EXPRESSION OF THE MESSENGER-RNA OF 7 METABOTROPIC GLUTAMATE RECEPTORS(MGLUR1 TO MGLUR7) IN THE RAT RETINA - AN IN-SITU HYBRIDIZATION STUDYON TISSUE-SECTIONS AND ISOLATED CELLS

We have studied the expression of mRNAs for seven metabotropic glutama te receptors (mGluR1-7) in the retina of the adult rat by in situ hybr idization with tissue sections and isolated cells using [alpha(35)S]dA TP-labelled oligonucleotide probes. Hybridization revealed the express ion of six of the metabotropic receptor mRNAs, mGluR1, 2 and 4-7, in t he retina, white mGluR3 was not detected. Each of the expressed recept or mRNAs showed a distinct pattern of expression. In the outer nuclear layer, corresponding to photoreceptor somata, no labelling was detect ed. In the outer part of the inner nuclear layer, putative horizontal cells were labelled for mGluR5. More proximal in this layer, correspon ding to the position of bipolar cell somata, there was strong labellin g for mGluR6. A small number of bipolar cells were also labelled for m GluR5 and mGluR7. In situ hybridization with isolated cells showed tha t mGluR6 was expressed by rod bipolar cells. Subsets of amacrine cells , with cell bodies along the border between the inner nuclear layer an d the inner plexiform layer, were positive for mGluR1, 2, 4 and 7, sug gesting considerable heterogeneity of these receptors among amacrine c ells. None of the seven metabotropic receptor mRNAs was expressed in i solated Muller glial cells. In the ganglion cell layer, virtually ever y ganglion cell and displaced amacrine cell was labelled for mGluR1 an d mGluR4. Some cells in this layer (-20% of the total), most likely bo th 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 retin a than indicated by previous physiological and pharmacological investi gations.