G protein coupling profile of mGluR6 and expression of G alpha proteins in retinal ON bipolar cells

Metabotropic glutamate receptor 6 (mGluR6) is a group III, pertussis toxin (PTX)-sensitive G protein coupled mGluR that plays a specialized role in the retina. Retinal ON bipolar cells, which receive direct glutamatergic input from photoreceptor cells, express mGluR6 as their primary postsynaptic glutamate receptor. Activation of mGluR6 in these cells initiates an intracellular signaling cascade ultimately leading to inhibition of a cation channel and cell hyperpolarization. The primary mediator of this pathway in vivo is G alpha(o), but the potential roles of other G proteins from the G alpha(i/o) family in the regulation of this or other signaling pathways in ON bipolar cells are unclear. To determine which specific G proteins from the G alpha(i/o) family are able to couple to mGluR6, a G alpha reconstitution system was employed using PTX-insensitive G alpha mutants expressed with mGluR6 in PTX-treated sympathetic neurons from the rat superior cervical ganglion (SCG). The efficiency of coupling to mGluR6 was G(oa) > G(ob), G(i1) > G(i2), G(i3), whereas no coupling was observed with G alpha(z), nor with the retinal G alpha proteins, rod (GNAT2) or cone (GNAT1) transducin (G alpha(Tr-R), G alpha(Tr-C)). Finally, the expression of G alpha proteins determined to couple with mGluR6 was examined in rat ON bipolar cells using single cell RT-PCR. Co-expression of mGluR6 message was used to distinguish ON from OFF bipolar cells. Expression of G alpha(o) was detected in every ON bipolar cell examined. Message for G alpha(i1), which coupled moderately to mGluR6, was not detected in ON bipolar cells, nor was G alpha(i3), which coupled to mGluR6 in only a few cells but on average did not exhibit statistically significant coupling. Finally, though G alpha(i2) was detectable in ON bipolar cells, its coupling to mGluR6 in the SCG system was not significant. Together, these data indicate that signaling through mGluR6 in mammalian ON bipolar cells is highly focused, apparently acting through a single G alpha protein subtype.