Expression of group one metabotropic glutamate receptor subunit mRNAs in neurochemically identified neurons in the rat neostriatum, neocortex, and hippocampus

Metabotropic glutamate receptors (mGluRs) can be divided into three groups based on sequence homology and pharmacology. We studied expression of group I mGluRs (mGluR1 and mGluR5) in identified neurons of the rat neostriatum, neocortex, and hippocampus using in situ hybridization. Tissue sections were hybridized with radiolabeled RNA probes for mGluR1 or mGluR5 and digoxygenin labeled RNA probes detecting somatostatin (SOM), preproenkephalin (ENK), preprotachykinin (SP), glutamic acid decarboxylase 67 (GAD(67)), parvalbumin (PARV), or choline acetyltransferase (ChAT) mRNA. In the striatum, mGluR1 hybridization signal was observed in all six neuronal populations. The strongest signal was found in SP-positive neurons, with a lower signal in ENK-positive neurons. All striatal interneurons were labeled less intensely than ENK- and SP-positive projection neurons. For striatal mGluR5 mRNA, both SP- and ENK-positive projection neurons were intensely labeled, but only GAD(67)-positive interneurons exhibited a significant signal. In the neocortex and hippocampus, mGluR1 and mGluR5 hybridization signals were studied in SOM-, GAD(67)-, and PARV-positive neurons. Hybridization signal for mGluR1 mRNA was intense in SOM-positive neurons of the cortex, CA1, CA3, and dentate gyrus, and weaker in GAD(67)-positive neurons of CA3 and dentate gyrus. MGluR5 signals were intensely labeled in SOM-, GAD(67)- and PARV-positive neuronal populations of the cortex and hippocampus. SOM-positive neurons were more intensely labeled in the hippocampus than cortex.