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Stephan C. Neuhauss, Marion Haug, Matthias Gesemann; mGluR6 In The Zebrafish Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6019.
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Metabotropic glutamate receptors (mGluRs) have been identified at all synapses of the vertebrate retina, where they likely regulate neurotransmitter release. The only example of an mGluR functioning in direct synaptic transmission is mGluR6. It is expressed on ON-bipolar cell dendrites and mediates the ON-response in the mammalian retina which provides information about brighter than background stimuli to the ganglion cells. Data from lower vertebrates propose a division of labor of the ON-bipolar cell response: mGluR6 is suggested to be fully responsible for the scotopic ON-response, whereas the photopic ON-response is involves mGluR6 together with a glutamate transporter. We use the cone dominant retina of zebrafish to assess this hypothesis and to define the functional role of mGluR6 in teleost vision.
Expression of mGluR6 in larval zebrafish retina was assessed by RNA in situ hybridization and immunohistochemistry. A functional analysis of mGluR6b was accomplished by measuring ERG and OKR on morpholino antisense injected larvae.
Phylogenetic analysis of mGluR6 reveals the occurrence of two zebrafish mGluR6 paralogs, mGluR6a and -6b. Interestingly, we located the RNA of both mGluR6 paralogs predominantly in retinal ganglion cells of larval zebrafish and in the inner part of the inner nuclear layer (INL). Only one paralog, mGluR6a, is expressed in the region where bipolar cell bodies are located. An immunohistochemical analysis of mGluR6a is consistent with the RNA pattern and shows a postsynaptic location in the outer plexiform layer, a saining in two layers of the IPL and in single cells around it. Behavioral analysis after depletion of mGluR6b with morpholino antisense injected larvae revealed a decreased ERG b-wave indicating a decrease in ON-bipolar cell response but had no effect on general visual performance detected with OKR measurements.
Those results provide evidence that mGluR6b plays a crucial role in the zebrafish cone ON-bipolar cell pathway. The diminished but not erased b-wave supports the hypothesis of two receptors involved in the photopic ON-pathway.
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