Effects of altered photoperiod and temperature on expression levels of gonadotrophin subunit mRNAs in the female stinging catfish Heteropneustes fossilis

Decoding the effect of photoperiodic cues in transducing kisspeptin-melatonin circuit during the pubertal onset in common carp

This study unravels the intricate interplay between photoperiod, melatonin, and kisspeptin to orchestrate the pubertal onset of Common carp. Female fingerlings exposed to long days (LD) exhibited a hormonal crescendo, with upregulated hypothalamic-pituitary-ovarian (HPO) axis genes (kiss1, kiss1r, kiss2, gnrh2, gnrh3) and their downstream targets (lhr, fshr, ar1, esr1). However, the expression of the melatonin receptor (mtnr1a) diminished in LD, suggesting a potential inhibitory role. This hormonal symphony was further amplified by increased activity of key transcriptional regulators (gata1, gata2, cdx1, sp1, n-myc, hoxc8, plc, tac3, tacr3) and decreased expression of delayed puberty genes (mkrn1, dlk1). In contrast, short days (SD) muted this hormonal chorus, with decreased gnrh gene and regulator expression, elevated mtnr1a, and suppressed gonadal development. In in-vitro, estradiol mimicked the LD effect, boosting gnrh and regulator genes while dampening mtnr1a and melatonin-responsive genes. Conversely, melatonin acted as a conductor, downregulating gnrh and regulator genes and amplifying mtnr1a. Our findings illuminate the crucial roles of melatonin and kisspeptin as opposing forces in regulating pubertal timing. LD-induced melatonin suppression allows the kisspeptin symphony to flourish, triggering GnRH release and, ultimately, gonadal maturation. This delicate dance between photoperiod, melatonin, and kisspeptin orchestrates common carp's transition from juvenile to reproductive life.

Day length regulates gonadotrope proliferation and reproduction via an intra-pituitary pathway in the model vertebrate Oryzias latipes

In seasonally breeding mammals and birds, the production of the hormones that regulate reproduction (gonadotropins) is controlled by a complex pituitary-brain-pituitary pathway. Indeed, the pituitary thyroid-stimulating hormone (TSH) regulates gonadotropin expression in pituitary gonadotropes, via dio2-expressing tanycytes, hypothalamic Kisspeptin, RFamide-related peptide, and gonadotropin-releasing hormone neurons. However, in fish, how seasonal environmental signals influence gonadotropins remains unclear. In addition, the seasonal regulation of gonadotrope (gonadotropin-producing cell) proliferation in the pituitary is, to the best of our knowledge, not elucidated in any vertebrate group. Here, we show that in the vertebrate model Japanese medaka (Oryzias latipes), a long day seasonally breeding fish, photoperiod (daylength) not only regulates hormone production by the gonadotropes but also their proliferation. We also reveal an intra-pituitary pathway that regulates gonadotrope cell number and hormone production. In this pathway, Tsh regulates gonadotropes via folliculostellate cells within the pituitary. This study suggests the existence of an alternative regulatory mechanism of seasonal gonadotropin production in fish.

Regulation of steroid production and key genes in catfish Heteropneustes fossilis using recombinant gonadotropins

The two gonadotropins, FSH and LH, stimulate growth and development of the gonads through gonadal biosynthesis of steroid hormones and growth factors. To date, cDNA sequences encoding gonadotropin subunits have been isolated and characterized from a large number of fish species. Recently, we successfully cloned and characterized gonadotropins (LHβ, FSHβ, and GPα) from the pituitary glands of the catfish, Heteropneustes fossilis. In the present study, we describe herein the production of recombinant stinging catfish, H. fossilis (hf) FSH (rhfFSH) and LH (rhfLH) using the methylotrophic yeast P. pastoris expression system. We further explored the hypothesis that the recombinant gonadotropins can modulate the hypothalamus-pituitary-ovarian (HPO) axis genes (avt, it, gnrh2, kiss2, and cyp19a1a) and regulate their transcriptional profile and steroid levels in relation to their annual developmental stage during preparatory and pre-spawning phases under in-vitro conditions. We found that the different concentrations of recombinant rhfFSH and rhfLH significantly stimulated E2 levels in the preparatory and prespawning season, and also upregulated gonadal aromatase gene expression in a dose dependent manner. Our results demonstrate that the yeast expression system produced biologically active recombinant catfish gonadotropins, enabling the study of their function in the catfish.

Interactive effects of anthropogenic environmental drivers on endocrine responses in wildlife

Anthropogenic activity has created unique environmental drivers, which may interact to produce unexpected effects. My aim was to conduct a systematic review of the interactive effects of anthropogenic drivers on endocrine responses in non-human animals. The interaction between temperature and light can disrupt reproduction and growth by impacting gonadotropins, thyroid hormones, melatonin, and growth hormone. Temperature and endocrine disrupting compounds (EDCs) interact to modify reproduction with differential effects across generations. The combined effects of light and EDCs can be anxiogenic, so that light-at-night could increase anxiety in wildlife. Light and noise increase glucocorticoid release by themselves, and together can modify interactions between individuals and their environment. The literature detailing interactions between drivers is relatively sparse and there is a need to extend research to a broader range of taxa and interactions. I suggest that incorporating endocrine responses into Adverse Outcome Pathways would be beneficial to improve predictions of environmental effects.

Expression profile of kisspeptin2 and gonadotropin-releasing hormone2 mRNA during photo-thermal and melatonin treatments in the female air-breathing catfish Heteropneustes fossilis

In seasonally breeding vertebrates, extrinsic factors like photoperiod and temperature are major determinants, controlling the annual reproductive cycle. In teleosts, kisspeptin, which occurs in two molecular forms: kisspeptin1 (Kiss1) and kisspetin2 (Kiss2), has been reported to alter gonadotropin (Lh and Fsh) secretion but its effect on gonadotropin-releasing hormone (Gnrh) secretion is not unequivocally proved. In the catfish Heteropneustes fossilis, we isolated and characterized kiss2 and gnrh2 cDNAs and the present work reports effects of altered photo-thermal conditions and melatonin (MT, a pineal hormone) on their expressions in the brain. The exposure of the catfish to long photoperiod (LP, 16 h light) at normal temperature (NT) or high temperature (HT, 28 °C) at normal photoperiod (NP) for 14 or 28 days stimulated both kiss2 and gnrh2 expression in both gonad resting and preparatory phases with the combination of LP + HT eliciting maximal effects. Short photoperiod (SP, 8 h light) under NT or HT altered the gene expression according to the reproductive phase and temperature. MT that mediates photo-thermal signals to the brain inhibited brain kiss2 and gnrh2 gene expression in the NP + HT, LP + NT, and SP + NT groups. The altered photo-thermal conditions elicited changes in steroidogenic pathway as evident from changes in plasma E₂, progesterone, and testosterone levels. The results show that brain kiss2-gnrh2 signaling is involved in photo-thermal-mediated mechanisms controlling reproduction.

Examination of methods for manipulating serum 17β-Estradiol (E2) levels by analysis of blood E2 concentration in medaka (Oryzias latipes)

It is widely known that reproduction in vertebrates is regulated by the hypothalamus-pituitary-gonadal (HPG) axis. Although the mechanism of the HPG axis has been well documented in mammals, it cannot be always applied to that in non-mammalian species, which is a great disadvantage in understanding reproduction of vertebrates in general. Recently, transgenic and genome editing tools have rapidly been developed in small teleosts, and thus these species are expected to be useful for the understanding of general mechanism of reproduction in vertebrates. One of the major sex steroid hormones in female vertebrates 17β-Estradiol (E2) plays crucial roles in the formation of sexual dimorphism and the HPG axis regulation. In spite of the importance of E2 in reproductive regulation, only a few studies have analyzed blood E2 levels in small teleosts that are easily amenable to genetic manipulation. In the present study, we analyzed blood E2 concentration in medaka and demonstrated that female medaka show diurnal changes in blood E2 concentration. We then examined the best method for manipulating the circulating E2. First, we found that ovariectomy (OVX) drastically removes endogenous E2 in a day in female medaka. We examined different methods for E2 administration and revealed that feeding administration of E2-containing food is the most convenient and physiological method for mimicking the diurnal E2 changes of female medaka. On the other hand, the medaka exposed to E2 containing water showed high blood E2 concentrations, which exceeds those of environmental water, suggesting that E2 may cause bioconcentration.

Photoperiodic regulation of plasma gonadotropin levels in previtellogenic greater amberjack (Seriola dumerili)

In Seriola species, exposure to a long photoperiod regime is known to induce ovarian development. This study examined photoperiodic effects on pituitary gene expression and plasma levels of follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) in previtellogenic greater amberjack (Seriola dumerili). The fish were exposed to short (8L:16D) or long (18L:6D) photoperiod. The water temperature was maintained at 22 °C. Compared with the short-photoperiod group, plasma Fsh levels were higher on days 10 and 30 in the long-photoperiod group, but plasma Lh levels did not significantly differ. On day 30, pituitary Fsh- and Lh-β subunit gene expressions were also higher in the long-photoperiod group than the short-photoperiod group, whereas α-subunit gene expressions were higher on days 20 and 30. Throughout the experiment, average gonadosomatic index and plasma E2 levels did not significantly differ between the two groups. This study clearly demonstrated that a long photoperiod induced Fsh release in the previtellogenic fish followed by upregulation of pituitary Fsh and Lh subunit gene expressions. An increase in plasma Fsh levels may be a key factor that mediates the photoperiodic effect on the initiation of ovarian development.

Reproductive stage- and sex-dependant effects of neurohypophyseal nonapeptides on gonadotropin subunit mRNA expression in the catfish Heteropneustes fossilis: An in vitro study

In the present study, in vitro effects of synthetic vasotocin (VT), isotocin (4Ser, 8Ile- oxytocin; ITb) and the recently cloned IT gene paralog product (8Val-Isotocin, ITa) were studied on the expression of pituitary gonadotropin (GtH) subunit mRNA levels. In male pituitaries of early (preparatory phase) and late (prespawning phase) recrudescing catfish, Heteropneustes fossilis, VT (10 nM, 100 nM and 1000 nM) stimulated fshβ expression dose-dependently. But in females, the dose-dependent effect was found only in the preparatory phase. In males, VT stimulated lhβ expression only at higher doses. In females, VT produced a significant dose-dependent increase of the lhβ expression only in the prespawning phase. VT stimulated the expression of gpα, dose-dependently in the preparatory phase in males and in the prespawning phase in females. The incubation of the pituitaries with ITb did not alter the fshβ expression in either sex in both preparatory and prespawning phases. In males, ITb stimulated the expression of lhβ and gpα only at the highest concentration (1000 nM) in both phases. In females, ITb stimulated both lhβ and gpα expression only at 1000 nM in the preparatory phase and dose-dependently in the prespawning phase. The incubation of the pituitaries with ITa produced effects similar to ITb on the expression of fshβ, lhβ, and gpα. The results show that the basic peptide VT modulates both fshβ and lhβ expressions, which are influenced by the sex and reproductive stage. The neutral peptide ITA/ITb exerts an insignificant effect on the fshβ expression regardless of sex or season. Both VT and ITa/ITb elicit a significant effect on the lhβ expression in late recrudescent phase especially in females.