Increased gonadotropin levels in goldfish do not result in alterations in circulating thyroid hormone levels

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.

Response of the thyroid axis and appetite-regulating peptides to fasting and overfeeding in goldfish (Carassius auratus)

The thyroid axis is a major regulator of metabolism and energy homeostasis in vertebrates. There is conclusive evidence in mammals for the involvement of the thyroid axis in the regulation of food intake, but in fish, this link is unclear. In order to assess the effects of nutritional status on the thyroid axis in goldfish, Carassius auratus, we examined brain and peripheral transcripts of genes associated with the thyroid axis [thyrotropin-releasing hormone (TRH), thyrotropin-releasing hormone receptors (TRH-R type 1 and 2), thyroid stimulating hormone beta (TSHβ), deiodinase enzymes (DIO2, DIO3) and UDP-glucoronsyltransferase (UGT)] and appetite regulators [neuropeptide Y (NPY), proopiomelanocortin (POMC), agouti-related peptide (AgRP) and cholecystokinin (CCK)] in fasted and overfed fish for 7 and 14 day periods. We show that the thyroid axis responds to overfeeding, with an increase of brain TRH and TSHβ mRNA expression after 14 days, suggesting that overfeeding might activate the thyroid axis. In fasted fish, hepatic DIO3 and UGT transcripts were downregulated from 7 to 14 days, suggesting a time-dependent inhibition of thyroid hormone degradation pathways. Nutritional status had no effect on circulating levels of thyroid hormone. Central appetite-regulating peptides exhibited temporal changes in mRNA expression, with decreased expression of the appetite-inhibiting peptide POMC from 7 to 14 days for both fasted and overfed fish, with no change in central NPY or AgRP, or intestinal CCK transcript expression. Compared to control fish, fasting increased AgRP mRNA expression at both 7 and 14 days, and POMC expression was higher than controls only at 7 days. Our results indicate that nutritional status time-dependently affects the thyroid axis and appetite regulators, although no clear correlation between thyroid physiology and appetite regulators could be established. Our study helps to fill a knowledge gap in current fish endocrinological research on the effects of energy balance on thyroid metabolism and function.

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.

Teleost Metamorphosis: The Role of Thyroid Hormone

In most teleosts, metamorphosis encompasses a dramatic post-natal developmental process where the free-swimming larvae undergo a series of morphological, cellular and physiological changes that enable the larvae to become a fully formed, albeit sexually immature, juvenile fish. In all teleosts studied to date thyroid hormones (TH) drive metamorphosis, being the necessary and sufficient factors behind this developmental transition. During metamorphosis, negative regulation of thyrotropin by thyroxine (T4) is relaxed allowing higher whole-body levels of T4 that enable specific responses at the tissue/cellular level. Higher local thyroid cellular signaling leads to cell-specific responses that bring about localized developmental events. TH orchestrate in a spatial-temporal manner all local developmental changes so that in the end a fully functional organism arises. In bilateral teleost species, the most evident metamorphic morphological change underlies a transition to a more streamlined body. In the pleuronectiform lineage (flatfishes), these metamorphic morphological changes are more dramatic. The most evident is the migration of one eye to the opposite side of the head and the symmetric pelagic larva development into an asymmetric benthic juvenile. This transition encompasses a dramatic loss of the embryonic derived dorsal-ventral and left-right axis. The embryonic dorsal-ventral axis becomes the left-right axis, whereas the embryonic left-right axis becomes, irrespectively, the dorsal-ventral axis of the juvenile animal. This event is an unparalleled morphological change in vertebrate development and a remarkable display of the capacity of TH-signaling in shaping adaptation and evolution in teleosts. Notwithstanding all this knowledge, there are still fundamental questions in teleost metamorphosis left unanswered: how the central regulation of metamorphosis is achieved and the neuroendocrine network involved is unclear; the detailed cellular and molecular events that give rise to the developmental processes occurring during teleost metamorphosis are still mostly unknown. Also in flatfish, comparatively little is still known about the developmental processes behind asymmetric development. This review summarizes the current knowledge on teleost metamorphosis and explores the gaps that still need to be challenged.

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.

Flatfish metamorphosis: a hypothalamic independent process?

Anuran and flatfish metamorphosis are tightly regulated by thyroid hormones that are the necessary and sufficient factors that drive this developmental event. In the present study whole mount in situ hybridization (WISH) and quantitative PCR in sole are used to explore the central regulation of flatfish metamorphosis. Central regulation of the thyroid in vertebrates is mediated by the hypothalamus-pituitary-thyroid (HPT) axis. Teleosts diverge from other vertebrates as hypothalamic regulation in the HPT axis is proposed to be through hypothalamic inhibition although the regulatory factor remains enigmatic. The dynamics of the HPT axis during sole metamorphosis revealed integration between the activity of the thyrotrophes in the pituitary and the thyroid follicles. No evidence was found supporting a role for thyroid releasing hormone (trh) or corticotrophin releasing hormone (crh) in hypothalamic control of TH production during sole metamorphosis. Intriguingly the results of the present study suggest that neither hypothalamic trh nor crh expression changes during sole metamorphosis and raises questions about the role of these factors and the hypothalamus in regulation of thyrotrophs.

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.

Thyroid hormones in male reproductive development: evidence for direct crosstalk between the androgen and thyroid hormone axes

Thyroid hormones (THs) exert a broad range of effects on development in vertebrate species, demonstrating connections in nearly every biological endocrine system. In particular, studies have shown that THs play a role in sexual differentiation and gonadal development in mammalian and non-mammalian species. There is considerable evidence that the effects of THs on reproductive development are mediated through the female hormonal axis; however, recent findings suggest a more direct crosstalk between THs and the androgen axis. These findings demonstrate that THs have considerable influence in the sexual ontogeny of male vertebrates, through direct interactions with select sex-determining-genes and regulation of gonadotropin production in the hypothalamus-pituitary-gonad axis. THs also regulate androgen biosynthesis and signaling through direct and indirect regulation of steroidogenic enzyme expression and activity. Novel promoter analysis presented in this work demonstrates the potential for direct and vertebrate wide crosstalk at the transcriptional level in mice (Mus musculus), Western clawed frogs (Silurana tropicalis) and medaka (Oryzias latipes). Cumulative evidence from previous studies; coupled with novel promoter analysis suggests mechanisms for a more direct crosstalk between the TH and male reproductive axes across vertebrate species.

Thyrotropic activity of recombinant human glycoprotein hormone analogs and pituitary mammalian gonadotropins in goldfish (Carassius auratus): insights into the evolution of thyrotropin receptor specificity

Thyrotropin (TSH) is a pituitary glycoprotein hormone heterodimer that binds to its G-protein coupled receptor (TSH-R) at the thyroid to promote the synthesis and secretion of thyroid hormone. Very little is known about TSH-TSH-R interactions in teleost fish. Mammalian gonadotropins have been reported to have an intrinsic ability to activate teleost fish TSH-Rs, suggesting the TSH-R in teleost fish is more promiscuous than in other vertebrates. In this study we utilized the goldfish T(4)-release response and recombinant human TSH analogs as in vivo tools to evaluate the structural constraints on hormone-receptor interactions. We found that four positively charged lysines substituted for neutral or negatively charged amino acids within positions 11-20 of the glycoprotein hormone subunit α (GSUα) significantly increased biological activity of hTSH in fish, as it does in mammals. We further found that bovine follicle stimulating hormone but not luteinizing hormone, whose GSUα subunits also contain four lysine or arginine amino acid residues in the N-terminal portion of GSUα, was thyrotropic in goldfish, suggesting gonadotropin β subunit contributes to the heterothyrotropic activity. Though recombinant human FSH did not produce a dose-dependent increase in T(4), thyrotropic activity could be acquired with the addition of positively charged amino acids at the N-terminal portion of its GSUα, confirming the importance of the charge on those amino acids for activation of the goldfish TSH-R. These studies demonstrate that mammalian glycoprotein hormone analogs can be utilized to evaluate the conservation of receptor binding and activation mechanisms between fish and mammals.

Thyrotropin in teleost fish

Thyrotropin (TSH), a pituitary glycoprotein hormone that stimulates the thyroid gland, has been cloned and sequenced from over a dozen teleost fish species. Although TSH is established as a primary driver of systemic thyroid status in mammals, its importance in the regulation of fish thyroid function is still uncertain. We review recent studies indicating that TSH structure is highly conserved across species representing six teleost families. These studies have found TSH messenger RNA consistently expressed in teleost pituitary tissue, although ectopic expression, particularly in gonads, has also been observed. They have also provided evidence for negative feedback inhibition of TSH expression by thyroid hormones, as well as stimulation by hypothalamic peptides. Descriptive studies have found increased TSHbeta expression associated with life history events thought to be promoted by thyroid hormones. These results, coupled with the discovery of a G-protein coupled TSH receptor in several teleost species, supports an active and conserved role for TSH in the regulation of teleost thyroid function. The relative importance of central pathways in regulating thyroid hormone provision to targets and the identity of a proposed thyrotropin-inhibiting factor in teleost fish are still unanswered questions whose resolution will be facilitated by development of methods to measure circulating TSH and its secretion from the pituitary gland.

Functional significance of a truncated thyroid receptor subtype lacking a hormone-binding domain in goldfish

Thyroid hormones are important mediators of growth and development in vertebrates and act by binding to a specific family of thyroid receptors (TRs). The TRs belong to the nuclear receptor superfamily, with two conserved regions, a DNA binding domain and a ligand binding domain (LBD). We recently demonstrated the presence of four TR subtypes in goldfish, two with complete DNA binding domains and LBDs (TRalpha-1 and TRbeta) and two novel forms including a transcript resembling TRalpha with variation in the LBD as well as a TRalpha-truncated (TRalpha-t) form lacking a LBD. To study the functional significance of TR subtypes, we first investigated the regulation of hepatic goldfish deiodinase type 3 (D3) by T3 and validated a bioassay in which D3 gene expression is up-regulated significantly in vivo and in vitro. Using short interfering RNA, TRalpha-1, TRbeta, or TRalpha-t was specifically knocked down and thyroid hormone-induced D3 gene expression was measured. Short interfering RNA against TRalpha-1 or TRbeta reduced the T3 induction of deiodinase gene expression to 50% or less than 25% of control (T3 treated) cells, respectively. Knocking down TRalpha-t alone, however, increased D3 expression 500-fold supporting the hypothesis that TRalpha-t plays a modulatory role in thyroid hormone-induced gene expression. Our results provide important insight into thyroid receptor biology in goldfish and a framework for the better understanding of thyroid receptor function in all vertebrates.

The effects of photoperiod and feeding on the diurnal rhythm of circulating thyroid hormones in the red drum, Sciaenops ocellatus

Available data in cyprinid and salmonid species indicate that nutrient intake sustains thyroidal rhythmicity and that time of feeding may influence the amplitude, but not the phase, of diurnal thyroid hormone cycles. Several experiments were conducted to characterize the nature of thyroidal rhythmicity in a more derived perciform teleost, the red drum. These studies were designed to test the following hypotheses: (1) that feeding time will alter the amplitude of the thyroid hormone rhythm without altering its phase and (2) that food deprivation will diminish the amplitude of the thyroid hormone rhythm. Circulating T(4) levels in this species exhibit high-amplitude diurnal rhythms, whereas circulating T(3) levels fluctuate within a more narrow range. Fish were reared under a 12L:12D photoperiod and fed 5% body weight once daily either at dawn or at dusk. Feeding time had no discernible effect on the phase of the T(4) cycle, but altered the amplitude of the cycle. Dawn-fed fish had significantly greater mean peak levels of T(4) than dusk-fed fish, although there was no difference in daily mean levels in both groups of fish. When red drum were deprived of food, significant declines in plasma glucose, HSI, and liver glycogen content occurred within 3 days. When red drum were sampled once per day after 3, 7, or 11 days of food deprivation there were no consistent changes in circulating T(4) and T(3) levels compared to those of fed controls. However, significant declines in circulating T(4) and T(3) levels in response to food deprivation were detected with a diurnal sampling protocol. Within 3 days of food deprivation, T(4) levels were significantly reduced compared to those in fed controls and not significantly different from T(4) levels after 10 days of food deprivation. T(3) levels exhibited a stepwise decline in circulating levels during food deprivation. These data indicate that both feeding time and nutrient status exert their effects on thyroid hormone rhythms by modifying the amplitude of these cycles. These data also underscore the importance of incorporating a consideration of endocrine rhythmicity into sampling protocols.

In vitro thyrotropin-releasing activity of corticotropin-releasing hormone-family peptides in coho salmon, Oncorhynchus kisutch

Investigations of hypothalamic regulation of fish thyrotropin (TSH) secretion and subsequent thyroid activity have been impeded by the lack of a reliable assay for TSH. Using a recently developed radioimmunoassay (RIA) for coho salmon TSH we employed an in vitro pituitary cell culture technique to examine regulation of TSH secretion by corticotropin-releasing hormone (CRH) family peptides [ovine CRH (oCRH), carp urotensin I (UI), and frog sauvagine (SV)] as well as thyrotropin-releasing hormone (TRH), salmon growth hormone-releasing hormone (sGHRH), and salmon gonadotropin-releasing hormone (sGnRH). At concentrations of 0.01 to 100 nM, TRH, sGHRH, and sGnRH did not stimulate TSH secretion from coho salmon pituitary cells. However, at these same concentrations, both oCRH and SV caused a significant and concentration-dependent increase in TSH secretion; whereas, UI was highly stimulatory at all concentrations tested. In a related experiment we examined the effect of alpha-helical CRF(9-41) on oCRH-stimulated TSH release by pituitary cells. alpha-Helical CRF(9-41) is an analogue of CRH that has been shown by others to antagonize the adrenocorticotropic hormone (ACTH)-releasing activity of CRH in goldfish. Preincubation of cells with 1 microM alpha-helical CRF(9-41) for 4 h caused a significant suppression of the TSH-releasing activity of oCRH at 1.0 and 10 nM concentrations. The results of these experiments demonstrate the potency of a CRH-like peptide in the hypothalamic regulation of TSH in fish and reveal similarities in the inhibition of the response of both the thyroid and interrenal axis of fish to alpha-helical CRF(9-41).

Hypothalamic regulation of the pituitary-thyroid axis in the tilapia Oreochromis mossambicus

Electrolytic lesioning of the preoptic area resulted in an increase in plasma thyroxine (T4) and reverse triiodothyronine (rT3) 10 days later; plasma triiodothyronine (T3) levels were not affected, so that there was also a significant decrease in the T3:T4, but not rT3:T4, ratios. No significant changes in T4, T3, or rT3 levels were observed in fish with lesions in either the anterior or posterior portions of the lateral tuberal nucleus. The pituitary contents of growth hormone and the two prolactins were not affected by any lesion. This indicates that the preoptic area may play a role in the inhibitory regulation of the pituitary-thyroid axis in Oreochromis mossambicus, presumably by way of effects on thyrotropin secretion.

Several hypothalamic peptides stimulate in vitro thyrotropin secretion by pituitaries of anuran amphibians

The effects of several hypothalamic peptides on hormone secretion by pituitaries of three species of anuran amphibians were investigated using in vitro techniques. Secretion of thyrotropic bioactivity (designated thyrotropin or TSH) was quantified by bioassay of the pituitary incubation medium using thyroxine (T4) production by paired thyroids from the same animals. Pituitaries from adult male Rana pipiens were cultured in medium alone, 10 or 100 ng/ml thyrotropin-releasing hormone (TRH), 1000 ng/ml ovine corticotropin-releasing hormone (oCRH), or 300 ng/ml synthetic mammalian gonadotropin-releasing hormone (mGnRH) (these represent approximately equimolar doses) for two 2-hr incubation periods. TSH secretion by control glands was nondetectable, but glands exposed to TRH increased their secretion of TSH in a dose-dependent manner. Both oCRH and mGnRH also stimulated significant increases in TSH. oCRH produced greater output of TSH than did the other two peptides and mGnRH was less active than TRH. Secretion of immunoreactive gonadotropin (GtH) was increased by mGnRH, but not by the other two peptides. Pituitaries from two other anuran species, Hyla regilla and Xenopus laevis, also responded to 100 ng/ml TRH by releasing TSH. These results provide the first unequivocal evidence that TRH can act directly on the anuran amphibian pituitary to stimulate the secretion of TSH, and suggest that the presence of functional TRH receptors on pituitary thyrotropes may be of greater phylogenetic antiquity than has been assumed previously. Furthermore, these data suggest the potential for multihormonal control of TSH secretion in frogs.

Luteinizing hormone-releasing hormone induces thyroxine release together with testosterone in the neotenic axolotl Ambystoma mexicanum

In male neotenic axolotls Ambystoma mexicanum plasma concentrations of thyroxine (T4) and testosterone were increased following intravenous injection of 10 micrograms luteinizing hormone-releasing hormone. A dose of 50 micrograms influenced only plasma T4 levels. This observation suggests for the first time that a hypothalamic hormone is capable of stimulating the thyroidal axis in the neotenic axolotl.

Luteinizing hormone-releasing hormone as a potent stimulator of the thyroidal axis in ranid frogs

Plasma concentrations of T4, measured by radioimmunoassay, were raised significantly 2 and 4 hr after intravenous injection of synthetic luteinizing hormone-releasing hormone (LHRH) in Rana ridibunda (1 and 10 micrograms on 2 consecutive days) and in Rana esculenta (10 micrograms). A dose of 1 microgram LHRH was not so effective as 50 micrograms synthetic thyrotropin-releasing hormone (TRH) when injected in Rana ridibunda in November. However 10 micrograms LHRH was equipotent to 50 micrograms TRH. In February somewhat less than half of the Rana temporaria group was responsive to LHRH. There is no clear indication that fluctuating plasma T3 concentrations were caused by LHRH or TRH. Preinjection levels of T3 and T4 were higher during the breeding season (April) in R. esculenta (resp. 35.4 +/- 1.4 pg/ml; 744 +/- 134 pg/ml; n = 22) compared to the basal concentrations in the very closely related Rana ridibunda (November) (resp. 15.2 +/- 1.1; 162 +/- 24 pg/ml; n = 28). Four days after removal of the pars distalis plasma T4 concentrations were significantly decreased in Rana esculenta, whereas T3 could stay longer in circulation. T3 and T4 content of the thyroids was not altered by the short-term hypophysectomy. Injection of 10 micrograms LHRH had no influence on plasma T4 nor testosterone concentrations in these frogs, contrary to the sham-ectomized animals in which plasma testosterone remained elevated longer than T4. The results suggest that the stimulatory effect of intravenous injected LHRH on thyroid (and gonadal) activity in the frog is primarily mediated through the hypophysis. They also point to a possible correlation between the gonadal and thyroidal axis.