Characterization of Genetically Targeted Neuron Types in the Zebrafish Optic Tectum

Department of Physiology, University of California San Francisco San Francisco, CA, USA.
Frontiers in Neural Circuits (Impact Factor: 2.95). 02/2011; 5:1. DOI: 10.3389/fncir.2011.00001
Source: PubMed

ABSTRACT The optically transparent larval zebrafish is ideally suited for in vivo analyses of neural circuitry controlling visually guided behaviors. However, there is a lack of information regarding specific cell types in the major retinorecipient brain region of the fish, the optic tectum. Here we report the characterization of three previously unidentified tectal cell types that are specifically labeled by dlx5/6 enhancer elements. In vivo laser-scanning microscopy in conjunction with ex vivo array tomography revealed that these neurons differ in their morphologies, synaptic connectivity, and neurotransmitter phenotypes. The first type is an excitatory bistratified periventricular interneuron that forms a dendritic arbor in the retinorecipient stratum fibrosum et griseum superficiale (SFGS) and an axonal arbor in the stratum griseum centrale (SGC). The second type, a GABAergic non-stratified periventricular interneuron, extends a bushy arbor containing both dendrites and axons into the SGC and the deepest sublayers of the SFGS. The third type is a GABAergic periventricular projection neuron that extends a dendritic arbor into the SGC and a long axon to the torus semicircularis, medulla oblongata, and anterior hindbrain. Interestingly, the same axons form en passant synapses within the deepest neuropil layer of the tectum, the stratum album centrale. This approach revealed several novel aspects of tectal circuitry, including: (1) a glutamatergic mode of transmission from the superficial, retinorecipient neuropil layers to the deeper, output layers, (2) the presence of interneurons with mixed dendrite/axon arbors likely involved in local processing, and (3) a heretofore unknown GABAergic tectofugal projection to midbrain and hindbrain. These observations establish a framework for studying the morphological and functional differentiation of neural circuits in the zebrafish visual system.

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Available from: Estuardo Robles, Jul 30, 2015
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    • "The tectal neuropil is a precisely laminated structure within which the RGC axons mostly target the superficial layers (Xiao et al., 2005): the stratum opticum (SO), right beneath the basement membrane, and the stratum fibrosum et griseum superficiale (SFGS). Classical Golgi studies in adult goldfish and genetic single-cell labeling in larval zebrafish revealed that the PVNs have a single dendritic shaft that extends into the tectal neuropil, often crossing multiple layers (Vanegas et al., 1974; Meek and Schellart, 1978; Scott and Baier, 2009; Nevin et al., 2010; Robles et al., 2011). "
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    ABSTRACT: Neural circuits in the vertebrate retina extract the direction of object motion from visual scenes and convey this information to sensory brain areas, including the optic tectum. It is unclear how computational layers beyond the retina process directional inputs. Recent developmental and functional studies in the zebrafish larva, using minimally invasive optical imaging techniques, indicate that direction selectivity might be a genetically hardwired property of the zebrafish brain. Axons from specific direction-selective (DS) retinal ganglion cells appear to converge on distinct laminae in the superficial tectal neuropil where they serve as inputs to DS postsynaptic neurons of matching specificity. In addition, inhibitory recurrent circuits in the tectum might strengthen the DS response of tectal output neurons. Here we review these recent findings and discuss some controversies with a particular focus on the zebrafish tectum's role in extracting directional features from moving visual scenes.
    Frontiers in Neural Circuits 06/2013; 7:111. DOI:10.3389/fncir.2013.00111 · 2.95 Impact Factor
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    • "While the broad function of the tectum is known: it converts a visuotopic map into motor commands that orient the head and body toward or away from a visual stimulus (for review see Nevin et al., 2010), the specific computations performed by local tectal circuits are not well understood. This is due to our relatively poor understanding of the essential circuit components: the various tectal cell types, their morphologies, functional properties and how cell types interact with one another (but see Robles et al., 2011; Gabriel et al., 2012 for examples). Expression of GECIs in single, identifiable cell types are well suited to addressing these gaps in our knowledge. "
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    ABSTRACT: Genetically encoded calcium indicators (GECIs) allow repeated, non-invasive measurements of neural activity in defined populations of neurons, but until recently GECIs based on single fluorescent proteins have been limited to the green region of the color spectrum. Recent efforts in protein engineering have expanded the color palette of GECIs. One of these new GECIs, the red RGECO, is spectrally separate from the traditional GFP-based sensors such as GCaMP, and therefore opens the way for simultaneous, multicolor imaging of neural activity. While RGECO has been shown to report spontaneous calcium fluctuations in neurons, the precise relationship of RGECO signal to evoked-neural activity is not known. Measurements of neural activity using RGECO in vivo have also not been reported. Using dissociated hippocampal neurons we performed a systematic analysis of two forms of RGECO- a cytosolic form and a presynaptically localized form generated by fusion of RGECO to the presynaptic protein, synaptophysin (SyRGECO). We find that RGECO and GCaMP3 are comparable in terms of dynamic range, signal-to-noise ratios and kinetics but that RGECO is a more reliable reporter of single action potentials. In terms of performance SyGCaMP3 and SyRGECO are comparable, and both are more sensitive reporters of activity than the cytosolic form of each probe. Using the zebrafish retinotectal system we show that SyRGECO and RGECO are can report neural activity in vivo and that RGECO expression permits detailed structural analysis of neuronal arbors. We have exploited these attributes to provide a morphological and functional description of tectal cells selective for motion along the vertical axis. These results open up the possibility of using zebrafish to functionally image genetically defined pre- and postsynaptic circuit components, separable by color, which will be a powerful approach to studying neural interactions in the brain.
    Frontiers in Neural Circuits 03/2013; 7:34. DOI:10.3389/fncir.2013.00034 · 2.95 Impact Factor
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    • "An obvious question is whether the functional retinotopic biases in RGC input is reflected in the postsynaptic tectal neurons. The rostro-caudal dendritic extent for at least three tectal cell types is well below the size of the RGC functional domains that we have identified (Robles et al., 2011). Such spatially restricted sampling of RGCs by tectal neurons suggests that the regional biases in direction and orientation selectivity could be preserved in the population of postsynaptic tectal neurons. "
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    ABSTRACT: How features of the visual scene are encoded in the population activity of retinal ganglion cells (RGCs) targeting specific regions of the brain is not well understood. To address this, we have used a geneti-cally encoded reporter of presynaptic function (SyGCaMP3) to record visually evoked activity in the population of RGC axons innervating the zebra-fish tectum. Using unbiased voxel-wise analysis of SyGCaMP3 signals, we identify three subtypes of direction-selective and two subtypes of orientation-selective retinal input. Composite parametric func-tional maps generated across many larvae show laminar segregation of direction-and orientation-selective responses and unexpected retinotopic biases in the distribution of functional subtypes. These findings provide a systematic description of the form, organization, and dimensionality of visual inputs to the brain and will serve as a platform for understanding emergent properties in tectal circuits associated with visually driven behavior.
    Neuron 10/2012; 76(2):317-324. DOI:10.1016/j.neuron.2012.08.040 · 15.98 Impact Factor
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