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D. Y. Tso, J. Schallek, G. McLellan, S. Viswanathan; Functional Retinal Imaging of Intrinsic Optical Signals in a Cat Model of Glaucoma. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2006. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the impact of glaucomatous damage on the properties of functionally-correlated intrinsic optical signals in the retina.
Two adult Siamese cats were obtained from a colony that has naturally-occurring, congenital open-angle glaucoma. Cats had moderate buphthalmos and elevated intraocular pressure (IOP). IOP and pattern ERG(PERG) recordings were taken at weekly intervals to monitor the condition of the eye. Additionally, the retina was examined with optical coherence tomography (OCT) to visualize the extent of degeneration. Cats were anesthetized and paralyzed in preparation for imaging. Intrinsic signals in the retina were imaged with cooled CCD coupled to a modified fundus camera. The apparatus allowed patterned visual stimuli (550nm) to be projected into the subject’s eye, while a dim near-infrared light (NIR, 700-900nm) served as interrogation light. Barrier filters were placed in front of the CCD so that only NIR reflectance signals were imaged.
Over the course of four months, the cats’ intraocular pressures averaged 30 and 31mmHg, respectively, which is above the "normal" range. The PERG at 5Hz showed a reduced response when compared to normal cats, consistent with a functional loss of ganglion cells. This finding corroborated data from OCT scans that showed thinning of the retinal nerve fiber layer. The intrinsic signals in the glaucoma cats were similar to those in normal cats. Signals showed coextensive response patterns with the visually stimulated region of retina. The magnitude of signals reduced when compared to normal cats, however the time course of signal growth and decay remained unchanged.
Despite reduced PERG function, elevated IOP and evidence for nerve fiber layer degeneration, the stimulus-evoked signals persist. This finding is consistent with our previous experiments using tetrodotoxin (TTX) in the cat retina that show intrinsic signals remain after ganglion cell suppression (We have recently confirmed this finding with TTX injections in the macaque monkey retina). Our interpretation is that ganglion cells are not the primary origin of intrinsic optical signals in the retina.
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