A Critical Review of Morphologic Findings and Data From 14 Toxicological Studies Involving Fish Exposures to Diclofenac

Impact of cadmium and diclofenac exposure on biochemical responses, transcriptome, gut microflora, and growth performance in grass carp (Ctenopharyngodonidella)

In recent years, the concentrations of cadmium (Cd) and diclofenac (DCF) in water have frequently exceeded the standard; however, the toxic effects of these two pollutants on grass carp under single and combined exposure are unknown. In this study, the concentrations of pollutants in different tissues were detected, and the toxicities of the two pollutants to grass carp under different exposure conditions were compared based on growth traits, biochemical responses, gut microbiome, and transcriptomes. Based on these findings, the brain showed the lowest levels of Cd and DCF accumulation. Oxidative stress and pathological damage were observed in the brain and intestines. Changes in the structure and abundance of the gut microflora affect the synthesis of neurotransmitters, such as GABA and steroids. Differentially expressed genes in the brain were enriched in circadian rhythm functions. The expression of PER, CLOCK,1L-1β, 1L-17, and other genes are related to the abundance of Akkermansia, which indicates that the disorder of gut microflora will affect the normal circadian rhythm of the brain. All indices in the recovery group showed an increasing trend. Overall, the toxicity of Cd and DCF showed antagonism, and a single exposure had a stronger effect on gut microorganisms and circadian rhythm, which provided a scientific basis for exploring the comprehensive effects of different pollutants.

Standardized tissue sampling guidelines for histopathological and molecular analyses of rainbow trout (Oncorhynchus mykiss) in ecotoxicological studies

In ecotoxicology, evaluation of toxicities and no observed effect concentrations (NOEC) of test compounds in experimental fish is commonly based on molecular-, biochemical- and analytical chemistry analyses of organ/tissue samples and the assessment of (histo-) pathological lesions. Standardization of organ/tissue sampling locations, sample numbers, and sample processing contributes to warrant the reproducibility and inter- and intra-study comparability of analysis results. The present article provides the first comprehensive tissue sampling guidelines specifically adapted to rainbow trout (Oncorhynchus mykiss) as a frequently used fish species in ecotoxicological studies. A broad spectrum of ~40 different organs and tissues is covered. Appropriate sampling locations, sample sizes and sample numbers for subsequent routine histopathological evaluation (all organs/tissue) and for molecular analyses (~30 organs/tissues) are described in detail and illustrated with schematic drawings and representative macroscopic and histological images. These field-proven sampling guidelines were developed based on the pertinent literature and practical experience in ecotoxicological fish studies. They are intended to serve as a standard reference for any routine ecotoxicological study using rainbow trout as a test system. A broad application of the featured tissue sampling procedures will help to improve the reproducibility of analyses and to reduce inter- and intra-study variability induced by sampling bias and (normal) inter-sample morphological variation, and will therefore provide a robust basis for reliable characterization of toxicity and NOEC identification of diverse test substances and aquatic pollutants.

Hazards of current concentration-setting practices in environmental toxicology studies

The setting of concentrations for testing substances in ecotoxicological studies is often based on fractions of the concentrations that cause 50% mortality (LC50 or LD50) rather than environmentally relevant levels. This practice can result in exposures to animals at test concentrations that are magnitudes of order greater than those experienced in the environment. Often, such unrealistically high concentrations may cause non-specific biochemical or morphologic changes that primarily reflect the near-lethal health condition of the animal subjects, as opposed to effects characteristic of the particular test compound. Meanwhile, it is recognized that for many chemicals, the toxicologic mode of action (MOA) responsible for lethality may differ entirely from the MOAs that cause various sublethal effects. One argument for employing excessively high exposure concentrations in sublethal studies is to ensure the generation of positive toxicological effects, which can then be used to establish safety thresholds; however, it is possible that the pressure to produce exposure-related effects may also contribute to false positive outcomes. The purpose of this paper is to explore issues involving some current usages of acute LC50 data in ecotoxicology testing, and to propose an alternative strategy for performing this type of research moving forward. Toward those ends, a brief literature survey was conducted to gain an appreciation of methods that are currently being used to set test concentrations for sublethal definitive studies.

Detection of Diclofenac-Induced Alterations in Rainbow Trout (Oncorhynchus mykiss) Using Quantitative Stereological Methods

In 2013, the nonsteroidal anti-inflammatory drug diclofenac (DCF) was included in the watch list for emerging pollutants under the European Union Water Framework Directive. Frequently, monitoring data revealed DCF concentrations in surface waters exceeding the proposed environmental quality standards of 0.04 µg L^-1 and 0.126 µg L^-1 . In recent literature, the possible effects of DCF on fish are discussed controversially. To contribute to a realistic risk assessment of DCF, a 28-day exposure experiment was carried on rainbow trout (Oncorhynchus mykiss). To warrant reliability of data, experiments were conducted considering the Criteria for Reporting and Evaluating Ecotoxicity Data. The test concentrations of DCF used (0.1, 0.5, 1, 5, 25, and 100 µg L^-1 ) also included environmentally relevant concentrations. The lowest-observed-effect concentration (LOEC) for a significant decrease in the plasma concentrations of the DCF biomarker prostaglandin E2 was 0.5 µg L^-1 (male fish). For objective evaluation of relevant histomorphological parameters of gills and trunk kidneys, unbiased quantitative stereological methods were applied. In the gills, significant increases in the thickness of the secondary lamella and in the true harmonic mean of barrier thickness in secondary lamellae were present at DCF concentrations of 25 µg L^-1 and 100 µg L^-1 . In the trunk kidneys, the absolute and relative volumes of nephrons were significantly decreased, paralleled by a significant increase of the volume of the interstitial renal tissue. With regard to quantitative histomorphological alterations in the trunk kidney, the observed LOEC was 0.5 µg L^-1 . The quantitative histomorphological analyses that were conducted allow identification and objective quantification of even subtle but significant morphological effects and thus provide an important contribution for the comparability of study results for the determination of no-observed-effect concentrations (NOEC). Environ Toxicol Chem 2023;42:859-872. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.