Expression of high-affinity neuronal and glial glutamate transporters in the rat optic nerve

Recent studies have revealed that a dynamic axon-glial signaling occurs in the rat optic nerve, which is devoid of synapses. This interaction is postulated to be mediated by non-vesicular release of glutamate via a reversal of high-affinity glutamate transporters. Here we examined the expression of glial glutamate transporters (GLAST and GLT-1) and a neuronal transporter (EAAC1) in the rat optic nerve. RT-PCR analysis revealed the presence of mRNAs for GLT-1 and GLAST, but not EAAC1. RNase protection assays showed that of the two glial transporters, mRNA for GLAST was expressed at much higher level than was GLT-1. A similar expression pattern was found in primary astrocyte culture cells. GLAST mRNA level in the optic nerve was comparable to that in the cerebellum. Developmentally, GLAST mRNA level was highest at P2 and dropped slightly by adulthood. Nerve transection resulted in little or no change in mRNA levels for GLAST and GLT-1 assayed at 4 to 14 days post-transection, but GLAST mRNA level was decreased at 64 days. Western blot analysis revealed that the rat optic nerve showed immunoreactivity to antibodies against GLT-1, GLAST, and EAAC1. In conclusion, we suggest that glial and neuronal transporters are present in the rat optic nerve, where dynamic axon-glial interaction has been known to occur. In particular, the unusually high level of expression of GLAST in the optic nerve suggests a possible role for this glial transporter in protecting optic nerves from neurotoxicity during postnatal development.