Interdependence of corticosterone and thyroid hormones in larval toads (Bufo boreas). I. Thyroid hormone-dependent and independent effects of corticosterone on growth and development

Influence of temperature on growth, development and thyroid metabolism of American bullfrog tadpoles (Lithobates catesbeianus) exposed to the herbicide tebuthiuron

The influence of temperature (25 and 32 °C) on the negative effects of the herbicide tebuthiuron (TBU, 0, 10, 50 and 200 ng.L-1, 16 days) on thyroid function and metamorphosis of Lithobates catesbeianus tadpoles was evaluated. Metamorphosis was accelerated by TBU exposure at 25 ºC, but delayed at 32 ºC with considerable losses of body mass. T3 and T4 levels were not altered. The highest TBU concentrarion at 25 ºC increased TR β and DIO3 transcript levels, which is consistent with development acceleration in tadpoles. At 32 ºC TR β transcript levels were lower than the values recorded at 25 ºC, and those tadpoles exposed to the highest TBU concentration presented increased diameter of thyroid follicles compared to controls at same temperature. This study evidences that TBU at environmentally realistic concentrations is able to disrupt thyroidogenesis in bullfrog tadpoles, impairing their development. These effects are influenced by temperature.

Chronic Exposure to Two Gestagens Differentially Alters Morphology and Gene Expression in Silurana tropicalis

Gestagens are active ingredients in human and veterinary drugs with progestogenic activity. Two gestagens-progesterone (P4), and the synthetic P4 analogue, melengestrol acetate (MGA)-are approved for use in beef cattle agriculture in North America. Both P4 and MGA have been measured in surface water receiving runoff from animal agricultural operations. This project aimed to assess the morphometric and molecular consequences of chronic exposures to P4, MGA, and their mixture during Western clawed frog metamorphosis. Chronic exposure (from embryo to metamorphosis) to MGA (1.7 µg/L) or P4 + MGA (0.22 µg/L P4 + 1.5 µg/L MGA) caused a considerable dysregulation of metamorphic timing, as evidenced by an inhibition of growth, narrower head, and lack of forelimb emergence in all animals. Molecular analysis revealed that chronic exposure to the mixture induced an additive upregulation of neurosteroid-related (GABA_A receptor subunit α6 (gabra6) and steroid 5-alpha reductase 1 (srd5α1) gene expression in brain tissue. Chronic P4 exposure (0.26 µg/L P4) induced a significant upregulation of the expression hypothalamic-pituitary-gonadal (HPG)-related genes (ipgr, erα) in the gonadal mesonephros complex (GMC). Our data suggest that exposure to P4, MGA, and their mixture induces multiple endocrine responses and adverse effects in larval Western clawed frogs. This study helps to better our understanding of the consequences of chronic gestagen exposure and suggests that the implications and risk of high gestagen use in beef cattle feeding operations may extend to the aquatic environment.

Metamorphic acceleration following the exposure to lithium and selenium on American bullfrog tadpoles (Lithobates catesbeianus)

To regulate the presence of contaminants in Brazilian water, the Brazilian Environmental Council (CONAMA) promulgates regulations regarding the concentrations of given compounds that are supposed to be safe for aquatic life. Considering these regulations, this study tested the effects of considered safe levels of lithium (2.5 mgL^-1) and selenium (0.01 mgL^-1), isolated and mixed, on the American bullfrog (Lithobates catesbeianus) tadpoles. The evaluation was done through the use of biomarkers of larval development as total wet weight (TWW), snout-vent-length (SVL), hind-limb-length (HLL), activity level (AL), histologic evaluation of the thyroid gland and the mortality rate. The tadpoles were allocated into four groups (n = 20 each): a control group (CT); a group exposed to lithium (LI), a group exposed to selenium (SE), and a group exposed to both lithium and selenium (SELI). The whole assay was carried out over 21 days, with two rounds of data collection (on 7th and 21st day) to evaluate the responses over time. A statistical reduction in the AL was observed in the tadpoles from the LI and SELI groups after 7 days of exposure, the same pattern was observed after 21 days. Histological analyses of the thyroid gland showed signs of up-regulation (i.e. statistic reduction in number and area of the follicles, as well a significant reduction in the area of the gland) in all exposed groups, which represents an endocrine response as an adaptative strategy to deal with polluted aquatic environment. The stress triggered by the polluted medium is discussed.

Morphological and histological abnormalities of the neotropical toad, Rhinella arenarum (Anura: Bufonidae) larvae exposed to dexamethasone

Dexamethasone (DEX) is a glucocorticoid highly effective as an anti-inflammatory, immunosuppressant and decongestant drug. In the present study, a preliminary acute toxicity test was assayed in order to determinate DEX median-lethal, lowest-observed-effect and the no-observed-effect concentrations (LC₅₀, LOEC and NOEC, respectively) on the common toad embryos (Rhinella arenarum). Also, morphological and histological abnormalities from five body larval regions, liver melanomacrophages (MM) and glutathione S-transferase (GST) activity were evaluated in the toad larvae to characterize the chronic sublethal effects of DEX (1-1,000 µg L^-L). Results of the acute test showed that the LC₅₀ of DEX at 96 h of exposure for the toad embryos (GS 18-20) was 10.720 mg L^-g, and the LOEC was 1 µg L^-g. In the chronic assay, the larval development and body length were significantly affected. DEX exposition also induced teratogenic effects. Most frequent external abnormalities observed in DEX-treated larvae included abdominal edema and swollen body, abnormal gut coiling and visceral congestion. Intestinal dysplasia was recurrent in cross-section of all DEX-treated larvae. Neural, conjunctive and renal epithelial cells were also affected. Significant increase in liver MM number and size, and GST activity levels were also registered in DEX treatments with respect to controls. The evaluation of a variety of biomarkers provided clear evidence of toad larvae sensitivity to DEX, and the ecotoxicological risk of these pharmaceuticals, commonly found in different water bodies worldwide on aquatic animals.

Effects of nonylphenols on embryonic development and metamorphosis of Xenopus laevis: FETAX and amphibian metamorphosis toxicity test (OECD TG231)

Nonylphenols (NPs) are a group of endocrine-disrupting surfactants that mimic estrogen. To determine the developmental toxicity and thyroid-disrupting effect of NPs, the effects of exposure to nonylphenol (NP), 4-nonylphenol (4-NP), and nonylphenol ethoxylate (NP-12) were examined according to the frog embryo teratogenesis assay-Xenopus (FETAX) and Organization for Economic Co-operation and Development test guidelines 231 (TG231). In FETAX, the LC₅₀ values of NP, 4-NP, and NP-12 were 59.14 mg/L, 10.13 mg/L, and 14.60 mg/L, respectively. At 10.0 mg/L, NP, 4-NP, and NP-12 significantly decreased the total length of tadpoles, and NP and 4-NP increased gut malformation and bent tails. In surviving tadpoles, the EC₅₀ values for malformation of NP, 4-NP, and NP-12 were 4.66, 6.51, and 13.08 mg/L, respectively. The teratogenic indices of NP, 4-NP, and NP-12 were 12.69, 1.56, and 1.08, respectively, suggesting the teratogenic potential of NP and 4-NP. In a range-finder assay for TG231, the 96-h LC₅₀ values of NP, 4-NP, and NP-12 were 2.0, 2.0, and 10.57 mg/L, respectively. When NF stage 51 larvae were exposed for 21 days, larval growth was inhibited by NP, 4-NP, and NP-12 at 0.67, 0.07, and 0.37 mg/L, respectively. 4-NP at 0.07 mg/L accelerated the developmental stage and significantly increased hind limb length, while 0.67 mg/L 4-NP delayed the developmental stage and decreased hind limb length, suggesting a bimodal effect of 4-NP on metamorphosis. NP and NP-12 at test concentrations did not alter the larval stage, but NP-12 at 0.37 mg/L significantly decreased total length and tail length, suggesting growth inhibition in larvae. The total colloid area of thyroid follicles was significantly increased by 0.07 mg/L 4-NP but not by NP and NP-12, suggesting that 4-NP may interfere with thyroid function. Together, the developmental toxicity of NPs was in the following order: 4-NP, NP-12, and NP. 4-NP may alter metamorphosis driven by thyroid hormones in X. laevis.

Insufficiency of Thyroid Hormone in Frog Metamorphosis and the Role of Glucocorticoids

Thyroid hormone (TH) is the most important hormone in frog metamorphosis, a developmental process which will not occur in the absence of TH but can be induced precociously by exogenous TH. However, such treatments including in-vitro TH treatments often do not replicate the events of natural metamorphosis in many organs, including lung, brain, blood, intestine, pancreas, tail, and skin. A potential explanation for the discrepancy between natural and TH-induced metamorphosis is the involvement of glucocorticoids (GCs). GCs are not able to advance development by themselves but can modulate the rate of developmental progress induced by TH via increased tissue sensitivity to TH. Global gene expression analyses and endocrine experiments suggest that GCs may also have direct actions required for completion of metamorphosis independent of their effects on TH signaling. Here, we provide a new review and analysis of the requirement and necessity of TH signaling in light of recent insights from gene knockout frogs. We also examine the independent and interactive roles GCs play in regulating morphological and molecular metamorphic events dependent upon TH.

Evolution of a complex phenotype with biphasic ontogeny: Contribution of development versus function and climatic variation to skull modularity in toads

The theory of morphological integration and modularity predicts that if functional correlations among traits are relevant to mean population fitness, the genetic basis of development will be molded by stabilizing selection to match functional patterns. Yet, how much functional interactions actually shape the fitness landscape is still an open question. We used the anuran skull as a model of a complex phenotype for which we can separate developmental and functional modularity. We hypothesized that functional modularity associated to functional demands of the adult skull would overcome developmental modularity associated to bone origin at the larval phase because metamorphosis would erase the developmental signal. We tested this hypothesis in toad species of the Rhinella granulosa complex using species phenotypic correlation pattern (P-matrices). Given that the toad species are distributed in very distinct habitats and the skull has important functions related to climatic conditions, we also hypothesized that differences in skull trait covariance pattern are associated to differences in climatic variables among species. Functional and hormonal-regulated modules are more conspicuous than developmental modules only when size variation is retained on species P-matrices. Without size variation, there is a clear modularity signal of developmental units, but most species have the functional model as the best supported by empirical data without allometric size variation. Closely related toad species have more similar climatic niches and P-matrices than distantly related species, suggesting phylogenetic niche conservatism. We infer that the modularity signal due to embryonic origin of bones, which happens early in ontogeny, is blurred by the process of growth that occurs later in ontogeny. We suggest that the species differing in the preferred modularity model have different demands on the orbital functional unit and that species contrasting in climate are subjected to divergent patterns of natural selection associated to neurocranial allometry and T3 hormone regulation.

Effect of corticosterone on larval growth, antipredator behaviour and metamorphosis of Hylarana indica

Corticosterone (CORT), a principal glucocorticoid in amphibians, is known to regulate diverse physiological processes including growth and metamorphosis of anuran tadpoles. Environmental stressors activate the neuroendocrine stress axis (hypothalamus-pituitary-interrenal axis, HPI) leading to an acute increase in CORT, which in turn, helps in coping with particular stress. However, chronic increase in CORT can negatively affect other physiological processes such as growth and metamorphosis. Herein, we studied the effect of exogenous CORT on larval growth, antipredator behaviour and metamorphic traits of Hylarana indica. Embryonic exposure to 5 or 20μg/L CORT did not affect their development, hatching duration as well as larval growth and metamorphosis. Exposure of tadpoles to 10 or 20μg/L CORT throughout larval development caused slower growth and development leading to increased body mass at stage 37. However, body and tail morphology of tadpoles was not affected. Interestingly, larval exposure to 5, 10 or 20μg/L CORT enhanced their antipredator response against kairomones in a concentration-dependent manner. Further, larval exposure to increasing concentrations of CORT resulted in the emergence of heavier froglets at 10 and 20μg/L while, delaying metamorphosis at all concentrations. Interestingly, the heavier froglets had shorter hindlimbs and consequently shorter jump distances. Tadpoles exposed to 20μg/L CORT during early, mid or late larval stages grew and developed slowly but tadpole morphology was not altered. Interestingly, exposure during early or mid-larval stages resulted in an enhanced antipredator response. These individuals metamorphosed later but at higher body mass while SVL was unaffected.

Are sex steroids essential for gonadal differentiation of the ornate frog, Microhyla ornata?

Knowledge about sensitivities and responses of amphibian larvae to sex steroids and the chemicals alike is the first step towards understanding and assessing the effect of diverse chemicals that interfere with gonadal development and other endocrine functions. Herein, we used Microhyla ornata to determine the role of sex steroids on its gonad differentiation and sex ratio. Our results show that the exposure to increasing concentrations of estradiol-17β throughout larval development did not affect gonad differentiation resulting in 1:1 sex ratio at metamorphosis. But, females emerging from estradiol-17β treatment had larger ovaries with larger sized follicles. Further, testes of some males contained lumens, the number of which was dose dependent. Similarly, exposure to testosterone propionate had negligible effects on gonad differentiation. However, the mean diameter of the largest follicles was smaller in treated ovaries. Treatment of tadpoles with tamoxifen had no effect on gonad differentiation and ovary development while testicular development was accelerated at the highest concentration. Similarly, treatment of tadpoles with cyproterone acetate had little effect on gonad differentiation as well as development, hence the sex ratios at the end of metamorphosis. Further, in tadpoles exposed to increasing concentrations of formestane, gonad differentiation was normal resulting in 1:1 sex ratio. Thus, in M. ornata, both estradiol and testosterone are essential for the development of ovaries and testes respectively but, they are not critical to gonadal differentiation. Hence, the effects of sex steroids and other endocrine disrupting chemicals could be species-specific; different species may have differential sensitivities to such chemicals.

Pattern of gonadal differentiation and development up to sexual maturity in the frogs, Microhyla ornata and Hylarana malabarica: A comparative study

Gonadogenesis was studied in Microhyla ornata (Family: Microhylidae) and Hylarana malabarica (Family: Ranidae) up to sexual maturity. Indifferent gonads of M. ornata directly differentiated into either testes or ovaries while those of H. malabarica differentiated into ovaries in all the individuals followed by testicular differentiation in males through an ovarian phase. In some tadpoles of M. ornata, formation of a central cavity at Gosner stage 27 marked ovary differentiation while meiosis was initiated at stage 29. Folliculogenesis was evident at stage 39. Vitellogenesis was initiated in females 9 months post-metamorphosis that attained maturity around 11 months after the completion of metamorphosis. Gonads of males with uniformly distributed germ and somatic cells remained undifferentiated until stage 41. Germ and somatic cells reorganized into seminiferous cords at stage 42. One month after completing metamorphosis, testes contained seminiferous tubules while those of 3 months old males exhibited all spermatogenic stages. In H. malabarica, germ cells entry into meiosis marked ovary differentiation at stage 29 while, ovarian cavity was discernable around stage 35. Post-metamorphosis, ovaries of 1-6 month old females contained pre-diplotene oocytes. Females were immature even 1 year after the completion of metamorphosis. In all the tadpoles, ovaries with central cavity and meiocytes were present up to the completion of metamorphosis. Gonads of prospective males displayed an obliterating ovarian cavity along with degenerating oocytes at the end of metamorphosis. Germ and somatic cells reorganized into seminiferous cords in males 3 months after the completion of metamorphosis. Testes of 4 months old males exhibited distinct seminiferous tubules while those of 6 months old exhibited meiosis. All spermatogenic stages were observed in testes of 9 months old males indicating maturity.

Corticosteroid signaling in frog metamorphosis

Stress in fetal and larval life can impact later health and fitness in humans and wildlife. Long-term effects of early life stress are mediated by altered stress physiology induced during the process of relaying environmental effects on development. Amphibian metamorphosis has been an important model system to study the role of hormones in development in an environmental context. Thyroid hormone (TH) is necessary and sufficient to initiate the dramatic morphological and physiological changes of metamorphosis, but TH alone is insufficient to complete metamorphosis. Other hormones, importantly corticosteroid hormones (CSs), influence the timing and nature of post-embryonic development. Stressors or treatments with CSs delay or accelerate metamorphic change, depending on the developmental stage of treatment. Also, TH and CSs have synergistic, antagonistic, and independent effects on gene regulation. Importantly, the identity of the endogenous corticosteroid hormone or receptor underlying any gene induction or remodeling event has not been determined. Levels of both CSs, corticosterone and aldosterone, peak at metamorphic climax, and the corticosteroid receptors, glucocorticoid and mineralocorticoid receptors, have wide expression distribution among tadpole tissues. Conclusive experiments to identify the endogenous players have been elusive due to difficulties in experimental control of corticosteroid production and signaling. Current data are consistent with the hypothesis that the two CSs and their receptors serve largely overlapping functions in regulating metamorphosis and synergy with TH. Knowledge of the endogenous players is critical to understanding the basic mechanisms and significance of corticosteroid action in regulating post-embryonic development in environmental contexts.

Crosstalk between the thyroid hormone and androgen axes during reproductive development in Silurana tropicalis

The objectives of this study were to examine the effects of potassium perchlorate (KClO4) treatment on androgen- and thyroid hormone (TH)-related transcript levels during gonadogenesis in the frog Silurana tropicalis. Androgen- and TH-related gene expression was examined in gonad-mesonephros complex (GMC) and liver tissues at stage NF 56 and stage NF 60. These stages of development coincide with the period of sexual differentiation. Real-time RT-PCR analyses revealed that androgen- and TH-related transcript levels in the GMC and liver of stage NF 56 and NF 60 frogs are responsive to KClO4 exposure during prometamorphosis. An increase in srd5α2 mRNA levels in hepatic tissues of KClO4-treated NF 56 tadpoles suggests an important role for hepatic tissues in androgen metabolism. Gene transcript differences highlight possible stage- and tissue-specific sensitivities to KClO4. A greater number of TH- and androgen-related transcriptional changes were discerned in the hepatic tissues compared to the gonads, and overall fewer transcriptional changes were observed in stage NF 60 tadpoles compared to stage NF 56 larvae. Perchlorate suppressed somatic and hind-limb development during the 96-d exposure period. Treatment with KClO4 had no significant effect on sex ratios, however a notable reduction in the percentage of males (33.3% M: 66.7% F) in the highest KClO4 concentration (107 μg/L) was observed. Overall, these findings suggest that KClO4 has secondary androgenic disrupting properties in addition to its known primary thyroid hormone-disrupting role.

Biological endpoints, enzyme activities, and blood cell parameters in two anuran tadpole species in rice agroecosystems of mid-eastern Argentina

Different biological variables of tadpoles, including survival, development and growth rates, and biomarkers [cholinesterases, glutathione-S-transferases (GST), and blood cell morphology] were evaluated in two anuran species, Scinax squalirostris (Hylidae) and Leptodactylus mystacinus (Leptodactylidae), using in situ experimental chambers in a rice field (RF) sprayed with insecticide Lambda-cyhalothrin (LTC) by aircraft in Santa Fe Province, Argentina. We found a significant decrease in body weight (0.62 ± 0.04 g) of L. mystacinus and an increased development rate of S. squalirostris in individuals from RF (41 ± 1; Gosner) with respect to individuals from the reference site (RS: 0.93 ± 0.04 g and 37 ± 0; respectively). In S. squalirostris, individuals from RF mean values of butyrylcholinesterase activities decreased at 48 (4.09 ± 0.32 nmol min-1 mg-1 of TP) and 96 h (3.74 ± 0.20 nmol min-1 mg-1 of TP), whereas inhibition of acetylcholinesterase was observed at 96 h (47.44 ± 2.78 nmol min-1 mg-1 of TP). In L. mystacinus from RF, an induction of acetylcholinesterase activity was observed at 96 h (36.01 ± 1.09 nmol min-1 mg-1 of TP). Glutathione-S-transferase levels varied between species, being higher in L. mystacinus individuals but lower in S. squalirostris from RF at 48 (272.29 ±11.78 and 71.87 ± 1.70 nmol min-1 mg-1 of TP; respectively) and 96 h (279.25 ± 13.06 and 57.62 ± 4.58 nmol min-1 mg-1 of TP, respectively). Blood cell parameters revealed a lower number of mitotic cells (MC: 0.36 ± 0.31%o for S. squalirostris and 0.08 ± 0.05 %o for L. mystacinus) and higher number of eosinophils (E: 3.45 ± 1.75 %o for S. squalirostris and 7.64 ± 0.98 %o for L. mystacinus) in individuals from the RF than in individuals from the RS (MC: 2.55 ± 0.74 %o for S. squalirostris and 1.87 ± 0.72%o for L. mystacinus; and E: 0.13 ± 0.09 for S. squalirostris and 3.20 ± 0.80 for L. mystacinus). Overall, our results demonstrate the existence of apparent differences in sensitivity between species in a series of sublethal responses to short-term exposure in RF after the application of Lambda-cyhalothrin. We suggest that the integral use of biological endpoints (development and growth) together with biomarkers (cholinesterase, GST, and blood cell parameters) may be a promising integral procedure for investigating pesticide exposure in wild frog populations.

Beyond synergy: corticosterone and thyroid hormone have numerous interaction effects on gene regulation in Xenopus tropicalis tadpoles

Hormones play critical roles in vertebrate development, and frog metamorphosis has been an excellent model system to study the developmental roles of thyroid hormone (TH) and glucocorticoids. Whereas TH regulates the initiation and rate of metamorphosis, the actions of corticosterone (CORT; the main glucocorticoid in frogs) are more complex. In the absence of TH during premetamorphosis, CORT inhibits development, but in the presence of TH during metamorphosis, CORT synergizes with TH to accelerate development. Synergy at the level of gene expression is known for three genes in frogs, but the nature and extent of TH and CORT cross talk is otherwise unknown. Therefore, to examine TH and CORT interactions, we performed microarray analysis on tails from Xenopus tropicalis tadpoles treated with CORT, TH, CORT+TH, or vehicle for 18 h. The expression of 5432 genes was significantly altered in response to either or both hormones. Using Venn diagrams and cluster analysis, we identified 16 main patterns of gene regulation due to up- or down-regulation by TH and/or CORT. Many genes were affected by only one of the hormones, and a large proportion of regulated genes (22%) required both hormones. We also identified patterns of additive or synergistic, inhibitory, subtractive, and annihilatory regulation. A total of 928 genes (17%) were regulated by novel interactions between the two hormones. These data expand our understanding of the hormonal cross talk underlying the gene regulation cascade directing tail resorption and suggest the possibility that CORT affects not only the timing but also the nature of TH-dependent tissue transformation.

Agricultural intensity in ovo affects growth, metamorphic development and sexual differentiation in the common toad (Bufo bufo)

Pollution was cited by the Global Amphibian Assessment to be the second most important cause of amphibian decline worldwide, however, the effects of the agricultural environment on amphibians are not well understood. In this study, spawn from Bufo bufo was taken from four sites in England and Wales with varying intensities of arable agriculture. Spawn was either placed in tanks containing aged tap water (ex-situ, five replicates) or in cages at the native site (caged, five replicates). Hatching success, abnormal tadpoles, and forelimb emergence were recorded during the larval stage. Individuals were also sampled at five time points (TP) during development (5-, 7-, 9-, 12-, 15-weeks post-hatch) and analysed for morphological parameters. The thyroids (TP2) and the gonads (TP3,4,5) were also analysed histologically. With the exception of the thyroid histopathology, all analysed endpoints were significantly different between ex-situ individuals reared under identical conditions from the different sites. In addition, intensity of arable agriculture had a negative effect on growth and development. At one site, despite distinct rearing conditions, a high level of intersex (up to 42%) and similar sex ratios were observed in both ex-situ and caged individuals. Taken together, these data suggest that maternal exposure and/or events in ovo had a much larger effect on growth, metamorphic development, and sexual differentiation in B. bufo than the ambient environment. This could have important implications for traditional exposure scenarios that typically begin at the larval stage. Intersex is reported for the first time in European amphibians in situ, highlighting the potential use of distinct populations of amphibians in fundamental research into the aetiology of specific developmental effects in wild amphibians.

Effects of nonylphenol on early embryonic development, pigmentation and 3,5,3'-triiodothyronine-induced metamorphosis in Bombina orientalis (Amphibia: Anura)

Nonylphenol (NP) is an estrogenic endocrine disruptor in many aquatic species. In an effort to highlight the developmental toxicity of NP in amphibians, we examined the effects of NP on the embryonic survival, tadpole growth, melanophore development and metamorphosis of a native Korean amphibian species, Bombina orientalis (Anura). When treated to fertilized eggs, 1 μM NP significantly decreased embryonic survival at 48 h post fertilization (p.f.), suggesting that 1 μM NP can exert systemic toxicity in B. orientalis embryos. In the surviving embryos, there were no significant differences in malformation rates between NP-treated embryos and controls at 240 h p.f., suggesting no or low teratogenicity of NP in B. orientalis embryos. Below LC(50) NP significantly decreased body growth and development of melanophores at 0.1 μM, suggesting that NP far below the LC(50) targets multiple developmental events in tadpoles of this frog species. In metamorphosis assay using the premetamorphic tadpoles (corresponding to Nieuwkoop Faber stage 53 in Xenopus laevis) exogenous 3,5,3'-triiodothyronine (T3)-induced tail resorption was significantly decreased by 1 μM NP. However, NP (0.1 and 1 μM)-only treatment did not affected total body T3 and T4 levels, suggesting that NP at tested concentrations inhibits thyroid hormones action but not the synthesis of hormones during metamorphosis.

Amphibians and agricultural chemicals: review of the risks in a complex environment

Agricultural landscapes, although often highly altered in nature, provide habitat for many species of amphibian. However, the persistence and health of amphibian populations are likely to be compromised by the escalating use of pesticides and other agricultural chemicals. This review examines some of the issues relating to exposure of amphibian populations to these chemicals and places emphasis on mechanisms of toxicity. Several mechanisms are highlighted, including those that may disrupt thyroid activity, retinoid pathways, and sexual differentiation. Special emphasis is also placed on the various interactions that may occur between different agro-chemicals and between chemicals and other environmental factors. We also examine the indirect effects on amphibian populations that occur when their surrounding pond communities are altered by chemicals.

Corticosteroids disrupt amphibian metamorphosis by complex modes of action including increased prolactin expression

Although thyroid hormones (TH) are the primary morphogens regulating amphibian metamorphosis, other hormones including corticosteroids are known to participate in this regulation. The present study investigated effects of corticosteroids on larval development of the amphibian Xenopus laevis. Premetamorphic tadpoles (stage 51) were treated with aldosterone (ALDO; 100 nM), corticosterone (B; 10, 100, 500 nM) and dexamethasone (DEX; 10, 100, 500 nM) for 21 days and organismal responses were assessed by gross morphology determining stage development, whole body length (WBL), and hind limb length (HLL). B and DEX reduced WBL and HLL and caused abnormal development including the lack of fore limb emergence while ALDO treatment showed no significant effect. Gene expression analyses using RT-PCR revealed up-regulation of prolactin (PRL) in brain, but down-regulation of type III deiodinase in tail tissue induced by the glucocorticoids B and DEX. Additionally, stromelysin-3 transcript in tail tissue was decreased by B. ALDO at 100 nM had no effect on mRNA expression, neither in brain nor in tail tissue. These findings indicate that corticosteroids modulate TH-dependent metamorphosis by complex mechanisms that even include indirect effects triggered by increased PRL mRNA expression.

Exposure to coal combustion residues during metamorphosis elevates corticosterone content and adversely affects oral morphology, growth, and development in Rana sphenocephala

Coal combustion residues (CCRs) are documented to negatively impact oral morphology, growth, and development in larval amphibians. It is currently unclear what physiological mechanisms may mediate these effects. Corticosterone, a glucocorticoid hormone, is a likely mediator because when administered exogenously it, like CCRs, also negatively influences oral morphology, growth, and development in larval amphibians. In an attempt to identify if corticosterone mediates these effects, we raised larval Southern Leopard Frogs, Rana sphenocephala, on either sand or CCR substrate and documented effects of sediment type on whole body corticosterone, oral morphology, and time to and mass at key metamorphic stages. Coal combustion residue treated tadpoles contained significantly more corticosterone than controls throughout metamorphosis. However, significantly more oral abnormalities occurred early in metamorphosis when differences in corticosterone levels between treatments were minimal. Overall, CCR-treated tadpoles took significantly more time to transition between key stages and gained less mass between stages than controls, but these differences between treatments decreased during later stages when corticosterone differences between treatments were greatest. Our results suggest endogenous increase in corticosterone content and its influence on oral morphology, growth and development is more complex than previously thought.

Programming neuroendocrine stress axis activity by exposure to glucocorticoids during postembryonic development of the frog, Xenopus laevis

Exposure to elevated glucocorticoids during early mammalian development can have profound, long-term consequences for health and disease. However, it is not known whether such actions occur in nonmammalian species, and if they do, whether the molecular physiological mechanisms are evolutionarily conserved. We investigated the effects of dietary restriction, which elevates endogenous corticosterone (CORT), or exposure to exogenous CORT added to the aquarium water of Xenopus laevis tadpoles on later-life measures of growth, feeding behavior, and neuroendocrine stress axis activity. Dietary restriction of prometamorphic tadpoles reduced body size at metamorphosis, but juvenile frogs increased food intake, showed catch-up growth through 21 d after metamorphosis, and had elevated whole-body CORT content compared with controls. Dietary restriction causes increased CORT in tadpoles, so to mimic this increase, we treated tadpoles with 100 nm CORT or vehicle for 5 or 10 d and then reared juvenile frogs to 2 months after metamorphosis. Treatment with CORT decreased body weight at metamorphosis, but juvenile frogs showed catch-up growth and had elevated basal plasma (CORT). Immunohistochemical analysis showed that CORT exposure as a tadpole led to decreased glucocorticoid receptor immunoreactivity in brain regions involved with stress axis regulation and in the anterior pituitary gland of juvenile frogs. The elevated CORT in juvenile frogs, which could result from decreased negative feedback owing to down-regulation of glucocorticoid receptor, may drive the hyperphagic response. Taken together, our findings suggest that long-term, stable phenotypic changes in response to elevated glucocorticoids early in life are an ancient and conserved feature of the vertebrate lineage.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Frogs and Its Role in Frog Development and Reproduction

Metamorphosis of the amphibian tadpole is a thyroid hormone (TH)-dependent developmental process. For this reason, the tadpole is considered to be an ideal bioassay system to identify disruption of thyroid function by environmental contaminants. Here we provide an in-depth review of the amphibian thyroid system with particular focus on the role that TH plays in metamorphosis. The amphibian thyroid system is similar to that of mammals and other tetrapods. We review the amphibian hypothalamic-pituitary-thyroid (HPT) axis, focusing on thyroid hormone synthesis, transport, and metabolism. We also discuss the molecular mechanisms of TH action, including the role of TH receptors, the actions of TH on organogenesis, and the mechanisms that underlie the pleiotropic actions of THs. Finally, we discuss methods for evaluating thyroid disruption in frogs, including potential sites of action, relevant endpoints, candidate protocols for measuring thyroid axis disruption, and current gaps in our knowledge. The utility of amphibian metamorphosis as a model for evaluating thyroid axis disruption has recently led to the development of a bioassay using Xenopus laevis.

Actions of glucocorticoids at a seasonal baseline as compared to stress-related levels in the regulation of periodic life processes

For decades, demands associated with the predictable life-history cycle have been considered stressful and have not been distinguished from stress that occurs in association with unpredictable and life-threatening perturbations in the environment. The recent emergence of the concept of allostasis distinguishes behavioral and physiological responses to predictable routines as opposed to unpredictable perturbations, and allows for their comparison within one theoretical framework. Glucocorticosteroids (GCs) have been proposed as important mediators of allostasis, as they allow for rapid readjustment and support of behavior and physiology in response to predictable and unpredictable demands (allostatic load). Much work has already been done in defining GC action at the high concentrations that accompany life-threatening perturbations. However, less is known about the role of GCs in relation to daily and seasonal life processes. In this review, we summarize the known behavioral and physiological effects of GCs relating to the predictable life-history cycle, paying particular attention to feeding behavior, locomotor activity and energy metabolism. Although we utilize a comparative approach, emphasis is placed on birds. In addition, we briefly review effects of GCs at stress-related concentrations to test the hypothesis that different levels of GCs play specific and distinct roles in the regulation of life processes and, thus, participate in the promotion of different physiological states. We also examine the receptor types through which GC action may be mediated and suggest mechanisms whereby different GC concentrations may exert their actions. In conclusion, we argue that biological actions of GCs at "non-stress" seasonal concentrations play a critical role in the adjustment of responses that accompany predictable variability in the environment and demand more careful consideration in future studies.

Environmental context determines nitrate toxicity in Southern toad (Bufo terrestris) tadpoles

Aquatic nitrate contamination has escalated over the past 50 years, primarily due to intensified fertilizer application and sewage production worldwide. Nitrate's role in the decline of amphibian populations remains unclear, although studies suggest that nitrate exposure affects larval development. We exposed Bufo terrestris tadpoles to environmentally relevant nitrate concentrations from Gosner stage 25 through forelimb emergence. Tadpoles were exposed to fluctuating (0-30 mg/L NO(3)-N, alternated three times per week) or constant nitrate concentrations (0, 5, 15, or 30 mg/L NO(3)-N), and effects were compared in two water types: natural spring water and reverse-osmosis filtered water, fortified with electrolytes (RO(e)). We measured growth and developmental rates, survival, time to metamorphosis, metamorph body size, hepatosomatic index, and whole-body thyroxine (T(4)) concentrations at forelimb emergence. Based on our observations, we reached three main conclusions: (1) in constant nitrate, tadpoles in RO(e) water grew faster, and were generally larger with higher or similar T(4) at metamorphosis than tadpoles raised in spring water, irrespective of nitrate concentration, (2) in fluctuating nitrate (flux), there were no differences in time to or size at metamorphosis in either water type relative to controls; however, mean T(4) concentrations in the flux treatment showed a reversed pattern in the two water types (SP>RO(e)) compared to the general pattern observed with constant nitrate exposure (RO(e)>SP), and (3) the effect of nitrate on growth and development depended on water type. In RO(e) water with high nitrate (RO(e)-30 mg/L NO(3)-N), tadpoles metamorphosed an average of 5 days (13%) earlier than control animals, but were similar in size to controls. However, spring water tadpoles reared in high nitrate (30 mg/L) delayed metamorphosis by 7 days (18%) compared to animals reared in SP-0, and 11 days (32%) compared to tadpoles raised in RO(e)-30. This delayed development allowed SP-30 tadpoles to reach a larger size that was more similar to metamorphs raised in RO(e) water. Based on information from other studies, we conclude that, in RO(e) water, tadpoles exhibited an expected stress response to nitrate (e.g. metamorphosed earlier as nitrate concentration increased). However, we suggest that, in spring water, tadpoles were exposed to additional stressors that decreased growth rate and thyroxine concentrations, and that this effect was modified by nitrate.

Pesticide mixtures, endocrine disruption, and amphibian declines: are we underestimating the impact?

Amphibian populations are declining globally at an alarming rate. Pesticides are among a number of proposed causes for these declines. Although a sizable database examining effects of pesticides on amphibians exists, the vast majority of these studies focus on toxicological effects (lethality, external malformations, etc.) at relatively high doses (parts per million). Very few studies focus on effects such as endocrine disruption at low concentrations. Further, most studies examine exposures to single chemicals only. The present study examined nine pesticides (four herbicides, two fungicides, and three insecticides) used on cornfields in the midwestern United States. Effects of each pesticide alone (0.1 ppb) or in combination were examined. In addition, we also examined atrazine and S-metolachlor combined (0.1 or 10 ppb each) and the commercial formulation Bicep II Magnum, which contains both of these herbicides. These two pesticides were examined in combination because they are persistent throughout the year in the wild. We examined larval growth and development, sex differentiation, and immune function in leopard frogs (Rana pipiens). In a follow-up study, we also examined the effects of the nine-compound mixture on plasma corticosterone levels in male African clawed frogs (Xenopus laevis). Although some of the pesticides individually inhibited larval growth and development, the pesticide mixtures had much greater effects. Larval growth and development were retarded, but most significantly, pesticide mixtures negated or reversed the typically positive correlation between time to metamorphosis and size at metamorphosis observed in controls: exposed larvae that took longer to metamorphose were smaller than their counterparts that metamorphosed earlier. The nine-pesticide mixture also induced damage to the thymus, resulting in immunosuppression and contraction of flavobacterial meningitis. The study in X. laevis revealed that these adverse effects may be due to an increase in plasma levels of the stress hormone corticosterone. Although it cannot be determined whether all the pesticides in the mixture contribute to these adverse effects or whether some pesticides are effectors, some are enhancers, and some are neutral, the present study revealed that estimating ecological risk and the impact of pesticides on amphibians using studies that examine only single pesticides at high concentrations may lead to gross underestimations of the role of pesticides in amphibian declines.

Effects of salinity on the survival, growth and development of tadpoles of the brown tree frog, Litoria ewingii

We investigated the effects of 4% seawater (sw), 8% sw, 12% sw and 16% sw (1.4 g NaCl L–1, 2.8 g NaCl L–1, 4.2 g NaCl L–1 and 5.6 g NaCl L–1, respectively) on survival, mass and development of larvae of the brown tree frog, Litoria ewingii. Salinity of 16% sw significantly decreased survival of tadpoles such that 39% of tadpoles in 16% sw treatment survived to metamorphosis compared with 92% in the control group (freshwater). Growth (mass) of 16% sw tadpoles (0.048 g ± 0.005 g) slowed significantly during early development compared with control tadpoles (0.105 g ± 0.004 g); however, there was no significant difference in final metamorphosis mass between 16% sw (0.192 g ± 0.008 g) and control tadpoles (0.226 ± 0.006 g). Time taken to reach metamorphosis was greater for 16% sw tadpoles (84 ± 1.8 days) than for control tadpoles (55 ± 0.84 days). Tadpoles at salinity concentrations of 4% sw, 8% sw and 12% sw were significantly heavier than control tadpoles at metamorphosis. Our results show that moderate levels of salinity (16% sw) are sufficient to significantly reduce survival and retard development of tadpoles of L. ewingii.

Stress Hormones: A Link between Maternal Condition and Sex‐Biased Reproductive Investment

In species where offspring fitness is sex-specifically influenced by maternal reproductive condition, sex allocation theory predicts that poor-quality mothers should invest in the evolutionarily less expensive sex. Despite an accumulation of evidence that mothers can sex-specifically modulate investment in offspring in relation to maternal quality, few mechanisms have been proposed as to how this is achieved. We explored a hormonal mechanism for sex-biased maternal investment by measuring and experimentally manipulating baseline levels of the stress hormone corticosterone in laying wild female European starlings (Sturnus vulgaris) and examining effects on sex ratio and sex-specific offspring phenotype adjustment. Here we show that baseline plasma corticosterone is negatively correlated with energetic body condition in laying starlings, and subsequent experimental elevation of maternal baseline plasma corticosterone increased yolk corticosterone without altering maternal condition or egg quality per se. Hormonal elevation resulted in the following: female-biased hatching sex ratios (caused by elevated male embryonic mortality), lighter male offspring at hatching (which subsequently grew more slowly during postnatal development), and lower cell-mediated immune (phytohemagglutinin) responses in males compared with control-born males; female offspring were unaffected by the manipulation in both years of the study. Elevated maternal corticosterone therefore resulted in a sex-biased adjustment of offspring quality favorable to female offspring via both a sex ratio bias and a modulation of male phenotype at hatching. In birds, deposition of yolk corticosterone may benefit mothers by acting as a bet-hedging strategy in stochastic environments where the correlation between environmental cues at laying (and therefore potentially maternal condition) and conditions during chick-rearing might be low and unpredictable. Together with recent studies in other vertebrate taxa, these results suggest that maternal stress hormones provide a mechanistic link between maternal quality and sex-biased maternal investment in offspring.

T-Screen as a tool to identify thyroid hormone receptor active compounds

The T-Screen represents an in vitro bioassay based on thyroid hormone dependent cell proliferation of a rat pituitary tumour cell line (GH3) in serum-free medium. It can be used to study interference of compounds with thyroid hormone at the cellular level, thus bridging the gap between limitations of assays using either isolated molecules (enzymes, transport proteins) or complex in vivo experiments with all the complex feedback mechanisms present. Compounds are tested both in the absence and presence of thyroid hormone (EC(50) concentration of T(3)) to test for both agonistic and antagonistic potency. Thyroid hormones (3,3'-5-triiodothyronine: T(3) and 3,3',5,5'-tetraiodothyroxine: T(4)) and compounds resembling the structure of thyroid hormones (3,3'-5-triiodothyroacetic acid: Triac; 3,3',5,5'-tetraiodothyroacetic acid: Tetrac) induced cell growth, with the rank order Triac > T(3) > Tetrac > T(4) (relative potency = 1.35 > 1 > 0.29 > 0.07), which is identical to published affinities of these compounds for nuclear thyroid hormone receptors. Exposure to 5,5'-diphenylhydantoin (DPH) in the presence of 0.25nM T(3) resulted in up to 60% decreased cell growth at 200μM DPH. No effect of DPH on basal metabolic activity of GH3 cells was observed at this concentration. Fentinchloride (IC(50) = 21nM) decreased cell growth induced by 0.25nM T(3), whereas parallel exposure to these concentrations in the absence of T(3) did not alter basal metabolic activities of GH3 cells. Apolar sediment extracts from the Dommel (34%) and Terneuzen (14%) decreased cell growth in the presence of 0.25nM T(3), whereas the extract from Hoogeveen increased cell growth (26%) and the extract from North Sea Channel had no effect. The T-Screen proved to be a fast and functional assay for assessing thyroid hormone receptor active potencies of pure chemicals or environmental mixtures.

Developmental Alterations and Osmoregulatory Physiology of a Larval Anuran under Osmotic Stress

Water salinity represents an environmental stress for many species. Amphibians are particularly sensitive because they are generally poor osmoregulators, and most species are completely absent from brackish and saline environments. We experimentally examined the effect of different salinity levels on larvae of the toad Bufo calamita L., a species that occupies freshwater ponds but can also breed in brackish ponds. Two independent experiments are reported here. In both experiments, tadpoles under saline conditions (ranging between 85 and 200 mOsm) showed a slower developmental rate, metamorphosing between 4 and 9 d later than the controls. Bufo calamita tadpoles reared in brackish water increased their osmolality and solute concentration (mainly sodium and chloride), decreased their levels of glucose, and decreased the total protein content, all measured from whole-animal extracts. Although most larval anurans are strictly ammoniotelic until the completion of metamorphosis, a few species exposed to dehydrating environments have evolved the ability to use urea as an osmolyte during the larval phase. The data presented here reveal that although B. calamita seems to be yet another exception to the rule of larval strict ammoniotelism, the tadpoles are not able to use urea as an osmolyte and rely on sodium-chloride balance instead. Preliminary immunoassays of thyroid hormone content suggest a possible decrease in hormone levels induced in water salinity conditions that correlate with a decreased developmental rate.

Distinctive gene profiles occur at key points during natural metamorphosis in the Xenopus laevis tadpole tail

Thyroid hormones (THs) are essential for tadpole metamorphosis into a juvenile frog; however, a complex interplay between additional hormones and signaling events also contributes to this dramatic developmental phase. A major mechanism of TH action is the nuclear receptor-mediated regulation of gene transcription of responsive genes. By using the precocious metamorphic model, several genes have been identified as TH responsive in the regressing tail. Many of these genes also exhibit altered expression during natural metamorphosis. Although identification of these genes provides insight into the mechanism whereby TH acts, complex interplay between TH and other hormones and the developmental stage-dependency of tissue responses contribute to the timing and coordination of metamorphic events. We investigated the temporal gene expression profile in Xenopus laevis tadpole tails from premetamorphosis through metamorphic climax by using a combination of a novel frog cDNA array containing 420 genes and quantitative real-time PCR. Seventy-nine genes were identified whose steady-state mRNA expression levels were altered in the tadpole tail during natural metamorphosis, of which 34 have previously been identified to be TH responsive in frogs or mammals. Of these genes, 75 clustered into 13 groups that displayed distinct developmental expression profiles. The levels of 28 transcripts were altered during premetamorphosis, 31 during prometamorphosis, and 43 with the onset of tail regression. This work establishes an important baseline for determining the mechanisms whereby tissues undergo differing metamorphic fates.

Small changes in whole-body corticosterone content affect larval Rana pipiens fitness components

In amphibians, large changes in tissue corticosterone content (caused by treatment with large doses of hormone) alter tadpole growth and development, but the effects of smaller changes on growth, development, behavior, and morphology are unknown. In the current study, we exposed pre-metamorphic Rana pipiens tadpoles to moderate doses (62 and 125 nM) of exogenous corticosterone by adding it to the rearing water. We then analyzed effects on growth, development, behavior, morphology, and the endogenous corticosterone response to exogenous adrenocorticotropic releasing hormone (ACTH). A 50% elevation in whole-body corticosterone content was associated with slowed growth and development, increased tail muscle depth, and a diminished corticosterone response to ACTH. Behavior was unaffected by corticosterone administration. Treatment with the corticoid synthesis inhibitor metyrapone (MTP) reduced whole-body corticosterone content by 50% and was associated with increased size at metamorphosis but no change in time to metamorphosis. Our findings support the hypothesis that corticoids can mediate growth, developmental, and morphological responses of tadpoles to changing environmental conditions. Our results also demonstrate that even small changes in corticosterone content can have important implications for amphibian fitness.

Role for corticoids in mediating the response of Rana pipiens tadpoles to intraspecific competition

Competition is known to decrease growth and development rate in tadpoles, but the physiological basis for this phenomenon is poorly understood. We hypothesized that competition results in increased production of stress hormones and that these hormones are responsible for the suppression of growth and development. To test this hypothesis, we measured whole-body corticosterone content in premetamorphic Leopard frog (Rana pipiens) tadpoles raised at two different population densities and three different food levels. Whole body corticosterone content was elevated in tadpoles subjected to either limited food (at low density) or high density. Within the low and intermediate food treatments, high density reduced tadpole growth and slowed development. Limited food slowed growth and development at all densities. Blocking corticoid synthesis by treating tadpoles with metyrapone (MTP) reversed the growth suppression caused by high density (tested in the intermediate food level treatment) but did not alter the effect of density on development rate. MTP treatment did not alter the depressive effect of limited resources on growth or development. Our results suggest that elevated corticoid biosynthesis mediates the negative effect of increased population density (i.e., increased intraspecific competition) on tadpole growth.

Influence of cortisol on the larval bullfrog thyroid axis in vitro and in vivo and on plasma and ocular melatonin

Adrenal (interrenal) steroids have an important role in amphibian development, antagonizing the metamorphic changes induced by the thyroid at first and then synergizing with the thyroid hormones as their level rises during metamorphosis. Because most of the studies of corticoids at metamorphosis have focused on peripheral tissues, we investigated the effect of cortisol (hydrocortisone; HC) in vitro and in vivo on the thyroid of Rana catesbeiana (bullfrog) tadpoles on 12:12 light/dark (LD) cycles. Plasma and ocular melatonin, which is altered by changes in thyroxine (T(4)) levels, were also assayed in some experiments. Thyroids from premetamorphic tadpoles secreted less T(4) into culture media when incubated with 10 micrograms/ml HC and 0.2 micrograms/ml ovine thyrotropin (TSH) than with TSH alone and when cultured in the absence of TSH following 5 days of 10-micrograms HC injections, indicating that HC inhibited the thyroid at young stages. The effect of 10 micrograms/ml HC at older stages was investigated by culturing thyroids and pituitaries separately on the first day in control or HC media and then incubating the thyroids on the second day in homologous pituitary-conditioned media as a bioassay for pituitary TSH. HC had no effect on baseline T(4) secretion by the thyroids of prometamorphic or climax tadpoles on the first day but increased T(4) secretion over the control on the second day. Thyroids cultured with TSH and HC showed no increase in T(4) secretion over the control TSH group on the second day, indicating that, in the previous experiments, HC had enhanced pituitary secretion of TSH, rather than the response of the thyroid to TSH. In vivo, 5 days of injections of 10 micrograms HC increased plasma T(4) at prometamorphosis and decreased it at climax. There was no marked effect of HC on plasma or ocular melatonin levels. The findings showed that the nature of the effect of HC on the thyroid axis changes during metamorphosis from inhibition at early stages to a positive influence at prometamorphosis and finally to a negative effect on the T(4) level in the plasma at climax.

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).

Stage-dependent changes in adrenal steroids and catecholamines during development in Xenopus laevis

Changes in adrenal hormones during the complete developmental cycle from egg to juvenile were investigated in the amphibian Xenopus laevis. Whole-body concentrations of the adrenal steroids corticosterone (B), and aldosterone (Aldo) were determined by radioimmunoassay and those of the adrenal catecholamines epinephrine (E), norepinephrine (NE), and dopamine (D) were determined by HPLC. In addition, the catecholamine-synthesizing enzymes tyrosine hydroxylase, dopamine beta-hydroxylase, and phenylethanolamine N-methyltransferase were immunocytochemically localized for the characterization of chromaffin adrenal cells. B and Aldo were not detectable in the whole body before hatching. B levels rose earlier than Aldo levels from stage 36 onward. B had already peaked at stage 46, whereas the largest amounts of Aldo were found at stage 54. After peaking, both steroids decreased gradually to 2.7 +/- 0.62 (B) and 0.4 +/- 0.1 (Aldo) ng/g body wt (mean +/- SEM, n = 10) in juvenile animals. E, NE, and D were detected just after hatching, when E and D showed an early peak at stage 40. E and NE increased moderately during development and demonstrated a sharp increase at the end of metamorphosis from stages 62 onward to 14.4 +/- 1.7 (E) and 34.1 +/- 4.67 (NE) ng/g body wt (mean +/- SEM, n = 6). Interestingly, D levels had a distinct pattern, because concentrations of D remained lower than those of NE and E over nearly the complete development, but showed a dramatic rise during the latest stages, reaching 707 +/- 54 ng/g body wt in juveniles. This dramatic shift in catecholamine levels was confirmed by immunocytochemistry in parallel. A large increase in chromaffin cells labeled with tyrosine hydroxylase immunoreactivity occurred in the latest developmental stages. The catabolic rates for all catecholamines in vivo were similar, which indicates that the different levels are due to various rates of synthesis. Thus, adrenal corticosteroids as well as catecholamines may have regulatory effects during premetamorphosis and metamorphic climax.

Histological examination of the effects of corticosterone in larvae of the western toad, Bufo boreas (Anura: Bufonidae), and the Oriental fire-bellied toad, Bombina orientalis (Anura: Discoglossidae)

The effects of corticosterone (CORT)-treatment on various tissues were examined in two species of anuran larvae, the discoglossid Bombina orientalis, and the bufonid Bufo boreas. Corticosterone was administered directly into aquarium water for 15 days. After treatment, histological analyses were conducted on skin, gut, spleen, thymus, and neural and muscle tissue. Corticosterone treatment prevented sloughing of the skin, which resulted in a build-up of stratum corneum, and inhibited the development of gland nests and the subsequent formation of dermal granular and mucous glands in both species. Corticosterone treatment also decreased epithelial folding in the gut and caused vesiculation of the gut epithelial cells. The thymus of CORT-treated animals was significantly reduced in size (P < .05) and cell density (P < .05), and the spleen of CORT-treated animals was completely involuted. The brain and pituitary of CORT-treated animals had a decreased cell density (P < .05) and many pyknotic cells. An examination of muscle revealed that muscle fibers of CORT-treated animals had a decreased cross-sectional area (P < .05). The dose of CORT used (1.1 microM) was within the range used in other studies in the literature and resulted in tissue levels within the range experienced by larvae at metamorphic climax. Thus, this study is appropriate to address the histological effects of CORT in experimental manipulations and the role of increasing CORT at metamorphic climax. The data suggest that increasing endogenous CORT at metamorphosis may be involved in degeneration of larval tissue, prior to regeneration, which is stimulated by thyroid hormones.

Interdependence of corticosterone and thyroid hormones in toad larvae (Bufo boreas). II. Regulation of corticosterone and thyroid hormones

Typically, the role of corticosterone(B) in metamorphosis is considered secondary to that of thyroid hormone, with B having only enhancing effects. In the current study, we demonstrate that the relationship between the thyroid hormones and B is much more complex and that thyroxine (T4) may depend on B for some of its functions. Tadpoles of the western toad (Bufo boreas) were treated with various combinations of corticosterone (B), thyroxine (T4), triiodothyronine (T3), a goitrogen (thiourea; Thio), and a corticoid synthesis inhibitor (metyrapone; MTP). Hormones were extracted from individual tadpoles and whole-body hormone levels determined by radioimmunoassay. B-treatment decreased the ratio of T4 to T3, suggesting that B increased the conversion of endogenous T4 to T3. In addition, B-treatment in combination with T4 resulted in high whole body-levels of T3. B also caused a decrease in whole body-thyroid hormone levels (T4 and T3), suggesting negative feedback on the hypothalamo-pituitary-thyroid axis and T3 had a similar effect, decreasing whole body-T4 levels. T4-treatment, but not T3, increased whole body-B levels and MTP-treatment in combination with T4 prevented the stimulatory effect of T4 on B production. MTP-treatment alone blocked all steroid metabolism of [3H]progesterone by the inter-renal in vitro, and lowered whole body-B levels three-fold in vivo. Thio-treatment reduced thyroid hormone levels and also resulted in decreased B. Finally, we suggest that these results demonstrate a system in which T4 may regulate its own potency: increasing T4 stimulates B production, which increases the conversion of T4 to its more active form T3.(ABSTRACT TRUNCATED AT 250 WORDS)