| dc.description | Although the release of the neurotransmitter glutamate onto retinal ganglion cell (RGC) dendrites is a normal part of retinal circuitry, prolonged exposure to glutamate is toxic to these retinal output neurons. A rise in intracellular calcium levels has been implicated as the trigger for glutamate excitotoxicity. In this thesis, I characterized the calcium dynamics of RGCs during brief (30 second) and prolonged (1 hour) glutamate exposures, investigated the relationship between calcium and excitotoxic death, examined the effect of adenosine on glutamate-induced calcium influx, and assessed the functional efficiency of retinal glutamate uptake. Using calcium imaging techniques on RGCs in both purified cultures and retinal wholemount preparations, I demonstrated that glutamate-induced calcium influx is mediated primarily by activation of NMDA-type glutamate receptors, but a smaller portion of the signal is mediated by voltage-gated calcium channel and AMPA/kainate-type glutamate receptor activation at saturating glutamate (100 muM) concentrations. One hour exposure to glutamate (10--1000 muM) killed 18--28% of the cultured RGCs, and RGC death was accompanied by delayed calcium deregulation. RGCs that displayed greater glutamate-evoked calcium signals were more likely to undergo calcium deregulation. Inhibition of NMDA receptors reduced calcium influx and significantly protected RGCs from glutamate excitotoxicity. The neuromodulator adenosine was shown to reversibly inhibit glutamate-induced calcium influx through an adenosine A1 receptor-mediated mechanism, supporting a role for adenosine as a modifier of RGC glutamatergic pathways. In retinal wholemounts, glutamate was much less effective than NMDA in eliciting an RGC calcium response (compared to isolated RGCs) due to the presence of glutamate transporters. Detectable calcium signals with micromolar glutamate concentrations could be obtained if retinal glutamate uptake was pharmacologically inhibited. Imaging RGC calcium dynamics in retinal wholemounts was used to functionally assess glutamate clearance mechanisms in a rat glaucoma model, and no significant defect in glutamate uptake was apparent using this novel methodology. The modulation of RGCs by adenosine and the rapid removal of extracellular glutamate by transporters represent two mechanisms through which glutamatergic signaling is regulated in the retina to counter the excitotoxic actions of this neurotransmitter. | en_US |
