UC Berkeley

Zebrafish have become a useful model for understanding olfactory circuitry and social behaviors. Combined with the benefits of optical transparency and fast developmental speed, the large clutch sizes of zebrafish larvae make this animal a desirable choice for large scale drug screens that may be affecting the olfactory system (McCarroll et al., 2016). Zebrafish olfactory anatomy consists of one olfactory epithelium (OE) that exposes at least five classes of olfactory sensory neurons (OSNs) to chemical ligands in the water (Ahuja et al., 2014; Wakisaka et al., 2017; Yoshihara, 2008). OSNs then project axons to the olfactory bulb, which then send projections to higher order brain regions (Yoshihara, 2014). In each step of the olfactory signaling process, ligand information is coded in the receptors, OSNs, and brain areas that each ligand, or suite of ligands, activates. However, to make comparisons with the mammalian olfactory systems, we need to understand how the circuitry of the zebrafish olfactory system differs from mammals. In mice, for instance, two different olfactory systems exist in the nose: the main olfactory epithelium (MOE) and the vomeronasal olfactory system (VNO). These two sensory systems contain OSNs that diverge in morphology and projection targets. Some are important in learned behaviors while others play a role in innate behaviors. In zebrafish, there is only one epithelium that contains a variety of cell types and projection targets. How does this system differentiate between learned and innate behaviors? Understanding why these differences have evolved this way might help us make better comparative studies between different animal groups. New behavioral assays and gene trap techniques have emerged as a powerful way to probe the structure and function of zebrafish olfactory behaviors (Koide et al., 2009; Yabuki et al., 2016). This dissertation examines the circuitry of the zebrafish olfactory system and innate behaviors using gene trapping, single cell RNA sequencing, immunohistochemistry, in situ hybridization, and novel behavioral assays. The following was determined: (1) A new class of unipolar olfactory sensory neuron (OSN) exists in zebrafish larvae that projects directly to the telencephalon and fish amygdala from the anterior olfactory epithelium (OE); (2) Two types of bipolar OSNs project to the mG2 and mG3 glomeruli of the olfactory bulb and express the ora4 and ora6 receptors respectively; (3) Larval zebrafish display thigmotaxis when exposed to fish skin extract; (4) This behavior is absent in fish with olfactory nerve lesions; and (5) Ablation of telencephalon projecting OSNs suggest that these neurons may play a role in thigmotaxis. These results provide a new look at how olfactory circuitry is organized in the larval zebrafish and how these circuits may play a role in innate fear behavior.