Assessment of sublethal ecotoxicity of solvents on larvae of a model native amphibian (Lithobates pipiens)

Does the Glucocorticoid Stress Response Make Toads More Toxic? An Experimental Study on the Regulation of Bufadienolide Toxin Synthesis

Chemical defense is a crucial component of fitness in many organisms, yet the physiological regulation of defensive toxin synthesis is poorly understood, especially in vertebrates. Bufadienolides, the main defensive compounds of toads, are toxic to many predators and other natural enemies, and their synthesis can be upregulated by stressors, including predation risk, high conspecific density, and pollutants. Thus, higher toxin content may be the consequence of a general endocrine stress response in toads. Therefore, we hypothesized that bufadienolide synthesis may be stimulated by elevated levels of corticosterone (CORT), the main glucocorticoid hormone of amphibians, or by upstream regulators that stimulate CORT production. To test these alternatives, we treated common toad tadpoles with exogenous CORT (exoCORT) or metyrapone (MTP, a CORT-synthesis inhibitor that stimulates upstream regulators of CORT by negative feedback) in the presence or absence of predation cues for 2 or 6 days, and subsequently measured their CORT release rates and bufadienolide content. We found that CORT release rates were elevated by exoCORT, and to a lesser extent also by MTP, regardless of treatment length. Bufadienolide content was significantly decreased by treatment with exoCORT for 6 days but was unaffected by exposure to exoCORT for 2 days or to MTP for either 6 or 2 days. The presence or absence of predation cues affected neither CORT release rate nor bufadienolide content. Our results suggest that changes in bufadienolide synthesis in response to environmental challenges are not driven by CORT but may rather be regulated by upstream hormones of the stress response.

Detection of imidacloprid and metabolites in Northern Leopard frog (Rana pipiens) brains

Neonicotinoids are a new type of highly water-soluble insecticide used in agricultural practices to eliminate pests. Neonicotinoids bind almost irreversibly to postsynaptic nicotinic acetylcholine receptors in the central nervous system of invertebrates, resulting in overstimulation, paralysis, and death. Imidacloprid, the most commonly used neonicotinoid, is often transported to nearby wetlands through subsurface tile drains and has been identified as a neurotoxin in several aquatic non-target organisms. The aim of the present study was to determine if imidacloprid could cross the blood-brain barrier in adult Northern Leopard frogs (Rana pipiens) following exposure to 0, 0.1, 1, 5, or 10 μg/L for 21 days. Additionally, we quantified the breakdown product of imidacloprid, imidacloprid-olefin, and conducted feeding trials to better understand how imidacloprid affects foraging behavior over time. Exposure groups had 12 to 313 times more imidacloprid in the brain relative to the control and breakdown products showed a dose-response relationship. Moreover, imidacloprid brain concentrations were approximately 14 times higher in the 10 μg/L treatment compared to the water exposure concentration, indicating imidacloprid can bioaccumulate in the amphibian brain. Reaction times to a food stimulus were 1.5 to 3.2 times slower among treatment groups compared to the control. Furthermore, there was a positive relationship between mean response time and log-transformed imidacloprid brain concentration. These results indicate imidacloprid can successfully cross the blood-brain barrier and bioaccumulate in adult amphibians. Our results also provide insights into the relationship between imidacloprid brain concentration and subsequent altered foraging behavior.

Frogs Respond to Commercial Formulations of the Biopesticide Bacillus thuringiensis var. israelensis, Especially Their Intestine Microbiota

It is generally believed that Bacillus thuringiensis var. israelensis (Bti) biopesticides are harmless to non-target organisms; however, new research shows controversial results. We exposed acutely and chronicallyLithobates sylvaticusandAnaxyrus americanus tadpoles until metamorphic climax to VectoBac 200G (granules) and VectoBac 1200L (aqueous suspension) at 300-20,000 ITU/L covering field-relevant concentrations and higher. The data show that the exposure parameters tested did not affect significantly the survival, total length, total weight, hepatosomatic index, gonadosomatic index, the expression of genes of interest (i.e., related to xenobiotic exposure, oxidative stress, and metamorphosis), and the intestine tissue layer detachment ofL. sylvaticusandA. americanus in a concentration-response pattern. In contrast, VectoBac 200G significantly increased the median time to metamorphosis ofL. sylvaticus tadpoles by up to 3.5 days and decreased the median by up to 1 day inA. americanus. VectoBac 1200L significantly increased the median time to metamorphosis ofL. sylvaticusandA. americanustadpoles by up to 4.5 days. Also, the exposure to VectoBac 200G and 1200L altered the intestine bacterial community composition inA. americanus at application rates recommended by the manufacturer, which led to an increase in the relative abundance of Verrucomicrobia, Firmicutes, Bacteroidetes, and Actinobacteria. Changes in the intestine microbiota might impact the fitness of individuals, including the susceptibility to parasitic infections. Our results indicate that the effect of Bti commercial products is limited; however, we recommend that Bti-spraying activities in amphibian-rich ecosystems should be kept minimal until there is more conclusive research to assess if the changes in the time to metamorphosis and microbiota can lead to negative outcomes in amphibian populations and, eventually, the functioning of ecosystems.

Organic solvents alter photophysiological and oxidative stress profiles of the coral Zoanthus sp. - Towards an optimization of ecotoxicological protocols

Coral reefs are declining, affected by climate change and escalating anthropogenic pressures, such as pollution or habitat alteration. Consequently, ecotoxicological assays with tropical corals have increased, specifically towards the study of emergent or persistent pollutants. However, standardized methodology to test for corals is non-existent, and their response to organic solvents, recurrently required in ecotoxicological appraisals, remains unknown. Therefore, we aimed to establish a threshold for the safe use of the selected solvents in ecotoxicological studies with these organisms. We assessed the oxidative stress response (antioxidant response and oxidative damage), cellular energy allocation and photophysiology of the photosynthetic coral Zoanthus sp. (Anthozoa, Hexacorallia) exposed to six doses of three different organic solvents (ethanol, methanol and dimethyl sulfoxide - DMSO). Our results suggest that the coral is more sensitive to methanol and DMSO than to ethanol. Methanol and DMSO LOEC were 0.01 mL L^-1 affecting maximum quantum yield (F_v/F_m) and glutathione S-transferase (GST) activity, respectively, while for ethanol was 0.03 mL L^-1, influencing F_v/F_m. Despite the higher tolerance of Zoanthus sp. to ethanol, 2.9 mL L^-1 of this organic solvent was the only treatment causing mortality. Based on these findings, thresholds for the use of organic solvents with tropical corals can now be adopted. Nevertheless, species specificities should not be overlooked.