Chronic dietary exposure to pyrolytic and petrogenic mixtures of PAHs causes physiological disruption in zebrafish--part II: behavior.

In the last 10 years, behavior assessment has been developed as an indicator of neurotoxicity and an integrated indicator of physiological disruption. Polycyclic aromatic hydrocarbon (PAH) release into the environment has increased in recent decades resulting in high concentrations of these compounds in the sediment of contaminated areas. We evaluated the behavioral consequences of long-term chronic exposure to PAHs, by exposing zebrafish to diets spiked with three PAH fractions at environmentally relevant concentrations. Fish were exposed to these chemicals from their first meal (5 days postfertilization) until they became reproducing adults (at 6 months old). The fractions used were representative of PAHs of pyrolytic (PY) origin and of two oils differing in composition (a heavy fuel oil (HO) and a light crude oil (LO)). Several tests were carried out to evaluate circadian spontaneous swimming activity, responses to a challenge (photomotor response), exploratory tendencies, and anxiety levels. We found that dietary PAH exposure was associated with greater mobility, lower levels of exploratory activity, and higher levels of anxiety, particularly in fish exposed to the HO fraction and, to a lesser extent, the LO fraction. Finally, our results indicate that PAH mixtures of different compositions, representative of situations encountered in the wild, can induce behavioral disruptions resulting in poorer fish performance.

Long-term dietary-exposure to non-coplanar PCBs induces behavioral disruptions in adult zebrafish and their offspring.

The use of polychlorinated biphenyls (PCBs) has been banned for several decades. PCBs have a long biological half-life and high liposolubility which leads to their bioaccumulation and biomagnification through food chains over a wide range of trophic levels. Exposure can lead to changes in animal physiology and behavior and has been demonstrated in both experimental and field analyses. There are also potential risks to high trophic level predators, including humans. A maternal transfer has been demonstrated in fish as PCBs bind to lipids in eggs. In this study, behavioral traits (exploration and free swimming, with or without challenges) of contaminated zebrafish (Danio rerio) adults and their offspring (both as five-day-old larvae and as two-month-old fish reared under standard conditions) were measured using video-tracking. Long-term dietary exposure to a mixture of non-coplanar PCBs was used to mimic known environmental contamination levels and congener composition. Eight-week-old fish were exposed for eight months at 26-28 °C. Those exposed to an intermediate dose (equivalent to that found in the Loire Estuary, ∑(CB)=515 ng g⁻¹ dry weight in food) displayed behavioral disruption in exploration capacities. Fish exposed to the highest dose (equivalent to that found in the Seine Estuary, ∑(CB)=2302 ng g⁻¹ dry weight in food) displayed an increased swimming activity at the end of the night. In offspring, larval activity was increased and two-month-old fish occupied the bottom section of the tank less often. These findings call for more long-term experiments using the zebrafish model; the mechanisms underlying behavioral disruptions need to be understood due to their implications for both human health and their ecological relevance in terms of individual fitness and survival.

Systematic screening of behavioral responses in two zebrafish strains.

Wild-type (WT) zebrafish are commonly used in behavioral tests, but the term WT is not a precise description, and corresponds to many different strains (e.g., AB, TU, WIK, and others). Previous studies compared the physiological, behavioral, or metabolic characteristics of different zebrafish strains (indigenous WT populations versus laboratory WT strains). AB and TU are widely used, but at least one study has demonstrated behavioral differences between them. To choose the most appropriate strain for our experiments, we systematically screened behavioral responses of AB and TU fish in several assays. We analyzed the locomotion activity and responses to a light/dark challenge in adults and larvae, and exploratory behavior and color conditioning in adults. Differences were observed for all tests, the strains displaying particular behavior depending on the tests. As larvae, TU displayed a wider activity range than AB larvae at the onset of locomotor behavior; as adults, TU were more reactive to sudden light transitions and recovered the swimming activity faster in T-maze or homebase release in novel tank tests, whereas AB fish had more contrasted circadian rhythms and performed better in color learning. Strain-specific behavior should be considered when designing experiments using behavior.