Glutamate-induced glutamine synthetase expression in retinal Muller cells after short-term ocular hypertension in the rat

PURPOSE. To determine the effect of intraocular pressure (IOP) elevation on glutamate-induced expression of glutamine synthetase (GS) in retinal Muller cells of rat eyes. METHODS. Six groups of three rats each were studied. Group I was a normal control group. In Group II, one eye received an intravitreal glutamate injection ( 75 nmoles) while the contralateral eye served as control. In Groups III and IV, IOP was raised in one eye by episcleral vein cauterization. Moderately elevated IOP was maintained for 1 day in Group III or 1 week in Group IV (35 +/- 1.9 - 45 +/- 5.2 mm Hg). An additional two groups of rats received bilateral intravitreal glutamate injections ( 75 nmoles) immediately ( Group V), or 6 days ( Group VI), after induction of IOP elevation in one eye. One day after glutamate injection the rats in all groups were killed, and the eyes enucleated and fixed. Retinas from left and right eyes of each animal were embedded together in LR White resin (Ted Pella, Redding, CA). Sections were processed for GS immunolabeling with antibodies to GS by two-stage immunogold labeling with silver enhancement. Images of labeled retinas from the two eyes were captured under identical light microscopic conditions and the GS immunoreactivity in Muller cells was compared between the left and right retinas in the same section by image analysis. An additional five rats were included in Group II and the retinas were analyzed by Western blot analysis to confirm immunohistochemical findings. RESULTS. Similar to the finding in the control group ( Group I), the GS immunoreactivity of the left and right eyes of Group III and IV remained unchanged even though the right eyes in the two groups had elevation of IOP lasting for 24 hours and 1 week, respectively. However, GS levels were significantly increased by 40 +/- 5.7% in normotensive eyes 24 hours after intravitreal injection of glutamate ( Group II). The rise in GS immunoreactivity was abolished in eyes with acute IOP elevation ( Group V). In contrast, when the eyes were exposed to high IOP for 1 week ( Group VI), the glutamate-induced increase in GS immunoreactivity was restored. CONCLUSIONS. Elevated levels of vitreal glutamate can increase the expression of GS in retinal Muller cells. This increase was blocked if IOP was acutely elevated for 24 hours but was restored if IOP remained elevated for 1 week. This finding suggests that moderate elevation of IOP causes only short-term functional changes of glutamate metabolism ( amidation) by retinal Muller cells. However, it is not known to what extent endogenous extracellular glutamate can regulate GS expression in normal eyes or in eyes with glaucoma.