Olfactory projectome in the zebrafish forebrain revealed by genetic single-neuron labelling

Abstract

Chemotopic odour representations in the olfactory bulb are transferred to multiple forebrain areas and translated into appropriate output responses. However, a comprehensive projection map of bulbar output neurons at single-axon resolution is lacking in vertebrates. Here we unravel a projectome of the zebrafish olfactory bulb through genetic single-neuron tracing and image registration. We show that five major target regions receive distinct modes of projections from olfactory bulb glomeruli. The central portion of posterior telencephalon receives non-selective, interspersed inputs from all glomeruli, whereas the ventral telencephalon is diffusely innervated by axons from particular glomerular clusters. The right habenula and posterior tuberculum (diencephalic nuclei) receive convergent inputs from restricted and all glomerular clusters, respectively. The bulbar recurrent projections are coarsely topographic. Thus, the primary chemotopic organization is transformed into distinct sensory representations in higher olfactory centres. These findings provide a framework to understand general principles as well as species-specific features in decoding of odour information.

Zebrafish Olfactory System

“…Within the Dp, the large core region samples intermingled inputs from all the glomerular clusters ( Fig. 5.4), thus transforming topographic information in the OB to broad and sparse representations (Miyasaka et al 2014). An optical imaging study showed that individual Dp neurons extract information about discrete combinations of odorant molecular features from ensembles of glomeruli to establish representations of higher-order olfactory objects ).…”

“…However, physiological and anatomical studies have begun to shed light on basic principles of odor information representation and computation in the secondary olfactory circuitry of zebrafish Miyasaka et al 2009;Blumhagen et al 2011). In particular, the most recent study combining a genetic single-neuron labeling method with the image registration system has uncovered a nearly comprehensive axon projection map from the OB to higher bran centers in zebrafish larvae (Miyasaka et al 2014).…”

“…These mutant fish will become available to the scientific community, which undoubtedly accelerate the zebrafish research in all fields of biology. In addition to these basic techniques, more advanced genetic methods have been developed in the zebrafish, such as the Tol2 transposon-mediated gene trap approach combined with the Gal4/UAS system (Asakawa et al 2008: Koide et al 2009), retrovirus-mediated large-scale enhancer trap screening (Ellingsen et al 2005), Cre/loxP-and/or Gal4/UAS-mediated singlecell mosaic labeling analysis (Sato et al 2007a;Miyasaka et al 2009;Miyasaka et al 2014), and TALEN-or CRISPR/Cas-mediated genome editing (Bedell et al 2012;Hwang et al 2013;Zu et al 2013). Thus, the zebrafish is one of the most useful vertebrate species with which we can perform both forward and reverse genetic analyses, similar to Drosophila melanogaster and Caenorhabditis elegans.…”

Attractant and repellent cues cooperate in guiding a subset of olfactory sensory axons to a well-defined protoglomerular target

“…The olfactory system is an attractive model for the study of axon targeting because each olfactory sensory neuron (OSN) assumes a specific identity as it chooses a single odorant receptor (OR) from a large gene repertoire and since OR choice is closely coordinated with axonal targeting (Alioto and Ngai, 2005;Zhang and Firestein, 2002). Although sensory neurons expressing a particular OR are broadly dispersed in the olfactory epithelium (OE), their axons converge upon one or a few reproducibly located OR-specific glomeruli in the olfactory bulb (OB); there, they synapse with the dendrites of second order neurons that communicate olfactory information deeper into the brain (Ramon y Cajal, 1892;Ressler et al, 1994;Vassar et al, 1994;Mombaerts et al, 1996;Sosulski et al, 2011;Miyasaka et al, 2014). The OR-specific glomerular map relates odorant experience to neuronal activity at specific locations within the OB, providing a basis for the identification and discrimination of odorants (Bozza et al, 2002).…”

Mixed input to olfactory glomeruli from two subsets of ciliated sensory neurons does not seem to impede relay neuron specificity in the crucian carp

“…Such restricted sub-compartmentalization within a glomerulus has been observed both in mammals (Treloar et al, 1996) and fish (Friedrich and Korsching, 1998), implying functional segregation within the glomeruli. Thus, the re-sorting concept of efferent axons to the telencephalon (Miyasaka et al, 2014) might be established at the site of olfactory glomeruli, as shown by the present study.…”