Expression of the mRNA of Seven Metabotropic Glutamate Receptors (mGluR1 to 7) in the Rat Retina. An In Situ Hybridization Study on Tissue Sections and Isolated Cells

We have studied the expression of mRNAs for seven metabotropic glutamate receptors (mGluR1–7) in the retina of the adult rat by in situ hybridization with tissue sections and isolated cells using [α³⁵S]dATP-labelled oligonucleotide probes. Hybridization revealed the expression of six of the metabotropic receptor mRNAs, mGluR1, 2 and 4–7, in the retina, while mGluR3 was not detected. Each of the expressed receptor mRNAs showed a distinct pattern of expression. In the outer nuclear layer, corresponding to photoreceptor somata, no labelling was detected. In the outer part of the inner nuclear layer, putative horizontal cells were labelled for mGluR5. More proximal in this layer, corresponding to the position of bipolar cell somata, there was strong labelling for mGluR6. A small number of bipolar cells were also labelled for mGluR5 and mGluR7. In situ hybridization with isolated cells showed that mGluR6 was expressed by rod bipolar cells. Subsets of amacrine cells, with cell bodies along the border between the inner nuclear layer and the inner plexiform layer, were positive for mGluR1, 2, 4 and 7, suggesting considerable heterogeneity of these receptors among amacrine cells. None of the seven metabotropic receptor mRNAs was expressed in isolated Müller glial cells. In the ganglion cell layer, virtually every ganglion cell and displaced amacrine cell was labelled for mGluR1 and mGluR4. Some cells in this layer (˜20% of the total), most likely both ganglion cells and displaced amacrine cells, were also labelled for mGluR2 and mGluR7. These findings suggest that metabotropic glutamate receptors are considerably more widespread among neurons in the retina than indicated by previous physiological and pharmacological investigations.