Developing behavioral assays to study dopamine-related disorders in zebrafish (Danio rerio)

Abstract

Summary Dopamine-related neurological disorders in human include Parkinson’s disease, drug and alcohol addiction, anxiety, depression, and schizophrenia. To understand the molecular and cellular basis of these disorders, it is highly desirable to establish appropriate animal models, especially in organisms that are amenable to genetic study. The overall goal is to use the zebrafish (zf), Danio rerio, as a vertebrate genetic model to identify genes and pathways that are important for the etiology as well as pathogenesis of these human disorders. The specific goal of this study was to develop simple and robust behavioral assays that model some aspects of these human disorders. Good assays are the foundation of successful mutagenesis screens. Employing a locomotor assay, several drugs were screened for interesting effects on zf fry. Ethanol and amphetamine turned out to induce hyperlocomotor activity at specific concentrations. The assays are simple and could be utilized in genetic screens looking for genes responsible for addiction-related behavior, although an amphetamine screen would be much less robust than an ethanol screen because of a less pronounced drug response. At higher doses, hypoactivity could also be utilized for screening procedures. Several other drugs were tested in the locomotor assay, but none were found with consistent features significantly different from placebo like ethanol or amphetamine. Cocaine is a possible exception. Furthermore, an open field and a light/dark assay were developed. Investigation with ethanol suggests that these assays may model anxiety. Both assays have potential to be employed in mutagenesis screens, with or without drug, but the light/dark assay might be preferred because it seems relatively robust and carrying out a screen would be quick. How successful a mutagenesis screen would be by employment of these behavioral assays remains to be elucidated, however genes involved in the etiology and pathogenesis of addiction and anxiety could eventually be discovered. The foundation of success rests on whether ethanol’s effect really reflects central nervous mediated behaviors, and that analogy to rodent behavior can be drawn for the open field and light/dark assay. In addition to development of behavior assays, the characteristics of fry with different genetic backgrounds including the too few mutant, which lack dopaminergic neurons were assessed with a hope to determine functional roles of dopaminergic pathways in fish. Large response differences were surprisingly seen between fry of different backgrounds, but no obvious differences were observed for the too few mutant as a consequence of the dopamine-deficiency. Summarizing, the goals for this specific study have been reached, although less variability and a few more positive findings would have been desirable.