Serge G. Rosolen

Guidelines for clinical electroretinography in the dog

These procedures described for the dog ERG were approved at the 1st European Conference on Veterinary Visual Electrophysiology in Vienna, Austria, May 30, 2000. Dr. Narfström was Chair of the Committee for a Harmonized ERG Protocol, appointed by the European College of Veterinary Ophthalmology (ECVO), and Dr. Ofri was secretary. The other coauthors are committee members. Guidelines for ERG procedures in other animal species for clinical and laboratory studies are planned for in the future and the present guidelines are planned to be revised on a biannual basis. A brief report of the recommended procedures is available in the Conference Proceedings book.

Late histological and functional changes in the P23H rat retina after photoreceptor loss

Several strategies have been proposed to restore useful vision following photoreceptor degeneration. However, a very few studies have investigated late anatomical changes and functional state of residual retinal neurons after complete photoreceptor loss. We investigated the progressive degeneration of retinal ganglion cells (RGCs) in P23H rats. The RGC multielectrode array recordings indicated lower firing rates, disappearance of broad-scale, and maintenance of short-scale pairwise correlations. Up to 11% of RGCs displayed repetitive and often correlated spike discharges, reminiscent of developmental rhythmic activity, which could be reversibly suppressed by blockade of the AMPA/kainite glutamate receptors. RGCs in P23H rats remain sensitive to local electrical stimulation, generating short-latency responses as in the normal retina. These results provide evidence that, despite the demonstrated RGC degeneration, remaining active RGCs maintain their basic physiological and network properties with some emerging functional changes such as the spontaneous rhythmic activity in late stages of the degenerative disease.

Recommendations for a toxicological screening ERG procedure in laboratory animals.

Electroretinography, using laboratory animals, is a commonly used technique for determining the retinal toxicity of chemical agents. In this paper, guidelines for performing this test are provided. The physiologic basis for visual testing is presented with attention to inter-species differences. Technical aspects of animal recordings are reviewed, including animal preparation, stimulation, signal conditioning, recording and data analysis. Finally, suggested protocols for recording in diurnal and nocturnal species are presented.

Retinal electrophysiology for toxicology studies : Applications and limits of ERG in animals and ex vivo recordings

Assessing retinal drug toxicity is becoming increasingly important as different molecules are now developed for the treatment of neurodegenerative diseases and vascular disorders. In pharmacology and toxicology, the electroretinogram (ERG) and the multielectrode array (MEA) recording techniques can be used to quantify the possible side effects of retino-active xenobiotics. Toxicity testing requires the use of rodent as well as non-rodent models for extrapolation to the human model when determining risk and safety. Animal species differ in their retinal anatomo-physiology: most rodents used in toxicology studies are essentially nocturnal species, whereas the non-rodent laboratory species normally used (e.g. dogs, pigs and monkeys) are diurnal. The ratio between the photoreceptor populations which varies from species to species, should be considered when designing the experiment protocol and the interpretation. The described ERG procedures are designed to comply with all applicable good laboratory practice standards. Use of these procedures should yield an acceptable level of intra- and inter-subject variability for compiling a historical database, and for detecting possible retinal toxicity in animal studies. They could therefore be used as specific and standardized tools for screening of potential retinotoxic molecules in drug discovery and development in order to compare methods and results with those obtained in human electrophysiological assessments. Recording of ganglion cell light responses on ex vivo retina with the MEA technique can further demonstrate how retino-active xenobiotics affect retinal visual information processing by eliminating potential obstacles related to bioavailability and blood barrier permeability.

Taurine: The comeback of a neutraceutical in the prevention of retinal degenerations

Taurine is the most abundant amino acid in the retina. In the 1970s, it was thought to be involved in retinal diseases with photoreceptor degeneration, because cats on a taurine-free diet presented photoreceptor loss. However, with the exception of its introduction into baby milk and parenteral nutrition, taurine has not yet been incorporated into any commercial treatment with the aim of slowing photoreceptor degeneration. Our recent discovery that taurine depletion is involved in the retinal toxicity of the antiepileptic drug vigabatrin has returned taurine to the limelight in the field of neuroprotection. However, although the retinal toxicity of vigabatrin principally involves a deleterious effect on photoreceptors, retinal ganglion cells (RGCs) are also affected. These findings led us to investigate the possible role of taurine depletion in retinal diseases with RGC degeneration, such as glaucoma and diabetic retinopathy. The major antioxidant properties of taurine may influence disease processes. In addition, the efficacy of taurine is dependent on its uptake into retinal cells, microvascular endothelial cells and the retinal pigment epithelium. Disturbances of retinal vascular perfusion in these retinal diseases may therefore affect the retinal uptake of taurine, resulting in local depletion. The low plasma taurine concentrations observed in diabetic patients may further enhance such local decreases in taurine concentration. We here review the evidence for a role of taurine in retinal ganglion cell survival and studies suggesting that this compound may be involved in the pathophysiology of glaucoma or diabetic retinopathy. Along with other antioxidant molecules, taurine should therefore be seriously reconsidered as a potential treatment for such retinal diseases.

A Low-Cost and Simple Imaging Technique of the Anterior and Posterior Segments: Eye Fundus, Ciliary Bodies, Iridocorneal Angle

PURPOSE The authors recently used topical endoscopy to image the mouse eye fundus. Here, they widened the field of application for this ophthalmologic tool, imaging both the posterior and the anterior eye segments in larger animals commonly encountered in research laboratories and veterinary clinics. METHODS Pupils were dilated, and local anesthetic and gel were applied to the animal cornea. The endoscopic probe was placed in contact with the cornea of conscious rats, sedated cats and dogs, anesthetized sheep, and nonhuman primates. RESULTS High-resolution digital images of the eye fundus were obtained in all investigated animals using the endoscopic probe along the eye axis. Arteriovenous filling time was monitored with fluorescein angiography in pigmented rats. The retinal periphery and ciliary bodies could be visualized with the probe placed at an oblique angle. The probe was inclined further to observe the iridocorneal angle such that the pectinate ligaments could be seen at high resolution in cats. The authors used the probe on eyes with retinal detachment, luxation of a cataractous lens, and pigment infiltration in the iridocorneal angle, demonstrating its potential use in eye diseases. CONCLUSIONS This topical endoscopic technique provides a unique tool for single eye examinations. The authors obtained a circular view of the anterior (iridocorneal angle) and the posterior (fundus) eye segments from all animal species studied. This technique is inexpensive and easy to use. It can be easily moved to the eye of the patient who cannot move to stand in front of classic apparatus, offering new opportunities in ophthalmology.

A practical method to obtain reproducible binocular electroretinograms in dogs

We present a simple method to record highly reproducible binocular 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 but also in research projects, such as toxicological assays, where the experimentation is performed on one eye while the other is kept as control.

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.

Comparison of the pig and feline models for full thickness corneal transplantation

PURPOSE The goal of this study was to report on the advantages and limitations of the pig and feline models for experimental in vivo corneal transplantation. METHODS Ten healthy domestic pigs and ten healthy cats were used. Full thickness penetrating keratoplasty was performed using autologous (eight cases), allogeneic (seven cases) or human xenogeneic (three cases) tissue. In two other cases, the inflammatory response to partial thickness trephination (without transplantation) was evaluated. Eyes were assessed daily before and after surgery by slit-lamp, pachymetry, and tonometry. A transparency score ranging from 0 (opaque graft) to 4 (clear graft) was used, based on the slit-lamp examination. Optical coherence tomography, histology, and electron microscopy were performed postmortem. RESULTS In the pig, the mean (±SD) transparency score for the eight full thickness grafts was 0.88 ± 0.99, ranging from 0 to 3. In the feline model, the mean transparency score for the seven uncomplicated grafts was 3.93 ± 0.19, ranging from 3.5 to 4. Both negative controls without endothelium remained opaque at all time. Intraoperative tendency for iris incarceration into the wound, rapid corneal swelling, suture cheese wiring, and postoperative intraocular inflammation were the main factors jeopardizing the functional success of the corneal transplant in the pig model. CONCLUSION Suboptimal functional results were obtained after full thickness corneal transplantation in the pig model, while in the feline model, the same protocol yielded uneventful surgeries and clear transplants, with functional results similar to those achieved in human subjects.

Impact of oral melatonin on the electroretinogram cone response.

Background In the eye, melatonin plays a role in promoting light sensitivity at night and modulating many aspects of circadian retinal physiology. It is also an inhibitor of retinal dopamine, which is a promoter of day vision through the cone system. Consequently, it is possible that oral melatonin (an inhibitor of retinal dopamine) taken to alleviate circadian disorders may affect cone functioning. Our aim was to assess the impact of melatonin on the cone response of the human retina using electroretinography (ERG). Methods Twelve healthy participants aged between 18 to 52 years old were submitted to a placebo-controlled, double-blind, crossover, and counterbalanced-order design. The subjects were tested on 2 sessions beginning first with a baseline ERG, followed by the administration of the placebo or melatonin condition and then, 30 min later, a second ERG to test the effect. Results Following oral melatonin administration, a significant decrease of about 8% of the cone maximal response was observed (mean 6.9 μV ± SEM 2.0; P = 0.0065) along with a prolonged b-wave implicit time of 0.4 ms ± 0.1, 50 minutes after ingestion. Conclusion Oral melatonin appears to reach the eye through the circulation. When it is administered at a time of day when it is not usually present, melatonin appears to reduce input to retinal cones. We believe that the impact of melatonin on retinal function should be taken into consideration when used without supervision in chronic self-medication for sleep or circadian disorder treatment.