Summary In vertebrate vision, the tetrachromatic larval zebrafish permits non-invasive monitoring and manipulating of neural activity across the nervous system in vivo during ongoing behavior. However, despite a perhaps unparalleled understanding of links between zebrafish brain circuits and visual behaviors, comparatively little is known about what their eyes send to the brain via retinal ganglion cells (RGCs). Major gaps in knowledge include any information on spectral coding and information on potentially critical variations in RGC properties across the retinal surface corresponding with asymmetries in the statistics of natural visual space and behavioral demands. Here, we use in vivo two-photon imaging during hyperspectral visual stimulation as well as photolabeling of RGCs to provide a functional and anatomical census of RGCs in larval zebrafish. We find that RGCs’ functional and structural properties differ across the eye and include a notable population of UV-responsive On-sustained RGCs that are only found in the acute zone, likely to support visual prey capture of UV-bright zooplankton. Next, approximately half of RGCs display diverse forms of color opponency, including many that are driven by a pervasive and slow blue-Off system—far in excess of what would be required to satisfy traditional models of color vision. In addition, most information on spectral contrast was intermixed with temporal information. Taken together, our results suggest that zebrafish RGCs send a diverse and highly regionalized time-color code to the brain.

In vertebrate vision, the tetrachromatic larval zebrafish permits non-invasive monitoring and manipulating of neural activity across the nervous system in vivo during ongoing behaviour. However, despite a perhaps unparalleled understanding of links between zebrafish brain circuits and visual behaviours, comparatively little is known about what their eyes send to the brain in the first place via retinal ganglion cells (RGCs). Major gaps in knowledge include any information on spectral coding, and information on potentially critical variations in RGC properties across the retinal surface to acknowledge asymmetries in the statistics of natural visual space and behavioural demands.Here, we use in vivo two photon (2P) imaging during hyperspectral visual stimulation as well as photolabeling of RGCs to provide the first eye-wide functional and anatomical census of RGCs in larval zebrafish.We find that RGCs' functional and structural properties differ across the eye and include a notable population of UV-responsive On-sustained RGCs that are only found in the acute zone, likely to support visual prey capture of UV-bright zooplankton. Next, approximately half of RGCs display diverse forms of colour opponency -long in excess of what would be required to satisfy traditional models of colour vision. However, most information on spectral contrast was intermixed with temporal information. To consolidate this series of unexpected findings, we propose that zebrafish may use a novel "dual-achromatic" strategy segregated by a spectrally intermediate background subtraction system. Specifically, our data is consistent with a model where traditional achromatic image-forming vision is mainly driven by longwavelength sensitive circuits, while in parallel UV-sensitive circuits serve a second achromatic system of foreground-vision that serves prey capture and, potentially, predator evasion.

Collybistin (CB) is a guanine nucleotide exchange factor selectively localized to γ-aminobutyric acid (GABA)ergic and glycinergic postsynapses. Active CB interacts with gephyrin, inducing the submembranous clustering and the postsynaptic accumulation of gephyrin, which is a scaffold protein that recruits GABA receptors (GABA Rs) at the postsynapse. CB is expressed with or without a src homology 3 (SH3) domain. We have previously reported the effects on GABAergic synapses of the acute overexpression of CB or CB in cultured hippocampal (HP) neurons. In the present communication, we are studying the effects on GABAergic synapses after chronic in vivo transgenic expression of CB2 or CB2 in neurons of the adult rat cerebral cortex. The embryonic precursors of these cortical neurons were in utero electroporated with CB or CB DNAs, migrated to the appropriate cortical layer, and became integrated in cortical circuits. The results show that: 1) the strength of inhibitory synapses in vivo can be enhanced by increasing the expression of CB in neurons; and 2) there are significant differences in the results between in vivo and in culture studies. J. Comp. Neurol. 525:1291-1311, 2017. © 2016 Wiley Periodicals, Inc.

It has been proposed that the combinatorial expression of γ‐protocadherins (Pcdh‐γs) and other clustered protocadherins (Pcdhs) provides a code of molecular identity and individuality to neurons, which plays a major role in the establishment of specific synaptic connectivity and formation of neuronal circuits. Particular attention has been directed to the Pcdh‐γ family, for which experimental evidence derived from Pcdh‐γ‐deficient mice shows that they are involved in dendrite self‐avoidance, synapse development, dendritic arborization, spine maturation, and prevention of apoptosis of some neurons. Moreover, a triple‐mutant mouse deficient in the three C‐type members of the Pcdh‐γ family (Pcdh‐γC3, Pcdh‐γC4, and Pcdh‐γC5) shows a phenotype similar to the mouse deficient in whole Pcdh‐γ family, indicating that the latter is largely due to the absence of C‐type Pcdh‐γs. The role of each individual C‐type Pcdh‐γ is not known. We have developed a specific antibody to Pcdh‐γC4 to reveal the expression of this protein in the rat brain. The results show that although Pcdh‐γC4 is expressed at higher levels in the embryo and earlier postnatal weeks, it is also expressed in the adult rat brain. Pcdh‐γC4 is expressed in both neurons and astrocytes. In the adult brain, the regional distribution of Pcdh‐γC4 immunoreactivity is similar to that of Pcdh‐γC4 mRNA, being highest in the olfactory bulb, dentate gyrus, and cerebellum. Pcdh‐γC4 forms puncta that are frequently apposed to glutamatergic and GABAergic synapses. They are also frequently associated with neuron‐astrocyte contacts. The results provide new insights into the cell recognition function of Pcdh‐γC4 in neurons and astrocytes.

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Collybistin (CB) is a guanine nucleotide exchange factor (GEF) selectively localized at GABAergic and glycinergic postsynapses. Analysis of mRNA shows that several isoforms of collybistin are expressed in the brain. Some of the isoforms have a SH3 domain (CBSH3+) and some have no SH3 domain (CBSH3−). The CBSH3+ mRNAs are predominantly expressed over CBSH3−. However, in an immunoblot study of mouse brain homogenates, only CBSH3+ protein isoforms were detected, proposing that CBSH3− protein might not be expressed in the brain. The expression or lack of expression of CBSH3− protein is an important issue because CBSH3− has a strong effect in promoting the postsynaptic clustering of gephyrin and GABA-A receptors (GABA A Rs). Moreover CBSH3− is constitutively active; therefore lower expression of CBSH3− protein might play a relatively stronger functional role than the more abundant but self-inhibited CBSH3+ isoforms, which need to be activated. We are now showing that: (a) CBSH3− protein is expressed in the brain; (b) parvalbumin positive (PV+) interneurons show higher expression of CBSH3− protein than other neurons; (c) CBSH3− is associated with GABAergic synapses in various regions of the brain and (d) knocking down CBSH3− in hippocampal neurons decreases the synaptic clustering of gephyrin and GABA A Rs. The results show that CBSH3− protein is expressed in the brain and that it plays a significant role in the size regulation of the GABAergic postsynapse.

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