CELLULAR-LOCALIZATION OF METABOTROPIC GLUTAMATE RECEPTORS IN THE MAMMALIAN OPTIC-NERVE - A MECHANISM FOR AXON-GLIA COMMUNICATION

It has been proposed that neurotransmitter signalling can occur betwee n axons and glia in the mammalian optic nerve in the absence of synapt ic specialisations, and that this may be glutamate mediated. Here, the cellular distribution of five metabotropic glutamate receptors (mGluR 's la, Ib, Ic, 2/3 and 5) have been assessed in the rat optic pathway using specific antibodies. Positive immunoreactivity is found for mGlu R2/3 and 5. Both are found in axons, although only mGluR5 is present i n the majority of these. Strong immunoreactivity for mGluR2/3 is found in cells in the optic pathway and thalamus. The cellular morphology a nd distribution is consistent with their being astrocytes. Examination of brain sections stained for mGluR2/3 is consistent with this notion , with many cells having end-feet processes terminating on blood vesse ls or the pial surface. The axonal immunoreactivity could represent th e presence of these receptors on axons, but it is more probable that t he receptor protein synthesised in the ganglion cell soma is being tra nsported to the cell terminal in sufficient concentration to be reveal ed by immunohistochemistry. The reason for the axon-astrocyte signalli ng is unclear, and may be associated with metabolic coupling, In devel opment, communication between axons and glia mediates a range of funct ions including pathway selection and myelination. It is probable that in the adult this form of signalling underpins a range of functions th at have yet to be described.