrine responses to stress in rainbow trout

Venlafaxine, a serotonin-norepinephrine reuptake inhibitor, is a widely prescribed antidepressant drug routinely detected in the aquatic environment. However, little is known about its impact on the physiology of nontarget organisms. We tested the hypothesis that venlafaxine perturbs brain monoamine levels and disrupts molecular responses essential for stress coping and feeding activity in fish. Rainbow trout (Oncorhynchus mykiss) were exposed to waterborne venlafaxine (0.2 and 1.0 μg/L) for 7 days. This treatment elevated norepinephrine, serotonin, and dopamine levels in the brain in a region-specific manner. Venlafaxine also increased the transcript levels of genes involved in stress and appetite regulation, including corticotropin releasing factor, pro-opiomelanocortin B, and glucose transporter type 2 in distinct brain regions of trout. The drug treatment reduced the total feed consumed per day, but did not affect the feeding behavior of the dominant and subordinate fish. However, the subordinate fish from the venlafaxine-exposed group had significantly higher plasma cortisol levels compared to the subordinate fish in the control group. Collectively, our results demonstrate that venlafaxine, at environmentally realistic levels, is a neuroendocrine disruptor, impacting the stress and feeding responses in rainbow trout. We propose the midbrain region as a key target for venlafaxine impact and the mode of action involves abnormal monoamine content in trout.

Crossover of the hypothalamic pituitary-adrenal/interrenal, -thyroid, and -gonadal axes in testicular development

Besides the well-known function of thyroid hormones (THs) for regulating metabolism, it has recently been discovered that THs are also involved in testicular development in mammalian and non-mammalian species. THs, in combination with follicle stimulating hormone, lead to androgen synthesis in Danio rerio, which results in the onset of spermatogenesis in the testis, potentially relating the hypothalamic-pituitary-thyroid (HPT) gland to the hypothalamic-pituitary-gonadal (HPG) axes. Furthermore, studies in non-mammalian species have suggested that by stimulating the thyroid-stimulating hormone (TSH), THs can be induced by corticotropin-releasing hormone. This suggests that the hypothalamic-pituitary-adrenal/interrenal gland (HPA) axis might influence the HPT axis. Additionally, it was shown that hormones pertaining to both HPT and HPA could also influence the HPG endocrine axis. For example, high levels of androgens were observed in the testis in Odonthestes bonariensis during a period of stress-induced sex-determination, which suggests that stress hormones influence the gonadal fate toward masculinization. Thus, this review highlights the hormonal interactions observed between the HPT, HPA, and HPG axes using a comparative approach in order to better understand how these endocrine systems could interact with each other to influence the development of testes.

Melanocortin 4 receptor becomes an ACTH receptor by coexpression of melanocortin receptor accessory protein 2

Melanocortin 2 receptor (MC2R) is the only canonical ACTH receptor. Its functional expression requires the presence of an accessory protein, known as melanocortin receptor 2 accessory protein 1 (MRAP1). The vertebrate genome exhibits a paralogue gene called MRAP2, which is duplicated in zebrafish (MRAP2a and MRAP2b), although its function remains unknown. In this paper, we demonstrate that MRAP2a enables MC4R, a canonical MSH receptor, to be activated by ACTH with a similar sensitivity to that exhibited by MC2R. Both proteins physically interact and are coexpressed in the neurons of the preoptic area, a key region in the control of the energy balance and hypophyseal secretion in fish. ACTH injections inhibit food intake in wild-type zebrafish but not in fish lacking functional MC4R. Both MRAP1 and MRAP2a are hormonally regulated, suggesting that these proteins are substrates for feed-back regulatory pathways of melanocortin signaling. Fasting has no effect on the central expression of MRAP2a but stimulates MRAP2b expression. This protein interacts and is colocalized with MC4R in the tuberal hypothalamic neurons but has no effect on the pharmacologic profile of MC4R. However, MRPA2b is able to decrease basal reporter activity in cell lines expressing MC4R. It is plausible that MRAP2b decreases the constitutive activity of the MC4R during fasting periods, driving the animal toward a positive energy balance. Our data indicate that MRAP2s control the activity of MC4R, opening up new pathways for the regulation of melanocortin signaling and, by extension, for the regulation of the energy balance and obesity.

The comparative endocrinology of feeding in fish: insights and challenges

Studying the endocrine regulation of food intake in fish can be challenging due to the diversity in appetite-regulating hormones and the diversity within the fish group itself. Studies show that although the structure of the hormones is relatively conserved among vertebrates, their functions might vary between fish and mammals as well as among fish species. In addition, feeding behavior and the action of appetite regulators can be largely modulated by the feeding and reproductive status of the fish as well as the environment in which they evolve. This review gives a brief perspective of the endocrine regulation of feeding in fish, some of the methods used, and challenges encountered when using a comparative approach.

A preliminary investigation of the role of melanin-concentrating hormone (MCH) and its receptors in appetite regulation of winter flounder (Pseudopleuronectes americanus)

In order to better understand the role of melanin-concentrating hormone (MCH) in the regulation of appetite in fish, the mRNAs of two forms of MCH, prepro-MCH and MCH2, and two forms of MCH receptors, MCH-R1 and MCH-R2, were isolated from winter flounder (Pseudopleuronectes americanus). In addition, the mRNA expressions of these peptides and their receptors were determined under fed and fasted conditions. Both MCHs are expressed in forebrain and midbrain, as well as peripheral tissues including gut and gonads. Both MCH-Rs are ubiquitously expressed in the brain and periphery. Fasting induced an increase in the expression levels of MCH and MCH-R1 mRNAs in optic tectum/thalamus and hypothalamus but had no effect on either MCH2 or MCH-R2 mRNA expressions. Our results suggest that MCH and MCH-R1, but not MCH2 and MCH-R2 might have a role in the regulation of appetite in flounder.

Effect of different glycaemic conditions on gene expression of neuropeptides involved in control of food intake in rainbow trout; interaction with stress

To assess mechanisms relating to food intake and glucosensing in fish, and their interaction with stress, we evaluated changes in the expression of orexigenic (NPY) and anorexigenic (POMC, CART and CRF) peptides in central glucosensing areas (hypothalamus and hindbrain) of rainbow trout subjected to normoglycaemic (control), hypoglycaemic (4 mg insulin kg(-1)) or hyperglycaemic (500 mg glucose kg(-1)) conditions for 6 h under normal stocking density (NSD; 10 kg fish mass m(-3)) or under stress conditions induced by high stocking density (HSD; 70 kg fish mass m(-3)). Hyperglycaemic NSD conditions resulted in decreased mRNA levels of NPY and increased levels of CART and POMC in the hypothalamus as well as increased mRNA levels of CART and CRF in the hindbrain compared with hypo- and normoglycaemic conditions. HSD conditions in normoglycaemic fish induced marked changes in the expression of all peptides assessed: mRNA levels of NPY and CRF increased and mRNA levels of POMC and CART decreased in the hypothalamus, whereas the expression of all four peptides (NPY, POMC, CART and CRF) decreased in the hindbrain. Furthermore, HSD conditions altered the response to changes in glycaemia of NPY and POMC expression in the hypothalamus and CART expression in the hypothalamus and the hindbrain. The results are discussed in the context of food intake regulation by glucosensor systems and their interaction with stress in fish.

Molecular cloning and characterization of two putative appetite regulators in winter flounder (Pleuronectes americanus): preprothyrotropin-releasing hormone (TRH) and preproorexin (OX)

cDNAs encoding for preproTRH and preproorexin were cloned in winter flounder, a species that undergoes a period of natural fasting during the winter. For both peptides, the deduced amino acid structure of the hormone precursor shows 30-70% similarities with their homologs in other fish species. RT-PCR studies show that these peptides are present not only in the brain, but also in several peripheral tissues, including gastrointestinal tract and testes. Fasting induced increases in both preproorexin and preproTRH expressions in the hypothalamus, but did not affect their expression levels in the telencephalon/preoptic area. In addition, the mRNA expressions of both preproorexin and preproTRH were higher in the winter than in the summer in both hypothalamus and telencephalon/preoptic area. Our results suggest that orexin and thyrotropin-releasing hormone (TRH) might have a role in the seasonal regulation of food intake in winter flounder.