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1From the Department of Physiology and the 2Neuroscience Program, Michigan State University, East Lansing, Michigan.
PURPOSE. The purpose of this study was to examine the effect that chronic elevation of intraocular pressure has on the intrinsic and visual response properties of parasol cells in the primate retina.
METHODS. A primate model of experimental glaucoma was combined with intracellular recording and staining techniques using an isolated retina preparation. Intrinsic electrical properties were examined by injection of depolarizing and hyperpolarizing currents. Visual responses were studied using drifting and counterphased gratings. Morphologic comparisons were made by injecting recorded cells with Neurobiotin and analyzing them quantitatively with a computer-based neuron reconstruction system.
RESULTS. Structurally, parasol cells from glaucomatous eyes had smaller somata and smaller, less complex dendritic arbors, resulting in a significant reduction in total dendrite length and surface area. Functionally, these neurons did not differ from normal in their mean resting membrane potentials, input resistances, or thresholds to electrical activation, but did differ in membrane time constants and spike duration. Parasol cells from both normal and glaucomatous eyes preferred low-spatial-frequency stimuli, but significantly fewer glaucoma-related cells were driven visually—in particular, by patterned stimuli. Glaucomatous cells also did not respond as well to visual stimuli presented at increased temporal frequencies.
CONCLUSIONS. Ganglion cells in the glaucomatous eye retain most of their normal intrinsic electrical properties, but are less responsive, both spatially and temporally, to visual stimuli. The reduction in visual responsiveness most likely results from significant changes in dendritic architecture, which affects their level of innervation by more distal retinal neurons.
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