Covariation of the simultaneously recorded c-wave and standing potential of the human eye

Electrophysiology in pigment epithelial changes

The c-wave of the ERG and the slow SP variations reflect mainly the activity of the pigment epithelium. However, both potentials are dependent upon the photoreceptors and/or the inner retina as well. In pigment epithelial abnormalities the c-wave is reduced or abolished, and the slow SP variations, d.c. recorded directly or investigated with the EOG, reduced or abolished as well.

Studies on acute and late stages of experimental central retinal artery occlusion in the Cynomolgus monkey. II. Influence on the cyclic changes in the amplitude of the c-wave of the ERG and in the standing potential of the eye

The slow (2-3/h) oscillations of the c-wave amplitude and of the standing potential of the eye (SP) were studied in the Cynomolgus monkey during the acute and late stages after experimentally induced (laser photocoagulation) occlusion of the central retinal artery (OCRA). Whereas the healthy control eyes showed large cyclic variations in both the c-wave amplitude and the SP, no oscillations in the c-wave amplitude were observed in the OCRA eyes at any stage, and the SP oscillations were barely detectable in these eyes. OCRA causes morphological damage to the inner retina but not to the pigment epithelium-photoreceptor complex, which generates the positive component of the c-wave, and where the SP is also believed to originate. The findings of the present study strongly indicate that the SP and c-wave oscillations are related, and that they are either dependent on an intact inner retina or that the pigment epithelium-photoreceptor complex is functionally affected by OCRA. If not taken into consideration, the marked difference in oscillations between the OCRA eye and the healthy eye may be a major source of error when comparing c-waves from the two eyes, and it seems that some of the conflicting results reported by others can be explained in this way.

From basic to clinical research: a journey with the retina, the retinal pigment epithelium, the cornea, age-related macular degeneration and hereditary degenerations, as seen in the rear view mirror

PURPOSE

This Acta Ophthalmologica Award and Gold Medal Honorary Lecture (the Lundsgaard Gold Medal Honorary Lecture) reviews some of the work I have carried out with my mentors and many of my wonderful collaborators and research students over more than 40 years, also including related work by other groups. It concentrates on the basic electrophysiology and ultrastructure of the retina and the retinal pigment epithelium (RPE), as well as covering basic and clinical aspects of the cornea, contact lenses, age-related macular degeneration (AMD) and hereditary diseases.

METHODS

The review describes research performed using light and electron microscopy, basic and clinical electrophysiology, genetics and biochemistry in animal experiments and in research on patients. It also outlines clinically used techniques, such as laser and photodynamic treatment and scanning laser ophthalmoscopy.

RESULTS

The paper reports on the following subjects: the mechanisms behind some of the electrical potentials originating in the retina and the RPE and the use of these potentials in hereditary diseases; corneal receptors for lectins and presumably for bacteria; the turnover of the photoreceptor outer segment and the formation of lipofuscin, including the relation of these processes to AMD; certain treatments for AMD, and hereditary degenerations in animal models, such as the RPE65 gene mutation in Briard dogs, which makes them a model of Leber's congenital amaurosis. The dogs are now treated successfully with gene therapy in the USA, and a clinical trial is in preparation.

CONCLUSIONS

During the last 40 years we have had the good fortune to experience a dramatic growth in knowledge and understanding within ophthalmic science of basic mechanisms. Huge progress has been made in diagnostics and clinical ophthalmological treatments, much to the benefit of our patients. Even a small contribution made by my group to these developments has been well worth the effort, particularly as scientific work is not just deeply satisfying: it is also fun!

Objective functional assessment of age‐related maculopathy: a special application for the multifocal electroretinogram

This paper gives a brief review of methods that assess objectively function in age-related maculopathy (ARM) with emphasis on a newer method, the multifocal electroretinogram (mfERG). In contrast to other electrophysiological tests, such as the full-field and focal electroretinogram (ERG) or the electro-oculogram (EOG), which measure summed responses from various cells from larger areas of the retina, the multifocal electroretinogram maps function locally with a resolution as small as four degrees within the central 30 degrees. By using different paradigms it can measure local cone- and rod-mediated functional impairment at early and late stages of ARM. This improved mapping and higher resolution of the posterior pole compared to other objective methods might lead to earlier detection of ARM. Its usefulness has been demonstrated in documenting the effects of treatment after established laser treatments, such as photodynamic therapy (PDT) and in documenting function after retinal pigment epithelial transplantation, a possible future treatment in late neovascular ARM.

Concentration-dependent effects of dopamine on the direct current electroretinogram of pigmented rabbits during prolonged intermittent recording

The direct-current (DC) electroretinogram (ERG) was studied in 24 pigmented rabbits. Four experiments were performed, each including six animals. One eye was injected intravitreally with 0.1 ml dopamine (DA) with an estimated concentration in the vitreous body of 0.0025 mM, 0.025 mM, 0.25 mM and 2.5 mM, respectively. The contralateral eye was injected with the same amount of saline. Following the injection the animals were dark adapted for 30 min and then exposed to repeated light stimuli of low intensity for almost 3 hours (series I: 1 stimulus per 3 min, 10 s duration, light intensity 6.8 x 10(2) lux). After another 30 min period of dark adaptation repeated light stimuli of high intensity were presented to the eyes (series II: 1 stimulus per 70 s, 10 s duration, light intensity 6.8 x 10(4) lux) for 33 min. In the control eyes, a slow increase with time of the a-, b- and c-wave amplitudes was observed during series I. During series II, the a- b- and c-wave amplitudes were markedly reduced between the first and the second light stimulus, but subsequently grew to a peak. The behavior of the ERG in the eyes injected with dopamine was not different from that observed in the control eyes at the lowest concentration of the drug. At higher concentrations the b- and c-wave amplitudes were reduced compared with the control eyes, and did not show the slow increase with time observed in these eyes during series I. Peak responses observed during series II in the control eyes was increasingly suppressed in the eyes treated with dopamine. Results of ERG recordings suggest that dopamine influences retinal adaptation in rabbits in a dose dependent manner.

Long-term behavior and intra-individual stability of the direct current electroretinogram and of the standing potential in the albino rabbit eye

The direct current electroretinogram (ERG) and the standing potential (SP) were studied in seven albino rabbits under general anesthesia. Identical experiments were performed on 2 consecutive days. After 30 min of dark adaptation, repeated light stimuli of maximal intensity of the system were presented to the eyes. The interstimulus interval was 70 s, and stimulus duration 10 s. Each experiment lasted for almost 3 h. In the first experiment, the b- and c-wave amplitudes measured in response to the second light stimulus were markedly reduced compared to those recorded in response to the first stimulus. Both amplitudes then recovered. The b-wave attained a peak about 20 min after the start of light stimulation. The peak was followed by a trough about 20 min later, and the amplitude then slowly increased. Following the minimum recorded during the second light stimulus, the c-wave amplitude reached a peak about 14 min after the start of stimulation. A trough in the amplitude occurred 20 min later. The amplitude then slowly increased to the end value, which was higher than the initial level. The a-wave behaved similarly to the b-wave, but the changes in most cases did not attain statistical significance. A minimum in the SP occurring at the second light stimulus was followed by a peak about 13 min after the start of light stimulation, and then by a trough about 17 min later. In the second experiment, performed one day after the first, the development of the a-, b-, and c-wave amplitudes and of the SP was similar to that observed during the first experiment, and no statistically significant differences between the two experiments were found. The reactions of the ERG and the SP were thus very stable between identical experiments performed on two consecutive days.

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.

Oscillatory potentials in the retina: what do they reveal

This chapter is an overview of current knowledge on the oscillatory potentials (OPs) of the retina. The first section describes the characteristics of the OPs. The basic, adaptational, pharmacological and developmental characteristics of the OPs are different from the a- and b-waves, the major components of the electroretinogram (ERG). The OPs are most easily recorded in mesopic adaptational conditions and reflect rapid changes of adaptation. They represent photopic and scotopic processes, probably an interaction between cone and rod activity in the retina. The OPs are sensitive to disruption of inhibitory (dopamine, GABA-, and glycine-mediated) neuronal pathways and are not selectively affected by excitatory amino acids. The earlier OPs are associated with the on-components and the late OPs with the off-components in response to a brief stimulus of light. The postnatal appearance of the first oscillatory activity is preceded by the a- and b-waves. The earlier OPs appear postnatally prior to, and mature differently from, the later ones. The second section deals with present views on the origin of the OPs. These views are developed from experimental studies with the vertebrate retina including the primate retina and clinical studies. Findings favor the conclusion that the OPs reflect neuronal synaptic activity in inhibitory feedback pathways initiated by the amacrines in the inner retina. The bipolar (or the interplexiform) cells are the probable generators of the OPs. Dopaminergic neurons, probably amacrines (or interplexiform cells), are involved in the generation of the OPs. The earlier OPs are generated in neurons related to the on-pathway of the retina and the later ones to the off-channel system. Peptidergic neurons may be indirectly involved as modulators. The individual OPs seem to represent the activation of several retinal generators. The earlier OPs are more dependent on an intact rod function and the later ones on an intact cone system. Thus, the OPs are good indicators of neuronal adaptive mechanisms in the retina and are probably the only post-synaptic neuronal components that can be recorded in the ERG except when structured stimuli are used. The last section describes the usefulness of the oscillatory response as an instrument to study the postnatal development of neuronal adaptation of the retina. In this section clinical examples of of the sensitivity of the OPs for revealing early disturbance in neuronal function in different retinal diseases such as pediatric, vascular and degenerative retinopathies are also given.

Electrophysiology in some animal and human hereditary diseases involving the retinal pigment epithelium

The present paper surveys slow electrophysiological responses recorded by a d.c. technique in some hereditary eye diseases involving the retinal pigment epithelium (RPE) in animals (English setter dogs and Polish Owczarec Nizinny (PON) dogs with ceroid lipofuscinosis and Briard dogs with a slowly progressive rod-cone dystrophy associated with RPE inclusions) and in humans (Best's disease). The electroretinogram c-wave was typically either decreased in amplitude, lacking or replaced by a negative wave. These c-wave changes could be seen at fairly early stages of disease, when the a- and b-waves of the electroretinogram were still within normal limits.

Changes in the Direct-Current Electroretinogram of Albino Rabbits during Prolonged Intermittent Recording

The study investigated the pattern of the direct-current electroretinogram (ERG) of albino rabbits during prolonged intermittent recording, and whether different initial dark adaptation periods or starting the experiments at different times influenced the results. We examined 27 rabbits under general anesthesia, in three experiments, each comprising nine animals. Five series (experiments 1 and 3) or four series (experiment 2) of ten repeated light stimuli were presented to the eyes with 30 minutes of dark adaptation before experiments 1 and 3, and 90 minutes before experiment 2. The dark adaptation of experiments 1 and 2 began at 10.30 a.m. and that of experiment 3 at 3.30 p.m. The interval between consecutive series of light stimuli was 33 minutes. Stimulus intensity was 680 lux, stimulus duration 10 seconds, and the interval between stimuli 3 minutes. The mean b-wave amplitude of the ten recordings in each series of stimuli increased up to the series beginning 3.5 hours (experiments 1 and 2) or 2.5 hours (experiment 3) after the start of dark adaptation. The mean c-wave amplitude increased throughout experiments 1 and 3, and up to the series beginning 3.5 hours after the start of dark adaptation in experiment 2. The mean a-wave amplitude was more stable. It seemed irrelevant for the long-term development of the mean ERG amplitudes whether the eye was dark adapted (experiment 2) or exposed to repeated light stimuli (experiments 1 and 3) during the first part of the experiment, and whether the experiments started in the morning or in the afternoon.

A cyclic adenosine monophosphate agonist elevates the b- and c-waves of the rabbit direct-current electroretinogram

The effects of the stable cyclic adenosine monophosphate analogue adenosine 3',5'-cyclic monophosphorothioate Sp-isomer (Sp-cAMPS) on the direct-current electroretinogram and the standing potential of the eye were studied. Corneal recordings were obtained from unilaterally vitrectomized albino rabbit eyes during alternating intravitreal perfusions with Sp-cAMPS and a control solution (Pharmacia eye irrigating solution). The contralateral eye was used as a control. To evaluate further the effects on the c-wave, in vivo intraretinal microelectrode measurements were made during simultaneous intravitreal perfusion of Sp-cAMPS and irrigating solution, respectively. Sp-cAMPS in concentrations of 1, 10 and 100 microM was tested by corneal direct-current electroretinography. There was no significant effect on the a-wave amplitude. The b-wave amplitude was reversibly elevated at an Sp-cAMPS concentration of 100 microM was tested by corneal direct-current electroretinography. There was no significant effect on the a-wave amplitude. The b-wave amplitude was reversibly elevated at an Sp-cAMPS concentration of 100 microM (p < 0.01, n = 7). The c-wave amplitude was reversibly elevated at an Sp-cAMPS concentration of 100 microM (p < 0.01, n = 7). The c-wave amplitude was reversibly elevated at a concentration of 10 microM (p < 0.001, n = 8), and this effect was more pronounced at 100 microM (p < 0.001, n = 7). The SP increased reversibly at a concentration of 100 microM (p < 0.001, n = 7). Microelectrode recordings were performed with Sp-cAMPS at a concentration of 100 microM. The recordings showed significant increases in both the transepithelial potential (p < 0.01, n = 3) and the slow PIII (p < 0.01, n = 3). The effects of Sp-cAMPS on the b-wave as well as the two components of the c-wave suggest influences on both the inner retina and the retinal pigment epithelium of the rabbit eye.

Effects of prolonged uniocular dark adaptation on the direct-current electroretinogram of pigmented and albino rabbits

The direct-current electroretinogram of seven pigmented and seven albino rabbits was recorded from both eyes for almost 4 h in response to repeated identical light stimuli. Stimulus duration was 10 s, light intensity was 6.8 x 10(2) lux, and the interval between the beginning of succeeding light stimuli was 3 min. The dark-adaptation period preceding light stimulation was 30 min for one of the eyes ('unoccluded eye') and 150 min for the contralateral eye ('occluded eye'), which was patched during the first part (117 min) of the experiment. In pigmented animals, the b- and c-wave amplitudes of the unoccluded eye slowly increased during the first part of the experiment but not significantly during the second. The a-wave amplitude was not significantly changed. After removal of the cover, the a- and b-wave amplitudes of the occluded eye immediately attained but not exceed the level of those in the unoccluded eye, irrespective of the light adaptation induced by the stimulus flashes previously presented to the unoccluded eye. (Control experiments on six pigmented rabbits confirmed that stimuli identical to those used in the main part of the study caused a light adaptation, since a decrease in a- and b-wave amplitudes occurred after the first light stimulus following an initial dark-adaptation period of 2 h for both eyes). In albino rabbits, electroretinogram responses were clearly discernible in the occluded eye also during the first part of the experiment, probably because of transillumination of the head. In other respects, the results were essentially similar to those of pigmented animals. The observation that occluded eyes did not dark adapt better, as judged by the electroretinogram responses, than contralateral eyes given repeated light adaptive stimuli may indicate the presence of a mechanism for transfer of adaptation information between the eyes.

Adrenergic effects on the corneal and intraretinal direct-current electroretinogram and on the standing potential of albino rabbit eyes

This study was undertaken to investigate further the responsiveness of the albino rabbit retinal pigment epithelium and the inner retina to adrenergic agents as reflected in changes of the direct-current electroretinogram and of the standing potential of the eye. After unilateral vitrectomy on albino rabbits, a continuous intraocular perfusion with a reference solution was established. The reference solution was then alternated with the test solution. The direct-current electroretinogram and the standing potential were recorded from both eyes with a scleral contact lens and a reference electrode connected to matched calomel half-cells. An in vivo experimental technique that allows intraocular perfusion of a test substance and simultaneous intraretinal microelectrode measurements was also used. The alpha-adrenergic agonist phenylephrine (0.04 microM, n = 8) produced a reversible increase in c-wave amplitude (48%, p < 0.001) and also a small increase in b-wave amplitude (12%, p < 0.002). There was no significant influence on the a-wave amplitude. The standing potential was elevated at 1694 +/- 362 microV (mean +/- SEM) (p < 0.002). The alpha 2-adrenergic agonist clonidine caused similar effects on the electroretinogram, although at a higher concentration (40 microM, n = 5), with an elevation of the c-wave (25%, p < 0.001) and a small b-wave increase (12% p < 0.002). No significant influence on the a-wave or on the standing potential was found. Intraretinal direct-current electroretinogram-recordings during intraocular perfusion with phenylephrine showed an increase in transepithelial potential (p < 0.004; n = 6), accompanied by a reduction of the slow PIII (p < 0.0035; n = 6). The c-wave increase resulting from alpha-adrenergic stimulation seems to be generated partly across the retinal pigment epithelium, with an increase in transepithelial potential, combined with a reduction of the slow PIII. The elevation of the b-wave amplitude, together with the influence on the slow PIII, suggests alpha-adrenergic effects also on the inner retina. The experimental technique used in this study with intraocular perfusion after vitrectomy and simultaneous intraretinal direct-current recordings seems to be a practicable method for studies of the influence of pharmacologic agents on the retina and the retinal pigment epithelium.

Electro-oculogram changes in patients with ocular hypertension and primary open-angle glaucoma

Recent evidence suggests that retinal hypoxia and ischemia affect the standing potential of the eye and the activity of the photoreceptors. To test whether chronically elevated intraocular pressure would produce similar effects, we measured electro-oculograms in two groups of patients: ocular hypertensive patients and patients with primary open-angle glaucoma. There were significant differences among the average electro-oculogram ratios of these groups compared to age-similar controls. The control observers had an average light-peak/dark-trough ratio of 2.86, the ocular hypertensive patients had an average ratio of 2.44, and the patients with primary open-angle glaucoma had an average ratio of 2.07. This indicates that long-term elevations in intraocular pressure can decrease the light peak of the electro-oculogram, even in patients with no other evidence of glaucomatous damage. This deficit may have its origins in the sensitivity of the outer retina to choroidal ischemia.

The c-wave of the direct-current electroretinogram and the standing potential of the albino rabbit eye in response to repeated series of light stimuli with different interstimulus intervals

The direct-current electroretinogram and the standing potential of the eye of seven albino rabbits were recorded in response to repeated light stimuli, which were presented in four consecutive series. The intervals between the beginning of succeeding stimuli were 8 minutes, 4 minutes, 2 minutes and 70 seconds (series 1, 2, 3 and 4, respectively). Stimulus duration (10 seconds) and light intensity (6.8 x 10(4) lux) were constant during the experiments. The series lasted for 36-40 minutes, and each was preceded by 30 minutes of dark adaptation. During series 1, the end amplitudes of the a-, b- and c-waves were not significantly changed compared with the initial levels. During series 2, 3 and 4, the a-, b- and c-wave amplitudes were markedly reduced immediately after the first electroretinogram recording. The a- and b-waves then recovered to a limited extent, but the c-wave was more fully restored. A slight peak in the c-wave amplitude could be discerned 16-20 minutes after the start of recording. A decrease in the standing potential was seen 50-54 seconds after the start of light stimulation during all four series, and a peak occurred 12-16 minutes after the start of recording. The similarity in behavior between the c-wave and the standing potential suggests the operation of a pigment epithelial mechanism behind the more complete recovery of the c-wave amplitude. When electroretinogram amplitudes and standing potential levels are discussed, and when one experiment is compared with another one, it is important that adaptational and stimulus conditions, as well as time course, are well controlled and clearly specified.

Alpha-1-adrenergic modulation of K and Cl transport in bovine retinal pigment epithelium

Intracellular microelectrode techniques were used to characterize the electrical responses of the bovine retinal pigment epithelium (RPE)-choroid to epinephrine (EP) and several other catecholamines that are putative paracrine signals between the neural retina and the RPE. Nanomolar amounts of EP or norepinephrine (NEP), added to the apical bath, caused a series of conductance and voltage changes, first at the basolateral or choroid-facing membrane and then at the apical or retina-facing membrane. The relative potency of several adrenergic agonists and antagonists indicates that EP modulation of RPE transport begins with the activation of apical alpha-1-adrenergic receptors. The membrane-permeable calcium (Ca2+) buffer, amyl-BAPTA (1,2-bis(o-aminophenoxy)-ethane-N,N,N',N' tetraacetic acid) inhibited the EP-induced voltage and conductance changes by approximately 50-80%, implicating [Ca2+]i as a second messenger. This conclusion is supported by experiments using the Ca2+ ionophore A23187, which mimics the effects of EP. The basolateral membrane voltage response to EP was blocked by lowering cell Cl, by the presence of DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid) in the basal bath, and by current clamping VB to the Cl equilibrium potential. In the latter experiments the EP-induced conductance changes were unaltered, indicating that EP increases basolateral membrane Cl conductance independent of voltage. The EP-induced change in basolateral Cl conductance was followed by a secondary decrease in apical membrane K conductance (approximately 50%) as measured by delta [K]o-induced diffusion potentials. Decreasing apical K from 5 to 2 mM in the presence of EP mimicked the effect of light on RPE apical and basolateral membrane voltage. These results indicate that EP may be an important paracrine signal that provides exquisite control of RPE physiology.

The c-wave of the direct-current-recorded electroretinogram and the standing potential of the albino rabbit eye in response to repeated series of light stimuli of different intensities

In 10 experiments on five albino rabbits, the direct-current electroretinogram and the standing potential of the eye were recorded in response to repeated light stimuli (duration, 10 s; interval, 70 s), presented in four series, each consisting of 25 light flashes. Light intensities were, in order of presentation to the eyes, 3, 2, 1 and 0 log rel units (series I, II, III and IV, respectively) below the maximum output of the system. Thirty minutes of dark adaptation preceded each series. At the end of series I, the mean amplitudes of the b- and c-waves were higher and that of the a-wave relatively unchanged compared with the corresponding initial amplitudes. During series II-IV, there was a marked decrease in mean a- and b-wave amplitudes between the first and the following electroretinogram responses, and at the end of the three series, the amplitudes were still significantly reduced compared with the corresponding initial values. The mean c-wave amplitude was also markedly decreased immediately after the first electroretinogram recording, but it later recovered to a large extent. A peak in the c-wave amplitude was discerned about 14-18 minutes after the start of the recordings. A standing potential minimum during the second light stimulus was followed by a peak after about 10-13 minutes. The partially parallel behavior of the c-wave and the standing potential suggests the operation of a pigment epithelial mechanism behind the recovery of the c-wave amplitude. The final amplitudes of the b- and c-waves, and to a large extent also of the a-wave, were about the same irrespective of stimulus intensity. The adaptational processes in the rabbit appear to be more complicated than was previously thought. When electroretinogram amplitudes and standing potential levels are discussed and when one experiment is compared with another one, it is important that adaptational and stimulus conditions, as well as time course, are well controlled and clearly specified.

Light and dark induced variations of the c-wave voltage of the chicken eye after treatment with sodium aspartate

Light and dark-induced variations of the ERG c-wave voltage were recorded in control chickens and after intravitreal injection of Na aspartate, a treatment whose main effect is to functionally disconnect the pigment epithelium-photoreceptor complex from second order neurons. After aspartate, the fast light rise which characterizes this preparation is no longer observed; it is substituted for by a potential variation of much slower time course and of lower magnitude. The data totally confirm previous findings obtained through an indirect EOG technique and suggest the participation of inner retinal layers in the generation of the light peak in the chicken eye.

Influence of isobutylmethylxanthine on the direct current electroretinogram of albino rabbit eyes

The direct current electroretinogram and the standing potential of the eye were recorded from both eyes of unilaterally vitrectomized albino rabbits. The effect of intraocular irrigation with the nonselective phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) was compared with that of a balanced salt control solution. IBMX (0.5 mM) produced a reversible increase in the amplitudes of the a-wave (19%, p less than 0.02), b-wave (21%, p less than 0.001) and c-wave (12%, p less than 0.01) of the electroretinogram. A small elevation of the standing potential was found (0.4 +/- 0.2 mV), but this increase was not statistically significant (p greater than 0.05). The results indicate a primary and principal influence of IBMX on the photoreceptors.

Effects of intravitreal perfusion with dopamine in different concentrations on the DC electroretinogram and the standing potential of the albino rabbit eye

The direct current electroretinogram and the standing potential were recorded from both eyes of 23 albino rabbits during intraocular perfusion of one of the eyes, which was vitrectomized, with a physiologic reference solution (PHS). PHS was then replaced by a test solution containing dopamine dissolved in PHS. The fluids were subsequently alternated (PHS-dopamine-PHS). During irrigation with 0.25-0.5 mM dopamine (11 rabbits) the c-wave amplitude was 140% higher (p less than 0.001) and during irrigation with 25 mM dopamine (6 rabbits) 85% lower (p less than 0.01) than it was during the corresponding initial perfusion with PHS. The simultaneously recorded b-wave amplitude was reduced (0.25-0.5 mM: -22%, p less than 0.001; 25 mM: -69%, p less than 0.001) and the SP level increased (0.25-0.5 mM: +2375 microV, p less than 0.01; 25 mM: +2530 microV, p less than 0.05) compared with the values obtained during the corresponding preceding irrigation with PHS. Thus the changes in the b- and c-wave amplitudes during perfusion with dopamine were dependent on the concentration of the drug. In the contralateral control eye (23 rabbits) the c-wave amplitude was 21% higher (p less than 0.001), the b-wave amplitude 14% higher (p less than 0.001) and the standing potential 1007 microV higher (p less than 0.001) during intravitreal perfusion with dopamine in the other eye than during the preceding irrigation with PHS in that eye, possibly as a result of increasing dark adaptation.

Corneal D.C. recordings of slow ocular potential changes such as the ERG c-wave and the light peak in clinical work. Equipment and examples of results

A set-up for D.C. recordings of slow ocular potentials such as the c-wave of the electroretinogram (ERG) as well as the fast oscillation (FO), the light peak (LP) and the dark trough (DT) in both clinical and experimental work is described. It includes matched calomel half-cells connected by saline-agar bridges to a corneal contact lens on the eye and a reference chamber on the forehead, a low-drift differential-input D.C. amplifier, an A/D converter, a computer, a thermoprinter, a flexible disc memory, a plotter, and a device for light stimulation controlled by the computer. Examples of the usefulness of the set-up in clinical work are shown in the form of D.C. c-wave ERGs of normal subjects as well as of patients with vitelliform macular degeneration, choriocapillaris atrophy, and retinitis pigmentosa. The direct corneal recording of the FO and LP is demonstrated as well. The different origins of the standing potential (SP) of the eye, the ERG c-wave, the FO and the LP are reviewed briefly.

Effects of intraocular irrigation with melatonin on the c-wave of the direct current electroretinogram and on the standing potential of the eye in albino rabbits

The direct current electroretinogram (ERG) and the standing potential (SP) were recorded from both eyes of 14 albino rabbits during intraocular perfusion of one of the eyes, which was vitrectomized, with a recently developed eye irrigation solution (PHS) produced by Pharmacia Ophthalmics. PHS was then replaced by a test solution containing melatonin dissolved in PHS (0.002 microM-200 microM). The fluids were subsequently alternated (PHS - melatonin - PHS). During uniocular irrigation with melatonin the mean c-wave amplitude and SP level of the intact control eye were increased (c-wave +24%, p less than 0.01; SP +0.65 mV, p less than 0.05) compared with the values during the initial perfusion with PHS. In contrast, the c-wave amplitude of the irrigated eye was markedly decreased in many rabbits during perfusion with melatonin compared with the initial PHS, but the mean reduction was small and not statistically significant. The mean SP level was reduced (-1.54 mV, p less than 0.001). This difference between the eyes probably depends on the route by which melatonin reaches the retinal pigment epithelium and thus whether it primarily affects the apical (as in the irrigated eye) or the basal (as in the control eye) pigment epithelial membrane. A peak in the b-wave amplitude was observed in both eyes during uniocular irrigation with melatonin when compared with the amplitude measured during the initial perfusion with PHS (irrigated eye: +27%, p less than 0.001; control eye +18%, p less than 0.002).

Mechanisms of azide induced increases in the c-wave and standing potential of the intact cat eye

The c-wave of the ERG and the standing potential of the eye both undergo increases in amplitude following intravenous infusions of sodium azide (NaN3), as first shown by Noell [Am. J. Physiol. 170, 217-238 (1952); U.S.A.F. School of Aviation Medicine, Project No. 21-1201-0004 (1953)]. We have studied the mechanism of these changes in the intact cat eye. Intraretinal and intracellular retinal pigment epithelial (RPE) cell recordings show that most of the change occurs at the RPE, but that there is a small direct effect on the neural retina. The increase of standing potential is caused by a depolarization of the basal membrane of the RPE, and the increase in c-wave amplitude results from a decrease in basal membrane resistance that accompanies the depolarization. This relation between basal membrane potential and resistance is similar to that observed during hypoxia and during the light peak of the d.c. ERG.

The normal c-wave amplitude in rabbits

We examined some of the problems arising when c-waves are employed in pharmacological experiments with pigmented rabbits. The cyclic variations of the c-wave are smaller in rabbit than in other species. Under our experimental conditions the c-wave amplitudes in some rabbits reached an equilibrium under intermittent stimuli, while in others they increased with time and did not reach a constant level within 3 h. The c-wave amplitude varied with the standing potential during dark adaptation. However interindividual variations made it difficult to standardize the c-waves by this change. Since the c-wave amplitudes of the right and left eyes were similar, the contralateral eye should be used as a control when drugs are administered topically. Interindividual variations of the c-wave amplitude were smaller under dim light. This method is useful in chronic pharmacological experiments when drugs are administered systemically.

Standing potential and c-wave during changes in PO2 and flow in the perfused cat eye

Changing flow rate (F) in arterially perfused cat eyes concomitantly alters the supply of O2 and greatly affects electrical signals: increasing F increases the standing potential (SP) and decreasing F induces the opposite. Corresponding effects on c-wave are more variable. To separate changes in supply of O2 from changes in F we performed two types of experiments: (1) changing supply of O2 while keeping F constant; this was done by switching to a perfusate of different PO2: and (2) changing F while keeping the supply of O2 constant; this was done by switching to hypoxic or hyperoxic perfusates and adjusting F to keep the b-wave amplitude constant. Results (1): Decrease in PO2 left the SP essentially unaffected, but increased the c-wave. Increase in PO2 tended to increase the SP and to decrease the c-wave. Results (2): Isolated decrease in F did not affect the SP consistently and left the c-wave unchanged. Isolated increase in F often increased the SP, but also left the c-wave unchanged. Covariation between SP and amplitude to the c-wave was absent during hyperoxia.

The DC-ERG as a highly sensitive measure of effects of prostaglandins

A few microliters of a prostaglandin preparation (PGE2 or PGF2 alpha) were injected via the ora serrata into the posterior vitreous of one of the eyes in albino rabbits. The fellow eye received an equal volume of saline intravitreally and served as control. The DC electroretinogram (ERG) and the standing potential of the eye (SP) were recorded directly with corneal contact lenses, very stable calomel electrodes, and under very constant anesthesia. The b- and c-wave amplitudes increased in response to 0.1 and 1.0 microgram of PGE2 and PGF2 alpha, respectively. At medium doses there was an increase in the b- and c-wave amplitudes followed by a long-lasting reduction. At very high, nonphysiological doses of PGE2, b- and c-wave amplitudes decreased as compared with the control eye. Prostaglandins modify inflammatory reactions, influence ion transport across membranes, modulate synaptic transmission, and regulate blood flow to various organs. Effects of extremely low doses on the retina and pigment epithelium might indicate a transmitter-like nature of prostaglandins. The present experimental model might be of use in studies of inflammatory eye disease, prostaglandin inhibitors, and characteristics of the pigment epithelial membranes.

Combined effects of DL-alpha-aminoadipic acid with sodium iodate, ethyl alcohol, or light stimulation on the ERG c-wave and on the standing potential of albino rabbit eyes

Albino rabbits were treated with intravitreal injections of DL-alpha-aminoadipic acid (alpha-AAA) into one eye (0.1 ml of a 0.15 M solution) and 0.1 ml of saline into the contralateral eye. Thirteen to fourteen hours later the DC electroretinogram (ERG) and/or the standing potential (SP) were recorded. (1) In eight of nine animals the c-wave amplitude of alpha-AAA injected eyes was increased compared with that of control eyes. Following intravenous injection of Sodium Iodate (40 mg/kg in 2% solution) the c-waves of both eyes were rapidly replaced by negative potentials. In 8 of 9 animals the amplitude reduction was more marked in alpha-AAA-treated eyes than in control eyes, but the final amplitude was higher in the former than in the latter. The SP was reduced with difference in curve form but not significantly in amplitude between the eyes. (2) In nine other rabbits iv.-injected ethyl alcohol (0.4 g/kg in 20% solution) provoked a transient increase of SP level and c-wave amplitude in control eyes and smaller but similar changes in alpha-AAA injected eyes. (3) In another five animals the SP was recorded following a step from darkness to continuous light stimulation. The light peak was less pronounced in alpha-AAA treated eyes than in control eyes.

Hereditary retinal degeneration in the Abyssinian cat: correlation of ophthalmoscopic and electroretinographic findings

Ophthalmoscopic and electroretinographic (ERG) findings were correlated in a group of Abyssinian cats affected by a slowly progressive and hereditary retinal degenerative disease. According to ophthalmoscopic findings the disease was divided into stages. At stage 1 and 2 retinal changes were minor; showing a gray discoloration most often in the peripheral and midperipheral tapetal fundus. At stage 3 discoloration was generalized and there was marked vascular attenuation. A generalized retinal atrophy was found at stage 4. ERG recordings showed an abnormally depressed stimulus response curve for the b-wave at stage 1 of disease when 30-Hz cone flicker responses were indistinguishable from normal. With progression of disease there was a successive decrease of a- and b-wave amplitudes before there was a significant reduction also of the c-wave amplitude (first seen at stage 3). The ERG was nonrecordable at stage 4. These findings suggest that the photoreceptors are affected primarily by the disease, before there is a functional involvement also of the pigment epithelium. The rod system seems to be affected early in the disease as compared with the cone system. A staging of the disease by ophthalmoscopy correlated more to the function of the rods than to that of the cones.

Progressive retinal atrophy in the Abyssinian cat: studies of the DC-recorded electroretinogram and the standing potential of the eye

DC-recorded electroretinography (ERG) and direct recordings of the standing potential (SP) were performed on a group of normal cats and Abyssinian cats affected by a hereditary retinal degenerative disease with similarities to human retinitis pigmentosa. A significant reduction of a- and b-wave amplitudes was found at an early stage of disease at a time when there were no major alterations in the c-wave and SP. At later stages both the c-wave and the SP oscillations were significantly reduced or absent. These findings indicate a primary photo-receptor disorder. Threshold studies for the scotopic b-wave showed a loss of retinal sensitivity early in the disease at a time when 30 Hz flicker responses were normal, which could indicate an earlier involvement of the rods than of the cones. There were no major alterations in the timing of the ERG in the affected animals tested.

Toxic ocular effects of two fibrinolytic drugs. An experimental electroretinographic study on albino rabbits

The toxic effects on rabbit eyes of 2 intravitreally injected fibrinolytic substances at different concentrations were studied with repeated clinical observations and registrations of the DC ERG. The fellow, control eye of each animal was injected with saline. Urokinase (Ukidan, Serono) (13 rabbits) initially produced aqueous flare (64%), iris hyperaemia (36%) vitreous opacities (27%) and small retinal haemorrhages (18%). 2-3 months after the injection cataract (50%), vitreous opacities (25%) and retinal changes (13%) were observed. The highest dose (10 000 Ploug units) caused reduction of the ERG b-wave, as a sign of retinal toxicity. Tissue activator (D-44, Centre d'immunologie et de biologie Pierre Fabre) (10 rabbits) produced marked aqueous flare (initially 100%, after 2 weeks 50%) and pronounced, persistent vitreous opacities (25% after 2-3 months). At the late stage corneal blood vessels (38%) and cataract (38%) were also found, but only in eyes injected with the highest dose (1000 units), which was retinotoxic as judged by the ERG (reduced b- and c-waves).

The relationship between the c-wave and light response of the rabbit eye

The rabbit c-wave and light response (LR) of the standing potential were recorded simultaneously while presenting the eye with light flashes at 15 sec intervals. The LR peaked about 15 min after the onset of flashes, but the c-wave amplitude remained constant after the first min of stimulation. Elimination of pigment epithelial activity with intravenous sodium iodate blocked both the c-wave and LR. Elimination of inner retinal activity with intravitreal aspartic acid blocked neither. These results are consistent with those from other mammals insofar as the pigment epithelium is required for both c-wave and LR, but differ from other species in two respects: (1) the rabbit LR is not sensitive to aspartate; (2) the rabbit c-wave does not vary in amplitude with the LR.

Studies on experimentally induced retinal degeneration. 1. Effect of lipid peroxides on electroretinographic activity in the albino rabbit

Lipid hydroperoxides (LHP) have been synthesized and purified from linoleic, linolenic, arachidonic and docosahexaenoic acids, using soybean lipoxygenase and oxygen. Intravitreal injections into the eyes of mature, albino rabbits produced an early and then progressive decrease in the amplitude of a-, b- and c-waves of the ERG. Depending upon the amount and activity of the LHP preparation, ERG's were markedly decreased in amplitude (greater than 50%) within 4 days following the injection and by 12 days, the activity from peroxide treated eyes was essentially nonrecordable. In preliminary studies, these effects were less pronounced in adult pigmented rabbits of similar age, however, a younger pigmented rabbit was only slightly less susceptible to damage than the albino animals. In other experiments, peroxidized native phospholipids, malonaldehyde, hydrogen peroxide and sodium iodate were also shown to be cytotoxic, but not all were as toxic as the LHP. In contrast, retinol, vitamin A acetate and retinoic acid had no effect upon ERG activity, nor did the parent fatty-acid compounds or the borate buffer in which they were injected. These studies confirm previous reports where indirect production of lipid peroxides caused retinal degeneration. The present report extends these observations to demonstrate that when the retina and RPE are exposed to a sample of purified LHP, retinal function is altered in an irreversible way. We also demonstrate that a metabolic by-product (malonaldehyde) is likewise cytotoxic. However, the mechanisms by which the parent LHP and/or metabolites might act could be quite different. This new animal model should prove useful in evaluating further the ultrastructural changes which are observed during peroxidative damage of the retina in vivo, as well as in evaluating the therapeutic approaches to these problems of retinal degeneration.

Examination of the fast oscillation of the corneoretinal potential under clinical conditions

The "fast oscillation" of the corneoretinal potential can be precisely examined by means of cyclic light stimuli and an automatic measuring device. This new procedure is carried out during the period of adaptation as part of the usual EOG-test. Normal values and some clinical findings are presented. The fast oscillation may provide additional information when compared with EOG and ERG results.

The C-wave of the rabbit electroretinogram during dark-adaptation and the steady-state

C-waves of the rabbit ERF under urethane anesthesia were elicited by a Grass photostimulator during dark adaptation and under steady-state conditions. After strong pre-adaptation with light, the amplitude and time-to-peak of the c-wave increased in parallel with the a- and b-waves, reaching a maximum of 90-130 min. In the dark-adapted steady-state, repetitive stimuli at intervals ranging from 15 sec to 5 min elicited a stable c-wave response except for a small dip in amplitude that occurred 2-6 min after initiating the shortest interval (15 sec) flashes. Although the changes in light and dark used in these experiments should have elicited shifts in the standing potential, no fluctuations of the c-wave amplitude were evident with a time course corresponding to a light response of the standing potential.

Early effects of sodium iodate on the directly recorded standing potential of the eye and on the c-wave of the DC registered electroretinogram in albino rabbits

The early effects of intravenously administered sodium iodate (NaIO3) on the directly recorded standing potential (SP) of the eye and on the c- and b-waves of the DC registered ERG were studied in 8 anaesthetized albino rabbits. In 5 of 6 animals obtaining 40 mg NaIO3/kg bwt. the SP decreased immediately following the injection, and had attained a level 3.5 - 4 mV below the original one after 1 h. The c-wave declined rapidly and 6 min after the injection it was replaced by a large cornea-negative potential. The b-wave was relatively unchanged except in one animal. In 2 rabbits treated with 30 mg NaIO3/kg btw. and in the 6th animal obtaining 40 mg NaIO3/kg bwt. an SP increase instead of a decrease was seen, and the c-wave was more slowly (about 22 min after the injection) replaced by the cornea-negative potential. The b-wave was somewhat increases. These results demonstrate the dose-related and inter-individual variability in the SP reaction to NaIO3 and are in good agreement with the well-known ultrastructural pigment epithelial injury and c-wave changes produced by this substance.

The ERG c-wave in vitelliruptive macular degeneration (VMD)

D.c. ERG registrations and EOG recordings were obtained from six patients with vitelliruptive macular degeneration (VMD). In all cases the EOG was highly pathological, but the alpha- and beta-waves of the ERG were normal. This is typical of VMD, which starts as a generalized defect of the pigment epithelium. Four patients showed no evidence of a c-wave. The other patients demonstrated small c-waves, but only under certain stimulus conditions. Thus, varying stimulus durations, intensities and frequencies are sometimes needed to decide whether or not ERG c-waves can be elicited in different diseases or suspected disorders. The findings are in agreement with the presence of a generalized pigment epithelial defect in VMD, since the major positive component of the c-wave is generated by the pigment epithelium-receptor complex.

New approaches to ophthalmic electrodiagnosis by retinal oscillatory potential, drug-induced responses from retinal pigment epithelium and cone potential

New clinical methods are proposed to assess (1) neuronal activities of the retinal inner layers including amacrine cells by means of the oscillatory potential, (2) photopic function through the rapid decay in the off-response and (3) activities of the retinal pigment epithelium through susceptibility of the standing potential of the eye to osmotic stress and to Diamox. These new methods are able to reveal otherwise undetectable retinal disorders.