An electroretinogram protocol for toxicological screening in the canine model

Evaluation of a convenient method of assessing rodent visual function in safety pharmacology studies: effects of sodium iodate on visual acuity and retinal morphology in albino and pigmented rats and mice

INTRODUCTION

We have evaluated the ability of a semi-automated, optomotor reflex method to assess drug-induced visual dysfunction, in albino and pigmented rats and mice.

METHODS

Male Han Wistar (HW) and Long Evans (LE) rats and mice (CD-1 and C57BL/6) were tested in a chamber formed by 4 computer monitors displaying a rotating vertical grating, to elicit head-tracking movements. The highest visible grating frequency was taken as the threshold of visual acuity, in cycles per degree (c/d). Animals received an intravenous infusion of either sodium iodate (50mg/kg) or 0.9% w/v NaCl (aq). They were tested 2h later, then re-tested daily for a further 3 days. The time course of the effect was assessed in HW rats over a 6-week period, including electron microscopy, and immunohistochemical analysis of markers of injury and repair in the retina.

RESULTS

Baseline visual acuities for HW and LE rats were 0.355 ± 0.007 and 0.530 ± 0.004 c/d, respectively, and 0.296 ± 0.003 c/d and 0.370 ± 0.001 c/d for CD-1 and C57BL/6 mice, respectively (n=10 for each). In HW rats there was a dramatic loss of visual acuity 2h after administration of sodium iodate (0.021 ± 0.021 c/d; P<0.001). Less dramatic decreases in visual acuity were seen in LE rats and in the two mouse strains. In HW rats, visual acuity was restored after 4 weeks. This paralleled the histopathological recovery of the peripheral retina, whereas the central retina did not recover.

DISCUSSION

The method proved to be very convenient, and the stability of visual acuity in vehicle control rats over a 6-week period also demonstrated its suitability for inclusion in long-term toxicity studies. Both albino and pigmented mice and rats are suitable for assessment of retinotoxicity using this method, but albino rats are the most sensitive to sodium iodate.

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.

Multifocal chorioretinal lesions in Borzoi dogs

OBJECTIVES

To identify the prevalence of Borzoi chorioretinopathy in western Canada, characterize lesions with fluorescein angiography, determine if lesions were progressive, clarify the association of progressive retinal atrophy and investigate the etiology.

MATERIALS AND METHODS

Serial ophthalmic examination, fundus photography, electroretinography, and fluorescein angiography were used to evaluate Borzoi dogs with lesions of Borzoi chorioretinopathy. Pedigree analysis and test breeding of two affected dogs were completed to determine the heritability of Borzoi chorioretinopathy.

RESULTS

One hundred three Borzoi dogs were examined between 1998 and 2003. Focal, peripheral, tapetal, hyper-reflective and pigmented areas consistent with focal retinal degeneration and RPE pigmentation were identified in 12 dogs between 7 months and 7 years of age. Seven males and five female dogs were affected. Ophthalmoscopy and fundus photography over 5 years revealed individual lesions that did not progress or coalesce in 12 affected dogs. Electroretinography of affected and normal Borzoi dogs confirmed that retinal function was similar in normal and affected dogs up to 7 years of age. Fluorescein angiography was performed in three affected dogs and confirmed intact blood-ocular barriers, focal retinal pigment epithelium hypertrophy, and focal absence of choroiocapillaris corresponding to chronic, focal lesions. Pedigree analysis precluded simple dominant, X-linked dominant, or X-linked recessive inheritance. One male dog from the test-bred litter developed bilateral lesions at 14 months of age. Simple recessive, polygenetic, and acquired etiologies of these lesions cannot be ruled out at this time.

CONCLUSIONS

Borzoi chorioretinopathy is an acquired condition that initially manifests as focal retinal edema and loss of choriocapillaris and tapetum. With time the retina degenerates becoming hyper-reflective and with RPE hyper-pigmentation and clumping within the borders of the tapetal lesions. Choriocapillaris remains hypofluorescent on fluorescein angiography. Progressive retinal atrophy was excluded as an etiology of multifocal chorioretinopathy in Borzois dogs. This condition is not inherited by simple autosomal dominant or sex-linked modes of inheritance.

Clinical electrophysiology in veterinary ophthalmology--the past, present and future

The aim of this review is to introduce the reader to the world of clinical veterinary electroretinography. An important indication for ERG recordings in the dog is the early diagnosis of progressive retinal atrophy, an inherited form of photoreceptor degeneration, analogous to retinitis pigmentosa in humans. In most of the 20 canine breeds in which the disease has been studied electrophysiologically, changes in the ERG appear long before the appearance of clinical signs. This early diagnosis is a vital tool in efforts to eradicate the disease through preventive breeding. Pre-operative screening of canine cataract patients is another common indication for electroretinography in the dog. The ERG is also used to diagnose inherited and nutritional photoreceptor degenerations in the cat, and retinal disorders in a number of other animal species. The abundance of animal species (and breeds) seen by the veterinary ophthalmologist lends additional importance to the problem of a harmonized ERG recording protocol. The European College of Veterinary Ophthalmologists has set up a special committee to formulate guidelines for such a protocol. International meetings and wetlabs are also being organized as part of an effort to improve the quality of electrophysiological diagnosis that veterinary ophthalmologists provide their patients.

Absence of neurovisual effects due to tissue and blood cholinesterase depression in a chronic disulfoton feeding study in dogs

Technical grade disulfoton (DiSyston) was fed to Beagle dogs (four animals per sex and treatment level) at nominal concentrations of 0, 0.5, 4 and 12 ppm for 1 year. The purpose of this study was to characterize the potential general and neurovisual toxicity according to routine Environmental Protection Agency (EPA) guideline requirements, and by use of ancillary ocular and neurologic tests established in this Laboratory. Ophthalmological tests included: ocular tissue cholinesterase and histopathology, electroretinography (ERG), tracking, refractivity, intraocular pressure and pachymetry (corneal thickness) measurements. Neurological examinations included; peripheral and cranial reflex tests, task performance tests, gait and behavioral observations, and rectal temperature measurements. Plasma, erythrocyte and corneal cholinesterase were significantly depressed at 4 and 12 ppm in both sexes. Brain cholinesterase was depressed at 4 and 12 ppm in females. Retinal cholinesterase was depressed at 4 ppm in females and at 12 ppm in males. Ciliary body cholinesterase was depressed at 12 ppm in both sexes. Despite these cholinergic effects, there were no ophthalmologic findings in measurements of ERG, tracking, refractivity, intraocular pressure or pachymetry. There were no clinical neurology findings related to compound administration. We conclude that 0.5 ppm was a no-observable effect level (NOEL), and effects were limited to cholinesterase changes that had no detectable physiologic impact. This study demonstrates that special mechanistic investigations incorporated within guideline studies, enhances scientific integrity and can minimize the need for dedicated organ system studies.

Neural retinal contribution to the slow negative potential of the canine electroretinogram

To establish the normal waveform of the electrical responses from canine eyes, electroretinograms and the light peak were recorded in hybrid and beagle dogs under general anesthesia and artificial ventilation. The neural retinal and retinal pigment epithelial components were pharmacologically isolated by intravitreal glutamate injection and systemic sodium iodate administration, respectively. The a- and b-waves elicited by either flash or rectangular stimuli, the oscillatory potentials elicited by flash stimuli and the light peak elicited either by a single maintained illumination or by repetitive stimuli were almost identical with those of other vertebrates thus far studied. In contrast, in response to rectangular (several-second duration) stimuli, the c-wave was usually absent and was replaced by a slow cornea-negative potential that had a time course similar to that of the c-wave in other species. This slow negative potential was elicited at such low stimulus intensities that the a-wave was absent, was deepened by an intravenous administration of sodium iodate, was not affected by an intravitreal injection of sodium glutamate and was shallowed during the light peak. These results suggest that the slow negative potential that replaces the c-wave consists of large slow PIII, small retinal pigment epithelial c-wave and negligible contribution from the late photoreceptor potential and the dc component of the PII.

The effects of physostigmine on the electroretinogram in the beagle dog

The purpose of the study was to correlate electroretinogram (ERG) parameters with increasing levels of plasma, erythrocyte and ocular tissue cholinesterase inhibition using the beagle dog as a model for human neurovisual toxicity. The anticholinesterase compound physostigmine was administered at various steady-state intravenous infusion rates based on pharmacokinetic estimates of plasma and red blood cell cholinesterase inhibition. The most sensitive parameter was the b-wave amplitude of the rod response, which was significantly depressed compared to pretreatment at all levels of acute cholinesterase depression. The overall maximal ERG response demonstrated a trend of declining a- and b-wave amplitudes, which corresponded with the increased levels of cholinesterase depression, but these differences were not significant. The depression of the electroretinogram rod and cone amplitudes appeared to parallel plasma cholinesterase inhibition more closely than erythrocyte cholinesterase activity. Ocular tissue cholinesterase activity was significantly depressed in the retina (70%), cornea (60%) and dorsal rectus extraocular muscle (46%). Electroretinography may be a useful physiological tool for evaluating the ocular toxicity of certain chemicals or pharmaceuticals associated with cholinesterase biomarker activity.