Pattern and flash electroretinogram following increased intraocular pressure in pigeons

The pattern ERG in chicks - Stimulus dependence and optic nerve section

The chick is widely used in studies of eye growth regulation and myopia. The aim of this study was to explore the utility of pattern (p)ERG as a tool to assess retinal function in such studies. Effects of optical defocus and diffusing blur, manipulations used to alter eye growth experimentally, were evaluated. PERGs were recorded from White-Leghorn chickens, using a checkerboard pattern, including 8 spatial frequencies (0.05-2.2c/d SF), 13 contrast levels (1-100%), and 8 temporal reversal frequencies (0.5-20Hz). The acute effects of defocus and diffusing blur were examined. Flash- and pERGs were also recorded from chicks that underwent monocular optic nerve section (ONS), to explore the contribution of retinal ganglion cells (RGCs). Measurements were made up to 6weeks post-ONS, complemented with SD-OCT imaging. In normal chicks, the response to 1Hz, 100% contrast stimuli showed positive- and negative-going waveforms at 43ms (P1) and 75ms (N95), respectively, with 0.06-0.1c/d SF eliciting the largest P1 amplitudes of 21.9±2.5μV. Contrast levels above 5% yielded measurable P1 responses. Responses were transient and monophasic for 0.5-5Hzreversal rates, with higher temporal frequencies yielding steady state responses. Defocus and diffusing blur decreased pERG amplitude across all SFs. pERG responses remained normal after ONS, despite the loss of RGCs. In conclusion, chicks show robust pERG responses, which are attenuated by defocus and diffusing blur. The pERG response is not affected by ONS, suggesting that RGCs do not contribute to the chick pERG.

Pattern electroretinography in a rat model of ocular hypertension: functional evidence for early detection of inner retinal damage

With the increasing use of the rat as an animal model for glaucoma and for the evaluation of neuroprotective treatments, there is a need for a sensitive test of retinal ganglion cell (RGC) function in this species. The aims of this study were to detect functional abnormalities of the inner retina in a rat model of high intraocular pressure (IOP) using the pattern electroretinogram (PERG), and to correlate them with morphometric analysis of RGC survival and the functional integrity of the inner retina. Unilateral ocular hypertension was induced in 17 Lewis rats through laser photocoagulation. Pattern ERGs were recorded prior to lasering and 3 weeks later, using a series of shifting patterns of decreasing spatial frequency projected directly onto the animals' fundus. IOP was measured at the same intervals, and the number of surviving RGCs estimated. Low amplitude PERG signals could be recorded in response to a narrow grating of 0.368 cycles per degree (cpd), and increased with stimulus size. Lasering caused mean (+/-s.d.) IOP to increase significantly from 18.3+/-4.5 (baseline) to 29.8+/-8.8 mmHg within 3 weeks (p<0.0001). At this time, PERG amplitudes were significantly reduced (p<0.05), declining an average of 45% compared to the normotensive, control eyes. No outer retinal damage was observed, but the mean number of RGCs decreased significantly (p<0.001), from 2 525.0+/-372.4 to 1 542.8+/-333.8 cells per mm2. This decrease in RGC number was significantly (p=0.03) correlated the decrease in PERG amplitude. The correlation between functional integrity of the inner retina and the rat PERG was further demonstrated by intravitreal tetrodotoxin injections, which temporarily abolished the PERG but did not affect outer retinal activity, reflected in the flash ERG. The evidence for early functional deficits, combined with tonometry and documentation of correlated ganglion cells loss, confirms the sensitivity of this diagnostic tool and the validity and importance of this animal model in glaucoma research.

Pattern electroretinogram and optic nerve section in pigeons

We studied the effect of optic nerve section on the pattern electroretinogram (PERG) of 10 pigeons. Base line PERG and flash ERG were measured prior to surgery and then one optic nerve was cut. We measured PERG at various time intervals up to 24 weeks after the section. There was a sharp drop in PERG amplitude immediately following the section. It gradually returned to normal levels in the following 24 weeks. Significant decreases in time to peak were noted at 1 and 16 weeks for 0.5 cycle per degree checks and at 1, 2 and 9 weeks for 0.25 cycle per degree checks. The ERG generated by a TV stimulus did not change in amplitude but had a shorter time to peak at 24 weeks for a high intensity stimulus and at 16, 18, 24 weeks for a low intensity stimulus. Light and electron microscopic sections (at 24 weeks) revealed an almost complete loss of nerve fibers in the optic nerve. Retinal sections revealed only a few surviving ganglion cells and a few nerve fibers. This suggests that in pigeons, PERG is not generated by ganglion cells.