INVESTIGATION OF THE DOPAMINERGIC NEURONS OF THE ZEBRAFISH OLFACTORY BULB THROUGH IMMUNOHISTOCHEMISTRY, BEHAVIOURAL ANALYSIS AND EFFECTS OF ENVIRONMENTAL TOXINS

Abstract

The sense of smell is integral to almost every aspect of life and its loss can be detrimental. Therefore, understanding how the olfactory system functions is essential to investigating potential solutions for its dysfunction. Zebrafish present a unique opportunity for study due to the relative simplicity of their already extensively examined olfactory system. However, the zebrafish olfactory bulb contains other less studied cells, such as dopaminergic (tyrosine hydroxylase positive; TH+) interneurons, which are of particular interest, as they potentially modulate communication between various regions/glomeruli of the bulb, although it is unknown whether or how this occurs. The purpose of this thesis was to characterize these interneurons and examine their role in olfactory behaviour. First, I described the normal anatomy of the TH+-cells in the olfactory bulb and changes during development. I identified four distinct cell types/subtypes which appear to have specific functions and noted differences in the lateral and medial regions of the bulb, namely the uniquely dopaminergic innervation to the medial olfactory tract. Then, I developed a successful appetitive olfactory learning paradigm, which trained fish to respond to an odour after four days of conditioning. I then used the apparatus and baseline food odour response to demonstrate that manganese exposure causes reduced and inappropriate responses to different odours. In addition, I showed that manganese exposure reduced the number of TH+-cells in the olfactory bulb by 50% and disrupted the olfactory sensory neurons (OSNs) of the bulb. Prompted by the observed disruption of the OSNs, I investigated the isolated effects of metronidazole (Mtz), an antibiotic used in a new chemogenetic ablation technique claiming to specifically target only TH+-cells. I found that Mtz alone not only caused a 30% reduction in the number of TH+-cells in the bulb and variably disrupted OSNs but also caused reduced responses to odourants. Collectively, these findings indicated that TH+-cells in the olfactory bulb have several different morphologies and seem to uniquely innervate the medial olfactory tract. Furthermore, specific ablation for investigating behavioural changes is difficult, as various methods cause stress and adversely affect other olfactory cells.