Mutually Exclusive Glomerular Innervation by Two Distinct Types of Olfactory Sensory Neurons Revealed in Transgenic Zebrafish

Abstract

The olfactory epithelium of fish contains two major types of olfactory sensory neurons (OSNs) that are distinct morphologically (ciliated vs microvillous) and possibly functionally. Here, we found that these OSNs express different sets of signal transduction machineries: the ciliated OSNs express OR-type odorant receptors, cyclic nucleotide-gated channel A2 subunit, and olfactory marker protein (OMP), whereas the microvillous OSNs express V2R-type receptors and transient receptor potential channel C2 (TRPC2). To visualize patterns of axonal projection from the two types of OSNs to the olfactory bulb (OB), we generated transgenic zebrafish in which spectrally distinct fluorescent proteins are expressed in the ciliated and microvillous OSNs under the control of OMP and TRPC2 gene promoters, respectively. An observation of whole-mount OB in adult double-transgenic zebrafish revealed that the ciliated OSNs project axons mostly to the dorsal and medial regions of the OB, whereas the microvillous OSNs project axons to the lateral region of the OB. A careful histological examination of OB sections clarified that the axons from the two distinct types of OSNs target different glomeruli in a mutually exclusive manner. This segregation is already established at very early developmental stages in zebrafish embryos. These findings clearly demonstrate the relationships among cell morphology, molecular signatures, and axonal terminations of the two distinct types of OSNs and suggest that the two segregated neural pathways are responsible for coding and processing of different types of odor information in the zebrafish olfactory system.

Coordinated shift of olfactory amino acid responses and V2R expression to an amphibian water nose during metamorphosis

“…Thus the correlation between V2R and TRPC2 expression is retained in a newly formed organ [4], the middle cavity. In the same vein, the distribution of amino acid-responsive cells between the middle cavity and the principal cavity closely parallels the distribution of V2R-C-expressing cells, consistent with the hypothesis that V2Rs expressed in microvillous neurons mediate the amino acid-responses in amphibians, similar to what has been shown for teleost fish [15].…”

“…Interestingly, a similar segregation between ciliated and microvillous neurons is found in the single olfactory surface of teleost fish, although the relative positions are inversed, with microvillous neurons occupying apical positions [15].…”

Foxg1Is Required for Development of the Vertebrate Olfactory System

“…Ciliated olfactory neurons were monitored using in situ hybridization of OMP mRNA at 48 hpf (Celik et al, 2002;Sato et al, 2005). Although the olfactory placodes in Foxg1 morpholinoinjected fish appear mostly intact, we nonetheless observed a decrease in OMP expression in Foxg1 morphants (Fig.…”

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

“…The resulting dendrogram categorized 55 neurons into four large groups; neurons innervating the same glomerular cluster did not always fall into the same group. The first group consisted mostly of three classes of output neurons, whose glomerular clusters (vaG, maG and dG) are innervated by OSNs that express G-protein a subunit G olf , olfactory marker protein and presumably OR-type olfactory receptors (presumptive ciliated OSNs) 25,31,32 . Each neuronal pair with highest similarity in this group often belonged to different glomerular clusters.…”