Expression of kainate-selective glutamate receptor subunits in glial cells of the adult bovine white matter.

It is now well established that glial cells, like neurons, express a large variety of non-NMDA glutamate receptors. However, knowledge about the subunits forming the receptors and the types of glial cells which express them is incomplete. The aim of this study was to determine the presence and distribution of kainate-preferring glutamate receptor subunits in glial cells in the white matter of adult cattle. To this end, reverse transcription and polymerase chain reaction (RT-PCR) analysis of mRNA extracted from the corpus callosum, fornix and optic nerve of brains of adult cattle was carried out using specific primers that allowed selective amplification of the kainate transcripts for all five subunits were detected. Restriction digestion of the amplified products confirmed the specificity of the PCR amplifications and showed that the extent of Q/R editing in the GluR6 subunit was lower than that described in the rat whole brain. Immunohistochemical experiments revealed that GluR5-7 and KA2 subunits appeared in approximately 50% of astrocytes and 40% of oligodendrocytes. In astrocytes the receptor subunits were located throughout the cell body and processes, whereas in oligodendrocytes they were mostly restricted to the soma. Double-labelling studies demonstrated that in the white matter areas studied GluR5-7 and KA2 subunits are coexpressed in the majority of macroglial cell endowed with kainate receptors. In addition, astrocytes, but not oligodendrocytes, expressing kainate-selective receptors were found to possess the GluR1 subunit, an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunit which has previously been found in this macroglial cell type. Taken together, these results provide evidence for the presence and molecular diversity of kainate-selective receptor subunits in glial cells of the adult white matter and for their coexpression with AMPA receptor subunits. The widespread distribution of kainate receptors in glial cells of the white matter suggests that these receptors may be important in axon-glia and/or glia-to-glia signalling.