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R.D. Glickman, W.R. Elliott, H. Zwick, H. Rentmeister–Bryant; The Pattern ERG in the Rat Snake Exhibits Frequency–Doubling as an Acute Response to Laser Retinal Injury . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5691.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The Great Plains rat snake, Elaphe guttata emoryi, is being used as an animal model to study the morphological and cellular responses of the retina to laser injury. Related studies of retinal laser injuries in this animal have examined oxidative and immunological responses. Here, we report the effects of acute laser injuries on retinal function, assessed with the pattern–evoked electroretinogram (PERG). Methods: The PERG was recorded from ketamine–xylazine anesthetized animals. A small incision was made in the spectacle (the clear specialized scale which covers the snake eye) near the periphery of the optical zone, in order to permit a needle electrode to contact the cornea. Recordings were made with a physiological amplifier with 10,000X gain, and 1Hz – 3 kHz bandpass. The PERG was elicited with counterphased square and sine wave gratings presented either on a conventional CRT, or by direct projection into the snake’s eye with a scanning laser ophthalmoscope (SLO). While imaging the fundus with the SLO, an array of 5 lesions was made with a Nd:VO4 laser (532 nm) that delivered 50 mW per 10–msec pulse. Lesions were 75 to 100 µm in diameter. PERG responses were recorded before and after laser exposure, and were subjected to Fourier analysis. Results: The normal snake PERG had a maximum amplitude response to gratings of 0.25–0.5 cpd, counterphased at 2–4 cps. The waveform of this response was roughly in synch with the counterphase frequency (i.e. shifts/sec) of the grating, and the real amplitude component determined by Fourier analysis was highest at this frequency. Immediately following the placement of the laser lesions, a new response waveform appeared at twice the counterphase frequency (frequency doubling), which was also revealed by an increase in the real Fourier amplitude component at the doubled frequency. The frequency doubling declined slowly over the next hour, but did not entirely vanish. Conclusions: Although we have found that the rat snake has an all–cone retina, there is little information about ganglion cell physiology, and its relation to the PERG, in this species. The appearance of frequency doubling in the PERG suggests that a network of linearly summating ("X"–like) ganglion cells was disrupted by the laser lesions. The frequency doubling response may be a sensitive measure that can be used to evaluate potential treatments for laser eye injuries.
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