Chronic, dietary polybrominated diphenyl ether exposure affects survival, growth, and development of Rana pipiens tadpoles

Inefficacy of dietary test substance administration in Amphibian Metamorphosis Assay (AMA) studies

Traditionally, the Amphibian Metamorphosis Assay (AMA; OECD TG 231) is performed by exposing Xenopus laevis tadpoles to test substances dissolved in laboratory water. Recently, the use of dietary administration has been proposed to combat poorly soluble test substances in ecotoxicologically-based regulatory endocrine disruption (ED) studies, specifically the AMA warranting an investigation into the efficacy of dietary administration. An efficacy study comprised of two phases: 1) evaluation of the physical influence of the loading process via solvent and 10, 1, and 0.1 mg/l test substance or surrogate (sunflower oil, SFO) on the Sera® Micron Nature (SMN) diet, and 2) performance of a modified AMA in which Nieuwkoop and Faber (NF) stage 51 X. laevis larvae were exposed to dechlorinated tap water using one concentration of the SFO in the diet for 21 days, was performed. In phase 1, the addition of acetone or acetone with bis(2-propylheptyl) phthalate (DPHP) or SFO to SMN with subsequent solvent purge altered the diet reducing the density of the liquified diet and dietary pellet size following centrifugation indicative of alteration of the physical properties of the diet. Treatments used in the modified AMA were acetone alone and 0.1 mg/l SFO dissolved in acetone. These treatments were evaluated against an SMN benchmark using standard AMA endpoints. Both the acetone-treated SMN and 0.1 mg/l SFO-treated diets significantly reduced survival rates, 67 and 70% relative to the SMN benchmark (100%), decreased developmental stage distribution and snout-vent length-normalized hind limb length relative to the SMN benchmark, and slightly increased the prevalence and severity of thyroid follicular cell hypertrophy. Although the acetone-treated diets may have impacted the hypothalamo-pituitary-thyroid axis, clinical signs of gastrointestinal impaction and tail flexure were also observed in the acetone-treated diets, but not the SMN diet alone. Ultimately, test substance exposure via the diet in an AMA study can produce results that may confound data interpretation, which suggests that the traditional aqueous exposure route is generally more appropriate.

Effects of cadmium exposure on thyroid gland and endochondral ossification in Rana zhenhaiensis

Discovery of elevated concentrations of cadmium in the natural environment has increased awareness because of their potential threats. Amphibians are negatively affected due to their moderate sensitivity to cadmium. Here, we conduct acute and subchronic toxicity tests to examine whether, and to what extent, cadmium exposure disturbs metamorphosis, growth, and kinetic ability of Rana zhenhaiensis. We set different concentration treatment groups for the subchronic toxicity test (0, 10, 40, 160 μg Cd L^-1). Our findings demonstrate that cadmium exposure reduces growth parameters and the cumulative metamorphosis percent of R. zhenhaiensis. Decreases in follicular size and follicular epithelial cell thickness of thyroid gland are found in the treatment group. Further, subchronic exposure to cadmium decreases ossification ratio of hindlimbs in all treatment. Also, adverse effects of cadmium exposure on aquatic tadpoles can result in the reduced physical parameters and weak jumping ability in adult frogs. In this sense, our study suggests that cadmium adversely influences body condition and metamorphosis of R. zhenhaiensis, damages thyroid gland and impairs endochondral ossification. Meanwhile, we speculated that cadmium-damaged thyroid hormones inhibit skeletal development, resulting in the poor jumping ability, which probably leads to reduced survival of R. zhenhaiensis.

The Effects of Dietary Polybrominated Diphenyl Ether Exposure and Rearing Temperature on Tadpole Growth, Development, and Their Underlying Processes

Depression of growth rate due to polybrominated diphenyl ethers (PBDEs) has been documented in birds, mammals, amphibians, and fish at single temperatures. However, the underlying energetic mechanism for this effect and how it might change in relation to changing environmental temperature remain unstudied. We used a simple energy budget to address hypotheses regarding effects of PBDEs on tadpole (Lithobates pipiens) growth: that reductions in growth are linked to increased respiratory costs, reductions in digestive performance, differences in body composition, reductions in food intake, or a combination of these factors. From 18 days postfertilization (dpf) until 42 dpf, tadpoles were exposed dietarily to a pentabromodiphenyl ether mixture (DE-71^TM ) at a concentration of 100 ng DE-71/g wet mass under a rearing temperature of either 22 or 27 °C. After 20 days of PBDE exposure, total PBDEs in tadpoles averaged 148.4 ng/g wet mass, with no differences by rearing temperature and approximately 50% higher than in their diet; controls not fed PBDE had levels <1 ng/g. Exposure to PBDE resulted in reductions in body length, mass, and development compared to controls, independent of rearing temperature; PBDE had no effect on measures of body composition, dry matter digestibility, or oxygen consumption. A simple energy budget using data from the present study revealed that a 10% decrease in feeding rate could explain the lower mass gain of tadpoles exposed to PBDE. Growth depression by PBDE could be due to (1) direct inhibition of growth processes by PBDE that indirectly decreases total energy demand and food intake, and (2) direct inhibition of food intake. Future studies to disentangle these possible pathways of PBDE effects are warranted. Environ Toxicol Chem 2021;40:3181-3192. © 2021 SETAC.

Effects of Environmental Contaminants at Great Bay National Wildlife Refuge on Anuran Development, Gonadal Histology, and Reproductive Steroidogenesis: A Comparison of In Situ and Laboratory Exposures

Previous monitoring at Great Bay National Wildlife Refuge (NWR), Newington, New Hampshire documented high prevalence of amphibian malformations at sites contaminated with potential endocrine active compounds. In the present study, a combination of in situ and laboratory experiments were used to determine whether contaminants present in the sites affect amphibian growth and reproductive development. Wood frog (Rana sylvatica) tadpoles were exposed in situ at four sites (Ferry Way, Beaver Pond, Lower Peverly, and Stubbs Pond) at Great Bay NWR and northern leopard frog (Rana pipiens) tadpoles were exposed in the lab to sediments collected from three sites (Beaver Pond, Ferry Way, Stubbs Pond) at Great Bay NWR as well as a positive (estradiol) and negative control. High mortality was observed at Stubbs Pond and extended larval period at Beaver Pond in the in situ exposure. Only three malformations were noted in the lab experiment, whereas there was a 63% prevalence of rounded femurs in Beaver Pond metamorphs in the in situ exposure. Only 2.4% (5 of 207) of R. sylvatica metamorphs exhibited abnormal reproductive development, whereas intersex metamorphs occurred in treatments and controls in the lab experiment at rates as high as 26%. Reproductive development was more advanced and estradiol to androgen ratios reduced in male metamorphs from Beaver Pond in both the in situ and lab exposures. DDT, PCBs, and PAHs were detected in sediments at Great Bay NWR at concentrations that exceed regulatory or guidance values, with concentrations of PAHs being highest at Lower Peverly Pond and DDT highest at Stubbs Pond. The effects on anuran development may be attributable to the primary contaminants-DDT and PCBs-acting on the thyroid and gonadal axes.

Warmer temperature increases toxicokinetic elimination of PCBs and PBDEs in Northern leopard frog larvae (Lithobates pipiens)

We studied the temperature dependence of accumulation and elimination of two polychlorinated biphenyls (PCBs; PCB-70 and PCB-126) and a commercial mixture of congeners of polybrominated diphenyl ethers (PBDEs; DE-71™)) in Northern leopard frog (Lithobates pipiens) tadpoles. We reared tadpoles at 18, 23, or 27 °C for 5.3 or up to 13.6 weeks (longer at cooler temperature where development is slower) on diets containing the toxicants, each at several different toxicant concentrations, and compared tissue concentrations as a function of food concentration and rearing temperature. Following > 1 month of accumulation, tissue concentrations of all three toxicants in exposed tadpoles were linearly related to dietary concentrations as expected for first order kinetics, with no significant effect of rearing temperature.We also raised free-swimming L. pipiens tadpoles for 14 days on foods containing either toxicant at 18 or 27 °C during an accumulation phase, and then during depuration (declining toxicant) phase of 14 days we provided food without toxicants and measured the decline of toxicants in tadpole tissue. All the congeners were eliminated faster at warmer rearing temperature, as expected. Using Arrhenius' equation, we calculated that the apparent activation energy for elimination of both PCB congeners by tadpoles was 1.21 eV (95% confidence interval 0.6-1.8 eV). We discuss how this value was within the range of estimates for metabolic reactions generally (range 0.2 - 1.2 eV), which might include metabolic pathways for biotransformation and elimination of PCBs. Furthermore, we discuss how the lack of an effect of rearing temperature on tadpole near-steady-state tissue residue levels suggests that faster elimination at the warmer temperature was balanced by faster uptake, which is plausible considering the similar temperature sensitivities (i.e., activation energies) of all these processes. Although interactions between toxicants and temperature can be complex and likely toxicant-dependent, it is plausible that patterns observed in tadpoles might apply to other aquatic organisms. Published data on depuration in 11 fish species eliminating 8 other organic toxicants indicated that they also had similar apparent activation energy for elimination (0.82 ± 0.12 eV; 95% confidence interval 0.56 - 1.08 eV), even though none of those studied toxicants were PCBs or PBDEs. Additional research on toxicant-temperature interactions can help improve our ability to predict toxicant bioaccumulation in warming climate scenarios.

Polybrominated Diphenylether (DE-71) Exposure Skews Phenotypic Sex Ratio, and Alters Steroid Hormone Levels and Steroidogenic Enzyme Activities in Juvenile Silurana tropicalis

The impact of the brominated flame-retardant mixture, DE-71, on gonadal steroidogenesis during sexual differentiation in Silurana tropicalis was examined. A partial lifecycle study exposing S. tropicalis to varying concentrations of DE-71 (0.0, 0.65, 1.3, 2.5, and 5.0 μg/L [nominal]) was conducted from early gastrula-stage embryo to 150 d post-metamorphosis (dpm). Exposure of S. tropicalis to DE-71 induced liver necrosis and induced abnormal ovary development characterized by previtellogenic oocyte necrosis and arrested development of vitellogenic oocytes in females in a concentration-dependent manner. Decreased mean plasma DHT and T, gonad T, and increased mean plasma E2 levels were found in 150 dpm DE-71-treated male S. tropicalis compared to controls. Plasma E2 levels in females were not significantly altered compared to control S. tropicalis, although lower plasma and gonad T were detected. Mean gonadal CYP 19 aromatase activity in both male and female S. tropicalis exposed to DE-71 was not appreciably affected. Decreased mean male 5α-reductase and CYP17 activities in both male and females were observed compared to control frogs. Overall, these studies suggested that PBDE exposure induced liver necrosis and abnormal ovary development; and reduced circulating and gonadal androgens resulting in a phenotypic skew in sex ratio toward the female sex in S. tropicalis.

Endocrine Disruption Alters Developmental Energy Allocation and Performance in Rana temporaria

Environmental change exposes wildlife to a wide array of environmental stressors that arise from both anthropogenic and natural sources. Many environmental stressors with the ability to alter endocrine function are known as endocrine disruptors, which may impair the hypothalamus-pituitary-thyroid axis resulting in physiological consequences to wildlife. In this study, we investigated how the alteration of thyroid hormone (TH) levels due to exposure to the environmentally relevant endocrine disruptor sodium perchlorate (SP; inhibitory) and exogenous L-thyroxin (T4; stimulatory) affects metabolic costs and energy allocation during and after metamorphosis in a common amphibian (Rana temporaria). We further tested for possible carry-over effects of endocrine disruption during larval stage on juvenile performance. Energy allocated to development was negatively related to metabolic rate and thus, tadpoles exposed to T4 could allocate 24% less energy to development during metamorphic climax than control animals. Therefore, the energy available for metamorphosis was reduced in tadpoles with increased TH level by exposure to T4. We suggest that differences in metabolic rate caused by altered TH levels during metamorphic climax and energy allocation to maintenance costs might have contributed to a reduced energetic efficiency in tadpoles with high TH levels. Differences in size and energetics persisted beyond the metamorphic boundary and impacted on juvenile performance. Performance differences are mainly related to strong size-effects, as altered TH levels by exposure to T4 and SP significantly affected growth and developmental rate. Nevertheless, we assume that juvenile performance is influenced by a size-independent effect of achieved TH. Energetic efficiency varied between treatments due to differences in size allocation of internal macronutrient stores. Altered TH levels as caused by several environmental stressors lead to persisting effects on metamorphic traits and energetics and, thus, caused carry-over effects on performance of froglets. We demonstrate the mechanisms through which alterations in abiotic and biotic environmental factors can alter phenotypes at metamorphosis and reduce lifetime fitness in these and likely other amphibians.

Contaminant and Environmental Influences on Thyroid Hormone Action in Amphibian Metamorphosis

Aquatic and terrestrial environments are increasingly contaminated by anthropogenic sources that include pharmaceuticals, personal care products, and industrial and agricultural chemicals (i. e., pesticides). Many of these substances have the potential to disrupt endocrine function, yet their effect on thyroid hormone (TH) action has garnered relatively little attention. Anuran postembryonic metamorphosis is strictly dependent on TH and perturbation of this process can serve as a sensitive barometer for the detection and mechanistic elucidation of TH disrupting activities of chemical contaminants and their complex mixtures. The ecological threats posed by these contaminants are further exacerbated by changing environmental conditions such as temperature, photoperiod, pond drying, food restriction, and ultraviolet radiation. We review the current knowledge of several chemical and environmental factors that disrupt TH-dependent metamorphosis in amphibian tadpoles as assessed by morphological, thyroid histology, behavioral, and molecular endpoints. Although the molecular mechanisms for TH disruption have yet to be determined for many chemical and environmental factors, several affect TH synthesis, transport or metabolism with subsequent downstream effects. As molecular dysfunction typically precedes phenotypic or histological pathologies, sensitive assays that detect changes in transcript, protein, or metabolite abundance are indispensable for the timely detection of TH disruption. The emergence and application of 'omics techniques-genomics, transcriptomics, proteomics, metabolomics, and epigenomics-on metamorphosing tadpoles are powerful emerging assets for the rapid, proxy assessment of toxicant or environmental damage for all vertebrates including humans. Moreover, these highly informative 'omics techniques will complement morphological, behavioral, and histological assessments, thereby providing a comprehensive understanding of how TH-dependent signal disruption is propagated by environmental contaminants and factors.

Warmer temperature modifies effects of polybrominated diphenyl ethers on hormone profiles in leopard frog tadpoles (Lithobates pipiens)

Amphibian populations have been declining, and climate change and exposure to environmental contaminants are thought to be involved. Higher water temperature accelerates larval development; however, its combined effects with contaminants and their influence on hormones during metamorphosis are poorly understood. The authors investigated changes in whole-body triiodothyronine (T3) and corticosterone concentrations in developing leopard frogs reared at 23 °C and 28 °C on diets with 0 ng g^-1 , 6 ng g^-1 , and 37 ng g^-1 of a technical mixture of polybrominated diphenyl ethers (PBDE; DE-71) from 10 d to 44 d (premetamorphosis to late climax; Gosner Stages 28 to 46). Unlike controls, PBDE-exposed tadpoles (6 ng g^-1 ) reared at 23 °C failed to show any increase in T3 concentrations throughout metamorphosis, and exposed tadpoles reared at 28 °C showed a lower peak at climax compared to controls. Corticosterone levels progressively increased throughout metamorphosis, but the levels were higher in PBDE-exposed tadpoles compared to controls at both temperatures. At the warmer temperature, corticosterone increase occurred earlier (at early climax) in controls and exposed tadpoles compared to tadpoles reared at the cooler temperature (late climax), coinciding with the faster development observed at 28 °C. Tadpoles reared at 28 °C were longer and developed faster than tadpoles reared at 23 °C. At both temperatures, PBDE exposure decreased T3 and increased corticosterone concentrations, which can potentially impair developing tadpoles. Environ Toxicol Chem 2017;36:120-127. © 2016 SETAC.

Dietary exposure to polybrominated diphenyl ether 47 (BDE-47) inhibits development and alters thyroid hormone-related gene expression in the brain of Xenopus laevis tadpoles

Few studies have investigated the thyroid-disrupting effects of polybrominated diphenyl ethers (PBDEs) across multiple levels of biological organization in anurans, despite their suitability for the screening of thyroid disruptors. Therefore, the present study evaluated the effects of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) on development, thyroid histology and thyroid hormone-related gene expression in Xenopus laevis exposed to 0 (control), 50 (low), 500 (medium) or 5000μg BDE-47/g food (high) for 21days. Only the high dose of BDE-47 hindered growth and development; however, thyroid hormone-associated gene expression was downregulated in the brains of tadpoles regardless of dose. These results show that BDE-47 disrupts thyroid hormone signaling at the molecular and whole-organism levels and suggest that gene expression in the brain is a more sensitive endpoint than metamorphosis. Furthermore, the altered gene expression patterns among BDE-47-exposed tadpoles provide insight into the mechanisms of PBDE-induced thyroid disruption and highlight the potential for PBDEs to act as neurodevelopmental toxicants.

Larval exposure to polychlorinated biphenyl 126 (PCB-126) causes persistent alteration of the amphibian gut microbiota

Interactions between gut microbes and anthropogenic pollutants have been under study. The authors investigated the effects of larval exposure to polychlorinated biphenyl 126 (PCB-126) on the gut microbial communities of tadpoles and frogs. Frogs treated with PCBs exhibited increased species richness in the gut and harbored communities significantly enriched in Fusobacteria. These results suggest that anthropogenic pollutants alter gut microbial populations, which may have health and fitness consequences for hosts.

In situ effects of pesticides on amphibians in the Sierra Nevada

For more than 20 years, conservationists have agreed that amphibian populations around the world are declining. Results obtained through laboratory or mesocosm studies and measurement of contaminant concentrations in areas experiencing declines have supported a role of contaminants in these declines. The current study examines the effects of contaminant exposure to amphibians in situ in areas actually experiencing declines. Early larval Pseudacris regilla were translocated among Lassen Volcanic, Yosemite and Sequoia National Parks, California, USA and caged in wetlands in 2001 and 2002 until metamorphosis. Twenty contaminants were identified in tadpoles with an average of 1.3-5.9 (maximum = 10) contaminants per animal. Sequoia National Park, which had the greatest variety and concentrations of contaminants in 2001, also had tadpoles that experienced the greatest mortality, slowest developmental rates and lowest cholinesterase activities. Yosemite and Sequoia tadpoles and metamorphs had greater genotoxicity than those in Lassen during 2001, as determined by flow cytometry. In 2001 tadpoles at Yosemite had a significantly higher rate of malformations, characterized as hemimelia (shortened femurs), than those at the other two parks but no significant differences were observed in 2002. Fewer differences in contaminant types and concentrations existed among parks during 2002 compared to 2001. In 2002 Sequoia tadpoles had higher mortality and slower developmental rates but there was no difference among parks in cholinesterase activities. Although concentrations of most contaminants were below known lethal concentrations, simultaneous exposure to multiple chemicals and other stressors may have resulted in lethal and sublethal effects.

Dietary exposure of BDE-47 and BDE-99 and effects on behavior, bioenergetics, and thyroid function in juvenile red-eared sliders (Trachemys scripta elegans) and common snapping turtles (Chelydra serpentina)

Juvenile red-eared sliders (Trachemys scripta elegans) and snapping turtles (Chelydra serpentina) were fed food dosed with brominated diphenyl ether-47 (BDE-47) or BDE-99 for 6 mo beginning approximately 9 mo posthatch. During the exposure period, measurements of growth, bioenergetics, and behavior were made; thyroid function and accumulation were quantified postexposure. Whole-body concentrations of both congeners were lower in red-eared sliders compared with snapping turtles after 6 mo of exposure. Snapping turtles receiving BDE-47 had significantly elevated standard metabolic rates after 3 mo and 4 mo of exposure (p = 0.014 and p = 0.019, respectively). When exposed to BDE-99, red-eared sliders were slower to right themselves after having been inverted (p < 0.0001). Total glandular thyroxine concentrations were significantly reduced in red-eared sliders exposed to BDE-47 (mean control, 8080 ng/g; mean BDE-47, 5126 ng/g; p = 0.034). These results demonstrate that dietary exposure to BDE-47 and BDE-99 can elicit a suite of responses in 2 species of turtles, but that the red-eared slider appears to be a more sensitive species to the measured end points.

Immunomodulation in post-metamorphic northern leopard frogs, Lithobates pipiens, following larval exposure to polybrominated diphenyl ether

Pollutants and disease are factors implicated in amphibian population declines, and it is hypothesized that these factors exert a synergistic adverse effect, which is mediated by pollutant-induced immunosuppression. Polybrominated diphenyl ethers (PBDEs) are ubiquitous pollutants that can exert immunotoxicity, making them of interest to test effects on amphibian immune function. We orally exposed Lithobates (Rana) pipiens tadpoles to environmentally realistic levels (0-634 ng/g wet diet) of a pentabromodiphenyl ether mixture (DE-71) from as soon as they became free-swimming through metamorphic climax. To assess adaptive immune response in juvenile frogs, we used an enzyme-linked immunosorbent assay to measure specific IgY production following immunization with keyhole limpet hemocyanin (KLH). Specific KLH antibody response was significantly decreased in juvenile frogs that had been exposed to PBDEs as tadpoles. When assessing innate immune responses, we found significantly different neutrophil counts among treatments; however, phagocytic activity of neutrophils was not significantly different. Secretion of antimicrobial skin peptides (AMPs) nonsignificantly decreased with increasing PBDE concentrations, and no significant effect of PBDE treatment was observed on efficacy of AMPs to inhibit chytrid fungus (Batrachochytrium dendrobatidis) growth. Our findings demonstrate that environmentally realistic concentrations of PBDEs are able to alter immune function in frogs; however, further research is needed to determine how these alterations impact disease susceptibility in L. pipiens.

Toxicokinetics of polybrominated diphenyl ethers across life stages in the northern leopard frog (Lithobates pipiens)

Polybrominated diphenyl ethers (PBDEs), a class of flame retardants, are bioaccumulative toxins that can biomagnify in food webs. However, little is known about the toxicokinetics of total and congener-specific BDEs in lower vertebrates. The authors exposed northern leopard frog (Lithobates (Rana) pipiens) tadpoles to diets containing DE-71 (a pentabromodiphenyl ether mixture (0 ng/g as control, 71.4 ng/g, and 634 DE-71 ng/g wet mass)) for 50 d, followed by a period of depuration during which they were fed only undosed (control) food. After 28 d, tadpoles eliminated over 94% of the ΣPBDEs from their tissues (t½  = 5.9 ± 1.9 d) with no significant differences in elimination rates for the predominant congeners. Elimination of BDE-99 was independent of dose, indicating first-order kinetics. It did not fit a biexponential model significantly better than a monoexponential model, indicating single-compartment elimination. To compare developmental life-stage kinetics following larval exposure, the authors collected individuals at the beginning and end of metamorphosis and at 70 d postmetamorphosis. During metamorphosis, total-body residues per individual did not significantly change, implying little to no elimination. After 70 d, juvenile frogs eliminated 89.7% of the ΣPBDEs from their tissues, and BDE-47 was eliminated at a faster rate (t½ = 17.3 d) than BDE-99 and BDE-100 (t½  = 63.0 d and 69.3 d, respectively). Because the kinetics of PBDEs in L. pipiens differed among life stages, developmental life stage-especially for species that undergo metamorphosis-should be considered when determining the toxicity of persistent organic pollutants.

Effects of chronic polybrominated diphenyl ether exposure on gonadal development in the northern leopard frog, Rana pipiens

Polybrominated diphenyl ethers (PBDEs) are bioaccumulative, persistent organic pollutants used as flame retardants in consumer goods. Concentrations of PBDEs in North American wildlife have been increasing for decades and been shown to have estrogenic effects on sexual development. No studies, however, have examined the effects of PBDEs on the sexual development of North American frogs at ecologically relevant concentrations. This study examined the effects of five dietary concentrations of DE-71 (0, 1.1, 6.1, 71.4, and 634 ng ΣPBDEs/g diet), a technical PBDE mixture, on the gonadal development of the northern leopard frog, Rana pipiens. Tadpoles were exposed chronically from the time they became free-swimming until metamorphosis. Frogs were killed either at metamorphic climax or 10 weeks after completing metamorphosis, processed for histology, and examined for alterations in sexual development. The experimental group exposed to PBDEs at 1.1 ng/g had a significantly larger proportion of females compared with the expected 50:50 sex ratio. At 10 weeks post-metamorphosis, male frogs exposed to 6.1 and 71.4 ng/g had significantly smaller testes, but all other measure of gonadal development tested showed no effects. No intersex or increased incidence of gonadal abnormality were detected. These findings indicate that PBDEs may disrupt sexual differentiation in frogs at low, environmentally relevant concentrations.