Gonadotropin-releasing hormone stimulates thyroid activity in a freshwater murrel, Channa gachua (ham.), and Carps, Catla catla (ham.) and Cirrhinus mrigala (ham.)

Molecular characterization and expression analysis of thyroid hormone receptors in protogynous rice field eel, Monopterus albus

Thyroid hormones (THs) play important roles in growth, development, morphogenesis, reproduction, and so on. They are mainly meditated by binding to thyroid hormone receptors (TRs) in vertebrates. As important members of the nuclear receptor superfamily, TRs and their ligands are involved in many biological processes. To investigate the potential roles of TRs in the gonadal differentiation and sex change, we cloned and characterized the TRs genes in protogynous rice field eel (Monopterus albus). In this study, three types of TRs were obtained, which were TRαA, TRαB and TRβ, encoding preproproteins of 336-, 409- and 415-amino acids, respectively. Multiple alignments of the three putative TRs protein sequences showed they had a higher similarity. Tissue expression analysis showed that TRαA mainly expressed in the gonad, while TRαB and TRβ in the brain. During female-to-male sex reversal, the expression levels of all the three TRs showed a similar trend of increase followed by a decrease in the gonad. Intraperitoneal injection of triiodothyronine (T3) stimulated the expression of TRαA and TRαB, while it had no significant change on the expression of TRβ in the ovary. Gonadotropin-releasing hormone analogue (GnRHa) injection also significantly upregulated the expression levels of TRαA and TRαB after 6 h, while it had no significant effect on TRβ. These results demonstrated that TRs were involved in the gonadal differentiation and sex reversal, and TRα may play more important roles than TRβ in reproduction by the regulation of GnRHa in rice field eel.

Molecular biological, physiological, cytological, and epigenetic mechanisms of environmental sex differentiation in teleosts: A systematic review

Human activities have been exerting widespread stress and environmental risks in aquatic ecosystems. Environmental stress, including temperature rise, acidification, hypoxia, light pollution, and crowding, had a considerable negative impact on the life histology of aquatic animals, especially on sex differentiation (SDi) and the resulting sex ratios. Understanding how the sex of fish responds to stressful environments is of great importance for understanding the origin and maintenance of sex, the dynamics of the natural population in the changing world, and the precise application of sex control in aquaculture. This review conducted an exhaustive search of the available literature on the influence of environmental stress (ES) on SDi. Evidence has shown that all types of ES can affect SDi and universally result in an increase in males or masculinization, which has been reported in 100 fish species and 121 cases. Then, this comprehensive review aimed to summarize the molecular biology, physiology, cytology, and epigenetic mechanisms through which ES contributes to male development or masculinization. The relationship between ES and fish SDi from multiple aspects was analyzed, and it was found that environmental sex differentiation (ESDi) is the result of the combined effects of genetic and epigenetic factors, self-physiological regulation, and response to environmental signals, which involves a sophisticated network of various hormones and numerous genes at multiple levels and multiple gradations in bipotential gonads. In both normal male differentiation and ES-induced masculinization, the stress pathway and epigenetic regulation play important roles; however, how they co-regulate SDi is unclear. Evidence suggests that the universal emergence or increase in males in aquatic animals is an adaptation to moderate ES. ES-induced sex reversal should be fully investigated in more fish species and extensively in the wild. The potential aquaculture applications and difficulties associated with ESDi have also been addressed. Finally, the knowledge gaps in the ESDi are presented, which will guide the priorities of future research.

Thyroid Hormones Deficiency Impairs Male Germ Cell Development: A Cross Talk Between Hypothalamic-Pituitary-Thyroid, and—Gonadal Axes in Zebrafish

In vertebrates, thyroid hormones are critical players in controlling different physiological processes such as development, growth, metabolism among others. There is evidence in mammals that thyroid hormones are also an important component of the hormonal system that controls reproduction, although studies in fish remain poorly investigated. Here, we tested this hypothesis by investigating the effects of methimazole-induced hypothyroidism on the testicular function in adult zebrafish. Treatment of fish with methimazole, in vivo, significantly altered zebrafish spermatogenesis by inhibiting cell differentiation and meiosis, as well as decreasing the relative number of spermatozoa. The observed impairment of spermatogenesis by methimazole was correlated with significant changes in transcript levels for several genes implicated in the control of reproduction. Using an in vitro approach, we also demonstrated that in addition to affecting the components of the brain-pituitary-peripheral axis, T3 (triiodothyronine) also exerts direct action on the testis. These results reinforce the hypothesis that thyroid hormones are an essential element of multifactorial control of reproduction and testicular function in zebrafish and possibly other vertebrate species.

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.

Thyroid hormone actions on male reproductive system of teleost fish

Thyroid hormones (THs) play important roles in the regulation of many biological processes of vertebrates, such as growth, metabolism, morphogenesis and reproduction. An increasing number of studies have been focused on the involvement of THs in the male reproductive system of vertebrates, in particular of fish. Therefore, this mini-review aims to summarize the main findings on THs role in male reproductive system of fish, focusing on sex differentiation, testicular development and spermatogenesis. The existing data in the literature have demonstrated that THs exert their roles at the different levels of the hypothalamic-pituitary-gonadal (HPG) axis. In general a positive correlation has been shown between THs and fish reproductive status; where THs are associated with testicular development, growth and maturation. Recently, the molecular mechanisms underlying the role of THs in spermatogenesis have been unraveled in zebrafish testis. THs promote germ cell proliferation and differentiation by increasing a stimulatory growth factor of spermatogenesis produced by Sertoli cells. In addition, THs enhanced the gonadotropin-induced androgen release in zebrafish testis. Next to their functions in the adult testis, THs are involved in the gonadal sex differentiation through modulating sex-related gene expression, and testicular development via regulation of Sertoli cell proliferation. In conclusion, this mini-review showed that THs modulate the male reproductive system during the different life stages of fish. The physiological and molecular mechanisms showed a link between the thyroid and reproduction, suggesting a possibly co-evolution and interdependence of these two systems.

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

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

Reproductive biomarkers to identify endocrine disruption in Clarias gariepinus exposed to 4-nonylphenol

The present study investigated the hormones concentrations and gonads alterations of Clarias gariepinus caused by sublethel concentrations of 4-nonylphenol (0, 0.05, 0.08 and 0.1 mg/l). The changes in the activities of the hormones after exposure to these sublethel doses of 4-nonylpenol referred to endocrine disruption in Clarias gariepinus in association with histopathological changes in reproductive tissues. The levels of thyroid stimulating hormone (TSH), triiodothyronine (T3), total thyroxine (T4), follicle stimulating hormone (FSH), luteinizing hormone (LH) and testosterone concentrations significantly decreased (P<0.05) in the treated fish in comparison with control. 17-β-estradiol increased significantly (P<0.05) with 4-nonylphenol concentrations increase. Reduction in the gonadosomatic index was evident with increase of sublethal doses of 4-nonylphenol. The histopathological changes of NP-treated were recorded in gonads of Clarias gariepinus reflecting their sensitivity to NP-estrogenic like effects.

Thyroid hormones as a potential early biomarker of exposure to 4-nonylphenol in adult male shubunkins (Carassius auratus)

Nonylphenol, an estrogenic-like compound, can induce vitellogenin synthesis in males and immature Teleostean species, but little is known about its effects on thyroid hormones balance. The present study evaluated the potential effects of a single acute exposure to nonylphenol (i.p. injected) on the thyroid and reproductive axis of 250 shubunkins (Carassius auratus). Plasma levels of thyroid hormones were quantified immunoenzymatically by ELISA assay. Nonylphenol induced a significant decrease of thyroxin levels, whereas no effect on triiodothyronine concentrations was detected. No histopathological changes were detected in thyroid or testes. The toxicological data confirmed that nonylphenol exerts an estrogenic effect on male fish. In addition, nonylphenol was suspected to inhibit the thyroid hormones balance, suggesting the thyroid should be included among the other endocrine glands susceptible to endocrine disruption.

The brain–pituitary–gonad axis in male teleosts, with special emphasis on flatfish (Pleuronectiformes)

The key component regulating vertebrate puberty and sexual maturation is the endocrine system primarily effectuated along the brain-pituitary-gonad (BPG) axis. By far most investigations on the teleost BPG axis have been performed on salmonids, carps, catfish and eels. Accordingly, earlier reviews on the BPG axis in teleosts have focused on these species, and mainly on females (e.g. 'Fish Physiology, vol. IXA. Reproduction (1983) pp. 97'; 'Proceedings of the Fourth International Symposium on the Reproductive Physiology of Fish. FishSymp91, Sheffield, UK, 1991, pp. 2'; 'Curr. Top. Dev. Biol. 30 (1995) pp. 103'; 'Rev. Fish Biol. Fish. 7 (1997) pp. 173'; 'Proceedings of the Sixth International Symposium on the Reproductive Physiology of Fish. John Grieg A/S, Bergen, Norway, 2000, pp. 211'). However, in recent years new data have emerged on the BPG axis in flatfish, especially at the level of the brain and pituitary. The evolutionarily advanced flatfishes are important model species both from an evolutionary point of view and also because many are candidates for aquaculture. The scope of this paper is to review the present status on the male teleost BPG axis, with an emphasis on flatfish. In doing so, we will first discuss the present understanding of the individual constituents of the axis in the best studied teleost models, and thereafter discuss available data on flatfish. Of the three constituents of the BPG axis, we will focus especially on the pituitary and gonadotropins. In addition to reviewing recent information on flatfish, we present some entirely new information on the phylogeny and molecular structure of teleost gonadotropins.

Structure, expression, and cluster organization of genes encoding gonadotropin-releasing hormone receptors found in the neural complex of the ascidian Ciona intestinalis

Gonadotropin-releasing hormone (GnRH) controls reproductive functions in vertebrates. In the present study, two distinct homologues (Ci-GnRHR1 and Ci-GnRHR2) of the vertebrate GnRH receptor (GnRHR) were identified by cDNA cloning from the neural complex of the ascidian Ciona intestinalis. The Ci-GnRHR1 and Ci-GnRHR2 genes are closely linked to each other and form a cluster with another GnRHR-like gene in the genome. Ci-GnRHR1 and Ci-GnRHR2 are more closely related to vertebrate GnRHRs than to the GnRHR-related protein of Drosophila. The phylogenetic analysis indicates that vertebrates and urochordates independently acquired multiple GnRHRs by gene duplications that occurred during the evolution of each lineage. A voltage clamp of Xenopus oocytes injected with synthetic Ci-GnRHR1 mRNA revealed inward currents in response to an ascidian form of GnRH, suggesting that Ci-GnRHR1 is a bona-fide GnRHR. Expression patterns of Ci-GnRHR1 and Ci-GnRHR2 suggest that a GnRH signaling system is involved in regulation of neuronal and reproductive processes as well as in other physiological functions in ascidians.

Gonadotropin-releasing hormone neuronal development during the sensitive period of temperature sex determination in the pejerrey fish, Odontesthes bonariensis

The development of gonadotropin-releasing hormone (GnRH) neurons was studied in relation to the sensitive period of thermolabile sex determination in the pejerrey Odontesthes bonariensis, an atherinid fish from South America. Fish were raised from hatching at three different temperatures: 17 degrees C (100% females), 24 degrees C (70% females), and 29 degrees C (100% males). Three groups of immunoreactive GnRH (ir-GnRH) neurons were identified at the terminal nerve ganglion (TNG), the midbrain tegmentum (MT), and the preoptic area (POA). Immunoreactive GnRH (ir-GnRH) neurons were identified in the TNG at hatching (day 0) and in the MT at day 3 at all the experimental temperatures. In the POA ir-GnRH neurons were identified in the nucleus preopticus periventricularis simultaneously with the first appearance of ir-GnRH fibers in the pituitary on days 11, 14, and 17 for larvae kept at 29, 24, and 17 degrees C, respectively. The number of ir-GnRH neurons in the TNG did not show any statistical difference between temperatures. The number of ir-GnRH neurons in the MT increased in number during the experiment for larvae kept at 17 and 24 degrees C but decreased between days 17 and 31 in larvae kept at 29 degrees C. The number of ir-GnRH neurons in the POA increased during development with a peak at day 28 for all temperatures studied and the magnitude of this peak showed a correlation with incubation temperature. These results reinforce the notion that the hypothalamus-pituitary-gonadal axis is active during sex determination in pejerrey suggesting a possible role of the central nervous system and GnRH in this process. It is also suggested that GnRH neurons located in the preoptic area might be the physiological transducers of temperature during the temperature sensitive period in this species.

Evolutionary aspects of cellular communication in the vertebrate hypothalamo-hypophysio-gonadal axis

This review emphasizes the comparative approach for developing insight into knowledge related to cellular communications occurring in the hypothalamus-pituitary-gonadal axis. Indeed, research on adaptive phenomena leads to evolutionary tracks. Thus, going through recent results, we suggest that pheromonal communication precedes local communication which, in turn, precedes communication via the blood stream. Furthermore, the use of different routes of communication by a certain mediator leads to a conceptual change related to what hormones are. Nevertheless, endocrine communication should leave out of consideration the source (glandular or not) of mediator. Finally, we point out that the use of lower vertebrate animal models is fundamental to understanding general physiological mechanisms. In fact, different anatomical organization permits access to tissues not readily approachable in mammals.

Gonadotropin-releasing hormone (GnRH): from fish to mammalian brains

This work deals with a family of neuropeptides, gonadotropin-releasing hormone (GnRH), that play a key role in the development and maintenance of reproductive function in vertebrates. 2. Until now, a total of 16 GnRH structural variants have been isolated and characterized from vertebrate and protochordate nervous tissue. All vertebrate species already investigated have at least two GnRH forms coexisting in the central nervous system. However, it is now well accepted that three forms of GnRH in early and late evolved bony fishes are present. 3. In these cases, cGnRH-II is expressed by midbrain neurons, a species-specific GnRH is present mainly in the preoptic area and the hypothalamus, and sGnRH is localized in the terminal nerve ganglion (TNG). In this context it is possible to think that three GnRH forms and three GnRH receptor (GnRH-R) subtypes are expressed in the central nervous system of a given species. 4. Then it is possible to propose three different GnRH lineages expressed by distinct brain areas in vertebrates: (1) the conserved cGnRH-II or mesencephalic lineage; or (2) the hypothalamic or "releasing" lineage whose primary structure has diverged by point mutations (mGnRH and its orthologous forms: hrGnRH, wfGnRH, cfGnRH, sbGnRH, and pjGnRH); and (3) the telencephalic sGnRH form. Also different GnRH nomenclatures are discussed.

The effects of thyroxine or a GnRH analogue on thyroid hormone deiodination in the olfactory epithelium and retina of rainbow trout, Oncorhynchus mykiss, and sockeye salmon, Oncorhynchus nerka

Using low (0.5nM) substrate levels we determined the activities of thyroxine (T4) outer-ring deiodination (ORD), T4 inner-ring deiodination (T4IRD) and 3,5,3(')-triiodothyronine (T3) IRD activities in the olfactory epithelium (OLF) and retina (RET) of laboratory-held immature 1-year-old rainbow trout and immature 2.5-year-old sockeye salmon. In both species all three deiodination activities were detected in OLF and RET. For OLF, no particular pathway predominated and activities were similar to those of brain. For RET, T3IRD activity was greater than T4ORD activity and in sockeye RET T3IRD activity exceeded that of liver. Trout immersion for 6 weeks in 100ppm T4 increased plasma T4 levels 3-fold and plasma T3 levels by 50% and caused the anticipated autoregulatory responses in brain and liver deiodination ( downward arrow T4ORD, upward arrow T4IRD, and upward arrow T3IRD); OLF deiodination and RET T4ORD activity were unaltered but RET T4IRD and T3IRD activities increased dramatically. Two injections of a GnRH analogue (20 microgkg(-1)) into sockeye increased plasma T3 levels but not T4 levels and decreased RET T4IRD and T3IRD activities without changing liver, brain, or OLF deiodination. We conclude that in salmonids the main TH deiodination pathways occur in OLF but show no regulation by T4 or GnRH. In contrast, T3IRD activity predominates in RET and can be regulated by T4 and GnRH, suggesting that for RET plasma may be the major T3 source. These findings have implications for thyroidal regulation of sensory functions during salmonid diadromous migrations.

Gonadotropin-releasing hormone-immunoreactive neurons and associated nicotinamide adenine dinucleotide phosphate-diaphorase-positive neurons in the brain of a teleost, Rhodeus amarus

Using combined nicotinamide adenine dinucleotide phosphate-diaphorase (NADPHd) histochemistry and salmon gonadotropin-releasing hormone (sGnRH) immunocytochemistry, it is reported for the first time that possible potential contacts occur between the nitric oxide (NO)- and the GnRH-containing neurons in the brain of a freshwater teleost, Rhodeus amarus. GnRH-immunoreactive (ir) neurons were observed in the olfactory nerve (OLN), olfactory bulb (OB), medial olfactory tract (MOT), ventral telencephalon (VT), nucleus preopticus periventricularis (NPP), nucleus lateralis tuberis (NLT), and midbrain tegmentum (MT). Although NADPHd neurons were widely distributed in the brain, only those having an association with GnRH-ir neurons are described. Based on the nature of the association between the GnRH and the NADPHd neurons, the former were classified into three types. The Type I GnRH neurons were characterized by the presence of NADPHd-positive granules in the perikarya and processes and occurred in the OLN, OB, MOT, and VT. The Type II GnRH neurons, having soma-soma or soma-process contacts with the NADPHd neurons, were restricted to the MT; the long processes of NADPHd cells crossed over either the perikarya or the thick processes of GnRH cells. However, the Type III GnRH neurons, found in the NPP and NLT, did not show direct contact, but a few NADPHd fibers were present in the vicinity. The terminal-soma contacts in the olfactory system and the VT and the soma-soma contacts in the MT represent the sites of possible potential contacts indicating a direct NO involvement in GnRH function, although NO action by diffusion remains possible. NO may influence the NPP and NLT GnRH cells by diffusion only, since a direct contact was not observed.