Glucocorticoid receptor exhibits sexually dimorphic expression in the medaka brain

Temperature- and genotype-dependent stress response and activation of the hypothalamus-pituitary-interrenal axis during temperature-induced sex reversal in pejerrey Odontesthes bonariensis, a species with genotypic and environmental sex determination

In the pejerrey Odontesthes bonariensis (Atheriniformes, Atherinopsidae), exposure to high and low temperatures during the critical period of sex determination (CPSD) induce testicular and ovarian differentiation, respectively, regardless of the presence or not of the sex determining gene amhy, which is crucial for testis formation only at intermediate, sexually neutral temperatures. In this study we explored the existence of genotype-specific signaling of Crh (Corticotropin Releasing Hormone) family genes and their associated carrier protein, receptors, and other stress-related genes in response to temperature during the CPSD and the potential involvement of the central nervous system via the hypothalamus-pituitary-interrenal (HPI) axis in the sex determination of this species. The Crh family genes crhb, uts1, ucn3, the receptor crhr1 and the stress-related genes gr1, gr2, nr3c2 were transiently upregulated in the heads of pejerrey larvae during the CPSD by high temperature alone or in combination with other factors. Only crhr2 transcript abundance was not influenced by temperature but independently by time and genotype. In most cases, mRNA abundance was higher in the XX heads compared to that of XY individuals. The mRNAs of some of these genes were localized in the hypothalamus of pejerrey larvae during the CPSD. XX larvae also showed higher whole-body cortisol titers than the XY, downregulation of cyp19a1a and upregulation of the testis-related genes amhy/amha in trunks (gonads) and were 100% masculinized at the high temperature. In contrast, at the low temperature, crhbp and avt were upregulated in the heads, particularly the former in XY larvae. cyp19a1a and amhy/amha were up- and downregulated, respectively, in the gonads, and fish were 100% feminized. Signaling via the HPI axis was observed simultaneously with the first molecular signs of ongoing sex determination/differentiation in the gonads. Overall, the results strongly suggest a temperature-dependent, genotype-specific regulatory action of the brain involving the Crh family of stress-related genes on the process of environmental sex determination of pejerrey.

Sexually dimorphic distribution of kiss1 and kiss2 in the brain of yellowtail clownfish, Amphiprion clarkii

Kisspeptin system was shown to be a key factor in mediating social stress and reproduction. Yellowtail clownfish, Amphiprion clarkii, is a hermaphrodite fish, whose sex determination and gonadal development are affected by the social status of individuals. The yellowtail clownfish is a fantastic animal model to explore sex determination, but the social status and precise distribution of kiss mRNAs in the brain of this species are unknown. Hererin, a novel in situ hybridization technique, RNAscope, was used to investigate the distribution of kiss1 and kiss2 expressions in the brain of yellowtail clownfish. The coronal planes of brain showed that the kiss1 signal was mainly present in dorsal habenular nucleus (NHd) and kiss2 mRNA was widely expressed in telencephalon, midbrain, and hypothalamus, especially in dorsal part of the nucleus of the lateral recess (NRLd). Additionally, kiss1 and kiss2 signals have sexually dimorphic distribution. The kiss1 mRNA was distributed in NHd, the telencephalon, and lateral part of the diffuse nucleus of the inferior lobe (NDLIl) of females but in NHd and NDLIl of males. kiss2 signals were stronger in females than that in males. The distribution of kiss1 and kiss2 neurons in NHd of habenula and NRLd of hypothalamus may suggest that kiss genes associate environmental signaling and reproductive function in yellowtail clownfish.

Gene duplication of C-type natriuretic peptide-4 (CNP4) in teleost lineage elicits subfunctionalization of ancestral CNP

The diversified natriuretic peptide (NP) family, consisting of four CNPs (CNP1-4), ANP, BNP, and VNP, has been identified in the eel. Here, we successfully cloned additional cnp genes from the brain of eel (a basal teleost) and zebrafish (a later branching teleost). The genes were identified as paralogues of cnp4 generated by the third round of whole genome duplication (3R) in the teleost lineage, thereby being named eel cnp4b and zebrafish cnp4-like, respectively. To examine the histological patterns of their expressions, we employed a newly developed in situ hybridization (ISH) chain reaction using short hairpin DNAs, in addition to conventional ISH. Eel cnp4b was expressed in the medulla oblongata, while mRNAs of eel cnp4a (former cnp4) were localized in the preoptic area. In the zebrafish brain, cnp4-like mRNA was undetectable, while the known cnp4 was expressed in both the preoptic area and medulla oblongata. Together with the different mRNA distribution of cnp4a and cnp4b in eel peripheral tissues determined by RT-PCR and ISH, it is suggested that subfunctionalization by duplicated cnp4s in ancestral teleosts has been retained only in basal teleosts. Intriguingly, cnp4b-expressing neurons in the glossopharyngeal-vagal motor complex of the medulla oblongata were co-localized with choline acetyltransferase, suggesting an involvement of Cnp4b in swallowing and respiration functions that are modulated by the vagus. Since teleost Cnp4 is an ortholog of mammalian CNP, the identified localization of teleost Cnp4 will contribute to future studies aimed at deciphering the physiological functions of CNP.

Predisposition of Women to Cardiovascular Diseases: A Side-Effect of Increased Glucocorticoid Signaling During the COVID-19 Pandemic?

The novel coronavirus disease 2019 (COVID-19) pandemic has created a significant health crisis worldwide. To mitigate this disease's spread, "social distancing" and "shelter in place" have been implemented. While these actions have been critical to controlling the pandemic, they have short- and long-term mental health consequences due to increased stress. There is a strong association between mental stress and cardiovascular disease (CVD). Young women (pre-menopausal) are at high risk of developing CV events in response to mental stress compared to age-matched men. The mechanisms underlying women's increased reactivity and response to stress are mostly unknown. The present review summarizes the known physiological consequences of mental stress in women's CV health and the latest molecular findings of the actions of the primary stress hormones, glucocorticoids, on the CV system. The current data suggest a clear link between psychological stress and heart disease, and women have an increased sensitivity to the harmful effects of stress hormone signaling imbalances. Therefore, it is expected that with the given unprecedented levels of stress associated with the COVID-19 pandemic, women's CV health will be significantly compromised. It is critical to widen our understanding of the direct contribution of mental stress to CVD risk in women and to identify biochemical markers with predictive value for CVD in female patients with/without cardiovascular conditions who have experienced significant mental stress during the current pandemic.

Kiss2 but not kiss1 is involved in the regulation of social stress on the gonad development in yellowtail clownfish, Amphiprion clarkii

The yellowtail clownfish (Amphiprion clarkii) is a hermaphrodite fish, whose sex differentiation and gonad development are closely related to its social status. The kisspeptin/KissR system is regarded as a key factor mediating social stress on reproductive regulation. In order to understand the effects of social rank stress on the yellowtail clownfish gonadal differentiation, full-length cDNAs of two paralogous genes encoding kisspeptin (kiss1 and kiss2) and KissR (kissr2 and kissr3) were cloned and characterized. The results of real-time PCR showed that kiss1 was primarily expressed in the hypothalamus, and kiss2/kissr2 were abundantly expressed in the liver, while kissr3 was almost exclusively concentrated in the cerebellum and pituitary. Moreover, both Kiss1-10 and Kiss2-10 peptides could initiate downstream signaling pathways by interacting with cognate receptors expressed in eukaryotic cells. Among the three social status groups, the mRNA levels of kiss2 in the hypothalamus and pituitary as well as kissr2 in the pituitary were significantly higher in subordinate individuals (nonbreeders) than dominate individuals (females and males); while the mRNA levels of kissr3 in the hypothalamus and gonad were low in subordinate individuals. Furthermore, the plasma estradiol (E₂) and testosterone (T) levels were higher in subordinate than dominate individuals. This study shows that kiss2 is involved in the regulation of social stress on the gonad development in the yellowtail clownfish, but not kiss1.

Introducing the Amphibious Mudskipper Goby as a Unique Model to Evaluate Neuro/Endocrine Regulation of Behaviors Mediated by Buccal Sensation and Corticosteroids

Some fish have acquired the ability to breathe air, but these fish can no longer flush their gills effectively when out of water. Hence, they have developed characteristic means for defense against external stressors, including thirst (osmolarity/ions) and toxicity. Amphibious fish, extant air-breathing fish emerged from water, may serve as models to examine physiological responses to these stressors. Some of these fish, including mudskipper gobies such as Periophthalmodon schlosseri, Boleophthalmus boddarti and our Periophthalmus modestus, display distinct adaptational behaviors to these factors compared with fully aquatic fish. In this review, we introduce the mudskipper goby as a unique model to study the behaviors and the neuro/endocrine mechanisms of behavioral responses to the stressors. Our studies have shown that a local sensation of thirst in the buccal cavity—this being induced by dipsogenic hormones—motivates these fish to move to water through a forebrain response. The corticosteroid system, which is responsive to various stressors, also stimulates migration, possibly via the receptors in the brain. We suggest that such fish are an important model to deepen insights into the stress-related neuro/endocrine-behavioral effects.

Metabolic control of teleost reproduction by leptin and its complements: Understanding current insights from mammals

Reproduction is expensive. Hence, reproductive physiology is sensitive to an array of endogenous signals that provide information on metabolic and nutritional sufficiency. Although metabolic gating of reproductive function in mammals, as evidenced by studies demonstrating delayed puberty and perturbed fertility, has long been understood to be a function of energy sufficiency, an understanding of the endocrine regulators of this relationship have emerged only within recent decades. Peripheral signals including leptin and cortisol have long been implicated in the physiological integration of metabolism and reproduction. Recent studies have begun to explore possible roles for these two hormones in the regulation of reproduction in teleost fishes, as well as a role for leptin as a catabolic stress hormone. In this review, we briefly explore the reproductive actions of leptin and cortisol in mammals and teleost fishes and possible role of both hormones as putative modulators of the reproductive axis during stress events.

Molecular characterization and expression patterns of glucocorticoid receptor (GR) genes in protandrous hermaphroditic yellowtail clownfish, Amphiprion clarkii

Sexual differentiation and ovotestis development are closely associated with cortisol levels, the principal indicator of stress, via the glucocorticoid receptor (GR) in teleosts. Thus, GR is regarded as a mediator to expound the relationship between social stress and gonad development. In the present study, two gr genes (gr1 and gr2) were cloned and analyzed from a protandrous hermaphroditic teleost, the yellowtail clownfish (Amphiprion clarkii). GR1 was found to display a conserved nine-amino-acid insert, WRARQNTDG, between two zinc finger domains. The phylogenetic tree of GR showed that yellowtail clownfish GR1 and GR2 are clustered to teleost GR1 and teleost GR2 separately, and differ from tetrapod GR. The result of real-time PCR revealed that high-level gr1 was mainly distributed in the cerebellum, hypothalamus and heart. The gr2 gene was abundant in the pituitary and liver of females and nonbreeders, while gr2 was mainly detected in the medulla oblongata and middle kidney of males. Moreover, GRs can be expressed in cultured eukaryotic cells and functionally interact with dexamethasone (exogenous glucocorticoid), thereby triggering downstream signaling pathways of different potentials. GR1 and GR2 can be activated by 10 nM dexamethasone treatment in HEK-293T cells. Notably, real-time PCR analysis among three social status groups demonstrated that gr2 expression was the highest in the hypothalamus of nonbreeders, but gr1 was no difference. We speculate that social stress would increase the expression of gr2 gene expression in the hypothalamus to inhibit sexual development. These data provide evidence of social stress involving reproductive regulation, which may help to elucidate the underlying mechanism of sex differentiation and change.

Seasonal regulation of the lncRNA LDAIR modulates self-protective behaviours during the breeding season

To cope with seasonal environmental changes, animals adapt their physiology and behaviour in response to photoperiod. However, the molecular mechanisms underlying these adaptive changes are not completely understood. Here, using genome-wide expression analysis, we show that an uncharacterized long noncoding RNA (lncRNA), LDAIR, is strongly regulated by photoperiod in Japanese medaka fish (Oryzias latipes). Numerous transcripts and signalling pathways are activated during the transition from short- to long-day conditions; however, LDAIR is one of the first genes to be induced and its expression shows a robust daily rhythm under long-day conditions. Transcriptome analysis of LDAIR knockout fish reveals that the LDAIR locus regulates a gene neighbourhood, including corticotropin releasing hormone receptor 2, which is involved in the stress response. Behavioural analysis of LDAIR knockout fish demonstrates that LDAIR affects self-protective behaviours under long-day conditions. Therefore, we propose that photoperiodic regulation of corticotropin releasing hormone receptor 2 by LDAIR modulates adaptive behaviours to seasonal environmental changes.

The Amphibious Mudskipper: A Unique Model Bridging the Gap of Central Actions of Osmoregulatory Hormones Between Terrestrial and Aquatic Vertebrates

Body fluid regulation, or osmoregulation, continues to be a major topic in comparative physiology, and teleost fishes have been the subject of intensive research. Great progress has been made in understanding the osmoregulatory mechanisms including drinking behavior in teleosts and mammals. Mudskipper gobies can bridge the gap from aquatic to terrestrial habitats by their amphibious behavior, but the studies are yet emerging. In this review, we introduce this unique teleost as a model to study osmoregulatory behaviors, particularly amphibious behaviors regulated by the central action of hormones. Regarding drinking behavior of mammals, a thirst sensation is aroused by angiotensin II (Ang II) through direct actions on the forebrain circumventricular structures, which predominantly motivates them to search for water and take it into the mouth for drinking. By contrast, aquatic teleosts can drink water that is constantly present in their mouth only by reflex swallowing, and Ang II induces swallowing by acting on the hindbrain circumventricular organ without inducing thirst. In mudskippers, however, through the loss of buccal water by swallowing, which appears to induce buccal drying on land, Ang II motivates these fishes to move to water for drinking. Thus, mudskippers revealed a unique thirst regulation by sensory detection in the buccal cavity. In addition, the neurohypophysial hormones, isotocin (IT) and vasotocin (VT), promote migration to water via IT receptors in mudskippers. VT is also dipsogenic and the neurons in the forebrain may mediate their thirst. VT regulates social behaviors as well as osmoregulation. The VT-induced migration appears to be a submissive response of subordinate mudskippers to escape from competitive and dehydrating land. Together with implications of VT in aggression, mudskippers may bridge the multiple functions of neurohypophysial hormones. Interestingly, cortisol, an important hormone for seawater adaptation and stress response in teleosts, also stimulates the migration toward water, mediated possibly via the mineralocorticoid receptor. The corticosteroid system that is responsive to external stressors can accelerate emergence of migration to alternative habitats. In this review, we suggest this unique teleost as an important model to deepen insights into the behavioral roles of these hormones in relation to osmoregulation.

Expression of isotocin is male-specifically up-regulated by gonadal androgen in the medaka brain

Oxytocin, a mammalian neuropeptide primarily synthesised in the supraoptic and paraventricular nuclei of the hypothalamus, mediates a variety of physiological and behavioural processes, ranging from parturition and lactation to affiliation and prosociality. Multiple studies in rodents have shown that the expression of the oxytocin gene (Oxt) is stimulated by oestrogen, whereas androgen has no apparent effect. However, this finding is not consistent across all studies, and no study has examined sex steroid regulation of Oxt or its orthologues in other animals. In the present study, we show that, in the teleost fish, medaka (Oryzias latipes), the expression of the isotocin gene (it), the teleost orthologue of Oxt, in the parvocellular preoptic nuclei (homologous to the mammalian supraoptic nucleus) is male-specifically up-regulated by gonadal androgen, whereas it expression in the magnocellular/gigantocellular preoptic nuclei (homologous to the mammalian paraventricular nucleus) is independent of sex steroids in both sexes. None of the it-expressing neurones appear to co-express androgen receptors, suggesting that the effect of androgen on it expression is indirect. We found that the expression of a kisspeptin gene, kiss2, in the male brain is dependent on gonadal androgen, raising the possibility that the androgen-dependent expression of it may be mediated by kiss2 neurones. Our data also show that the isotocin peptide synthesised in response to androgen is axonally transported to the posterior pituitary to act peripherally. Given that levels of it expression are higher in females than in males, androgen may serve to compensate for the female-biased it expression to ensure a role for isotocin that is equally important for both sexes. These results are unexpectedly quite different from those reported in rodents, indicating that the regulatory role of sex steroids in Oxt/it expression has diverged during evolution, possibly with accompanying changes in the role of oxytocin/isotocin.

Sex-dependent programming effects of prenatal glucocorticoid treatment on the developing serotonin system and stress-related behaviors in adulthood

Prenatal stress and overexposure to glucocorticoids (GC) during development may be associated with an increased susceptibility to a number of diseases in adulthood including neuropsychiatric disorders, such as depression and anxiety. In animal models, prenatal overexposure to GC results in hyper-responsiveness to stress in adulthood, and females appear to be more susceptible than males. Here, we tested the hypothesis that overexposure to GC during fetal development has sex-specific programming effects on the brain, resulting in altered behaviors in adulthood. We examined the effects of dexamethasone (DEX; a synthetic GC) during prenatal life on stress-related behaviors in adulthood and on the tryptophan hydroxylase-2 (TpH2) gene expression in the adult dorsal raphe nucleus (DRN). TpH2 is the rate-limiting enzyme for serotonin (5-HT) synthesis and has been implicated in the etiology of human affective disorders. Timed-pregnant rats were treated with DEX from gestational days 18-22. Male and female offspring were sacrificed on the day of birth (postnatal day 0; P0), P7, and in adulthood (P80-84) and brains were examined for changes in TpH2 mRNA expression. Adult animals were also tested for anxiety- and depressive- like behaviors. In adulthood, prenatal DEX increased anxiety- and depressive- like behaviors selectively in females, as measured by decreased time spent in the center of the open field and increased time spent immobile in the forced swim test, respectively. Prenatal DEX increased TpH2 mRNA selectively in the female caudal DRN at P7, whereas it decreased TpH2 mRNA selectively in the female caudal DRN in adulthood. In animals challenged with restraint stress in adulthood, TpH2 mRNA was significantly lower in rostral DRN of prenatal DEX-treated females compared to vehicle-treated females. These data demonstrated that prenatal overexposure to GC alters the development of TpH2 gene expression and these alterations correlated with lasting behavioral changes found in adult female offspring.