Genetically induced retinal degeneration leads to changes in metabotropic glutamate receptor expression

In the retina, neurotransmission from photoreceptors to ON-cone and rod bipolar cells is sign reversing and mediated by the metabotropic glutamate receptor mGluR6, which converts the light-evoked hyperpolarization of the photoreceptors into depolarization of ON bipolar cells. The Royal College of Surgeons (RCS) rat retina undergoes progressive photoreceptor loss due to a genetic defect in the pigment epithelium cells. The consequences of photoreceptor loss and the concomitant loss of glutamatergic input to second-order retinal neurons on the expression of the metabotropic glutamate receptor was investigated in the RCS rat retina from early stages of photoreceptor degeneration (P17) up to several months after complete rod and cone degeneration (P120). The expression of the gene encoding mGluR6 was studied by in situ hybridization in the retina, using an [(35)S]dATP-labeled oligonucleotide probe. In congenic control and RCS retina, we found mRNA expression of mGluR6 receptor only in the outer half of the inner nuclear layer (INL) on emulsion-coated retinal sections. Quantitative analysis of the hybridization signal obtained from the autoradiographic films revealed decreased expression levels of the mGluR6 mRNA at early stages of photoreceptor degeneration (P17). On the contrary, increased expression levels were observed at late stages of degeneration (P60 and P120) in RCS compared to congenic control retina. In conclusion, our data demonstrate that the metabotropic glutamate receptor-6 mRNA levels are altered in the young and adult RCS rat retina and suggest that the genetically induced degeneration of photoreceptors affects the expression of this receptor by the INL retinal neurons.