Olfactory toxicity in zebrafish : Assessing the effects of three commonly used pesticides and cadmium on the olfactory system of Danio rerio

Volz, Sina; Hollert, Henner (Thesis advisor); Schäffer, Andreas (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

Fish are highly dependent on olfaction since a variety of indispensable behaviors are mediated via this sensory modality. Situated in cavities at the dorsal part of the cranium, the fish olfactory epithelium (OE), which contains the olfactory sensory neurons (OSNs) responsible for odorant detection, is in almost direct contact to the ambient water and thus readily accessible for both odorants and dissolved contaminants. In the last decades, an increasing number of studies demonstrated that numerous pollutants, among them metals and pesticides, are able to impair olfaction in environmentally relevant concentrations, thereby disrupting pivotal behaviors. However, the toxic mechanisms underlying olfactory dysfunction largely remain unknown and despite the paramount significance of olfaction for fish, olfactory endpoints have not yet been incorporated into the environmental risk assessment of chemicals. The objective of the present Ph.D. thesis was to investigate the impact of three commonly used pesticides (chlorpyrifos, linuron, and permethrin) and cadmium (Cd) on the olfactory system of zebrafish (Danio rerio). In doing so, this thesis further aimed at contributing to the establishment of zebrafish as a model for evaluating olfactory toxicity. In order to enhance the understanding of the toxic mechanisms underlying olfactory dysfunctions, the effect of contaminants on the olfactory system was analyzed at different levels of biological organization. Employing quantitative real time polymerase chain reaction, it was shown that Cd, chlorpyrifos, and permethrin differentially alter the expression of OSN marker genes in the OE. Furthermore, Cd and chlorpyrifos induced genes associated with stress in the OE, but only Cd increased the expression of such genes in the olfactory bulb (OB). The assessment of second messengers with central roles in olfactory signal transduction showed that linuron decreased the level of cyclic adenosine monophosphate (cAMP) in the OE. The influence of the pollutants on the olfactory mediated endocrine response of male zebrafish to the pheromone prostaglandin F2α (PGF2α) was examined by analyzing 11 ketotestosterone (11 KT) plasma levels. The pesticides did not cause a statistically significant impairment of this endocrine response, whereas male zebrafish exposed to Cd displayed a downward trend of 11 KT levels. The assessment of the antipredator response following treatment with the respective contaminant demonstrated that Cd and chlorpyrifos significantly disrupted this crucial behavior. While it recovered from chlorpyrifos induced deficits following 48 h of depuration, the antipredator response did not recover in fish previously exposed to Cd. The suitability of zebrafish early life stages as alternative models for olfactory toxicity assessments was examined by studying contaminant induced effects on OSN marker gene expression, cell death in the olfactory placode, and olfactory evoked swimming activity. In this process, no statistically significant alterations were detected. Since water chemistry is known to markedly influence acute metal toxicity, its impact on Cd induced olfactory impairment was investigated, thereby showing that hardness, pH, and dissolved organic carbon substantially modified the latter. The research presented in the thesis at hand demonstrated that chlorpyrifos and Cd disrupted the antipredator response, an olfactory mediated behavior essential for fish survival. The results from the gene expression study strongly supported the assumption that Cd is taken up into the OE and OB, causing oxidative stress in these organs. By contrast, oxidative stress may not be a predominant factor of chlorpyrifos induced olfactory dysfunction. The decreasing trend observed for the endocrine response to PGF2α following Cd treatment may further point at a potential impact on reproduction and needs to be subject of additional research. Zebrafish proved to be an excellent model for the assessment of olfactory toxicity, in particular with respect to the investigation of mechanisms underlying an olfactory dysfunction. However, the suitability of its early life stages as alternatives for olfactory toxicity testing could not be confirmed based on the data obtained in this thesis. Overall, the work contributed to the establishment of zebrafish as a model for the study of olfactory toxicity and underlined the importance of the incorporation of olfactory endpoints into the environmental risk assessment of chemicals.

Institutions

• Department of Biology [160000]
• Chair of Environmental Biology and Chemodynamics [162710]