Neuron types in the zebrafish optic tectum labeled by an id2b transgene

Abstract

The larval zebrafish optic tectum has emerged as a prominent model for understanding how neural circuits control visually guided behaviors. Further advances in this area will require tools to monitor and manipulate tectal neurons with cell type specificity. Here, we characterize the morphology and neurotransmitter phenotype of tectal neurons labeled by an id2b:gal4 transgene. Whole‐brain imaging of stable transgenic id2b:gal4 larvae revealed labeling in a subset of neurons in optic tectum, cerebellum, and hindbrain. Genetic mosaic labeling of single neurons within the id2b:gal4 expression pattern enabled us to characterize three tectal neuron types with distinct morphologies and connectivities. The first is a neuron type previously identified in the optic tectum of other teleost fish: the tectal pyramidal neuron (PyrN). PyrNs are local interneurons that form two stratified dendritic arbors and one stratified axonal arbor in the tectal neuropil. The second tectal neuron type labeled by the id2b:gal4 transgene is a projection neuron that forms a stratified dendritic arbor in the tectal neuropil and an axon that exits tectum to form a topographic projection to torus longitudinalis (TL). A third neuron type labeled is a projection neuron with a nonstratified dendritic arbor and a descending axonal projection to tegmentum. These findings establish the id2b:gal4 transgenic as a useful tool for future studies aimed at elucidating the functional role of tectum, TL, and tegmentum in visually guided behaviors.

Anatomy and Connectivity of the Torus Longitudinalis of the Adult Zebrafish

“…Noticeably, a particular class of retinal ganglion cells shows bistratified axonal projections that reach both the SFGS6 and SGC (Robles et al, 2014). These results match with the distribution of the dendritic fields of type X cells described for the zebrafish adult (present results) and larva (DeMarco et al, 2019). Therefore, it seems that zebrafish type X cells probably receive direct retinal information, among other types of inputs.…”

“…These neurons showed a clear bistratified dendritic lamination in the SGC and the SFGS, which roughly coincides with the morphology of the type X tectal cells described after Golgi staining in the goldfish (Meek and Schellart, 1978). A recent study in zebrafish larva has shown that type X cells are labeled by id2b transgene, being able to observe the axon running along TL (DeMarco et al, 2019). In the carp tectum, the toropetal cells reported with retrograde labeling (Ito et al, 2003) appear to be more heterogeneous in appearance and location than those reported here in zebrafish, with cells located in the SFGS, SGC and SAC (Ito et al, 2003;present results).…”

“…The zebrafish pyramidal cells were succinctly described with the Golgi method (Sajovic and Levinthal, 1982) and appear to correspond to the type I tectal neurons described in goldfish also with Golgi methods (Meek and Schellart, 1978). Recently, pyramidal cells have been described in zebrafish larva based on the expression of id2b transgene (DeMarco et al, 2019). Based on general cell morphology and cell body location (in the SGC and SAC, not in the SFGS), these larval cells do not seem to correspond with the pyramidal (type I) cells reported here.…”

“…To our knowledge, only two studies have described aspects of the toro-tectal circuit in zebrafish larva. One describes tectal cell types (including type X cells) based on transgene expression (DeMarco et al, 2019), while the other reports neural responses in the TL as a result of visual stimulation (Portugues et al, 2014).…”

The zebrafish visual system transmits dimming information via multiple segregated pathways

“…id2b:gal4 BAC transgene(DeMarco, Xu, Baier, & Robles, 2019): Tg (id2b:gal4) × Tg(UAS:GCaMP6s). Visual stimuli were presented to a single eye via an OLED screen equipped with magenta filters to minimize interference with GCaMP6s fluorescence detection(Figure 1a,b).…”