Role of serotonin in fish reproduction

Environmental fluoxetine promotes skin cell proliferation and wound healing

This study investigates the effects of environmentally-relevant concentrations of fluoxetine (FLX, commercial name: Prozac) on wound healing. Pollution of water systems with pharmaceutical and personal care products, including antidepressants such as FLX and other selective serotonin reuptake inhibitors, is a growing environmental concern. Environmentally-relevant FLX concentrations are known to impact physiological functions and behaviour of aquatic animals, however, the effects of exposure on humans are currently unknown. Using a combination of human skin biopsies and a human keratinocyte cell line, we show that exposure to environmental FLX promotes wound closure. We show dose-dependent increases in wound closure with FLX concentrations from 125 ng/l. Using several -omics and pharmaceutical approaches, we demonstrate that the mechanisms underlying enhanced wound closure are increased cell proliferation and serotonin signalling. Transcriptomic analysis revealed 350 differentially expressed genes after exposure. Downregulated genes were enriched in pathways related to mitochondrial function and metabolism, while upregulated genes were associated with cell proliferation and tissue morphogenesis. Kinase profiling showed altered phosphorylation of kinases linked to the MAPK pathway. Consistent with this, phosphoproteomic analyses identified 235 differentially phosphorylated proteins after exposure, with enriched GO terms related to cell cycle, division, and protein biosynthesis. Treatment of skin biopsies and keratinocytes with ketanserin, a serotonin receptor antagonist, reversed the increase in wound closure observed upon exposure. These findings collectively show that exposure to environmental FLX promotes wound healing through modulating serotonin signalling, gene expression and protein phosphorylation, leading to enhanced cell proliferation. Our results justify a transition from the study of behavioural effects of environmental FLX in aquatic animals to the investigation of effects of exposure on wound healing in aquatic and terrestrial animals, including direct impacts on human health.

Genome-wide identification of Gα family in grass carp (Ctenopharyngodon idella) and reproductive regulation functional characteristics of Cignaq

BACKGROUND

The Gα family plays a crucial role in the complex reproductive regulatory network of teleosts. However, the characterization and function of Gα family members, especially Gαq, remain poorly understood in teleosts. To analyze the characterization, expression, and function of grass carp (Ctenopharyngodon idella) Gαq, we identified the Gα family members in grass carp genome, and analyzed the expression, distribution, and signal transduction of Gαq/gnaq. We also explored the role of Gαq in the reproductive regulation of grass carp.

RESULTS

Our results showed that the grass carp genome contains 27 Gα genes with 46 isoforms, which are divided into four subfamilies: Gαs, Gαi/o, Gαq/11, and Gα12/13. The expression level of Cignaq in the testis was the highest and significantly higher than in other tissues, followed by the hypothalamus and brain. The luteinizing hormone receptor (LHR) was mainly localized to the nucleus in grass carp oocytes, with signals also present in follicular cells. In contrast, Gαq signal was mainly found in the cytoplasm of oocytes, with no signal in follicular cells. In the testis, Gαq and LHR were co-localized in the cytoplasm. Furthermore, the grass carp Gαq recombinant protein significantly promoted Cipgr expression.

CONCLUSIONS

These results provided preliminary evidence for understanding the role of Gαq in the reproductive regulation of teleosts.

Effect of immunization against OPN5 on the reproductive performance in Shan Partridge ducks under different photoperiods

Photoperiod is an important environmental factor that influences seasonal reproduction behavior in birds. Birds translate photoperiodic information into neuroendocrine signals through deep brain photoreceptors (DBPs). OPN5 has been considered candidate DBPs involved in regulating seasonal reproduction in birds. We found that OPN5 could mediate light to regulate the follicle development in ducks. In this study, we further verified the effect of OPN5 on follicular development in Shan Partridge ducks by immunizing against the extracellular domain (ECD) of OPN5. We investigated the specific regulatory mechanism of photoperiod mediated by OPN5 on the reproductive activity of ducks. The trial randomly divided 120 Shan Partridge ducks into 3 groups with different treatments: the immunization of OPN5 group was done at d0, d15, d30, and d40 with 1 mL of vaccine containing OPN5 protein (thus containing 1, 1, 0.5, and 0.5 mg of OPN5-KLH protein), and the control group (CS and CL groups) was injected at the same time with the same dose of OPN5-uncontained blank vaccine. The group of CS (900 lux), OPN5 (600 lux), and CL (600 lux) lasted for 40 d in 12 L:12 D photoperiods, respectively. Then, the groups of CS, OPN5, and CL subsequently received 12 L:12 D, 12 L:12 D, and 17 L:7 D light treatments for 33 d, respectively. The ducks were caged in 3 constant rooms with the same feeding conditions for each group, free water, and limited feeding (150 g per duck each day). Duck serum and tissue samples were collected at d 40, d 62, and d 73 (n = 12). It was found that before prolonged light, the group of immunization (group OPN5) and the group of strong light intensity (group CS) were higher than the group of CL in egg production. Subsequent to prolonged light, the group CL in egg production rose about the same as the group immunization, while the strong light group (group CS) was lower. Group OPN5 increased the ovarian index of ducks, and both the immunization of group OPN5 and group CL (extended light) increased the thickness of the granular layer and promoted the secretion of E2, P4, LH, and PRL hormones. Compared with group CS, group CL and OPN5 increased the mRNA level and protein expression of OPN5 in the hypothalamus on d 62 and d 73 (P < 0.05). The gene or protein expression patterns of GnRH, TRH, TSHβ, DIO2, THRβ, VIP, and PRL were positively correlated with OPN5, whereas the gene expression patterns of GnIH and DIO3 were negatively correlated with OPN5. The results showed that immunization against OPN5 could activate the corresponding transmembrane receptors to promote the expression of OPN5, up-regulate the expression of TSHβ and DIO2, and then regulate the HPG axis-related genes to facilitate the follicular development of Shan Partridge ducks. In addition, in this experiment, prolonging the photoperiod or enhancing the light intensity could also enhance follicle development, but the effect was not as significant as immunizing against OPN5. Our results will offer beneficial data and more supportive shreds of evidence in favor of elucidating the role of OPN5 in relation to photoperiods and reproduction.

Chronic exposure to tris (2-chloroethyl) phosphate (TCEP) induces brain structural and functional changes in zebrafish (Danio rerio): A comparative study on the environmental and LC50 concentrations of TCEP

Tris (2-chloroethyl) phosphate (TCEP) is a crucial organophosphorus flame retardant widely used in many industrial and commercial products. Available reports reported that TCEP could cause various toxicological effects on organisms, including humans. Unfortunately, toxicity data for TCEP (particularly on neurotoxicity) on aquatic organisms are lacking. In the present study, Danio rerio were exposed to different concentrations of TCEP for 42 days (chronic exposure), and oxidative stress, neurotoxicity, sodium, potassium-adenosine triphosphatase (Na+, K+-ATPase) activity, and histopathological changes were evaluated in the brain. The results showed that TCEP (100 and 1500 µg L-1) induced oxidative stress and significantly decreased the activities of antioxidant enzymes (SOD, CAT and GR) in the brain tissue of zebrafish. In contrast, the lipid peroxidation (LPO) level was increased compared to the control group. Exposure to TCEP inhibited the acetylcholinesterase (AChE) and Na+,K+-ATPase activities in the brain tissue. Brain histopathology after 42 days of exposure to TCEP showed cytoplasmic vacuolation, inflammatory cell infiltration, degenerated neurons, degenerated purkinje cells and binucleate. Furthermore, TCEP exposure leads to significant changes in dopamine and 5-HT levels in the brain of zebrafish. The data in the present study suggest that high concentrations of TCEP might affect the fish by altering oxidative balance and inducing marked pathological changes in the brain of zebrafish. These findings indicate that chronic exposure to TCEP may cause a neurotoxic effect in zebrafish.

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.

A PITX2-HTR1B pathway regulates the asymmetric development of female gonads in chickens

All female vertebrates develop a pair of ovaries except for birds, in which only the left gonad develops into an ovary, whereas the right gonad regresses. Previous studies found that the transcription factor Paired-Like Homeodomain 2 (PITX2), a key mediator for left/right morphogenesis in vertebrates, was also implicated in asymmetric gonadal development in chickens. In this study, we systematically screened and validated the signaling pathways that could be targeted by Pitx2 to control unilateral gonad development. Integrated chromatin immunoprecipitation sequencing (ChIP-seq) and RNA sequencing (RNA-seq) analyses indicated that Pitx2 directly binds to the promoters of genes encoding neurotransmitter receptors and leads to left-biased expression of both serotonin and dopamine receptors. Forcibly activating serotonin receptor 5-Hydroxytryptamine Receptor 1B (HTR1B) signaling could induce ovarian gene expression and cell proliferation to partially rescue the degeneration of the right gonad. In contrast, inhibiting serotonin signaling could block the development of the left gonad. These findings reveal a PITX2-HTR1B genetic pathway that guides the left-specific ovarian growth in chickens. We also provided new evidence showing neurotransmitters stimulate the growth of nonneuronal cells during the early development of reproductive organs well before innervation.

Short daylight photoperiod alleviated alarm substance-stimulated fear response of zebrafish

Photoperiod has been well-documented to be involved in regulating many activities of animals. However, whether photoperiod takes part in mood control, such as fear response in fish and the underlying mode(s) of action remain unclear. In this study, adult zebrafish males and females (Danio rerio) were exposed to different photoperiods, Blank (12 h light: 12 h dark), Control (12 h light: 12 h dark), Short daylight (SD, 6 h light: 18 h dark) and Long daylight (LD, 18 h light: 6 h dark) for 28 days. After exposure, fear response of the fish was investigated using a novel tank diving test. After alarm substance administration, the onset to higher half, total duration in lower half and duration of freezing in SD-fish were significantly decreased, suggesting that short daylight photoperiod is capable of alleviating fear response in zebrafish. In contrast, comparing with the Control, LD didn't show significant effect on fear response of the fish. Further investigation revealed that SD increased the levels of melatonin (MT), serotonin (5-HT) and dopamine (DA) in the brain while decreased the plasma level of cortisol comparing to the Control. Moreover, the expressions of genes in MT, 5-HT and DA pathways and HPI axis were also altered consistently. Our data indicated that short daylight photoperiod might alleviate fear response of zebrafish probably through interfering with MT/5-HT/DA pathways and HPI axis.

Investigating the genetic basis of vertebrate dispersal combining RNA-seq, RAD-seq and quantitative genetics

Although animal dispersal is known to play key roles in ecological and evolutionary processes such as colonization, population extinction and local adaptation, little is known about its genetic basis, particularly in vertebrates. Untapping the genetic basis of dispersal should deepen our understanding of how dispersal behaviour evolves, the molecular mechanisms that regulate it and link it to other phenotypic aspects in order to form the so-called dispersal syndromes. Here, we comprehensively combined quantitative genetics, genome-wide sequencing and transcriptome sequencing to investigate the genetic basis of natal dispersal in a known ecological and evolutionary model of vertebrate dispersal: the common lizard, Zootoca vivipara. Our study supports the heritability of dispersal in semi-natural populations, with less variation attributable to maternal and natal environment effects. In addition, we found an association between natal dispersal and both variation in the carbonic anhydrase (CA10) gene, and in the expression of several genes (TGFB2, SLC6A4, NOS1) involved in central nervous system functioning. These findings suggest that neurotransmitters (serotonin and nitric oxide) are involved in the regulation of dispersal and shaping dispersal syndromes. Several genes from the circadian clock (CRY2, KCTD21) were also differentially expressed between disperser and resident lizards, supporting that the circadian rhythm, known to be involved in long-distance migration in other taxa, might affect dispersal as well. Since neuronal and circadian pathways are relatively well conserved across vertebrates, our results are likely to be generalisable, and we therefore encourage future studies to further investigate the role of these pathways in shaping dispersal in vertebrates.

Sex-Inclined Piwi-Interacting RNAs in Serum Exosomes for Sex Determination in the Greater Amberjack (Seriola dumerili)

The greater amberjack (Seriola dumerili) is a gonochoristic fish with no sexual dimorphism in appearance, making sex identification difficult. Piwi-interacting RNAs (piRNAs) function in transposon silencing and gametogenesis and are involved in various physiological processes, including sex development and differentiation. Exosomal piRNAs can be indicators for the determination of sex and physiological status. In this study, four piRNAs were differentially expressed in both serum exosomes and gonads between male and female greater amberjack. Three piRNAs (piR-dre-32793, piR-dre-5797, and piR-dre-73318) were significantly up-regulated and piR-dre-332 was significantly down-regulated in serum exosomes and gonads of male fish, compared to female fish, consistent with the serum exosomal results. According to the relative expression of four marker piRNAs derived from the serum exosomes of greater amberjack, the highest relative expression of piR-dre-32793, piR-dre-5797, and piR-dre-73318 in seven female fish and that of piR-dre-332 in seven male fish can be used as the standard for sex determination. The method of sex identification can ascertain the sex of greater amberjack by blood collection from the living body, without sacrificing fish. The four piRNAs did not show sex-inclined expression in the hypothalamus, pituitary, heart, liver, intestine, and muscle tissue. A piRNA-target interaction network involving 32 piRNA-mRNA pairs was generated. Sex-related target genes were enriched in sex-related pathways, including oocyte meiosis, transforming growth factor-beta signaling pathway, progesterone-mediated oocyte maturation, and gonadotropin releasing hormone signaling pathway. These results provide a basis for sex determination in greater amberjack and improve our understanding of the mechanisms underlying sex development and differentiation in the species.

Sex-Specific Effects of Blood Serotonin on Reproductive Effort in a Small Passerine

AbstractLaboratory animal models have shown that blood serotonin levels reflect consistent individual differences in behavioral decision-making and maternal behavior. Serotonin could also help to understand intraspecific variation in reproductive strategies, although the mechanisms are poorly understood. In this study, the relationships of plasma serotonin with breeding parameters and parental behavior were examined in wild great tits (Parus major). Females who laid eggs earlier had higher levels of serotonin in the second half of the nestling period, while no significant relationship of serotonin with clutch size, brood size, and body size was detected. In males, serotonin levels were negatively related to clutch size and brood size and positively related to body size. The association of serotonin with provisioning behavior was sex specific, and acute fear stress induced by a predator presentation did not change this relationship. Food provisioning was positively related to size-corrected serotonin levels in females and negatively related to size-corrected serotonin levels in males. These results suggest that peripheral serotonin is a sensitive marker of parental behavior and reproductive effort in wild birds, while the mechanisms linking this neurotransmitter to reproduction are probably mediated by interplay between the serotonergic system, sex hormones, and other neurotransmitters.

The serotonin circuit that coordinates germline proliferation and egg laying with other reproductive functions in Caenorhabditis elegans

Behaviour and physiology are altered in reproducing animals, but neuronal circuits that regulate these changes remain largely unknown. Insights into mechanisms that regulate and possibly coordinate reproduction-related traits could be gleaned from the study of sex pheromones that can improve the reproductive success of potential mating partners. In Caenorhabditis elegans, the prominent male pheromone, ascr#10, modifies reproductive behaviour and several aspects of reproductive physiology in hermaphrodite recipients, including improving oocyte quality. Here we show that a circuit that contains serotonin-producing and serotonin-uptaking neurons plays a key role in mediating effects of ascr#10 on germline development and egg laying behaviour. We also demonstrate that increased serotonin signalling promotes proliferation of germline progenitors in adult hermaphrodites. Our results establish a role for serotonin in maintaining germline quality and highlight a simple neuronal circuit that acts as a linchpin that couples food intake, mating behaviour, reproductive output, and germline renewal and provisioning.

Adverse effects of herbicides in freshwater Neotropical fish: A review

Although herbicides have been developed to act on the physiological processes of plants, they are responsible for causing deleterious effects on animals. These chemical compounds are widely used throughout the world, but especially in countries that export agricultural products such as Central and South America, their use has increased in recent years. Aquatic environments are natural reservoirs of herbicides, which after being applied on crops, run off through the soil reaching rivers, lakes, and oceans. Fish are among the many organisms affected by the contamination of aquatic environments caused by herbicides. These animals play an important ecological role and are a major source of food for humans. However, few studies address the effects of herbicides on fish in this region. Thus, in the present review we discuss the morphophysiological and molecular consequences of herbicide exposure in Neotropical fish systems as well as how the environmental and land use characteristics in this region can influence the toxicity of these pollutants. A toxicity pathway framework was developed summarizing the mechanisms by which herbicides act and endpoints that need to be further investigated.

Low doses and lifecycle exposure of waterborne antidepressants in zebrafish model: A survey on sperm traits, reproductive behaviours, and transcriptome responses

Venlafaxine and citalopram have been commonly found in surface water and may disrupt fish reproduction, yet the long-term impact and the underlying mechanism are largely unknown. Here, zebrafish were exposed to 0.1-100 μg/L venlafaxine and citalopram for their entire life cycle from embryo to adult, respectively. After exposure for 180 days, the lowest observable effective concentration (LOEC) of venlafaxine and citalopram to significantly reduce the mean number of egg production in adults were 10 and 1 μg/L, respectively, whereas the fertilization rate displayed no significant changes. Further, we investigated the impacts of venlafaxine and citalopram in a reproductive context, including sperm quality and reproductive behaviour. In contrast, venlafaxine and citalopram exposure did not affect sperm quality but caused a reduction of reproductive behaviour (e.g., mating duration and mating interval) of adults exposed to 1-10 μg/L of the antidepressant. Transcriptomic profiling of the whole ovary revealed that lifecycle venlafaxine and citalopram exposure significantly affected the Na⁺/Cl^- dependent neurotransmitter transporters signaling. Moreover, immune system-mediated ovarian regeneration and creatine metabolism regulated energy metabolism were proposed as the novel mechanism in the observed effects. Taken together, our results highlight the risk of lifecycle venlafaxine and citalopram exposure to fish reproduction and provide novel perspectives for unveiling the mechanism of female reproductive dysfunction.

Serotonin system is partially involved in immunomodulation of Nile tilapia (Oreochromis niloticus) immune cells

Serotonin (5-hydroxytryptamine) is a well-known neurotransmitter affecting emotion, behavior, and cognition. Additionally, numerous immunomodulatory functions of serotonin have been discovered in mammals. However, the regulatory role of the serotonin system in fish immunity remains unclear. In this study, various serotonergic markers in Nile tilapia (Oreochromis niloticus) were identified and characterized. The involvement of the serotonin system during bacterial infection was investigated. Moreover, the expression characteristics and specific functions of serotonergic markers within Nile tilapia immune cells were also assessed. Overall, 22 evolutionarily conserved serotonergic marker genes in Nile tilapia were cloned and characterized. Transcriptional levels of these molecules were most abundant in the brain, and their transcripts were induced during Streptococcus agalactiae infection. Nevertheless, few serotonergic markers exist on Nile tilapia immune cells, and no distinct immunomodulation effect was observed during an immune response. The present study lays a theoretical foundation for further investigation of the immunological mechanisms in fish as well as the evolution of the serotonin system in animals.

Deletion of the Serotonin Receptor 7 Gene Changed the Development and Behavior of the Mosquito, Aedes aegypti

Serotonin (5-HT) plays a vital role in many physiological processes in insects, regulating physiological activities such as growth and movement through multiple 5-HT receptors (5-HTRs), which were potential targets for some new insecticides. However, the specific function of individual 5-HTRs in Ae. aegypti is still unclear. In this study, we investigated the function of the 5-HT7A receptor during Ae. aegypti development. 5-HTR7A transcripts were detected at all stages of development by real-time PCR. The results indicated that the gene expression was highest in the limbs (p < 0.01). We also generated 5-HTR7A mutant mosquitoes using CRISPR-mediated gene editing. The mutants had an abnormal phenotype at the larval stage, including an aberrant head-to-chest ratio and decreased motor activity. The mutant pupae developed abnormally, and most died (56.67%) (p < 0.0001). Using external stimuli to larvae and pupae with abnormal phenotypes, we found the mutant G1 and G2 generations responded to external stimuli in a longer time than the wild-type (WT) mosquitoes, and most of the mutants were 2 to 3 s slower than the WTs to respond to external stimuli (p < 0.01). Due to higher mortality, mutant larvae and pupae had fewer numbers than the WTs. The egg hatching rate of mutant G1 and G2 generations was lower than that of the WTs (p < 0.01). The expression level of 5-HTR7A in the mutants decreased by about 65% compared with the control group using real-time PCR (p < 0.05). In all, the 5-HT7A receptor plays an important role in the metamorphosis, development and motor function of Aedes aegypti.

Transcriptomic Profiling Revealed Signaling Pathways Associated with the Spawning of Female Zebrafish under Cold Stress

As one of the critical abiotic factors, temperature controls fish development and reproduction. However, the effects of low temperature on the transcriptional regulation of zebrafish reproduction remain largely unclear. In this study, the fecundity of zebrafish was examined after exposure to cold temperatures at 19.5 °C, 19 °C, 18.5 °C, or 18 °C. The temperature at 19 °C showed no significant influence on the fecundity of zebrafish, but temperature at 18.5 °C or 18 °C significantly blocked the spawning of females, suggesting the existence of a low temperature critical point for the spawning of zebrafish females. Based on these observations, the brains of anesthetized fish under cold stress at different cold temperatures were collected for high-throughput RNA-seq assays. Key genes, hub pathways and important biological processes responding to cold temperatures during the spawning of zebrafish were identified through bioinformatic analysis. The number of down-regulated and up-regulated genes during the temperature reduction from egg-spawning temperatures at 19.5 °C and 19 °C to non-spawning temperatures at 18.5 °C and 18 °C were 2588 and 2527 (fold change ≥ 1.5 and p-value ≤ 0.01), respectively. Venn analysis was performed to identify up- and down-regulated key genes. KEGG enrichment analysis indicated that the hub pathways overrepresented among down-regulated key genes included the GnRH signaling pathway, vascular smooth muscle contraction, C-type lectin receptor signaling pathway, phosphatidylinositol signaling system and insulin signaling pathway. GO enrichment analysis of down-regulated key genes revealed the most important biological processes inhibited under non-spawning temperatures at 18.5 °C and 18 °C were photoreceptor cell outer segment organization, circadian regulation of gene expression and photoreceptor cell maintenance. Furthermore, 99 hormone-related genes were found in the brain tissues of non-spawning and spawning groups, and GnRH signaling pathway and insulin signaling pathway were enriched from down-regulated genes related to hormones at 18.5 °C and 18 °C. Thus, these findings uncovered crucial hormone-related genes and signaling pathways controlling the spawning of female zebrafish under cold stress.

Antidepressants as Endocrine Disrupting Compounds in Fish

As antidepressant usage by the global population continues to increase, their persistent detection in aquatic habitats from municipal wastewater effluent release has led to concerns of possible impacts on non-target organisms, including fish. These pharmaceuticals have been marketed as mood-altering drugs, specifically targeting the monoaminergic signaling in the brain of humans. However, the monoaminergic systems are highly conserved and involved in the modulation of a multitude of endocrine functions in vertebrates. While most studies exploring possible impact of antidepressants on fish have focused on behavioural perturbations, a smaller spotlight has been placed on the endocrine functions, especially related to reproduction, growth, and the stress response. The purpose of this review is to highlight the possible role of antidepressants as endocrine disruptors in fish. While studies linking the effects of environmentally relevant levels of antidepressant on the endocrine system in fish are sparse, the emerging evidence suggests that early-life exposure to these compounds have the potential to alter the developmental programming of the endocrine system, which could persist as long-term and multigenerational effects in teleosts.

An Emerging Cross-Species Marker for Organismal Health: Tryptophan-Kynurenine Pathway

Tryptophan (TRP) is an essential dietary amino acid that, unless otherwise committed to protein synthesis, undergoes metabolism via the Tryptophan-Kynurenine (TRP-KYN) pathway in vertebrate organisms. TRP and its metabolites have key roles in diverse physiological processes including cell growth and maintenance, immunity, disease states and the coordination of adaptive responses to environmental and dietary cues. Changes in TRP metabolism can alter the availability of TRP for protein and serotonin biosynthesis as well as alter levels of the immune-active KYN pathway metabolites. There is now considerable evidence which has shown that the TRP-KYN pathway can be influenced by various stressors including glucocorticoids (marker of chronic stress), infection, inflammation and oxidative stress, and environmental toxicants. While there is little known regarding the role of TRP metabolism following exposure to environmental contaminants, there is evidence of linkages between chemically induced metabolic perturbations and altered TRP enzymes and KYN metabolites. Moreover, the TRP-KYN pathway is conserved across vertebrate species and can be influenced by exposure to xenobiotics, therefore, understanding how this pathway is regulated may have broader implications for environmental and wildlife toxicology. The goal of this narrative review is to (1) identify key pathways affecting Trp-Kyn metabolism in vertebrates and (2) highlight consequences of altered tryptophan metabolism in mammals, birds, amphibians, and fish. We discuss current literature available across species, highlight gaps in the current state of knowledge, and further postulate that the kynurenine to tryptophan ratio can be used as a novel biomarker for assessing organismal and, more broadly, ecosystem health.

OPN5 Regulating Mechanism of Follicle Development Through the TSH-DIO2/DIO3 Pathway in Mountain Ducks Under Different Photoperiods

Abstract: Photoperiod is an important environmental factor that influence seasonal reproduction behavior in bird. Birds translates photoperiodic information into neuroendocrine signals through deep brain photoreceptors (DBPs). OPN5 has been considered as candidate DBPs involving in regulation of seasonal reproduction in birds. However, little is known about the effect of OPN5 in non-seasonal breeding birds. Thus, we pondered on whether OPN5 regulating follicular development through TSH-DIO2/DIO3 system responds to different photoperiods in non-seasonal laying ducks. As an ideal non-seasonal breeding bird, a total of 120 mountain ducks were randomly divided into three groups and treated respectively to a different photoperiod: group S (8 L:16D), group C (17 L:7D), and group L (24 L:0D). The ducks were caged in a fully enclosed shelter with the same feeding conditions for each group, free water and limited feeding (150 g per duck each day). Samples were collected from each group at d 0, d 5, d 8, d 20, and d 35 (n = 8). The ducks in 24 h photoperiod had the highest laying rate and the lowest feed-to-egg ratio, while the ducks in 8 h photoperiod had the lowest laying rate and the highest feed-to-egg ratio. Long-day photoperiod for 24 h significantly increased the ovarian index and GnRH, LH, E2, and P4 levels in serum; short-day photoperiod for 8 h increased testosterone levels in serum. Compared with 8 h photoperiod, long-day photoperiod significantly or highly significantly increased the mRNA level and protein expression of OPN5 in the hypothalamus of long-day photoperiod on d 35 (p &lt; 0.05). The gene or protein expression patterns of GnRH, TRH, TSHβ, DIO2, THRβ, VIP, and PRL were positively correlated with OPN5, whereas the gene expression patterns of GnIH and DI O 3 were negatively correlated with OPN5. The results revealed that OPN5 mediated the effect of light on follicular development through the TSH-DIO2/DIO3 pathway, the expression of OPN5 increased with light duration and improved the efficiency of the HPG axis to promote follicular development in mountain ducks.

Is serotonin uptake by peripheral tissues sensitive to hypoxia exposure?

In the Gulf toadfish (Opsanus beta), the serotonin (5-HT) transporter (SERT) is highly expressed in the heart, and the heart and gill both demonstrate the capacity for SERT-mediated uptake of 5-HT from the circulation. Because 5-HT is a potent vasoconstrictor in fish, we hypothesized that hypoxia exposure may increase 5-HT uptake by these tissues-and increase excretion of 5-HT-to prevent branchial vasoconstriction that would hamper gas exchange. Spot sampling of blood, bile, and urine revealed that fish exposed to chronic hypoxia (1.83 ± 0.12 mg·L^-1 O₂ for 24-26 h) had 41% lower plasma 5-HT in the ventral aorta (immediately following the heart) than in the hepatic vein (immediately before the heart), suggesting enhanced cardiac 5-HT uptake during hypoxia. 5-HT concentrations in the bile were greater than those in the urine, but there were no effects of acute (1.31 ± 0.06 mg·L^-1 O₂ for 25 min) or chronic hypoxia on 5-HT levels in these fluids. In 5-HT radiotracer experiments, the presence of tracer in the bile decreased upon hypoxia exposure, but, surprisingly, neither acute nor chronic hypoxia-induced changes in [³H]5-HT uptake in the heart, gill, or other tissues. Given the likely impact of the hypoxia exposure on metabolic rate, future studies should examine the effects of a milder hypoxia exposure on 5-HT uptake into these tissues and the role of 5-HT degradation.

Photoperiodic Changes in Both Hypothalamus Neurotransmitters and Circulating Gonadal Steroids Metabolomic Profiles in Relation to Seasonal Reproduction in Male Quail

Both hypothalamic neurotransmitters and serum steroid hormones are impacted by photoperiod and have effects on physiology and seasonal reproductive. However, the relationship between circulating gonadal steroids and hypothalamic neurotransmitters underlying different photoperiod is still unclear. To further understand the crosstalk of neurotransmitters and steroids in seasonal reproduction, metabolic changes of 27 neurotransmitters concentrated in hypothalamus tissues and 42 steroids hormones in serum were assessed during two artificial photoperiodic programs. The results showed that photoperiod induce testicular atrophy and recrudescence. In L-to-S groups, significantly decreased levels of testosterone concentration were found in serum (P < 0.001) and increased 11-Dehydrocorticosterone (P < 0.05); Testosterone were almost undetectable at SD_14d. In addition, the hypothalamus exhibited significantly increased arginine and 4-aminobutyric acid (GABA) concentration and decreased serotonin and epinephrine content (P < 0.01 or P < 0.05). Accordingly, serum testosterone and androstenedione became detectable at LD_3d in the S-to-L group and were markedly increase at LD_7d. Furthermore, Serum androstenedione showed a significant increase with long light expose (P < 0.01). Additionally, the hypothalamus exhibited both significantly increased L.Tryptophan and phenylalanine concentration, as well as decreased L-glutamine and L-glutamine.acid content (P < 0.01 or P < 0.05). Serotonin metabolism showed significant differences between L-to-S group and S-to-L group. Furthermore, in the correlation analysis, serum testosterone had a positive correlation with 5-Hydroxyindole-3-acetic acid (5-HIAA), while Androstenedione was significantly negative with L.Tryptophan in L-to-S (P < 0.05). However, in S-to-L group, serum testosterone showed strong negative correlation with both serotonin and 5-HIAA (P < 0.05), but positive correlation with L.Tryptophan (P < 0.01), while Androstenedione was significantly negative correlation with both serotonin (P < 0.05) and L-Glutamine (P < 0.01). Photoperiod also had significant effects on the mRNA expression. We found significant differences in gene expression patterns of both serotonin signaling and steroid biosynthesis, while MAOB, NR5A1, and 3β-HSD showed an opposite tendency between two groups. Taken together, our results revealed that circulating gonadal steroids and hypothalamic neurotransmitters were significantly impact quail’s seasonal reproduction. Circulating gonadal steroids have different effects on neurotransmitter at different photoperiodism, which may coordinately influence the seasonal reproduction of quails.

Clozapine modulation of zebrafish swimming behavior and gene expression as a case study to investigate effects of atypical drugs on aquatic organisms

Mental illnesses affect more than 150 million people in Europe and lead to an increasing consumption of neuroactive drugs during the last twenty years. The antipsychotic compound, clozapine, is one of the most used psychotropic drugs worldwide, with potentially negative consequences for the aquatic environment. Hence, the objectives of the study presented here were the quantification of clozapine induced changes in swimming behavior of exposed Danio rerio embryos and the elucidation of the molecular effects on the serotonergic and dopaminergic systems. Yolk-sac larvae were exposed to different concentrations (0.2 mg/L, 0.4 mg/L, 0.8 mg/L, 1.6 mg/L, 3.2 mg/L and 6.4 mg/L) of clozapine for 116 h post-fertilization, and changes in the swimming behavior of the larvae were assessed. Further, quantitative real-time PCR was performed to analyze the expression of selected genes. The qualitative evaluation of changes in the swimming behavior of D. rerio larvae revealed a significant decrease of the average swimming distance and velocity in the light-dark transition test, with more than a 36% reduction at the highest exposure concentration of 6.4 mg/L. Furthermore, the total larval body length was reduced at the highest concentration. An in-depth analysis based on expression of selected target genes of the serotonin (slc6a4a) and dopamine (drd2a) system showed an upregulation at a concentration of 1.6 mg/L and above. In addition, a lower increase in expression was detected for biomarkers of general stress (adra1a and cyp1a2). Our data show that exposure to clozapine during development inhibits swimming activity of zebrafish larvae, which could, in part, be due to disruption of the serotonin- and dopamine system.

Quantitative lipidomics and spatial MS-Imaging uncovered neurological and systemic lipid metabolic pathways underlying troglomorphic adaptations in cave-dwelling fish

The Sinocyclocheilus represents a rare, freshwater teleost genus endemic to China that comprises the river-dwelling surface fish and the cave-dwelling cavefish. Using a combinatorial approach of quantitative lipidomics and mass-spectrometry imaging (MSI), we demonstrated that neural compartmentalization of lipid distribution and lipid metabolism are associated with the evolution of troglomorphic traits in Sinocyclocheilus. Attenuated DHA biosynthesis via the Δ4 desaturase pathway led to reductions in docosahexaenoic acid (DHA)-phospholipids in cavefish cerebellum. Instead, cavefish accumulates arachidonic acid (ARA)-phospholipids that may disfavor retinotectal arbor growth. Importantly, MSI of sulfatides, coupled with immunostaining of myelin basic protein and transmission electron microscopy images of hindbrain axons revealed demyelination in cavefish raphe serotonergic neurons. Demyelination in cavefish parallels the loss of neuroplasticity governing social behavior such as aggressive dominance. Outside the brain, quantitative lipidomics and qRT-PCR revealed systemic reductions in membrane esterified DHAs in the liver, attributed to suppression of genes along the Sprecher pathway (elovl2, elovl5, acox1). Development of fatty livers was observed in cavefish, likely mediated by an impeded mobilization of storage lipids, as evident in the diminished expressions of pnpla2, lipea, lipeb, dagla and mgll; and suppressed β-oxidation of fatty acyls via both mitochondria and peroxisomes, reflected in the reduced expressions of cpt1ab, hadhaa, cpt2, decr1 and acox1. These neurological and systemic metabolic adaptations serve to reduce energy expenditure, forming the basis of recessive evolution that eliminates non-essential morphological and behavioral traits, giving cavefish a selective advantage to thrive in caves where proper resource allocation becomes a major determinant of survival.

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.

Foodborne fluoxetine impacts the immune response in rainbow trout (Oncorhynchus mykkis)

The aim of this study was to reveal the effects of foodborne fluoxetine on morphological and condition profile, hematological profile, biochemical and oxidative stress indices on juvenile rainbow trout. The study was performed according to OECD Guideline No. 215. Fluoxetine was incorporated into Biomar 921 pellets at a dose of 0.047 mg/kg (environmental concentration), 0.577 mg/kg and 6.7 mg/kg. There was statistically significant change in hematological profile, including an increasing trend in neutrophil/lymphocyte ratio and a decreasing trend in the number of lymphocytes. Measurements of oxidative stress indicated decreased activity of the detoxifying enzyme glutathione-S-transferase in the liver and kidney. However, the measurement of GR, GPx, CAT, SOD activity, and TBARS showed no changes. Histopathological examination revealed damage to the proximal tubules of caudal kidney in exposed groups. This study confirms that fluoxetine has a significant effect on immune response.

Microbial and metabolomic mechanisms mediating the effects of dietary inulin and cellulose supplementation on porcine oocyte and uterine development

Background

Dietary fiber (DF) is often eschewed in swine diet due to its anti-nutritional effects, but DF is attracting growing attention for its reproductive benefits. The objective of this study was to investigate the effects of DF intake level on oocyte maturation and uterine development, to determine the optimal DF intake for gilts, and gain microbial and metabolomic insight into the underlying mechanisms involved.

Methods

Seventy-six Landrace × Yorkshire (LY) crossbred replacement gilts of similar age (92.6 ± 0.6 d; mean ± standard deviation [SD]) and body weight (BW, 33.8 ± 3.9 kg; mean ± SD) were randomly allocated to 4 dietary treatment groups (n = 19); a basal diet without extra DF intake (DF 1.0), and 3 dietary groups ingesting an extra 50% (DF 1.5), 75% (DF 1.75), and 100% (DF 2.0) dietary fiber mixture consisting of inulin and cellulose (1:4). Oocyte maturation and uterine development were assessed on 19 d of the 2nd oestrous cycle. Microbial diversity of faecal samples was analysed by high-throughput pyrosequencing (16S rRNA) and blood samples were subjected to untargeted metabolomics.

Results

The rates of oocytes showing first polar bodies after in vitro maturation for 44 h and uterine development increased linearly with increasing DF intake; DF 1.75 gilts had a 19.8% faster oocyte maturation rate and a 48.9 cm longer uterus than DF 1.0 gilts (P <  0.05). Among the top 10 microbiota components at the phylum level, 8 increased linearly with increasing DF level, and the relative abundance of 30 of 53 microbiota components at the genus level (> 0.1%) increased linearly or quadratically with increasing DF intake. Untargeted metabolic analysis revealed significant changes in serum metabolites that were closely associated with microbiota, including serotonin, a gut-derived signal that stimulates oocyte maturation.

Conclusions

The findings provide evidence of the benefits of increased DF intake by supplementing inulin and cellulose on oocyte maturation and uterine development in gilts, and new microbial and metabolomic insight into the mechanisms mediating the effects of DF on reproductive performance of replacement gilts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40104-021-00657-0.

Neuroendocrine Control of Reproduction in Teleost Fish: Concepts and Controversies

During the teleost radiation, extensive development of the direct innervation mode of hypothalamo-pituitary communication was accompanied by loss of the median eminence typical of mammals. Cells secreting follicle-stimulating hormone and luteinizing hormone cells are directly innervated, distinct populations in the anterior pituitary. So far, ∼20 stimulatory and ∼10 inhibitory neuropeptides, 3 amines, and 3 amino acid neurotransmitters are implicated in the control of reproduction. Positive and negative sex steroid feedback loops operate in both sexes. Gene mutation models in zebrafish and medaka now challenge our general understanding of vertebrate neuropeptidergic control. New reproductive neuropeptides are emerging. These include but are not limited to nesfatin 1, neurokinin B, and the secretoneurins. A generalized model for the neuroendocrine control of reproduction is proposed. Hopefully, this will serve as a research framework on diverse species to help explain the evolution of neuroendocrine control and lead to the discovery of new hormones with novel applications. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Advances in Reproductive Endocrinology and Neuroendocrine Research Using Catfish Models

Catfishes, belonging to the order siluriformes, represent one of the largest groups of freshwater fishes with more than 4000 species and almost 12% of teleostean population. Due to their worldwide distribution and diversity, catfishes are interesting models for ecologists and evolutionary biologists. Incidentally, catfish emerged as an excellent animal model for aquaculture research because of economic importance, availability, disease resistance, adaptability to artificial spawning, handling, culture, high fecundity, hatchability, hypoxia tolerance and their ability to acclimate to laboratory conditions. Reproductive system in catfish is orchestrated by complex network of nervous, endocrine system and environmental factors during gonadal growth as well as recrudescence. Lot of new information on the molecular mechanism of gonadal development have been obtained over several decades which are evident from significant number of scientific publications pertaining to reproductive biology and neuroendocrine research in catfish. This review aims to synthesize key findings and compile highly relevant aspects on how catfish can offer insight into fundamental mechanisms of all the areas of reproduction and its neuroendocrine regulation, from gametogenesis to spawning including seasonal reproductive cycle. In addition, the state-of-knowledge surrounding gonadal development and neuroendocrine control of gonadal sex differentiation in catfish are comprehensively summarized in comparison with other fish models.

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.

Revelation of candidate genes and molecular mechanism of reproductive seasonality in female rohu (Labeo rohita Ham.) by RNA sequencing

BACKGROUND

Carp fish, rohu (Labeo rohita Ham.) is important freshwater aquaculture species of South-East Asia having seasonal reproductive rhythm. There is no holistic study at transcriptome level revealing key candidate genes involved in such circannual rhythm regulated by biological clock genes (BCGs). Seasonality manifestation has two contrasting phases of reproduction, i.e., post-spawning resting and initiation of gonadal activity appropriate for revealing the associated candidate genes. It can be deciphered by RNA sequencing of tissues involved in BPGL (Brain-Pituitary-Gonad-Liver) axis controlling seasonality. How far such BCGs of this fish are evolutionarily conserved across different phyla is unknown. Such study can be of further use to enhance fish productivity as seasonality restricts seed production beyond monsoon season.

RESULT

A total of ~ 150 Gb of transcriptomic data of four tissues viz., BPGL were generated using Illumina TruSeq. De-novo assembled BPGL tissues revealed 75,554 differentially expressed transcripts, 115,534 SSRs, 65,584 SNPs, 514 pathways, 5379 transcription factors, 187 mature miRNA which regulates candidate genes represented by 1576 differentially expressed transcripts are available in the form of web-genomic resources. Findings were validated by qPCR. This is the first report in carp fish having 32 BCGs, found widely conserved in fish, amphibian, reptile, birds, prototheria, marsupials and placental mammals. This is due to universal mechanism of rhythmicity in response to environment and earth rotation having adaptive and reproductive significance.

CONCLUSION

This study elucidates evolutionary conserved mechanism of photo-periodism sensing, neuroendocrine secretion, metabolism and yolk synthesis in liver, gonadal maturation, muscular growth with sensory and auditory perception in this fish. Study reveals fish as a good model for research on biological clock besides its relevance in reproductive efficiency enhancement.

Constant darkness negatively affects the outcome of hormonally induced reproduction in cultured Eurasian perch females

This study aimed to assess the effect of constant darkness applied to fish during controlled breeding on reproductive traits in domesticated females of Eurasian perch. Based on the assumption that keeping fish in constant darkness during the reproduction operation may reduce stress, suspected to be responsible for variable spawning effectiveness in this species. Two conditions were assessed (16 h light per day [group 16L] and constant darkness [group 0L], two tank replicates per condition). The reproductive protocol involved a 7-day-long adaptation period for group 0L where photoperiod was reduced by 2.3 h a day down to constant darkness. After the adaptation period, two hormone injections (salmon gonadoliberin analogue) were applied to both groups: priming (10 µg/kg) and resolving (25 µg/kg) with a 7-day interval between them. During the study, morphometric indices were recorded and blood, brain, and pituitary samples were collected to assess stress markers and determine hypothalamic-pituitary-gonadal axis functioning via measuring blood plasma hormones, as well as gonadoliberin and gonadotropins (luteinising hormone [LH] and follicle-stimulating hormone [FSH]) transcript abundance (n = 7 for each group at each sampling point). In addition, kinetics of the final oocyte maturation (FOM) process, ovulation rate, and egg quality of each group was monitored (n = 12 for each group). The results indicated that there were no differences in terms of morphometry, FOM kinetics, and most stress indices between groups throughout the experiment, except haematocrit, which increased immediately following the acclimation period in fish kept in darkness. Constant darkness negatively affected plasma levels of 17α,20β-dihydroxy-4-pregnen-3-one (DHP) and LH transcript expression at the time of the second hormone injection. This indicated that exposure to constant darkness negatively affected priming of the hormonal dose applied, resulted in the disruption of ovulation, and reduced ovulation rates (50%) for group 0L, as compared to 16L (91%). The findings of this study clearly indicate that constant darkness may have significant deleterious effects on reproductive traits throughout out-of-season induced, hormonally supported, controlled reproduction. Therefore, we advise against the use of constant darkness when managing broodstock reproduction in domesticated Eurasian perch.

Genetic association with boldness and maternal performance in a free-ranging population of grey seals (Halichoerus grypus)

Individual variation in quantitative traits clearly influence many ecological and evolutionary processes. Moderate to high heritability estimates of personality and life-history traits suggest some level of genetic control over these traits. Yet, we know very little of the underlying genetic architecture of phenotypic variation in the wild. In this study, we used a candidate gene approach to investigate the association of genetic variants with repeated measures of boldness and maternal performance traits (weaning mass and lactation duration) collected over an 11- and 28-year period, respectively, in a free-ranging population of grey seals on Sable Island National Park Reserve, Canada. We isolated and re-sequenced five genes: dopamine receptor D4 (DRD4), serotonin transporter (SERT), oxytocin receptor (OXTR), and melanocortin receptors 1 (MC1R) and 5 (MC5R). We discovered single nucleotide polymorphisms (SNPs) in each gene; and, after accounting for loci in linkage disequilibrium and filtering due to missing data, we were able to test for genotype-phenotype relationships at seven loci in three genes (DRD4, SERT, and MC1R). We tested for association between these loci and traits of 180 females having extreme shy-bold phenotypes using mixed-effects models. One locus within SERT was significantly associated with boldness (effect size = 0.189) and a second locus within DRD4 with weaning mass (effect size = 0.232). Altogether, genotypes explained 6.52-13.66% of total trait variation. Our study substantiates SERT and DRD4 as important determinants of behaviour, and provides unique insight into the molecular mechanisms underlying maternal performance variation in a marine predator.

Expression of Repressor Element 1 Silencing Transcription Factor (REST) in Serotonin Neurons in the Adult Male Nile Tilapia (Oreochromis niloticus)

Repressor element-1 silencing transcription factor (REST) is highly expressed in the dorsal raphe where serotonin (5-hydroxytryptamine, 5-HT) neurons are located. REST works as a transcription factor for the 5-HT receptor and tryptophan hydroxylase two-gene expression. We hypothesized that REST is co-expressed in 5-HT neurons, which, if demonstrated, would be useful to understand the mechanism of 5-HT dysfunction-related disorders such as negative emotions and depression. Therefore, the present study was designed to examine the expression of the REST gene in the brain (forebrain, midbrain, and hindbrain) of adult male Nile tilapia (Oreochromis niloticus) using rt-PCR. Besides, using immunocytochemistry, co-localization of the REST gene was examined in 5-HT neurons and with neuronal-/glial-cell markers. We found a high expression of the REST gene in the midbrain region of the dorsal raphe, an area of 5-HT neurons. Double-label immunocytochemistry showed neuron-specific expression of REST co-localized in 5-HT neurons in the dorsal and ventral parts of the periventricular pretectal nucleus, paraventricular organ, and dorsal and medial raphe nucleus. Since midbrain 5-HT neurons express REST, we speculate that REST may control 5-HT neuronal activity related to negative emotions, including depression.

The serotonin 1A receptor modulates the social behaviour within groups of a cooperatively-breeding cichlid

The neurotransmitter serotonin (5-HT) reduces aggressive behaviour in a number of vertebrates, and the 5-HT_1A receptor is known to be involved in this regulation. However, the role of this receptor in the modulation of sociopositive behaviour remains largely unknown. Here we investigated the role of the 5-HT_1A receptor in the regulation of aggressive, submissive and affiliative behaviour in the cooperatively-breeding cichlid Neolamprologus pulcher. In two experiments, we performed intramuscular injections of a 5-HT_1A agonist (8-OH-DPAT) and antagonist (Way-100635) followed by recordings of social behaviour of injected fish within their social groups. We determined the concentrations and post-injection times when the drugs had the greatest effect on social behaviour. We recorded spontaneous social behaviour in both experiments. In the second experiment we also recorded behaviour after social groups received a territorial challenge by live presentations of either conspecifics or egg predators. The 5-HT_1A agonist caused an increase in aggression and a decrease in submission and affiliation, whereas the antagonist had the opposite effects. Thus, the 5-HT_1A receptor plays an important regulatory role not only for aggressive but also sociopositive behaviour.

Antidepressant exposure reduces body size, increases fecundity and alters social behavior in the short-lived killifish Nothobranchius furzeri

Social and mating behavior are fundamental fitness determinants in fish. Although fish are increasingly exposed to pharmaceutical compounds that may alter expression of such behavior, potential effects are understudied. Here, we examine the impact of lifelong exposure to two concentrations (0.7 and 5.3 μg/L) of the antidepressant fluoxetine on fecundity and social behavior (i.e. sociability and male-male aggression) in the turquoise killifish, Nothobranchius furzeri. When exposed to the highest concentration of fluoxetine (5.3 μg/L), fish were smaller at maturation but they more frequently engaged in mating. In addition, in both fluoxetine treatments females roughly doubled their overall fecundity while egg fertilization rates were the same for exposed and unexposed fish. Although aggression of male fish was not impacted by fluoxetine exposure, exposed male fish (5.3 μg/L) spent more time in the proximity of a group of conspecifics, which implies an increased sociability in these individuals. Overall, the results of this study indicate that exposure to fluoxetine may result in disrupted male sociability, increased mating frequency and an increased reproductive output in fish populations.

Verification of differentially expressed genes in relation to hydrostatic pressure in the brain of two wrasse species with high-tide preference in spawning

Fish that inhabit shallow water are exposed to periodic changes in tidal cues, including hydrostatic pressure (HP). The present study aimed at verifying differentially expressed genes (DEGs) in the brain of the threespot wrasse Halichoeres trimaculatus (tropical species) and the honbera wrasse Halichoeres tenuispinis (temperate species), both of which were exposed to HP at 30 kPa (possible high-tide stimuli in the field) or 1 kPa (low tide) for 3 or 6 h. A de novo assembly yielded 174,710 contigs (63,530 contigs were annotated) from the brain of threespot wrasse. Following RNA sequencing, quantitative PCR confirmed DEGs that were upregulated [AT atypical cadherin 2 (FAT2)] and downregulated [neuronal leucine-rich repeat protein 3 (LRRN3), dual specificity tyrosine phosphorylation-regulated kinase 1 (DYRK), mitogen-activated protein kinase kinase 1 (MAP2K1) and phosphoinositide 3 kinase (PI3K)]. The effect of HP on the transcription of these DEGs (except for MAP2K1) disappeared within 6 h, suggesting that HP is a transitory stimulus occurring at the beginning of the tidal cycle. Similar DEG transcription was observed in the brain of honbera wrasse maintained under HP for 6 h. In situ hybridization of the brain of the threespot wrasse revealed that strong signals of MPA2K1 were seen in the telencephalon, diencephalon and pituitary, whereas those of PI3K were seen in the telencephalon, diencephalon and medulla oblongata. This result suggests that these kinases are involved in sensory function (telencephalon), somatic and visceral function (medullar oblongata) and the neuroendocrine system (diencephalon and pituitary), all of which were related to changes in HP stimuli. Following HP exposure, the transcription of c-fos increased in the pituitary of honbera wrasse, suggesting that external stimuli directly or indirectly activate hormone synthesis at the hypothalamic-pituitary-gonadal axis. It is concluded that HP alters gene expression in relation to neural development and function in the central nervous system and plays a role in exerting tidal-related reproduction and feeding in wrasses.

A mutation in monoamine oxidase (MAO) affects the evolution of stress behavior in the blind cavefish Astyanax mexicanus

The neurotransmitter serotonin controls a variety of physiological and behavioral processes. In humans, mutations affecting monoamine oxidase (MAO), the serotonin-degrading enzyme, are highly deleterious. Yet, blind cavefish of the species Astyanax mexicanus carry a partial loss-of-function mutation in MAO (P106L) and thrive in their subterranean environment. Here, we established four fish lines, corresponding to the blind cave-dwelling and the sighted river-dwelling morphs of this species, with or without the mutation, in order to decipher the exact contribution of mao P106L in the evolution of cavefish neurobehavioral traits. Unexpectedly, although mao P106L appeared to be an excellent candidate for the genetic determinism of the loss of aggressive and schooling behaviors in cavefish, we demonstrated that it was not the case. Similarly, the anatomical variations in monoaminergic systems observed between cavefish and surface fish brains were independent from mao P106L, and rather due to other, morph-dependent developmental processes. However, we found that mao P106L strongly affected anxiety-like behaviors. Cortisol measurements showed lower basal levels and an increased amplitude of stress response after a change of environment in fish carrying the mutation. Finally, we studied the distribution of the P106L mao allele in wild populations of cave and river A. mexicanus, and discovered that the mutant allele was present - and sometimes fixed - in all populations inhabiting caves of the Sierra de El Abra. The possibility that this partial loss-of-function mao allele evolves under a selective or a neutral regime in the particular cave environment is discussed.

Brain-encysting trematodes (Euhaplorchis californiensis) decrease raphe serotonergic activity in California killifish (Fundulus parvipinnis)

Modulation of brain serotonin (5-HT) signalling is associated with parasite-induced changes in host behaviour, potentially increasing parasite transmission to predatory final hosts. Such alterations could have substantial impact on host physiology and behaviour, as 5-HT serves multiple roles in neuroendocrine regulation. These effects, however, remain insufficiently understood, as parasites have been associated with both increased and decreased serotonergic activity. Here, we investigated effects of trematode Euhaplorchis californiensis metacercariae on post-stress serotonergic activity in the intermediate host California killifish (Fundulus parvipinnis). This parasite is associated with conspicuous behaviour and increased predation of killifish by avian end-hosts, as well as inhibition of post-stress raphe 5-HT activity. Until now, laboratory studies have only been able to achieve parasite densities (parasites/unit host body mass) well below those occurring in nature. Using laboratory infections yielding ecologically relevant parasite loads, we show that serotonergic activity indeed decreased with increasing parasite density, an association likely indicating changes in 5-HT neurotransmission while available transmitter stores remain constant. Contrary to most observations in the literature, 5-HT activity increased with body mass in infected fish, indicating that relationships between parasite load and body mass may in many cases be a real underlying factor for physiological correlates of body size. Our results suggest that parasites are capable of influencing brain serotonergic activity, which could have far-reaching effects beyond the neurophysiological parameters investigated here.

Long-Term Pharmaceutical Contamination and Temperature Stress Disrupt Fish Behavior

Natural environments are subject to a range of anthropogenic stressors, with pharmaceutical pollution being among the fastest-growing agents of global change. However, despite wild animals living in complex multi-stressor environments, interactions between pharmaceutical exposure and other stressors remain poorly understood. Accordingly, we investigated effects of long-term exposure to the pervasive pharmaceutical contaminant fluoxetine (Prozac) and acute temperature stress on reproductive behaviors and activity levels in the guppy (Poecilia reticulata). Fish were exposed to environmentally realistic fluoxetine concentrations (measured average: 38 or 312 ng/L) or a solvent control for 15 months using a mesocosm system. Additionally, fish were subjected to one of three acute (24 h) temperature treatments: cold stress (18 °C), heat stress (32 °C), or a control (24 °C). We found no evidence for interactive effects of fluoxetine exposure and temperature stress on guppy behavior. However, both stressors had independent impacts. Fluoxetine exposure resulted in increased male coercive copulatory behavior, while fish activity levels were unaffected. Under cold-temperature stress, both sexes were less active and males exhibited less frequent reproductive behaviors. Our results demonstrate that long-term exposure to a common pharmaceutical pollutant and acute temperature stress alter fundamental fitness-related behaviors in fish, potentially shifting population dynamics in contaminated ecosystems.

Gut microbial metabolism of dietary fibre protects against high energy feeding induced ovarian follicular atresia in a pig model

To investigate the effects of dietary fibre on follicular atresia in pigs fed a high-fat diet, we fed thirty-two prepubescent gilts a basal diet (CON) or a CON diet supplemented with 300 g/d dietary fibre (fibre), 240 g/d soya oil (SO) or both (fibre + SO). At the 19th day of the 4th oestrus cycle, gilts fed the SO diet showed 112 % more atretic follicles and greater expression of the apoptotic markers, Bax and caspase-3, and these effects were reversed by the fibre diet. The abundance of SCFA-producing microbes was decreased by the SO diet, but this effect was reversed by fibre treatment. Concentrations of serotonin and melatonin in the serum and follicular fluid were increased by the fibre diet. Overall, dietary fibre protected against high fat feeding-induced follicular atresia at least partly via gut microbiota-related serotonin-melatonin synthesis. These results provide insight into preventing negative effects on fertility in humans consuming a high-energy diet.

The adverse effects of psychotropic drugs as an endocrine disrupting chemicals on the hypothalamic-pituitary regulation in male

Adverse effects of drugs on male reproductive system can be categorized as pre-testicular, testicular, and post-testicular. Pre-testicular adverse effects disrupt the hypothalamic-pituitary-gonadal (HPG) axis, generally by interfering with endocrine function. It is known that the HPG axis has roles in the maintenance of spermatogenesis and sexual function. The hypothalamus secretes gonadotropin-releasing hormone (GnRH) which enters the hypophyseal portal system to stimulate the anterior pituitary. The anterior pituitary secretes gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) which are vital for spermatogenesis, into the blood. The FSH stimulates the Sertoli cells for the production of regulatory molecules and nutrients needed for the maintenance of spermatogenesis, while the LH stimulates the Leydig cells to produce and secrete testosterone. Many neurotransmitters influence the hypothalamic-pituitary regulation, consequently the HPG axis, and can consequently affect spermatogenesis and sexual function. Psychotropic drugs including antipsychotics, antidepressants, and mood stabilizers that all commonly modulate dopamine, serotonin, and GABA, can affect male spermatogenesis and sexual function by impairment of the hypothalamic-pituitary regulation, act like endocrine-disrupting chemicals. Otherwise, studies have shown the relationship between decreased sperm quality and psychotropic drugs treatment. Therefore, it is important to investigate the adverse reproductive effects of psychotropic drugs which are frequently used during reproductive ages in males and to determine the role of the hypothalamic-pituitary regulation axis on possible pathologies.

Mapping of Morphine-Induced OPRM1 Gene Expression Pattern in the Adult Zebrafish Brain

Morphine is a potent analgesic opiate commonly used in treating pain, and it is also a substance of abuse and highly addictive. Hence, it is vital to discover the action sites of morphine in the brain to increase its efficacy of treatment. In the present study, we aimed at identifying comprehensive neuroanatomical locations that are sensitive to morphine in the adult zebrafish (Danio rerio). We performed in situ hybridization to localize the mu opioid receptor (oprm1) gene and to map the morphine sensitive brain areas using neuronal PAS domain-containing protein 4a (npas4a), an early gene marker. Real-time PCR was used to detect changes in mRNA levels of oprm1 and npas4a in control and acute morphine treated fish (2 mg/L; 20 min). Intense positive oprm1 signals were seen in the telencephalon, preoptic area, habenula, hypothalamic area and periventricular gray zone of the optic tectum. Acute morphine exposure significantly increased oprm1 and npas4a mRNA levels in the medial zone of dorsal telencephalon (Dm), ventral region of the ventral telencephalon (Vv), preoptic area, and in the hypothalamus but a decrease in oprm1 and npas4a signals in the dorsal habenula. This study provides a detailed map of oprm1 localization in the brain, which includes previously unreported oprm1 in the habenula of teleost. Presence of oprm1 in multiple brain sites implies multiple action targets of morphine and potential brain functions which could include reward, cognitive and negative emotions.

Identification, characterization, and expression analysis of a serotonin receptor involved in the reproductive process of the Pacific abalone, Haliotis discus hannai

Serotonin receptor (5-HT) is a biogenic amine acting as a neurotransmitter and neuromodulator that mediates various aspects of reproduction and gametogenesis. The full-length nucleotide sequence of Haliotis discus hannai encodes a protein of 417 amino acids with a predicted molecular mass of 46.54 kDa and isoelectric point of 8.94. The structural profile of 5-HT_Hdh displayed key features of G protein-coupled receptors, including seven hydrophobic transmembrane domains, putative N-linked glycosylation sites, and several phosphorylation consensus motifs. It shares the highest homology of its amino acid sequence with the 5-HT receptor from Haliotis asinina, and to lesser extent of human 5-HT receptor. The cloned sequence possesses two cysteine residues (Cys-115 and Cys-193), which are likely to form a disulfide bond. Phylogenetic comparison with other known 5-HT receptor genes revealed that the 5-HT_Hdh is most closely related to the 5-HT_Ha receptor. The three-dimensional structure of the 5-HT_Hdh showed multiple alpha helices which is separated by a helix-loop-helix (HLH) structure. Quantitative PCR demonstrated that the receptor mRNA was predominantly expressed in the pleuropedal ganglion. Significant differences in the transcriptional activity of the 5-HT_Hdh gene were observed in the ovary at the ripening stage. An exclusive expression was detected in pleuropedal ganglion, testis, and ovary at higher effective accumulative temperature (1000 °C). In situ hybridization showed that the 5-HT_Hdh expressing neurosecretory cells were distributed in the cortex of the pleuropedal ganglion. Our results suggest that 5-HT_Hdh synthesized in the neural ganglia may be involved in oocyte maturation and spawning of H. discus hannai.

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.

Do reproductive hormones control Gulf toadfish pulsatile urea excretion?

Gulf toadfish (Opsanus beta) can excrete the majority of their nitrogenous waste as urea in distinct pulses across their gill. Urea pulses are controlled by cortisol and serotonin (5-HT) and are believed to contain chemical signals that may communicate reproductive and/or social status. The objectives of this study were to determine if reproductive hormones are involved in controlling pulsatile urea excretion, and if toadfish respond to prostaglandins as a chemical signal. Specifically, 11-ketotestosterone (11-KT), estradiol (E2), and the teleost pheromone prostaglandin E₂ (PGE₂) were investigated. Castration during breeding season did not affect pulsatile urea excretion but serial injections of 11-KT outside of breeding season did result in a 48% reduction in urea pulse size in fish of both sexes. Injections of E2 and PGE₂, on the other hand, did not alter urea excretion patterns. Toadfish also did not pulse urea in response to waterborne exposure of PGE₂ suggesting that this compound does not serve as a toadfish pheromone alone. Toadfish have significantly higher plasma 5-HT during breeding season compared to the months following breeding season. Future research should focus on the composition of the chemical signal in toadfish and the potential importance of seasonal changes in plasma 5-HT in toadfish pulsatile urea excretion and teleost reproduction in general.

The Psychoactive Drug Escitalopram Affects Foraging Behavior in Zebrafish (Danio rerio)

Selective serotonin reuptake inhibitors (SSRIs) are psychoactive pharmaceuticals that have been detected intact in natural waters globally. Laboratory experiments have reported that several SSRIs inhibit fish foraging behavior, but data for the SSRI escitalopram are lacking. The objectives of the present study were to determine whether escitalopram affects feeding behavior in zebrafish and whether possible sex differences exist. We exposed female and male zebrafish (Danio rerio) to 0.00, 0.10, and 1.50 µg/L of escitalopram in flow-through tanks for a 3-wk exposure period. We used a video tracking system with high temporal and spatial resolution to collect data on zebrafish swimming patterns in test tanks containing a food source. The results show a more pronounced effect of escitalopram in males compared with females. At the assumed most environmentally relevant concentration (0.10 µg/L), male average feeding time/visit and maximum feeding duration were significantly reduced by 27 and 42%, respectively. In addition, male total feeding duration was also significantly reduced (by 73%) at the highest concentration (1.50 µg/L). In females, only the maximum feeding duration was significantly reduced (by 41%) in the 0.10 µg/L treatment group. Hence, we reject our initial hypothesis that female feeding behavior is more vulnerable to escitalopram. There was no effect of escitalopram on length or weight among the experimental groups. The present study demonstrates that escitalopram, like other SSRIs, can inhibit fish foraging behavior and therefore potentially disturb natural food chains. Finally, our study suggests that SSRIs can both be sex and behavior specific. Environ Toxicol Chem 2019;38:1902-1910. © 2019 SETAC.

The pharmaceutical pollutant fluoxetine alters reproductive behaviour in a fish independent of predation risk

Pharmaceutical pollutants constitute a major threat to wildlife because of their capacity to induce biological effects at low doses. One such pollutant is the antidepressant fluoxetine, which has been detected in surface waters globally at levels that recent studies suggest can alter physiology and behaviour in aquatic organisms. However, wildlife exposed to pharmaceutical contaminants are typically confronted with multiple stressors simultaneously, including predation risk, which is a particularly important natural stressor that can have direct (e.g. mortality) and indirect (e.g. changed prey behaviour) fitness effects. Accordingly, we investigated potential impacts of environmentally realistic fluoxetine exposure on reproductive behaviour in the guppy (Poecilia reticulata) under predation risk. Specifically, we tested whether fluoxetine exposure altered mating behaviour in male and female guppies in the presence of either a predatory spangled perch (Leiopotherapon unicolor) or a non-predatory rainbowfish (Melanotaenia splendida) control. We found that fluoxetine and the presence of a predatory spangled perch did not interact to affect reproductive behaviour. We also found that, independent of a predatory threat, fluoxetine exposure altered male mating strategy, with males in the high treatment conducting significantly more coercive 'sneak' copulations, whereas the number of courtship displays performed was not significantly affected. Moreover, while fluoxetine exposure did not significantly affect the amount of time that males and females spent following one another, we found that females, but not males, followed a potential partner less when in the presence of the predatory fish. Finally, both sexes reacted to the risk of predation by spending less time in close proximity to a predator than a non-predator. In combination, our findings highlight the capacity of fluoxetine to influence processes of sexual selection at field-realistic concentrations and emphasise the importance of considering multiple stressors when assessing impacts of pharmaceutical pollutants on the behaviour of wildlife.