mGluR3 and mGluR5 are the predominant metabotropic glutamate receptor mRNAs expressed in hippocampal astrocytes acutely isolated from young rats

The metabotropic glutamate receptors (mGluRs) that are expressed and not expressed on astrocytes in the brain have not been defined. While immunohistochemistry and in situ mRNA hybridization have been used on a limited basis to address this question, they do not readily enable the proportion of astrocytes expressing a particular mRNA or protein to be determined. Also, for many receptors, expression by cultured astrocytes does not reflect in situ expression. In this study, therefore, we examined expression of mRNA for all the mGluRs except mGluR6 by single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) in freshly isolated hippocampal astrocytes from postnatal day P1-10 rats, as an additional approach to address the question of which mGluRs are expressed on astrocytes in situ. The astrocytic nature of the cells was supported by simultaneously measuring mRNA for the astrocytic marker glial fibrillary acidic protein (GFAP) from the same cells. In these studies, the percentage of cells showing GFAP mRNA expression was the same as the percentage of cells showing immunocytochemical staining for GFAP. We found that only mGluR3 and mGluR5 mRNAs were significantly present in GFAP mRNA(+) cells. The mGluR5 PCR products were primarily of the "a" splice variant. mGluR1, 2, 4, 7, and 8 were very rarely or never detected. mGluR6 mRNA level was too low in whole rat brain and hippocampus to warrant examination. These results show that interpretation of effects involving mGluR3 or 5 activation in the hippocampus of young rats needs to also consider effects due to astrocytes.