Effects of serotonin on gonadotropin and growth hormone release from in vitro perifused goldfish pituitary fragments

Buspirone administration: Influence on growth, spawning, immune response, and stress in female goldfish (Carassius auratus)

The current study evaluated the impact of buspirone supplementation on the growth, physiology, stress response, spawning, and immunity in female goldfish (Carassius auratus). For this purpose, buspirone was dissolved in absolute methanol and sprayed onto the feed to create four experimental groups: B0 (control), B25 (25 mg kg-1), B50 (50 mg kg-1), and B100 (100 mg kg-1). Fish were fed their respective diets for 56 days and subjected to stress using the air exposure method at the end of the experiment. Growth performance analysis revealed that fish in the B100 group exhibited significantly higher final weight, weight gain, specific growth rate, and average daily gain than the other groups (P < 0.05). Plasma stress response indicated that cortisol levels were significantly lower in the B100 group after stress exposure, accompanied by a simultaneous decrease in glucose levels. The mucus stress response also showed lower cortisol and glucose levels in the B100 group compared to the other groups. Immunological analysis revealed significant increases in total protein, albumin, complement C3 and C4, and immunoglobulin M concentrations in both plasma and mucus of the B100 group (P < 0.05). Reproductive performance showed a notable enhancement in the number of eggs, fertilization rate, hatching rate, and survival rate in the B100 group compared to other groups (P < 0.05). Buspirone at higher concentrations, positively impacted various physiological aspects of goldfish, including growth, stress, immune activity, and reproductive performance. The significant improvements observed in growth parameters, cortisol levels, immunological markers, and reproductive outcomes suggest the potential of buspirone supplementation as a beneficial strategy in aquaculture practices.

Serotonin-immunoreactivity in the brain of the cichlid fish Oreochromis mossambicus

Serotonin (5-HT) is an evolutionarily conserved monoaminergic neurotransmitter found in the central nervous system and peripheral nervous system across invertebrates and vertebrates. Although the distribution of 5-HT-immunoreactive (5-HT-ir) neurons is investigated in various fish species, the organization of these neurons in cichlid fishes is poorly understood. These fish are known for their adaptability to diverse environments, food habits, and complex mating and breeding behaviors, including parental care. In this paper, we describe the organization of 5-HT-ir neurons in the brain of the cichlid fish Oreochromis mossambicus. Aggregations of 5-HT-ir neurons were spotted in the granule cell layer of the olfactory bulb and near the ventricular border in the preoptic area and magnocellular subdivisions of the nucleus preopticus. Although the presence of 5-HT-ir cells and fibers in the hypothalamic and thalamic regions, cerebellum, and raphe nuclei was comparable to that of other teleosts, the current study reveals the occurrence of 5-HT-ir cells and fibers for the first time in some areas, such as the nucleus posterior tuberis, nucleus oculomotorius, and nucleus paracommissuralis in the tilapia. While the presence of 5-HT-ir cells and fibers in gustatory centers suggests a role for serotonin in the processing of gustatory signals, distinctive pattern of 5-HT immunoreactivity was seen in the telencephalon, pretectal areas, mesencephalic, and rhombencephalic regions, suggesting a cichlid fish specific organization of the serotonergic system. In conclusion, the 5-HT system in the tilapia brain may serve several neuroendocrine and neuromodulatory roles, including regulation of reproduction and sensorimotor processes.

GABA-immunoreactive neurons in the Central Nervous System of the viviparous teleost Poecilia sphenops

Gamma-aminobutyric acid (GABA) functions as the primary inhibitory neurotransmitter within the central nervous system (CNS) of vertebrates. In this study, we examined the distribution pattern of GABA-immunoreactive (GABA-ir) cells and fibres in the CNS of the viviparous teleost Poecilia sphenops using immunofluorescence method. GABA immunoreactivity was seen in the glomerular, mitral, and granular layers of the olfactory bulbs, as well as in most parts of the dorsal and ventral telencephalon. The preoptic area consisted of a small cluster of GABA-ir cells, whereas extensively labelled GABA-ir neurons were observed in the hypothalamic areas, including the paraventricular organ, tuberal hypothalamus, nucleus recessus lateralis, nucleus recessus posterioris, and inferior lobes. In the thalamus, GABA-positive neurons were only found in the ventral thalamic and central posterior thalamic nuclei, whereas the dorsal part of the nucleus pretectalis periventricularis consisted of a few GABA-ir cells. GABA-immunoreactivity was extensively seen in the alar and basal subdivisions of the midbrain, whereas in the rhombencephalon, GABA-ir cells and fibres were found in the cerebellum, motor nucleus of glossopharyngeal and vagal nerves, nucleus commissuralis of Cajal, and reticular formation. In the spinal cord, GABA-ir cells and fibres were observed in the dorsal horn, ventral horn, and around the central canal. Overall, the extensive distribution of GABA-ir cells and fibres throughout the CNS suggests several roles for GABA, including the neuroendocrine, viscerosensory, and somatosensory functions, for the first time in a viviparous teleost.

Tenets in Microbial Endocrinology: A New Vista in Teleost Reproduction

Climate vulnerability and induced changes in physico-chemical properties of aquatic environment can bring impairment in metabolism, physiology and reproduction in teleost. Variation in environmental stimuli mainly acts on reproduction by interfering with steroidogenesis, gametogenesis and embryogenesis. The control on reproductive function in captivity is essential for the sustainability of aquaculture production. There are more than 3,000 teleost species across the globe having commercial importance; however, adequate quality and quantity of seed production have been the biggest bottleneck. Probiotics are widely used in aquaculture as a growth promoter, stress tolerance, pathogen inhibition, nutrient digestibility and metabolism, reproductive performance and gamete quality. As the gut microbiota exerts various effects on the intestinal milieu which influences distant organs and pathways, therefore it is considered to be a full-fledged endocrine organ. Researches on Gut-Brain-Gonad axis (GBG axis) and its importance on physiology and reproduction have already been highlighted for higher mammals; however, the study on fish physiology and reproduction is limited. While looking into the paucity of information, we have attempted to review the present status of microbiome and its interaction between the brain and gut. This review will address a process of the microbiome physiological mechanism involved in fish reproduction. The gut microbiota influences the BPG axis through a wide variety of compounds, including neuropeptides, neurotransmitter homologs and transmitters. Currently, research is being conducted to determine the precise process by which gut microbial composition influences brain function in fish. The gut-brain bidirectional interaction can influence brain biochemistry such as GABA, serotonin and tryptophan metabolites which play significant roles in CNS regulation. This review summarizes the fact, how microbes from gut, skin and other parts of the body influence fish reproduction through the Gut-Brain-Gonad axis.

A cross-species comparative approach to assessing multi- and transgenerational effects of endocrine disrupting chemicals

A wide range of chemicals have been identified as endocrine disrupting chemicals (EDCs) in vertebrate species. Most studies of EDCs have focused on exposure of both male and female adults to these chemicals; however, there is clear evidence that EDCs have dramatic effects when mature or developing gametes are exposed, and consequently are associated with in multigenerational and transgenerational effects. Several publications have reviewed such actions of EDCs in subgroups of species, e.g., fish or rodents. In this review, we take a holistic approach synthesizing knowledge of the effects of EDCs across vertebrate species, including fish, anurans, birds, and mammals, and discuss the potential mechanism(s) mediating such multi- and transgenerational effects. We also propose a series of recommendations aimed at moving the field forward in a structured and coherent manner.

The potential for adverse effects in fish exposed to antidepressants in the aquatic environment

Antidepressants are one of the most commonly prescribed pharmaceutical classes for the treatment of psychiatric conditions. They act via modulation of brain monoaminergic signaling systems (predominantly serotonergic, adrenergic, dopaminergic) that show a high degree of structural conservation across diverse animal phyla. A reasonable assumption, therefore, is that exposed fish and other aquatic wildlife may be affected by antidepressants released into the natural environment. Indeed, there are substantial data reported for exposure effects in fish, albeit most are reported for exposure concentrations exceeding those occurring in natural environments. From a critical analysis of the available evidence for effects in fish, risk quotients (RQs) were derived from laboratory-based studies for a selection of antidepressants most commonly detected in the aquatic environment. We conclude that the likelihood for effects in fish on standard measured end points used in risk assessment (i.e., excluding effects on behavior) is low for levels of exposure occurring in the natural environment. Nevertheless, some effects on behavior have been reported for environmentally relevant exposures, and antidepressants can bioaccumulate in fish tissues. Limitations in the datasets used to calculate RQs revealed important gaps in which future research should be directed to more accurately assess the risks posed by antidepressants to fish. Developing greater certainty surrounding risk of antidepressants to fish requires more attention directed toward effects on behaviors relating to individual fitness, the employment of environmentally realistic exposure levels, on chronic exposure scenarios, and on mixtures analyses, especially given the wide range of similarly acting compounds released into the environment.

Developmental aspects of the hypothalamic-pituitary network related to reproduction in teleost fish

The hypothalamic-pituitary-gonadal axis is the main system that regulates reproduction in vertebrates through a complex network that involves different neuropeptides, neurotransmitters, and pituitary hormones. Considering that this axis is established early on life, the main goal of the present work is to gather information on its development and the actions of its components during early life stages. This review focuses on fish because their neuroanatomical characteristics make them excellent models to study neuroendocrine systems. The following points are discussed: i) developmental functions of the neuroendocrine components of this network, and ii) developmental disruptions that may impact adult reproduction. The importance of the components of this network and their susceptibility to external/internal signals that can alter their specific early functions and/or even the establishment of the reproductive axis, indicate that more studies are necessary to understand this complex and dynamic network.

Serotonin (5-hydroxytryptamine)-immunoreactive neurons in the brain of the viviparous fish Gambusia affinis

The monoaminergic neurotransmitter serotonin (5-HT) acts as a neuromodulator and is associated with a wide range of functions in fish. In this investigation, 5-HT immunoreactivity was studied in the central nervous system (CNS) of the viviparous mosquitofish Gambusia affinis. 5-HT-immunoreactive (5-HT-ir) cells/fibres were observed throughout the subdivisions of ventral and dorsal telencephalon including the olfactory bulb. Several intensely stained 5-HT-ir cells and/or fibres were detected in different areas of the hypothalamus as well as the proximal pars distalis of the pituitary gland. 5-HT-ir cells were restricted to the dorsal and ventral part of the pretectal diencephalic cluster, but only fibres were detected in the anterior, ventromedial and posterior subdivisions of the thalamic nucleus and in the preglomerular complex. In the mesencephalon, 5-HT-ir perikarya, and fibres were seen in the optic tectum, midbrain tegmentum and torus semicircularis. A cluster of prominently labelled 5-HT-ir neurons was observed in the superior raphe nucleus, whereas numerous 5-HT-ir fibres were distributed throughout the rhombencephalic divisions. In addition, a bundle of rostrocaudally running 5-HT-ir fibres was noticed in the spinal cord. This is the first detailed neuroanatomical study in a viviparous teleost, reporting a widespread distribution of 5-HT-ir somata and fibres in the CNS. The results of this study provide new insights into the evolutionarily well conserved nature of the monoaminergic system in the CNS of vertebrates and suggest a role for 5-HT in regulation of several physiological, behavioural and neuroendocrine functions in viviparous teleosts.

Discovery of new receptors regulating luteinizing hormone and follicle-stimulating hormone secretion by bovine gonadotrophs to explore a new paradigm for mechanisms regulating reproduction

Previous studies in the 1960s and 1970s have reported that both gonadotropin-releasing hormone (GnRH) and estradiol-activated nuclear estrogen receptors regulate gonadotropin secretion in women. However, I had previously reported that gonadotroph function is regulated by complex crosstalk between several membrane receptors. RNA-seq had previously revealed 259 different receptor genes expressed in the anterior pituitary of heifers. However, the biological roles of most of these receptors remain unknown. I identified four new receptors of interest: G protein-coupled receptor 30 (GPR30), anti-Mullerian hormone (AMH) receptor type 2 (AMHR2), and G protein-coupled receptors 61 and 153 (GPR61 and GPR153). GPR30 rapidly (within a few minutes) mediates picomolar, but not nanomolar, levels of estradiol to suppress GnRH-induced luteinizing hormone (LH) secretion from bovine gonadotrophs, without decreasing mRNA expressions of the LHα, LHβ, or follicle-stimulating hormone (FSH) β subunits. GPR30 is activated by other endogenous estrogens, estrone and estriol. Moreover, GPR30 activation by zearalenone, a nonsteroidal mycoestrogen, suppresses LH secretion. AMHR2, activated by AMH, stimulates LH and FSH secretion, thus regulating gonadotrophs, where other TGF-β family members, including inhibin and activin, potentially affect FSH secretion. I also show that GPR61, activated by its ligand (recently discovered) significantly alters LH and FSH secretion. GPR61, GPR153, and AMHR2 co-localize with the GnRH receptor in unevenly dispersed areas of the bovine gonadotroph cell surface, probably lipid rafts. The findings summarized in this review reveal a new paradigm regarding the mechanisms regulating reproduction via novel receptors expressed on bovine gonadotrophs.

Seasonal Related Multifactorial Control of Pituitary Gonadotropin and Growth Hormone in Female Goldfish: Influences of Neuropeptides and Thyroid Hormone

Female reproduction is under multifactorial control of brain-pituitary-peripheral origin. The present study provides information on seasonal changes in circulating LH and GH concentrations, as well as transcript levels for a number of genes involved in the regulation of reproduction and growth in female goldfish. We also provide information on the effects of treatments with GnRH and/or GnIH, and their interaction with T3, at three stages of gonadal recrudescence. Maximum basal concentration of LH was observed at late recrudescence (Spring) while no seasonal changes in basal serum GH levels was detected. Serum LH and GH levels were stimulated by GnRH as expected, depending on the season. GnIH stimulated basal GH concentrations in gonadally regressed fish. GnIH inhibitory action on GnRH-induced LH response was observed in late, but not in mid recrudescence. T3 actions on basal and GnRH- or GnIH-induced GH secretion were generally inhibitory, depending on season. Administration of T3 attenuated GnRH-induced LH responses in mid and late stages of gonadal recrudescence, and the presence of GnIH abolished inhibitory actions of T3 in fish at mid recrudescence. Our results also demonstrated seasonal patterns in basal and GnRH- and/or GnIH-induced transcript levels for ERα, ERβI, FSHR, aromatase, TRαI, TRβ, IGF-I, and Vtg in the liver and ovary. However, there were no clear correlations between changes in transcript levels and circulating levels of LH and GH. The results support the hypothesis that GnRH, GnIH, and T3 are contributing factors in complex reciprocal control of reproduction and growth in goldfish.

Effects of Melatonin on Anterior Pituitary Plasticity: A Comparison Between Mammals and Teleosts

Melatonin is a key hormone involved in the photoperiodic signaling pathway. In both teleosts and mammals, melatonin produced in the pineal gland at night is released into the blood and cerebrospinal fluid, providing rhythmic information to the whole organism. Melatonin acts via specific receptors, allowing the synchronization of daily and annual physiological rhythms to environmental conditions. The pituitary gland, which produces several hormones involved in a variety of physiological processes such as growth, metabolism, stress and reproduction, is an important target of melatonin. Melatonin modulates pituitary cellular activities, adjusting the synthesis and release of the different pituitary hormones to the functional demands, which changes during the day, seasons and life stages. It is, however, not always clear whether melatonin acts directly or indirectly on the pituitary. Indeed, melatonin also acts both upstream, on brain centers that control the pituitary hormone production and release, as well as downstream, on the tissues targeted by the pituitary hormones, which provide positive and negative feedback to the pituitary gland. In this review, we describe the known pathways through which melatonin modulates anterior pituitary hormonal production, distinguishing indirect effects mediated by brain centers from direct effects on the anterior pituitary. We also highlight similarities and differences between teleosts and mammals, drawing attention to knowledge gaps, and suggesting aims for future research.

A Comparative Update on the Neuroendocrine Regulation of Growth Hormone in Vertebrates

Growth hormone (GH), mainly produced from the pituitary somatotrophs is a key endocrine regulator of somatic growth. GH, a pleiotropic hormone, is also involved in regulating vital processes, including nutrition, reproduction, physical activity, neuroprotection, immunity, and osmotic pressure in vertebrates. The dysregulation of the pituitary GH and hepatic insulin-like growth factors (IGFs) affects many cellular processes associated with growth promotion, including protein synthesis, cell proliferation and metabolism, leading to growth disorders. The metabolic and growth effects of GH have interesting applications in different fields, including the livestock industry and aquaculture. The latest discoveries on new regulators of pituitary GH synthesis and secretion deserve our attention. These novel regulators include the stimulators adropin, klotho, and the fibroblast growth factors, as well as the inhibitors, nucleobindin-encoded peptides (nesfatin-1 and nesfatin-1-like peptide) and irisin. This review aims for a comparative analysis of our current understanding of the endocrine regulation of GH from the pituitary of vertebrates. In addition, we will consider useful pharmacological molecules (i.e. stimulators and inhibitors of the GH signaling pathways) that are important in studying GH and somatotroph biology. The main goal of this review is to provide an overview and update on GH regulators in 2020. While an extensive review of each of the GH regulators and an in-depth analysis of specifics are beyond its scope, we have compiled information on the main endogenous and pharmacological regulators to facilitate an easy access. Overall, this review aims to serve as a resource on GH endocrinology for a beginner to intermediate level knowledge seeker on this topic.

Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications

In humans, as in the other mammals, the neuroendocrine control of reproduction is ensured by the brain-pituitary gonadotropic axis. Multiple internal and environmental cues are integrated via brain neuronal networks, ultimately leading to the modulation of the activity of gonadotropin-releasing hormone (GnRH) neurons. The decapeptide GnRH is released into the hypothalamic-hypophysial portal blood system and stimulates the production of pituitary glycoprotein hormones, the two gonadotropins luteinizing hormone and follicle-stimulating hormone. A novel actor, the neuropeptide kisspeptin, acting upstream of GnRH, has attracted increasing attention in recent years. Other neuropeptides, such as gonadotropin-inhibiting hormone/RF-amide related peptide, and other members of the RF-amide peptide superfamily, as well as various nonpeptidic neuromediators such as dopamine and serotonin also provide a large panel of stimulatory or inhibitory regulators. This paper addresses the origin and evolution of the vertebrate gonadotropic axis. Brain-pituitary neuroendocrine axes are typical of vertebrates, the pituitary gland, mediator and amplifier of brain control on peripheral organs, being a vertebrate innovation. The paper reviews, from molecular and functional perspectives, the evolution across vertebrate radiation of some key actors of the vertebrate neuroendocrine control of reproduction and traces back their origin along the vertebrate lineage and in other metazoa before the emergence of vertebrates. A focus is given on how gene duplications, resulting from either local events or from whole genome duplication events, and followed by paralogous gene loss or conservation, might have shaped the evolutionary scenarios of current families of key actors of the gonadotropic axis.

Metabolic stress leads to divergent changes in the ghrelinergic system in goldfish (Carassius auratus) gonads

Various endocrine factors that regulate energy homeostasis are also implicated in the reproductive physiology of mammals. However, the hormonal link between metabolism and reproduction in fish is poorly understood. Ghrelin is a multifunctional hormone with both metabolic and reproductive roles in vertebrates. Post-translational acylation by ghrelin-O-acyltransferase (GOAT) is critical for its biological actions. The expression of ghrelin, ghrelin or growth hormone secretagogue receptor (GHSR), and GOAT (which forms the ghrelinergic system) in fish under metabolic stress remains unclear. In this research, we used RT-qPCR and Western blot analysis to determine the expression of the ghrelinergic system in goldfish (during the reproductively active phase) hypothalamus and gonads under 7 and 28 days of fasting. We found a significant increase in preproghrelin mRNA expresson in the ovary, and GOAT mRNA expression in the testis of goldfish deprived of food for 7 days. In fish deprived of food for 28 days, preproghrelin, GHSR and GOAT mRNA expression was significantly increased in the hypothalamus of male goldfish. Such differences were not observed in the hypothalamus of female fish, and in the testis of 28 days fasted fish. Meanwhile, preproghrelin, GHSR, and GOAT expression (both mRNA and protein) was significantly increased in the ovary of female fish fasted for 28 days. Ghrelin has been shown to suppress oocyte maturation in fish. The upregulation of a system that has ovarian inbititory roles suggests a role for ghrelin in maintaining reduced reproductive capability during metabolically challenging periods.

Influence of brain plasmalogen changes on gonadotropin secretion from the cultured bovine anterior pituitary cells

We recently discovered that the orphan G-protein-coupled receptor (GPR) 61 colocalized with GnRH receptors (GnRHRs) on the surface of most of bovine gonadotrophs. A recent study suggested that ethanolamine plasmalogen (PI) is a ligand for GPR61 in mouse neuroblastoma. Therefore, this study evaluated the hypothesis that PI alters LH and FSH secretion from cultured bovine anterior pituitary (AP) cells. We prepared bovine AP cells from postpubertal heifers (26 mo old) and cultured the cells for 3.5 d. We treated the cells with increasing concentrations (0, 5, 50, 500, 5,000, 50,000, or 500,000 pg/mL) of phosphoethanolamine PI (PEPI) extracted from the bovine brain, or l-α-lysophosphatidylethanolamine PI (LEPI) extracted from the bovine brain, for 5 min before either no treatment or GnRH stimulation. The medium samples were harvested 2 h after culture for LH and FSH assays. Phosphoethanolamine PI (50-500 pg/mL) stimulated (P < 0.05) the basal secretion of FSH but not LH. Phosphoethanolamine PI at 50 pg/mL also enhanced (P < 0.05) GnRH-induced FSH secretion. However, higher doses (500-500,000 pg/mL) of PEPI suppressed GnRH-induced FSH secretion. Moreover, 50 to 500,000 pg/mL PEPI suppressed GnRH-induced LH secretion. None of the tested concentrations of LEPI showed any effect on basal or GnRH-induced LH or FSH secretion. Pretreatment with Sma and Mad pathway inhibitors suppressed FSH secretion induced by PEPI, whereas an extracellular signal-regulated kinase pathway inhibitor blocked the PEPI-induced suppression of GnRH-stimulated LH secretion. Therefore, PEPI, but not LEPI, extracted from the bovine brain, alters FSH and LH secretion from cultured AP cells. Further studies are required to decide whether PEPI binds to GPR61 and whether PEPI plays an important role in the control of gonadotropin secretion from gonadotrophs.

A novel treatment for height growth in patients with growth hormone insensitivity syndrome by cyproheptadine hydrochloride

BACKGROUND

Cyproheptadine HCl (CyproH) is an appetite-stimulating drug and while it was prescribed for a patient with growth hormone insensitivity syndrome (GHIS) for increasing appetite, his height growth was surprisingly increased. Therefore, the aim of this study was to investigate the effect of CyproH on growth parameters of the patients with GHIS.

PATIENTS AND DESIGN

Twenty patients were enrolled in two prospective cohorts at two different times. Fifteen cases were observed for 1.17 ± 1.3 years without treatment (observation period, OP). Then, CyproH was administered for 2.2 ± 2.7 years (treatment period, TP), and growth parameters were compared within these two periods. Five patients who did not receive any treatment for 1-8.24 years (4 ± 2.9) were the control group.

RESULTS

Height velocity (HV) increased from 1.88 ± 0.7 to 6.1 ± 0.8 cm/year and HV-SDS reached from -4.5 ± 0.74 to -0.21 ± 1.2 in OP and TP, respectively (P < .001), whereas HV and HV-SDS were 2.2 ± 1.1 cm/yr and -4.2 ± 1.2, respectively, in controls (P < .001). Height SDS was -7.0 ± 1.7 and increased to -6 ± 2.2 after treatment (P = .002). Gain in height was 2.3 ± 0.6 SDS in 5 patients who were treated for 5.4 ± 2.8 years. BMI-SDS was not significantly changed within two time periods and also in cases and controls.

CONCLUSION

CyproH caused height growth in the patients with GHIS, and therefore, this treatment can be considered as an alternative option to IGF-I injection.

The nonapeptide isotocin in goldfish: Evidence for serotonergic regulation and functional roles in the control of food intake and pituitary hormone release

Nonapeptides are a highly conserved family of peptides synthesized in the neuroendocrine brain and acting on central and peripheral receptors to regulate physiological functions in vertebrates. While the evolution of the two gene families of oxytocin-like and vasopressin-like nonapeptides and their receptors, as well as the neuroanatomy of their independent neuronal circuits have been well-characterized across vertebrate species, comparative studies on the physiological roles across vertebrates are lagging behind. In the current study, we focused on the comparative neuroendocrine functions and regulation of isotocin, the teleost homologue of mammalian oxytocin. Specifically, we address the hypothesis that isotocin exerts opposing effects on food intake and reproduction, which are well-established effects of its homologue oxytocin in mammalian species. Using goldfish, a well-characterized model of neuroendocrine regulation of both food intake and reproduction, we here showed that isotocin acts as an anorexigenic factor while exerting stimulatory effects on pituitary luteinizing hormone and growth hormone release. Given the dual inhibitory and stimulatory roles of serotonin on food intake and pituitary release of reproductive hormone in goldfish, we also investigated the potential crosstalk between both systems using immunohistochemistry and pharmacological approaches. Results provide neuroanatomical and pharmacological evidence for serotonergic regulation of magnocellular isotocinergic neurons in the preoptic area and pituitary. Together, these findings firstly provide the basis to investigate neuroendocrine cross-talk between serotonergic and nonapeptidergic systems in the regulation of both food intake and reproduction in goldfish, and secondly point to a conserved function of oxytocin-like peptides in the differential neuroendocrine control of both physiological processes in vertebrates.

Social hierarchy modulates responses of fish exposed to contaminants of emerging concern

Many organisms, including the fathead minnow (Pimephales promelas), a toxicological model organism, establish social hierarchies. The social rank of each male in a population is under the control of the hypothalamic-pituitary-gonadal (HPG) axis mainly through regulation of circulating androgen concentrations, which in turn drive the expression of secondary sex characteristics (SSCs). As dominant and subordinate males in an exposure study are initially under different physiological conditions (i.e., differing plasma androgen concentrations), we proposed that they belong to different subpopulations in the context of exposure to compounds that may interact with the HPG axis. Using a meta-analysis of our data from several previously published studies, we corroborated the hypothesis that social status, as indicated by SSCs, results in distinct clusters (eigenvalues >0.8 explaining >80% of variability) with differential expression of plasma vitellogenin, a commonly used biomarker of exposure to contaminants of emerging concern (CEC). Furthermore, we confirmed our predictions that exposure to estrogenic CECs would homogenize plasma vitellogenin response (E1: cluster mean SSC values decreased to 4.33 and 4.86 relative to those of control; E2: decreased to 4.8 and 5.37) across the social hierarchy. In contrast, serotonin-specific reuptake inhibitors expand this response range (cluster mean SSC increased to 5.21 and 6.5 relative to those of control). Our results demonstrated that social hierarchies in male fathead minnows result in heterogeneous responses to chemical exposure. These results represent a cautionary note for the experimental design of single-sex exposure studies. We anticipate our study to be a starting point for the re-evaluation of toxicological data analyses in single sex exposure experiments.

An AOP analysis of selective serotonin reuptake inhibitors (SSRIs) for fish

Pharmaceuticals and personal care products (PPCPs) are found in measureable quantities within the aquatic environment. Selective serotonin reuptake inhibitor (SSRI) antidepressants are one class of pharmaceutical compound that has received a lot of attention. Consistent with most PPCPs, the pharmacokinetics and physiological impacts of SSRI treatment have been well-studied in small mammals and humans and this, combined with the evolutionary conservation of the serotonergic system across vertebrates, allows for the read-across of known SSRI effects in mammals to potential SSRI impacts on aquatic organisms. Using an Adverse Outcome Pathway (AOP) framework, this review examines the similarities and differences between the mammalian and teleost fish SSRI target, the serotonin transporter (SERT; SLC6A4), and the downstream impacts of elevated extracellular serotonin (5-HT; 5-hydroxytryptamine), the consequence of SERT inhibition, on organ systems and physiological processes within teleost fish. This review also intends to reveal potentially understudied endpoints for SSRI toxicity based on what is known to be controlled by 5-HT in fish.

The SSRI fluoxetine exhibits mild effects on the reproductive axis in the cichlid fish Cichlasoma dimerus (Teleostei, Cichliformes)

Among the wide variety of pharmaceuticals released into the environment, Fluoxetine (FLX), a selective serotonin reuptake inhibitor, is one of the most prescribed for the treatment of major depression. It inhibits serotonin (5-HT) reuptake at the presinaptic membrane, increasing serotonergic activity. In vertebrates, including fish, the serotonergic system is closely related to the Hypothalamic Pituitary Gonadal (HPG) axis which regulates reproduction. As FLX can act as an endocrine disrupting compound (EDC) by affecting several reproductive parameters in fish, the aim of this study was to provide an integral assessment of the potential effect of FLX on the reproductive axis of the Neotropical freshwater fish Cichlasoma dimerus. Adult fish were intraperitoneally injected with 2 μg g^-1 FLX or saline every third day for 15 days. No significant differences were found on serotonergic turnover (5-HIAA/5-HT ratio). Pituitary βLH content in FLX injected females was significantly higher than control females; no significant differences were seen for βFSH content. Sex steroids remained unaltered, both in males and females fish, after FLX treatment. No plasma vitellogenin was induced in treated males. Some alterations were seen in testes of FLX injected males, such as the presence of foam cells and an acidophilic PAS positive, Alcian-Blue negative secretion in the lobular lumen. Although there is no clear consensus about the effect of this drug on reproductive physiology, these results indicate that FLX is acting as a mild EDC in adults of C. dimerus.

Effects of an antidepressant mixture on the brain serotonin and predation behavior of hybrid striped bass

Antidepressants have been found in measurable concentrations in final treated wastewater effluent and receiving waters throughout the world. Studies have shown that these concentrations are typically not overtly toxic, but the psychotropic mode of action of these chemicals warrants examination of their behavioral effects. Exposure of hybrid striped bass to the antidepressants fluoxetine or venlafaxine alone has been shown to cause decreased brain serotonin levels and increased time to capture prey at concentrations typically 1 to 2 orders of magnitude higher than environmentally relevant concentrations. In the present study, equally effective doses of fluoxetine and venlafaxine were used to perform a mixture study, using a toxic unit approach to determine whether these antidepressants may act in an additive manner at lower concentrations. The results indicated that mixtures of these antidepressants caused decreased brain serotonin and increased time to capture prey at concentrations lower than reported in previous studies. Low concentration mixtures caused an additive effect on brain serotonin levels and time to capture prey, whereas higher concentrations were less than additive. The results were consistent with the dose addition concept, with higher concentration mixtures potentially saturating the effects on serotonin in the brain. Results from the present study indicate that antidepressants have the potential to be additive on the biochemical and individual scale, which necessitates more robust analysis of antidepressant mixtures and their potential to act together in low concentration scenarios.

Prozac affects stickleback nest quality without altering androgen, spiggin or aggression levels during a 21-day breeding test

Pharmaceuticals are increasingly being used in human and veterinary medicine, and their presence in the aquatic environment may present a threat to non-target aquatic organisms. The selective serotonin reuptake inhibitor fluoxetine (Prozac) has been reported to affect diverse behaviours (feeding, aggression, and reproduction) and also the endocrine system (steroid biosynthesis pathway) in fish. To investigate these claims further, and in particular effects on androgen synthesis, male three-spined sticklebacks (Gasterosteus aculeatus) were exposed to fluoxetine at 0, 3.2, 10 and 32μg/L in a flow-through system for 21 days. Their sex was determined prior to exposure using a non-invasive method to collect DNA for determining the genetic sex, reported here for the first time. This was necessary as the exposure required males of a non-breeding status which had not developed secondary characteristics. Post exposure a number of biochemical (serotonin, steroid and spiggin levels) and apical (aggressive behaviour) endpoints were measured. No effects were detected on morphometric parameters, spiggin or androgen (11-ketotestosterone) levels. However, all fluoxetine-exposed male fish had higher cortisol levels in comparison to the control fish, although this effect only persisted throughout the whole exposure duration at the highest concentration (32μg/L). In addition, the ratio of 5-HIAA/5-HT (serotonin metabolite/serotonin) was significantly lower in the brains of males exposed to fluoxetine at all concentrations tested. Although we found no differences in the number of nests built by the males, the quality of the nests produced by the fluoxetine-exposed males was generally inferior consisting only of a basic, rudimentary structure. Males exposed to 32μg/L of fluoxetine displayed a delayed response to a simulated threat (rival male via own mirror image) and were less aggressive (number of bites and attacks) toward their mirror image, but these differences were not statistically significant. In summary, fluoxetine exposure resulted in reduced serotonergic activity in the male three-spined stickleback brain suggesting that the mechanism of action between humans and fish is at least partially conserved. Furthermore, this study provided additional evidence of cross-talk between the serotonergic and stress axes as demonstrated by the perturbations in cortisol levels. This potentially complex interaction at brain level may be responsible for the effects observed on nest quality, an endpoint with serious ecological consequences for this species. Finally, despite our hypothesis (an effect on steroid biosynthesis, based on limited literature evidence), we observed no effects of fluoxetine exposure (at the concentrations and duration employed) on male stickleback androgen levels.

Serotonergic outcome, stress and sexual steroid hormones, and growth in a South American cichlid fish fed with an L-tryptophan enriched diet

Reared animals for edible or ornamental purposes are frequently exposed to high aggression and stressful situations. These factors generally arise from conspecifics in densely breeding conditions. In vertebrates, serotonin (5-HT) has been postulated as a key neuromodulator and neurotransmitter involved in aggression and stress. The essential amino acid L-tryptophan (trp) is crucial for the synthesis of 5-HT, and so, leaves a gateway for indirectly augmenting brain 5-HT levels by means of a trp-enriched diet. The cichlid fish Cichlasoma dimerus, locally known as chanchita, is an autochthonous, potentially ornamental species and a fruitful laboratory model which behavior and reproduction has been studied over the last 15years. It presents complex social hierarchies, and great asymmetries between subordinate and dominant animals in respect to aggression, stress, and reproductive chance. The first aim of this work was to perform a morphological description of chanchita's brain serotonergic system, in both males and females. Then, we evaluated the effects of a trp-supplemented diet, given during 4weeks, on brain serotonergic activity, stress and sexual steroid hormones, and growth in isolated specimens. Results showed that chanchita's brain serotonergic system is composed of several populations of neurons located in three main areas: pretectum, hypothalamus and raphe, with no clear differences between males and females at a morphological level. Animals fed with trp-enriched diets exhibited higher forebrain serotonergic activity and a significant reduction in their relative cortisol levels, with no effects on sexual steroid plasma levels or growth parameters. Thus, this study points to food trp enrichment as a "neurodietary'' method for elevating brain serotonergic activity and decreasing stress, without affecting growth or sex steroid hormone levels.

Adrenergic signaling in teleost fish liver, a challenging path

Adrenergic receptors or adrenoceptors (ARs) belong to the huge family of G-protein coupled receptors (GPCRs) that have been well characterized in mammals primarily because of their importance as therapeutic drug targets. ARs are found across vertebrates and this review examines the path to identify and characterize these receptors in fish with emphasis on hepatic metabolism. The absence of reliable and specific pharmacological agents led investigators to define the fish hepatic AR system as relying solely on a β2-AR, cAMP-dependent signaling transduction pathway. The use of calcium-radiometric imaging, purified membranes for ligand-binding studies, and perifused rather than static cultured fish hepatocytes, unequivocally demonstrated that both α1- and β2-AR signaling systems existed in the fish liver consistent with studies in mammals. Additionally, the use of molecular tools and phylogenetic analysis clearly demonstrated the existence of multiple AR-types and -subtypes in hepatic and other tissues of a number of fish species. This review also examines the use of β-blockers as pharmaceuticals and how these drugs that are now in the aquatic environment may be impacting aquatic species including fish and some invertebrates. Clearly there is a large conservation of structure and function within the AR system of vertebrates but there remain a number of key questions that need to be addressed before a clear understanding of these systems can be resolved.

Increase in telencephalic dopamine and cerebellar norepinephrine contents by hydrostatic pressure in goldfish: the possible involvement in hydrostatic pressure-related locomotion

Fish are faced with a wide range of hydrostatic pressure (HP) in their natural habitats. Additionally, freshwater fish are occasionally exposed to rapid changes in HP due to heavy rainfall, flood and/or dam release. Accordingly, variations in HP are one of the most important environmental cues for fish. However, little information is available on how HP information is perceived and transmitted in the central nervous system of fish. The present study examined the effect of HP (water depth of 1.3 m) on the quantities of monoamines and their metabolites in the telencephalon, optic tectum, diencephalon, cerebellum (including partial mesencephalon) and vagal lobe (including medulla oblongata) of the goldfish, Carassius auratus, using high-performance liquid chromatography. HP affected monoamine and metabolite contents in restricted brain regions, including the telencephalon, cerebellum and vagal lobe. In particular, HP significantly increased the levels of dopamine (DA) in the telencephalon at 15 min and that of norepinephrine (NE) in the cerebellum at 30 min. In addition, HP also significantly increased locomotor activity at 15 and 30 min after HP treatment. It is possible that HP indirectly induces locomotion in goldfish via telencephalic DA and cerebellar NE neuronal activity.

Serotonin reuptake inhibitor citalopram inhibits GnRH synthesis and spermatogenesis in the male zebrafish

Selective serotonin reuptake inhibitors (SSRIs) are widely used antidepressants for the treatment of depression. However, SSRIs cause sexual side effects such as anorgasmia, erectile dysfunction, and diminished libido that are thought to be mediated through the serotonin (5-hydroxytryptamine, 5-HT) system. In vertebrates, gonadotropin-releasing hormone (GnRH) neurons play an important role in the control of reproduction. To elucidate the neuroendocrine mechanisms of SSRI-induced reproductive failure, we examined the neuronal association between 5-HT and GnRH (GnRH2 and GnRH3) systems in the male zebrafish. Double-label immunofluorescence and confocal laser microscopy followed by three-dimensional construction analysis showed close associations between 5-HT fibers with GnRH3 fibers and preoptic-GnRH3 cell bodies, but there was no association with GnRH2 cell bodies and fibers. Quantitative real-time PCR showed that short-term treatment (2 wk) with low to medium doses (4 and 40 μg/L, respectively) of citalopram significantly decreased mRNA levels of gnrh3, gonadotropins (lhb and fshb) and 5-HT-related genes (tph2 and sert) in the male zebrafish. In addition, short-term citalopram treatment significantly decreased the fluorescence density of 5-HT and GnRH3 fibers compared with controls. Short-term treatment with low, medium, and high (100 μg/L) citalopram doses had no effects on the profiles of different stages of spermatogenesis, while long-term (1 mo) citalopram treatment with medium and high doses significantly inhibited the different stages of spermatogenesis. These results show morphological and functional associations between the 5-HT and the hypophysiotropic GnHR3 system, which involve SSRI-induced reproductive failures.

Role of serotonin in fish reproduction

The neuroendocrine mechanism regulates reproduction through the hypothalamo-pituitary-gonadal (HPG) axis which is evolutionarily conserved in vertebrates. The HPG axis is regulated by a variety of internal as well as external factors. Serotonin, a monoamine neurotransmitter, is involved in a wide range of reproductive functions. In mammals, serotonin regulates sexual behaviors, gonadotropin release and gonadotropin-release hormone (GnRH) secretion. However, the serotonin system in teleost may also play unique role in the control of reproduction as the mechanism of reproductive control in teleosts is not always the same as in the mammalian models. In fish, the serotonin system is also regulated by natural environmental factors as well as chemical substances. In particular, selective serotonin reuptake inhibitors (SSRIs) are commonly detected as pharmaceutical contaminants in the natural environment. Those factors may influence fish reproductive functions via the serotonin system. This review summarizes the functional significance of serotonin in the teleosts reproduction.

Fluoxetine inhibits aggressive behaviour during parental care in male fighting fish (Betta splendens, Regan)

The increasing presence of aquatic contaminants, such as the pharmaceutical fluoxetine, has raised concerns over potentially disrupting effects on several aspects of fish reproduction. However, the effects of fluoxetine on reproductive and paternal behavior in fish remain understudied, particularly at environmentally relevant concentrations. In the current study, we therefore tested the hypothesis that waterborne fluoxetine at an environmentally relevant concentration (540 ng/l), disrupts specific reproductive and paternal behaviors in male Siamese fighting fish at distinct reproductive phases. A pre-post test design was adopted to investigate specific behavioral responses at the individual fish level in response to male conspecific intruders at two different distances from the nest across four distinct reproductive phases (before bubblenest construction, following bubblenest construction, after spawning and after hatching of the larvae). In the control specimens, the measured behaviours were not different between the spawning times and among the interactions in either distance to nest at the different reproduction phases. Our results indicate that fluoxetine specifically disrupts characteristic paternal territorial aggression behaviour only after spawning and hatching of the larvae, while male behaviour in previous reproductive phases is unaffected by fluoxetine exposure. Results of comparison between males at 1st spawning and specimens exposed to fluoxetine at 2nd spawning showed that the first reaction of the nest-holding males to the intruders, duration of fin spreading, number of bites, and 90° turn, and the frequency of sweeps were different between the spawning times after spawning or hatching of embryos. However, interaction of spawning time and reproduction phase was significant on biting behaviour. These results demonstrate that fluoxetine exposure at environmental concentrations negatively affects territorial defense behaviour in fighting fish during parental care after larval hatching, which may have possible implications on reproductive success and population dynamics.

Reprint of: Effects of the antidepressant venlafaxine on fish brain serotonin and predation behavior

Antidepressants that enter receiving waters through final treated wastewater effluent have exhibited relatively low acute toxicity in traditional fish tests at currently measured concentrations. However, the psychotropic mode of action of these compounds warrants examination of the behavioral effects these chemicals may have on aquatic organisms. Previous research has demonstrated that exposure to the antidepressant fluoxetine causes decreased brain serotonin levels in fish and results in a decreased ability to capture prey. Another antidepressant, venlafaxine, has been found at low μg/L concentrations in final treated wastewater effluent. The objective of this study was to quantify the effects of venlafaxine on fish predation behavior and determine if this effect was correlated with changes in brain neurotransmitter concentrations. The predator prey bioassay used hybrid striped bass (Morone saxatilis x Morone chrysops) as the predator and fathead minnows (Pimephales promelas) as prey. Bass were exposed to venlafaxine (0-500 μg/L) for a period of 6 days and then allowed to recover for 6 days. During both exposure and recovery, bass were fed four minnows every third day. The time to capture the minnows was quantified and compared among treatments to determine if there was an effect on predation behavior. Brain tissue was analyzed for serotonin, norepinephrine, and dopamine, to determine the relationship between exposure concentration, brain monoamine levels, and predation behavior. Results indicated that venlafaxine exposures increased time to capture prey 1 and 2 by day 6 for the 250 and 500 μg/L treatments. Time to capture prey 3 was increased for all venlafaxine treatments by day 6. Venlafaxine caused a statistically significant decrease in brain serotonin concentrations that initially decreased in a dose dependent manner before reaching a steady state by the end of exposures for all treatments. No significant, dose-dependent changes in dopamine or norepinephrine were seen. Brain serotonin alone did not adequately explain behavioral results. Serotonin response in other tissues as well as peripheral effects may have accounted for additional behavioral responses after brain serotonin reached a depressed steady state.

Effects of the antidepressant venlafaxine on fish brain serotonin and predation behavior

Antidepressants that enter receiving waters through final treated wastewater effluent have exhibited relatively low acute toxicity in traditional fish tests at currently measured concentrations. However, the psychotropic mode of action of these compounds warrants examination of the behavioral effects these chemicals may have on aquatic organisms. Previous research has demonstrated that exposure to the antidepressant fluoxetine causes decreased brain serotonin levels in fish and results in a decreased ability to capture prey. Another antidepressant, venlafaxine, has been found at low μg/L concentrations in final treated wastewater effluent. The objective of this study was to quantify the effects of venlafaxine on fish predation behavior and determine if this effect was correlated with changes in brain neurotransmitter concentrations. The predator prey bioassay used hybrid striped bass (Morone saxatilis x Morone chrysops) as the predator and fathead minnows (Pimephales promelas) as prey. Bass were exposed to venlafaxine (0-500 μg/L) for a period of 6 days and then allowed to recover for 6 days. During both exposure and recovery, bass were fed four minnows every third day. The time to capture the minnows was quantified and compared among treatments to determine if there was an effect on predation behavior. Brain tissue was analyzed for serotonin, norepinephrine, and dopamine, to determine the relationship between exposure concentration, brain monoamine levels, and predation behavior. Results indicated that venlafaxine exposures increased time to capture prey 1 and 2 by day 6 for the 250 and 500 μg/L treatments. Time to capture prey 3 was increased for all venlafaxine treatments by day 6. Venlafaxine caused a statistically significant decrease in brain serotonin concentrations that initially decreased in a dose dependent manner before reaching a steady state by the end of exposures for all treatments. No significant, dose-dependent changes in dopamine or norepinephrine were seen. Brain serotonin alone did not adequately explain behavioral results. Serotonin response in other tissues as well as peripheral effects may have accounted for additional behavioral responses after brain serotonin reached a depressed steady state.

Serotonin directly stimulates cortisol secretion from the interrenals in goldfish

While serotonin (5-HT) can stimulate the hypothalamic-pituitary-interrenal stress axis in fish, the specific site(s) of 5-HT action are poorly understood. In this study, goldfish (Carassius auratus) were injected intraperitoneally with either saline or the 5-HT1A/7 receptor agonist 8-OH-DPAT at a dose of 100 or 400 μg/kg body weight and sampled 1.5 and 8 h post-injection. Relative to unhandled controls, the saline and 100 μg/kg 8-OH-DPAT treatments elicited similar transient 5- to 7-fold increases in plasma cortisol and the 400 μg/kg 8-OH-DPAT dosage resulted in a sustained 16-fold increase in cortisol levels. Although the 5-HT1A receptor is expressed in the brain preoptic area (POA), the pituitary and the head kidney, neither the saline nor the 8-OH-DPAT treatments affected the mRNA abundance of POA corticotropin-releasing factor and pituitary pro-opiomelanocortin or plasma adrenocorticotropic hormone (ACTH) levels. To assess the direct actions of 5-HT on cortisol secretion relative to those of ACTH, head kidney tissue were superfused with 10(-7)M 5-HT, ACTH or a combined 5-HT/ACTH treatment. Overall, the ACTH and 5-HT/ACTH treatments resulted in higher peak cortisol and total cortisol release than in the 5-HT treatment but the response time to peak cortisol release was shorter in the combined treatment than in either the 5-HT or ACTH alone treatments. Both 8-OH-DPAT and cisapride, a 5-HT4 receptor agonist, also stimulated cortisol release in vitro and their actions were reversed by selective 5-HT1A and 5-HT4 receptor antagonists, respectively. Finally, double-labeling with anti-tyrosine hydroxylase and anti-5-HT revealed that the chromaffin cells of the head kidney contain 5-HT. Thus, in goldfish, 5-HT can directly stimulate cortisol secretion from the interrenals via multiple 5-HT receptor subtypes and the chromaffin cells may be involved in the paracrine regulation of cortisol secretion via 5-HT.

Relationships between radial glial progenitors and 5-HT neurons in the paraventricular organ of adult zebrafish - potential effects of serotonin on adult neurogenesis

In non-mammalian vertebrates, serotonin (5-HT)-producing neurons exist in the paraventricular organ (PVO), a diencephalic structure containing cerebrospinal fluid (CSF)-contacting neurons exhibiting 5-HT or dopamine (DA) immunoreactivity. Because the brain of the adult teleost is known for its neurogenic activity supported, for a large part, by radial glial progenitors, this study addresses the origin of newborn 5-HT neurons in the hypothalamus of adult zebrafish. In this species, the PVO exhibits numerous radial glial cells (RGCs) whose somata are located at a certain distance from the ventricle. To study relationships between RGCs and 5-HT CSF-contacting neurons, we performed 5-HT immunohistochemistry in transgenic tg(cyp19a1b-GFP) zebrafish in which RGCs are labelled with GFP under the control of the cyp19a1b promoter. We show that the somata of the 5-HT neurons are located closer to the ventricle than those of RGCs. RGCs extend towards the ventricle cytoplasmic processes that form a continuous barrier along the ventricular surface. In turn, 5-HT neurons contact the CSF via processes that cross this barrier through small pores. Further experiments using proliferating cell nuclear antigen or 5-bromo-2'-deoxyuridine indicate that RGCs proliferate and give birth to 5-HT neurons migrating centripetally instead of centrifugally as in other brain regions. Furthermore, treatment of adult zebrafish with tryptophan hydroxylase inhibitor causes a significant decrease in the number of proliferating cells in the PVO, but not in the mediobasal hypothalamus. These data point to the PVO as an intriguing region in which 5-HT appears to promote genesis of 5-HT neurons that accumulate along the brain ventricles and contact the CSF.

The toadfish serotonin 2A (5-HT(2A)) receptor: molecular characterization and its potential role in urea excretion

Based on early pharmacological work, the serotonin 2A (5-HT(2A)) receptor subtype is believed to be involved in the regulation of toadfish pulsatile urea excretion. The goal of the following study was to characterize the toadfish 5-HT(2A) receptor at a molecular level, to determine the tissues in which this receptor is predominantly expressed and to further investigate the pharmacological specificity of toadfish pulsatile urea excretion by examining the effect of ketanserin, a 5-HT(2A) receptor antagonist, on resting rates of pulsatile urea excretion. The full-length toadfish 5-HT(2A) receptor encodes a 496 amino acid sequence and shares 57-80% sequence identity to 5-HT(2A) receptors of other organisms, with 100% conservation among important ligand-binding residues. Toadfish 5-HT(2A) receptor mRNA expression was highest in the swim bladder and gonad, followed by the whole brain. All other tissues tested (esophagus, stomach, anterior intestine, posterior intestine, rectum, liver, kidney, heart, muscle and gill) had mRNA expression levels that were significantly less than whole brain. Toadfish 5-HT(2A) receptor mRNA expression within the brain was highest in the hindbrain, telencephalon and midbrain/diencephalon regions. Treatment with the 5-HT(2A) receptor antagonist, ketanserin, resulted in a significant decrease in the pulsatile component of spontaneous urea excretion due to a reduction in urea pulse size with no significant change in pulse frequency. These results lend further support for the 5-HT(2A) receptor in the regulation of pulsatile urea excretion in toadfish.

Effects of hydrostatic pressure on monoaminergic activity in the brain of a tropical wrasse, Halicoeres trimaculatus: possible implication for controlling tidal-related reproductive activity

Most wrasse species in tropical waters exhibit daily spawning synchrony with a preference for high tide. Fish perceive tidal rhythm cues through sensory organs and activate the brain-pituitary-gonadal endocrine axis for synchronous gonadal maturation, although how the tidal-related spawning cycle is controlled endogenously is not known. The purpose of this study was to examine whether hydrostatic pressure has an impact on brain monoamine levels and reproductive activities in the threespot wrasse Halichoeres trimaculatus. The contents of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in the brain were measured using high-performance liquid chromatography and an electrochemical detection system. Exposing the fish to hydrostatic pressure occurring at a 3-m depth (~30 kPa) resulted in an increase in 5-HIAA/5-HT over 3h and a decrease in DOPAC/DA over 6h. No changes in gonadosomatic index or oocyte diameter were observed between the groups when female fish were reared at 0-m and 3-m depth for 3h. Hydrostatic pressure did not alter pituitary mRNA abundance of follicle stimulating hormone-β or luteinizing hormone-β. However, in vitro culture of ovaries from pressurized fish in the presence of human chorionic gonadotropin resulted in an increase in 17α,20β-dihydroxy-4-pregnen-3-one in the medium. These results suggest that hydrostatic pressure activates oocyte maturation through brain monoaminergic activity in this tropical wrasse species.

Calcium and other signalling pathways in neuroendocrine regulation of somatotroph functions

Relative to mammals, the neuroendocrine control of pituitary growth hormone (GH) secretion and synthesis in teleost fish involves numerous stimulatory and inhibitory regulators, many of which are delivered to the somatotrophs via direct innervation. Among teleosts, how multifactorial regulation of somatotroph functions are mediated at the level of post-receptor signalling is best characterized in goldfish. Supplemented with recent findings, this review focuses on the known intracellular signal transduction mechanisms mediating the ligand- and function-specific actions in multifactorial control of GH release and synthesis, as well as basal GH secretion, in goldfish somatotrophs. These include membrane voltage-sensitive ion channels, Na(+)/H(+) antiport, Ca(2+) signalling, multiple pharmacologically distinct intracellular Ca(2+) stores, cAMP/PKA, PKC, nitric oxide, cGMP, MEK/ERK and PI3K. Signalling pathways mediating the major neuroendocrine regulators of mammalian somatotrophs, as well as those in other major teleost study model systems are also briefly highlighted. Interestingly, unlike mammals, spontaneous action potential firings are not observed in goldfish somatotrophs in culture. Furthermore, three goldfish brain somatostatin forms directly affect pituitary GH secretion via ligand-specific actions on membrane ion channels and intracellular Ca(2+) levels, as well as exert isoform-specific action on basal and stimulated GH mRNA expression, suggesting the importance of somatostatins other than somatostatin-14.

Selective uptake and biological consequences of environmentally relevant antidepressant pharmaceutical exposures on male fathead minnows

Antidepressant pharmaceuticals have been reported in wastewater effluent at the nanogram to low microgram-per-liter range, and include bupropion (BUP), fluoxetine (FLX), sertraline (SER), and venlafaxine (VEN). To assess the effects of antidepressants on reproductive anatomy, physiology, and behavior, adult male fathead minnows (Pimephales promelas) were exposed for 21 days either to a single concentration of the antidepressants FLX, SER, VEN, or BUP, or to an antidepressant mixture. The data demonstrated that exposure to VEN (305 ng/L and 1104 ng/L) and SER (5.2 ng/L) resulted in mortality. Anatomical alterations were noted within the testes of fish exposed to SER and FLX, both modulators of the neurotransmitter serotonin. Additionally, FLX at 28 ng/L induced vitellogenin in male fish--a common endpoint for estrogenic endocrine disruption. Significant alterations in male secondary sex characteristics were noted with single exposures. Effects of single compound exposures neither carried over, nor became additive in the antidepressant mixtures, and reproductive behavior was not affected. Analysis of brain tissues from the exposed fish suggested increased uptake of FLX, SER and BUP and minimal uptake of VEN when compared to exposure water concentrations. Furthermore, the only metabolite detected consistently in the brain tissues was norfluoxetine. Similar trends of uptake by brain tissue were observed when fish were exposed to antidepressant mixtures. The present study demonstrates that anatomy and physiology, but not reproductive behavior, can be disrupted by exposure to environmental concentrations of some antidepressants. The observation that antidepressant uptake into fish tissues is selective may have consequences on assessing the mode-of-action and effects of these compounds in future studies.

Pharmaceuticals as neuroendocrine disruptors: lessons learned from fish on Prozac

Pharmaceuticals are increasingly detected in a variety of aquatic systems. One of the most prevalent environmental pharmaceuticals in North America and Europe is the antidepressant fluoxetine, a selective serotonin reuptake inhibitor (SSRI) and the active ingredient of Prozac. Usually detected in the range below 1 μg/L, fluoxetine and its active metabolite norfluoxetine are found to bioaccumulate in wild-caught fish, particularly in the brain. This has raised concerns over potential disruptive effects of neuroendocrine function in teleost fish, because of the known role of serotonin (5-HT) in the modulation of diverse physiological processes such as reproduction, food intake and growth, stress and multiple behaviors. This review describes the evolutionary conservation of the 5-HT transporter (the therapeutic target of SSRIs) and reviews the disruptive effects of fluoxetine on several physiological endpoints, including involvement of neuroendocrine mechanisms. Studies on the goldfish, Carassius auratus, whose neuroendocrine regulation of reproduction and food intake are well characterized, are described and represent a reliable model to study neuroendocrine disruption. In addition, fish studies investigating the effects of fluoxetine, not only on reproduction and food intake, but also on stress and behavior, are discussed to complement the emerging picture of neuroendocrine disruption of physiological systems in fish exposed to fluoxetine. Environmental relevance and key lessons learned from the effects of the antidepressant fluoxetine on fish are highlighted and may be helpful in designing targeted approaches for future risk assessments of pharmaceuticals disrupting the neuroendocrine system in general.

Tryptophan hydroxylase: a target for neuroendocrine disruption

Tryptophan hydroxylase (TPH), the rate-limiting enzyme in serotonin (5-HT) synthesis, performs an essential role in the maintenance of serotonergic functions in the central nervous system (CNS), including regulation of the neuroendocrine system controlling reproduction. The results of recent studies in a teleost model of neuroendocrine disruption, Atlantic croaker, indicated that hypothalamic TPH is a major site of interference of hypothalamic-pituitary-gonadal function by environmental stressors. The effects of exposure to two different types of environmental stressors, low dissolved oxygen (hypoxia) and a polychlorinated biphenyl mixture (Aroclor 1254), on the stimulatory brain serotonergic system controlling reproductive neuroendocrine function in Atlantic croaker are reviewed. Exposure to both stressors produced decreases in TPH activity, which were accompanied by a fall in hypothalamic 5-HT and gonadotropin-releasing hormone (GnRH I) content in the preoptic-anterior hypothalamic area and were associated with reduction in luteinizing hormone (LH) secretion and gonadal development. Pharmacological restoration of hypothalamic 5-HT levels after exposure to both stressors also restored neuroendocrine and reproductive functions, indicating that the serotonergic system is an important site for hypoxia- and Aroclor 1254-induced inhibition of reproductive neuroendocrine functions. The mechanisms underlying downregulation of TPH activity by these stressors remain unclear but may involve alterations in hypothalamic antioxidant status. In support of this hypothesis, treatment with an antioxidant, vitamin E, was found to reverse the inhibitory effects of Aroclor 1254 on TPH activity. The results suggest that TPH is a major target for neuroendocrine disruption by diverse environmental stressors.

Waterborne fluoxetine disrupts the reproductive axis in sexually mature male goldfish, Carassius auratus

Fluoxetine (FLX) is a pharmaceutical acting as a selective serotonin reuptake inhibitor and is used to treat depression in humans. Fluoxetine and the major active metabolite norfluoxetine (NFLX) are released to aquatic systems via sewage-treatment effluents. They have been found to bioconcentrate in wild fish, raising concerns over potential endocrine disrupting effects. The objective of this study was to determine effects of waterborne FLX, including environmental concentrations, on the reproductive axis in sexually mature male goldfish. We initially cloned the goldfish serotonin transporter to investigate tissue and temporal expression of the serotonin transporter, the FLX target, in order to determine target tissues and sensitive exposure windows. Sexually mature male goldfish, which showed the highest levels of serotonin transporter expression in the neuroendocrine brain, were exposed to FLX at 0.54μg/L and 54μg/L in a 14-d exposure before receiving vehicle or sex pheromone stimulus consisting of either 4.3nM 17,20β-dihydroxy-4-pregnene-3-one (17,20P) or 3nM prostaglandin F₂(α) (PGF₂(α)). Reproductive endpoints assessed included gonadosomatic index, milt volume, and blood levels of the sex steroids testosterone and estradiol. Neuroendocrine function was investigated by measuring blood levels of luteinizing hormone, growth hormone, pituitary gene expression of luteinizing hormone, growth hormone and follicle-stimulating hormone and neuroendocrine brain expression of isotocin and vasotocin. To investigate changes at the gonadal level of the reproductive axis, testicular gene expression of the gonadotropin receptors, both the luteinizing hormone receptor and the follicle-stimulating hormone receptor, were measured as well as expression of the growth hormone receptor. To investigate potential impacts on spermatogenesis, testicular gene expression of the spermatogenesis marker vasa was measured and histological samples of testis were analyzed qualitatively. Estrogen indices were measured by expression and activity analysis of gonadal aromatase, as well as liver expression analysis of the estrogenic marker, esr1. After 14d, basal milt volume significantly decreased at 54μg/L FLX while pheromone-stimulated milt volume decreased at 0.54μg/L and 54μg/L FLX. Fluoxetine (54μg/L) inhibited both basal and pheromone-stimulated testosterone levels. Significant concentration-dependent reductions in follicle-stimulating hormone and isotocin expression were observed with FLX in the 17,20P- and PGF₂(α)-stimulated groups, respectively. Estradiol levels and expression of esr1 concentration-dependently increased with FLX. This study demonstrates that FLX disrupts reproductive physiology of male fish at environmentally relevant concentrations, and potential mechanisms are discussed.

Understanding the multifactorial control of growth hormone release by somatotropes: lessons from comparative endocrinology

Control of postnatal growth is the main, but not the only, role for growth hormone (GH) as this hormone also contributes to regulating metabolism, reproduction, immunity, development, and osmoregulation in different species. Likely owing to this variety of group-specific functions, GH production is differentially regulated across vertebrates, with an apparent evolutionary trend to simplification, especially in the number of stimulatory factors governing substantially GH release. Thus, teleosts exhibit a multifactorial regulation of GH secretion, with a number of factors, from the newly discovered fish GH-releasing hormone (GHRH) to pituitary adenylate cyclase-activating peptide (PACAP) but also gonadotropin-releasing hormone, dopamine, corticotropin-releasing hormone, and somatostatin(s) directly controlling somatotropes. In amphibians and reptiles, GH secretion is primarily stimulated by the major hypothalamic peptides GHRH and PACAP and inhibited by somatostatin(s), while other factors (ghrelin, thyrotropin-releasing hormone) also influence GH release. Finally, in birds and mammals, primary control of GH secretion is exerted by a dual interplay between GHRH and somatostatin. In addition, somatotrope function is modulated by additional hypothalamic and peripheral factors (e.g., ghrelin, leptin, insulin-like growth factor-I), which together enable a balanced integration of feedback signals related to processes in which GH plays a relevant regulatory role, such as metabolic and energy status, reproductive, and immune function. Interestingly, in contrast to the high number of stimulatory factors impinging upon somatotropes, somatostatin(s) stand(s) as the main primary inhibitory regulator(s) for this cell type.

Serotonin, social status and sex change in the bluebanded goby Lythrypnus dalli

In a variety of vertebrates, highly aggressive individuals tend to have high social status and low serotonergic function. In the sex changing fish Lythrypnus dalli, serotonin (5-HT) may be involved as a mediator between the social environment and the reproductive system because social status is a critical cue in regulating sex change. Subordination inhibits sex change in L. dalli, and it is associated with higher serotonergic activity in other species. We tested the hypothesis that high serotonergic activity has an inhibitory effect on sex change. In a social situation permissive to sex change, we administered to the dominant female implants containing the serotonin precursor 5-hydroxytryptophan (5-HTP). In a social situation not conducive to sex change, we administered either the serotonin synthesis inhibitor p-chlorophenylalanine (PCPA) or the 5-HT(1A) receptor antagonist p-MPPI. After three weeks we used HPLC to measure brain levels of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). We also performed PCPA, p-MPPI and fluoxetine injections in size-matched pairs of females to assess its effect on dominance status. Males and newly sex changed fish showed a trend for higher levels of 5-HIAA and 5-HT/5-HIAA ratio than females. The different implants treatments did not affect the probability of sex change. Interestingly, this species does not seem to fit the pattern seen in other vertebrates where dominant individuals have lower serotonergic activity than subordinates.

Effects of fluoxetine on the reproductive axis of female goldfish (Carassius auratus)

We investigated the effects of fluoxetine, a selective serotonin reuptake inhibitor, on neuroendocrine function and the reproductive axis in female goldfish. Fish were given intraperitoneal injections of fluoxetine twice a week for 14 days, resulting in five injections of 5 μg fluoxetine/g body wt. We measured the monoamine neurotransmitters serotonin, dopamine, and norepinephrine in addition to their metabolites with HPLC. Homovanillic acid, a metabolite in the dopaminergic pathway, increased significantly in the hypothalamus. Plasma estradiol levels were measured by radioimmunoassay and were significantly reduced approximately threefold after fluoxetine treatment. We found that fluoxetine also significantly reduced the expression of estrogen receptor (ER)β1 mRNA by 4-fold in both the hypothalamus and the telencephalon and ERα mRNA by 1.7-fold in the telencephalon. Fluoxetine had no effect on the expression of ERβ2 mRNA in the hypothalamus or telencephalon. Microarray analysis identified isotocin, a neuropeptide that stimulates reproductive behavior in fish, as a candidate gene affected by fluoxetine treatment. Real-time RT-PCR verified that isotocin mRNA was downregulated approximately sixfold in the hypothalamus and fivefold in the telencephalon. Intraperitoneal injection of isotocin (1 μg/g) increased plasma estradiol, providing a potential link between changes in isotocin gene expression and decreased circulating estrogen in fluoxetine-injected fish. Our results reveal targets of serotonergic modulation in the neuroendocrine brain and indicate that fluoxetine has the potential to affect sex hormones and modulate genes involved in reproductive function and behavior in the brain of female goldfish. We discuss these findings in the context of endocrine disruption because fluoxetine has been detected in the environment.

Molecular cloning, characterization and expression of two tryptophan hydroxylase (TPH-1 and TPH-2) genes in the hypothalamus of Atlantic croaker: down-regulation after chronic exposure to hypoxia

Recently we discovered that hypoxia causes marked impairment of reproductive neuroendocrine function in Atlantic croaker, a marine teleost, which is due to a decline in hypothalamic serotonergic activity. As a first step in understanding the molecular responses of the hypothalamic serotonergic system to hypoxia, we cloned and characterized the genes for the enzymes regulating the rate-limiting step in serotonin biosynthesis, tryptophan hydroxylase (TPH-1 and TPH-2) in the croaker brain. The full-length croaker TPH-1 and TPH-2 cDNAs contain open reading frames encoding proteins with 479 and 487 amino acids, respectively, which are highly homologous to the TPH-1 (76-93%) and TPH-2 (64-92%) proteins of other vertebrates. Croaker TPH-1 and TPH-2 mRNA expression was detected throughout the brain but was greatest in the hypothalamic region. Both Northern blot analysis and real-time PCR showed that TPH-1 (transcript size approximately 2.1 kb) and TPH-2 ( approximately 1.9 kb) mRNA levels were significantly decreased in the hypothalami of croaker exposed for 2 weeks to hypoxic conditions compared with those in fish exposed to normoxic conditions. Immunohistochemistry of hypothalamic neurons with TPH antibodies showed reduced expression of TPHs in hypoxia-exposed fish compared with normoxic fish. Western blot analysis confirmed that hypoxia caused a marked decline in hypothalamic TPH protein levels, which was associated with decreases in hypothalamic TPH enzyme activity and 5-hydroxytryptophan levels. These results suggest that TPH is a major site of hypoxia-induced down-regulation of serotonergic function in croaker brains. Moreover, they provide the first evidence that hypoxia decreases the expression of TPH transcripts in vertebrate brains.

The goldfish (Carassius auratus) as a model for neuroendocrine signaling

Goldfish (Carassius auratus) are excellent model organisms for the neuroendocrine signaling and the regulation of reproduction in vertebrates. Goldfish also serve as useful model organisms in numerous other fields. In contrast to mammals, teleost fish do not have a median eminence; the anterior pituitary is innervated by numerous neuronal cell types and thus, pituitary hormone release is directly regulated. Here we briefly describe the neuroendocrine control of luteinizing hormone. Stimulation by gonadotropin-releasing hormone and a multitude of classical neurotransmitters and neuropeptides is opposed by the potent inhibitory actions of dopamine. The stimulatory actions of gamma-aminobutyric acid and serotonin are also discussed. We will focus on the development of a cDNA microarray composed of carp and goldfish sequences which has allowed us to examine neurotransmitter-regulated gene expression in the neuroendocrine brain and to investigate potential genomic interactions between these key neurotransmitter systems. We observed that isotocin (fish homologue of oxytocin) and activins are regulated by multiple neurotransmitters, which is discussed in light of their roles in reproduction in other species. We have also found that many novel and uncharacterized goldfish expressed sequence tags in the brain are also regulated by neurotransmitters. Their sites of production and whether they play a role in neuroendocrine signaling and control of reproduction remain to be determined. The transcriptomic tools developed to study reproduction could also be used to advance our understanding of neuroendocrine-immune interactions and the relationship between growth and food intake in fish.