Functional assessment of glutamate clearance mechanisms in a chronic rat glaucoma model using retinal ganglion cell calcium imaging

Using optical imaging of retinal ganglion cell (RGC) calcium dynamics in living intact retinal wholemount preparations, we tested whether RGCs in an experimental rat glaucoma model were more sensitive to exogenously applied glutamate as a result of deficient glutamate clearance mechanisms. In contrast to post-natal rat RGCs in purified cultures, in which the calcium influx induced by 200 μM NMDA and 10 μM glutamate was approximately equivalent, application of up to 500 μM glutamate did not affect calcium levels in RGCs in retinal wholemounts, even though the RGCs responded to 200 μM NMDA. Glutamate (500 μM) did elicit a RGC calcium response in retinal wholemounts when glutamate transporters were inhibited pharmacologically with DL-threo-β-benzyloxyaspartate, confirming the presence of glutamate clearance mechanisms in this intact retina preparation. The effect of glutamate was then assessed on retinas from rats with chronically elevated intraocular pressure in one eye, produced by the injection of hypertonic saline into an episcleral vein. Application of up to 500 μM glutamate had no effect on RGC calcium levels, while millimolar concentrations of glutamate induced a calcium signal in RGCs that was indistinguishable from that in fellow control retinas. Therefore, there was no evidence for a global defect in glutamate uptake in this rat model of experimental glaucoma. Imaging glutamatergic calcium dynamics of RGCs in retinal wholemounts represents a novel methodology to probe glutamate transporter function and dysfunction in an intact CNS tissue system.