Interaction between thyroid hormones and gonadotropin inhibitory hormone in ex vivo culture of zebrafish testis: An approach to study multifactorial control of spermatogenesis

Arginine Vasotocin Directly Regulates Spermatogenesis in Adult Zebrafish (Danio rerio) Testes

The neuropeptide vasopressin is known for its regulation of osmotic balance in mammals. Arginine vasotocin (AVT) is a non-mammalian homolog of this neuropeptide that is present in fish. Limited information suggested that vasopressin and its homologs may also influence reproductive function. In the present study, we investigated the direct effect of AVT on spermatogenesis, using zebrafish as a model organism. Results demonstrate that AVT and its receptors (avpr1aa, avpr2aa, avpr1ab, avpr2ab, and avpr2l) are expressed in the zebrafish brain and testes. The direct action of AVT on spermatogenesis was investigated using an ex vivo culture of mature zebrafish testes for 7 days. Using histological, morphometric, and biochemical approaches, we observed direct actions of AVT on zebrafish testicular function. AVT treatment directly increased the number of spermatozoa in an androgen-dependent manner, while reducing mitotic cells and the proliferation activity of type B spermatogonia. The observed stimulatory action of AVT on spermiogenesis was blocked by flutamide, an androgen receptor antagonist. The present results support the novel hypothesis that AVT stimulates short-term androgen-dependent spermiogenesis. However, its prolonged presence may lead to diminished spermatogenesis by reducing the proliferation of spermatogonia B, resulting in a diminished turnover of spermatogonia, spermatids, and spermatozoa. The overall findings offer an insight into the physiological significance of vasopressin and its homologs in vertebrates as a contributing factor in the multifactorial regulation of male reproduction.

Gonadotropin-inhibitory hormone and its receptors in teleosts: Physiological roles and mechanisms of actions

Gonadotropin-inhibitory hormone (GnIH) was the first reported hypothalamic neuropeptide inhibiting reproduction in vertebrates. Since its discovery in the quail brain, its orthologs have been identified in a variety of vertebrate species and even protochordates. Depending on the species, the GnIH precursor polypeptides comprise two, three or four mature peptides of the RFamide family. It has been well documented that GnIH inhibits reproduction at the brain-pituitary-gonadal levels and participates in metabolism, stress response, and social behaviors in birds and mammals. However, most studies in fish have mainly been focused on the physiological roles of GnIH in the control of reproduction and results obtained are in some cases conflicting, leaving aside its potential roles in the regulation of other functions. In this manuscript we summarize the information available in fish with respect to the structural diversity of GnIH peptides and functional roles of GnIH in reproduction and other physiological processes. We also highlight the molecular mechanisms of GnIH actions on target cells and possible interactions with other neuroendocrine factors.

Dual effects of gonadotropin-inhibitory hormone on testicular development in prepubertal Minxinan Black rabbits (Oryctolagus cuniculus)

Gonadotropin-inhibitory hormone (GnIH) is a neurohormone that not only suppresses reproduction at the brain level but also regulates steroidogenesis and gametogenesis at the gonad level. However, its function in gonadal physiology has received little attention in rabbits. The main objective of this study was to evaluate the effects of GnIH on testicular development and function in prepubertal Minxinan Black rabbits (Oryctolagus cuniculus). In the present study, we investigated the serum reproductive hormone concentration, testicular parameters, morphology of seminiferous tubules, apoptosis of testicular cells, and expression of reproductive-related genes in male prepubertal Minxinan Black rabbits intraperitoneally administered with 0, 0.5, 5, or 50 μg quail GnIH-related peptides (qGnIH) for 10 days. Compared with the vehicle, administration with 5 μg of qGnIH downregulated the serum testosterone concentration and mRNA levels of spermatogenic genes (PCNA, FSHR, INHβA, HSF1, and AR) and upregulated the apoptosis rate of testicular cells; administration with 50 μg of qGnIH decreased the serum testosterone concentration and hypothalamic GnIH gene mRNA level and increased the serum LH concentration, pituitary LHβ gene mRNA level, testicular weight, gonadosomatic index (GSI), and spermatogenic cell layer thickness. It is concluded that GnIH could exert dual actions on testicular development depending on the male prepubertal rabbits receiving different intraperitoneal doses.

Impact of stress on male fertility: role of gonadotropin inhibitory hormone

Studies have implicated oxidative stress-sensitive signaling in the pathogenesis of stress-induced male infertility. However, apart from oxidative stress, gonadotropin inhibitory hormone (GnIH) plays a major role. The present study provides a detailed review of the role of GnIH in stress-induced male infertility. Available evidence-based data revealed that GnIH enhances the release of corticosteroids by activating the hypothalamic-pituitary-adrenal axis. GnIH also mediates the inhibition of the conversion of thyroxine (T4) to triiodothyronine (T3) by suppressing the hypothalamic-pituitary-thyroidal axis. In addition, GnIH inhibits gonadotropin-releasing hormone (GnRH), thus suppressing the hypothalamic-pituitary-testicular axis, and by extension testosterone biosynthesis. More so, GnIH inhibits kisspeptin release. These events distort testicular histoarchitecture, impair testicular and adrenal steroidogenesis, lower spermatogenesis, and deteriorate sperm quality and function. In conclusion, GnIH, via multiple mechanisms, plays a key role in stress-induced male infertility. Suppression of GnIH under stressful conditions may thus be a beneficial prophylactic and/or therapeutic strategy.

Thyroid axis participates in high-temperature-induced male sex reversal through its activation by the stress response

Environmental changes alter the sex fate in about 15% of vertebrate orders, mainly in ectotherms such as fish and reptiles. However, the effects of temperature changes on the endocrine and molecular processes controlling gonadal sex determination are not fully understood. Here, we provide evidence that thyroid hormones (THs) act as co-players in heat-induced masculinization through interactions with the stress axis to promote testicular development. We first demonstrated that the thyroid axis (through thyroid-related genes and T3 levels) is highly active in males during the gonadal development in medaka (Oryzias latipes). Similarly, T3 treatments promoted female-to-male sex reversal in XX embryos. Subsequently, embryonic exposure to temperature-induced stress up-regulated the genes related to the thyroid and stress axes with a final increase in T3 levels. In this context, we show that blocking the stress axis response by the loss of function of the corticotropin-releasing hormone receptors suppresses thyroid-stimulating hormone expression, therefore, heat-induced activation of the thyroid axis. Thus, our data showed that early activation of the stress axis and, in consequence, the TH axis, too, leaves us with that both being important endocrine players in inducing female-to-male reversal, which can help predict possible upcoming physiological impacts of global warming on fish populations.

NMR-based metabolomics applied to ecotoxicology with zebrafish (Danio rerio) as a prominent model for metabolic profiling and biomarker discovery: Overviewing the most recent approaches

Metabolomics is an innovative approach used in the medical, toxicological, and biological sciences. As an interdisciplinary topic, metabolomics and its relation with the environment and toxicological research are extensive. The use of substances, such as drugs and pesticides, contributes to the continuous releasing of xenobiotics into the environment, harming organisms and their habitats. In this context, fish are important bioindicators of the environmental condition and have often been used as model species. Among them, zebrafish (Danio rerio) presents itself as a versatile and straightforward option due to its unique attributes for research. Zebrafish proves to be a valuable model for toxicity assays and also for metabolomics profiling by analytical tools. Thus, NMR-based metabolomics associated with statistical analysis can reasonably assist researchers in critical factors related to discovering and validating biomarkers through accurate diagnosis. Therefore, this review aimed to report the studies that applied zebrafish as a model for (eco)toxicological assays and essentially utilized NMR-based metabolomics analysis to assess the biochemical profile and thus suggest the potential biological marker.

Cannabidiol improves Nile tilapia cichlid fish welfare

Cannabidiol (CBD) is a substance derived from Cannabis sativa, widely studied in medicine for controlling neural diseases in humans. Besides the positive effects on humans, it also presents anxiolytic proprieties and decreases aggressiveness and stress in mammals. Therefore, CBD has the potential to increase welfare in reared animals, as it seems to reduce negative states commonly experienced in artificial environments. Here, we tested the effect of different CBD doses (0, 1, 10 and 20 mg/kg) on aggressiveness, stress and reproductive development of the Nile tilapia (Oreochromis niloticus) a fish reared worldwide for farming and research purposes. CBD mixed with fish food was offered to isolated fish for 5 weeks. The 10 mg/kg dose decreased fish's aggressiveness over time, whereas 20 mg/kg attenuated non-social stress. Both doses decreased the baseline cortisol level of fish and increased the gonadosomatic index. However, CBD 1 and 10 mg/kg doses decreased the spermatozoa number. No CBD dose affected feeding ingestion and growth variables, showing that it is not harmful to meat production amount. Despite the effect on spermatozoa, CBD supplementation exhibits high potential to benefit animals' lives in artificial environments. Therefore, we showed for the first time that CBD could be used as a tool to increase non-mammal welfare, presenting a great potential to be explored in other husbandry and captivity species.

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.

Maternally-transferred thyroid hormones and life-history variation in birds

In vertebrates, thyroid hormones (THs) play an important role in the regulation of growth, development, metabolism, photoperiodic responses and migration. Maternally transferred THs are important for normal early phase embryonic development when embryos are not able to produce endogenous THs. Previous studies have shown that variation in maternal THs within the physiological range can influence offspring phenotype.

Given the essential functions of maternal THs in development and metabolism, THs may be a mediator of life‐history variation across species.

We tested the hypothesis that differences in life histories are associated with differences in maternal TH transfer across species. Using birds as a model, we specifically tested whether maternally transferred yolk THs covary with migratory status, developmental mode and traits related to pace‐of‐life (e.g. basal metabolic rate, maximum life span).

We collected un‐incubated eggs (n = 1–21 eggs per species, median = 7) from 34 wild and captive bird species across 17 families and six orders to measure yolk THs [both triiodothyronine (T3) and thyroxine (T4)], compiled life‐history trait data from the literature and used Bayesian phylogenetic mixed models to test our hypotheses.

Our models indicated that both concentrations and total amounts of the two main forms of THs (T3 and T4) were higher in the eggs of migratory species compared to resident species, and total amounts were higher in the eggs of precocial species, which have longer prenatal developmental periods, than in those of altricial species. However, maternal yolk THs did not show clear associations with pace‐of‐life‐related traits, such as fecundity, basal metabolic rate or maximum life span.

We quantified interspecific variation in maternal yolk THs in birds, and our findings suggest higher maternal TH transfer is associated with the precocial mode of development and migratory status. Whether maternal THs represent a part of the mechanism underlying the evolution of precocial development and migration or a consequence of such life histories is currently unclear. We therefore encourage further studies to explore the physiological mechanisms and evolutionary processes underlying these patterns.

Cylindrospermopsin directly disrupts spermatogenesis in isolated male zebrafish testis

Cylindrospermopsin (CYN) is a cytotoxin, and its documented effects in mammals include damage to several organs. CYN also has hormone-disrupting properties, including estrogenic activity, progesterone production inhibition, and apoptosis induction. While CYN has been reported to exert reproductive toxicity in mice, little is known about its effect on fish reproductive function. Using ex vivo organ culture, we investigated the direct action of CYN on the male reproductive system. Isolated zebrafish testis was exposed to 250, 500, and 1000 µg/L CYN for 24 h and 7 d, followed by histo-morphological analysis. The results demonstrate that exposure to CYN led to a decrease in cell types from all three phases of spermatogenesis in zebrafish testis. There were also significant changes in fshr, lhr, and igf3 transcript levels, as well as testosterone secretion following exposure to CYN. In summary, this study provides novel information on the adverse effects of CYN on testicular spermatogenesis and male reproduction in zebrafish. These results provide a framework for a better understanding of CYN toxicity and the mechanism underlying the adverse action of CYN on male reproduction in fish.