Zebrafish in the drug development process : characterization of the zebrafish oxytocin system

Zebrafish (Danio rerio) is a vertebrate model organism. It is suited for many phases of drug development process like toxicological studies. The major advantage of using zebrafish is the possibility to conduct high-throughput screens on a whole vertebrate animal. However, there is not as much knowledge about zebrafish as there is about other model organisms. Therefore there might be differences between zebrafish and humans that affect the use of zebrafish as a model in the drug development process. The purpose of this thesis was to characterize the structure of the zebrafish oxytocin system and assess the role of oxytocin on zebrafish behaviour. In humans defects in the oxytocin system have been linked to many psychiatric disorders like autism. If the mammalian and zebrafish oxytocin systems resembled each other functionally and structurally, it would enable the use of zebrafish as a model when studying the role of oxytocin in pathophysiology of diseases and also in oxytocin system related drug development. The structure and development of zebrafish oxytocin system was studied by staining adult zebrafish brain cryosections and larval brains with antibodies made against mammalian oxytocin. The specificity of the antibodies to recognize zebrafish oxytocin was determined by absorption and cross-reactivity controls. The role of oxytocin on zebrafish locomotion was studied by inhibiting the splicing of oxytocin messenger RNA with morpholino oligonucleotides (MOs). The MOs were used to address the relevance of the model in pharmacology, since the zebrafish oxytocin receptors have not been expressed and pharmacologically characterized. In zebrafish oxytocin was produced in the cells of the preoptic nucleus. There were thick oxytocin fibers towards the pituitary and also thinner fibers into areas in the telencephalon, diencephalon, mesencephalon and rhombencephalon. One of the MOs was able to inhibit the production of oxytocin with a dose that did not cause morphological abnormalities. The MO reduced the locomotor activity of the fish, but the specificity of the MO has to be determined. The structure of the zebrafish oxytocin system resembles mammalian oxytocin system in terms of the location of oxytocin cells and fiber projections. Therefore zebrafish seems a suitable model organism for oxytocin research. However, the structure of the zebrafish oxytocin receptor system and the effect of oxytocin on other behavioural aspects have to be determined in order to further evaluate the applicability of zebrafish for oxytocin research.