Recording the photopic electroretinogram from conscious adult Yucatan micropigs

Normative Data of Axial Length, Retinal Thickness Measurements, Visual Evoked Potentials, and Full-Field Electroretinography in Female, Wild-Type Minipigs

Purpose

The purpose of this study was to present normative data of optical coherence tomography (OCT), electrophysiological, and ocular biometry parameters and their correlation in minipigs.

Methods

Eighty-eight eyes of 44 minipigs underwent full-field electroretinogram (ERG) recording and ocular biometry. However, 10 eyes of 6 minipigs were excluded because of poor OCT image quality. The thickness of the retinal sublayers was measured on a vertical line at 5 locations with a 1 mm interval from the disc margin to the dorsal periphery and at 10 locations on the visual streak. Visual evoked potentials (VEPs) were measured in 15 eyes of 8 minipigs.

Results

All minipigs were female with a mean age and axial length of 13.83 ± 10.56 months and 20.33 ± 0.88 mm, respectively. The implicit time of the a-wave and b-wave in scotopic 3.0 ERGs was longer than that in photopic 3.0 ERG. The implicit time of the n2-wave and p2-wave in VEP was 25.67 ± 7.41 ms and 52.96 ± 10.38 ms, respectively. The total retinal layer (TRL) and nerve fiber layer (NFL) became thinner near the periphery. The inner retinal sublayers near the visual streak were thicker than those at other locations. Central TRL and NFL thickness on visual streak was 223.06 ± 23.19 µm and 74.03 ± 13.93 µm, respectively. The temporal TRL and NFL on the visual streak were thicker than those on the nasal side.

Conclusions

The normative electrophysiological and OCT parameters used in our study can be used as reference data in further pig studies.

Translational Relevance

This study presents normative data of minipigs, which are adequate animal models for preclinical studies.

The determination of dark adaptation time using electroretinography in conscious miniature Schnauzer dogs

The optimal dark adaptation time of electroretinograms (ERG's) performed on conscious dogs were determined using a commercially available ERG unit with a contact lens electrode and a built-in light source (LED-electrode). The ERG recordings were performed on nine healthy Miniature Schnauzer dogs. The bilateral ERG's at seven different dark adaptation times at an intensity of 2.5 cd·s/m² was performed. Signal averaging (4 flashes of light stimuli) was adopted to reduce electrophysiologic noise. As the dark adaptation time increased, a significant increase in the mean a-wave amplitudes was observed in comparison to base-line levels up to 10 min (p < 0.05). Thereafter, no significant differences in amplitude occured over the dark adaptation time. Moreover, at this time the mean amplitude was 60.30 ± 18.47 µV. However, no significant changes were observed for the implicit times of the a-wave. The implicit times and amplitude of the b-wave increased significantly up to 20 min of dark adaptation (p < 0.05). Beyond this time, the mean b-wave amplitudes was 132.92 ± 17.79 µV. The results of the present study demonstrate that, the optimal dark adaptation time when performing ERG's, should be at least 20 min in conscious Miniature Schnauzer dogs.

The ERG of the Beagle Dog: Evidence Associating a Post b-wave Negativity with the Tapetum Lucidum

As previously reported in the literature, the electroretinogram (ERG) of the Beagle dog includes a large post b-wave negativity, the origin of which is not yet established. In the course of our investigations on the electroretinogram in dogs, we examined two Beagle dogs (2 years apart) who had one eye devoid of a Tapetum Lucidum (TL). Photopic (cone-mediated) and scotopic (rod-mediated) ERGs were obtained according to the guidelines for clinical electroretinography in dog. In both dogs the short-latency ERG components (i.e. a- and b-waves) were found to be within the normal range in amplitude, peak time and morphology O.U. However, the large negative component that, in Beagle dogs, normally follow the b-wave was absent from the photopic as well as the scotopic signals obtained from the TL-free eye. Our results thus suggest a possible contribution of the TL to the ERG of Beagle dogs.

An Overview of Drug Development with Special Emphasis on the Role of Visual Electrophysiological Testing

Visual electrophysiological techniques, such as electroretinography (ERG) and visual evoked potentials (VEP) can provide useful information on the safety, efficacy, and proper dosing of chemical entities under development as new drug therapies. During post-marketing safety surveillance, a variety of visual electrophysiological measures can be used to objectively assess and document individual patient response to ophthalmic drugs and ocular or visual system side effects of non-ophthalmic drugs. In this paper, the discovery, exploratory development, full-development and post-marketing stages of drug development are briefly outlined. The potential role of visual electrophysiological techniques in each of these stages is described and discussed.

Comparing the photopic ERG i-wave in different species

The i-wave, a post b-wave component of the human photopic electroretinogram (ERG), is claimed to originate at the level of the retinal ganglion cells (RGC) or more distally. We investigated whether this wave is a feature common to all species. Photopic ERGs were obtained from the following species: Beagle dog, European cat, New Zealand white rabbit, Göttingen minipig, Cynomolgus monkey, Sprague-Dawley and brown Norway rats, Hartley guinea pig, and CD1 and C57BL6 mice. Results were compared with those obtained from normal human subjects. Except for rats and mice, all species yielded a well-demarcated i-wave, easily identifiable and separated from the a-b-wave complex by approximately 20 ms. Our sample suggests that the i-wave is a feature common to the photopic ERG of most species including humans. In view of its suggested origin, the i-wave would offer a unique opportunity to test, with the flash ERG, the functional integrity of the retinal ganglion cells in animals where use of a pattern stimulus is not always easily obtained.

A practical method to obtain reproducible binocular electroretinograms in dogs

We present a simple method to record highly reproduciblebinocular electroretinograms in dogs. Rod and cone electroretinograms were elicited in 60 Beagle dogs, with the use of two adjustable photostimulators, one directed at each eye, and maintained in position with the use of a special device. Data analysis revealed no significant differences in amplitudes between the right and the left eye for each animal and each recording session, thus attesting to the high reliability of our approach. In a few instances, however, small interocular timing differences were noted. The proposed approach could therefore be used not only in a clinical setup where the functional status of the two eyes is often needed to reach a diagnosis butalso in research projects, such as toxicological assays, where the experimentation is performed on one eye while the other is kept as control.

SLO angiography: arterio-venous filling times in monkey and minipig

Confocal scanning laser ophthalmoscope (cSLO) is a new technique which enables ocular fundus image recording and dynamic retinal angiography to be performed. The ocular fundus image is acquired sequentially, point by point, and is reconstructed on a video monitor at the rate of 25 images per second. The aim of this paper is to evaluate the feasibility of measuring retinal arterio-venous filling times (AVFT) with a I + Tech cSLO. Three young adult cynomolgus monkeys and three young adult Göttingen minipigs were used as experimental models. All animals were anesthetized using a zolazepam + tiletamine mixture injected intramuscularly; heart rate and rectal temperature were monitored and corneal irrigation was regularly performed. For all subjects, prior to examination, hematocrit and globe axial length were measured. The images were recorded, stabilized and analyzed. The retinal examination consisted of retinal images with 40 degrees field cSLO, retinal fluorescein angiography and arterio-venous 50% filling time measurements. For each subject all images were easily recorded while keeping the animals in a normally lighted room without having to use any additional optical device. AVFT using an I + Tech cSLO is easily performed in monkeys and minipigs. AVFT measurements in minipigs and monkeys are similar. These results suggest that minipigs can replace monkeys as an experimental species for AVFT investigations.

The electroretinogram and visual evoked potential of freely moving rats

The vascularised rat retina could be one of the most useful experimental objects in visual neuroscience to understand human visual physiological and pathological processes. We report here on a new method of implantation for studying the visual system of freely moving rats that provides a rat model for simultaneous recording at corneal and cortical level and is stable enough to record for months. We implanted light emitting diodes onto the skull behind the eyeball to stimulate the eye with flashes and to light adapt the retina with constant light levels. A multistrand, stainless steel, flexible fine wire electrode placed on the eyeball was used for electroretinogram recording and screw electrodes (left/right visual and parietal cortical) were used to record the visual evoked potential and the electroencephalogram. In the present report we focus on the new method of implantation for recording the corneal flash electroretinogram of normal, freely moving rats simultaneously with the visual evoked cortical potential showing examples in various visual experiments. We also introduce a program for retinogram and visual evoked potential analysis, which defines various measures (latencies, areas, amplitudes, and durations) and draw attention to the benefits of this method for those involved in visual, functional genomic, pharmacological, and human ophthalmologic research.