In the present study we measured
calcium-dependent, vesicular
glutamate release, and
calcium-independent, transport-mediated
glutamate release patterns in the vertebrate retina to better understand the sources of elevated
glutamate in neural tissue under ischemic conditions. A
potassium concentration of 40 mM, which mimics the extracellular
potassium concentration in the central nervous system during
ischemia, was applied 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-coupled device (CCD) camera. Following the elevation of external
potassium to 40 mM several bands of
glutamate fluorescence, reflecting the spatial distribution of
glutamate release, were observed. A
calcium-dependent cloud of
glutamate was observed in the inner plexiform layer, that was antagonized by bath-applied
nifedipine. A relatively dense
glutamate cloud (1-10 microM) was observed over the
ganglion cell layer, which was blocked by
dihydrokainate, a
glutamate transport antagonist. In contrast,
nifedipine, an inhibitor of
calcium-dependent
neurotransmitter release in the retina, failed to block the cloud of released
glutamate in the
ganglion cell layer. These data suggest that under pathological conditions in the eye where
glutamate levels are elevated surrounding retinal ganglion cells, such as observed in some forms of
glaucoma, a possible source of the elevated
glutamate is through a
glutamate transporter operating in a reversed direction. A likely candidate for mediating this reversed transport of
glutamate is the retinal Muller cell.