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MGLUR6 SIGNALING IN ON-BIPOLAR CELLS
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R.M. Duvoisin , C.W. Morgans , B.G. Jeffrey , R.L. Brown
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 Physiology & Pharmacology, Ophthalmology, Oregon National Primate Research Center, Oregon
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Health & Science University, Portland, OR, VCAPP, Washington State University, Pullman, WA, USA
In photoreceptor outer segments, photon absorption by rhodopsin is coupled to activation of the G protein
transducin, resulting in closure of cGMP-gated cation channels, hyperpolarization of photoreceptors, and
a decrease in glutamate release from synaptic terminals. In turn, ON-bipolar cells depolarize as a result of
the reduced stimulation of the metabotropic glutamate receptor, mGluR6, which signals via a G protein
(Go) to control the activity of a depolarizing current. Thus, the ON-pathway of the visual system is
established at the first retinal synapse between photoreceptors and ON-bipolar cells. We are interested in
identifying the components of the mGluR6 signaling pathway.

We hypothesized that the depolarizing response of ON-bipolar cells to light results from a rapid
inactivation of Go signaling and that regulator of G protein signaling (RGS) proteins, which accelerate the
GTPase activity of G proteins, are necessary. Consistent with this hypothesis, immunofluorescence
microscopy revealed that Gβ5, RGS7, RGS11, and R9AP are co-localized with mGluR6 in the tips of ON-
bipolar cell dendrites. Electroretinogram (ERG) recordings demonstrated a delay in the rising phase of the
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ERG b-wave in mice carrying a deletion mutation in RGS7 and lacking RGS11 (RGS7 /RGS11 ).
Patch-clamp recordings of chemically-simulated light responses of rod bipolar cells revealed a 25 ms
                                                                 Δ/Δ      -/-
delay in the onset of the ON-bipolar cell response in the RGS7 /RGS11 mouse compared to wild type.
This delay is shorter than predicted suggesting that some RGS7 function remains in the deletion mutant,
or that additional RGS proteins compensate for the absence of RGS11 and RGS7.

Downstream of Go, we sought to identify the channel generating the depolarizing current in ON-bipolar
cells. Using gene profiling of FACS-purified fluorescent ON-bipolar cells, we found that several TRP
channels, including TRPM1, are enriched in ON-bipolar cell preparations. Immunohistochemical analysis
demonstrated that TRPM1 is found in the retina where it is localized exclusively to the dendrites and cell
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bodies of rod and cone ON-bipolar cells. The ERG of TRPM1 mice had a normal a-wave, but no b-
wave, indicating a loss of bipolar cell signaling. Finally, patch-clamp recording from ON-bipolar cells
demonstrated that genetic deletion of TRPM1 abolished chemically-simulated light responses from rod
bipolar cells and dramatically altered the responses of cone ON-bipolar cells. [Supported by NIH grants
EY009534, EY018625, and EY019907]

				
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