TRANSPORT-MEDIATED RELEASE OF ENDOGENOUS GLUTAMATE IN THE VERTEBRATE RETINA

In the present study we measured calcium-dependent, vesicular glutamat e release, and calcium-independent, transport-mediated glutamate relea se patterns in the vertebrate retina to better understand the sources of elevated glutamate in neural tissue under ischemic conditions. A po tassium concentration of 40 mM, which mimics the extracellular potassi um concentration in the central nervous system during ischemia, was ap plied to the bathing medium of a retinal slice prepared from zebrafish . High external potassium evoked release of endogenous glutamate that was measured using a glutamate-specific fluorometric assay applied to the bath. The slice was visualized under 668 nm light using Normarski optics and fluorescent images were captured using a cooled charge-coup led device (CCD) camera. Following the elevation of external potassium to 40 mM several bands of glutamate fluorescence, reflecting the spat ial distribution of glutamate release, were observed. A calcium-depend ent cloud of glutamate was observed in the inner plexiform layer, that was antagonized by bath-applied nifedipine, A relatively dense glutam ate cloud (1-10 mu M) was observed over the ganglion cell layer, which was blocked by dihydrokainate, a glutamate transport antagonist. In c ontrast, nifedipine, an inhibitor of calcium-dependent neurotransmitte r release in the retina, failed to block the cloud of released glutama te in the ganglion cell layer. These data suggest that under pathologi cal conditions in the eye where glutamate levels are elevated surround ing retinal ganglion cells, such as observed in some forms of glaucoma , a possible source of the elevated glutamate is through a glutamate t ransporter operating in a reversed direction. A likely candidate for m ediating this reversed transport of glutamate is the retinal Muller ce ll.