Analysis of the response properties and light-integrating characteristics of the c-wave in the rabbit eye

Early Functional and Morphologic Abnormalities in the Diabetic Nyxnob Mouse Retina

Purpose

The electroretinogram c-wave is generated by the summation of the positive polarity hyperpolarization of the apical RPE membrane and a negative polarity slow PIII response of Müller glia cells. Therefore, the c-wave reduction noted in prior studies of mouse models of diabetes could reflect a reduction in the RPE component or an increase in slow PIII. The present study used a genetic approach to distinguish between these two alternatives.

Methods

Nyx^nob mice lack the ERG b-wave, revealing the early phase of slow PIII. To visualize changes in slow PIII due to diabetes, Nyx^nob mice were given streptozotocin (STZ) injections to induce diabetes or received vehicle as a control. After 1, 2, and 4 weeks of sustained hyperglycemia (>250 mg/dL), standard strobe flash ERG and dc-ERG testing were conducted. Histological analysis of the retina was performed.

Results

A reduced c-wave was noted at the 1 week time point, and persisted at later time points. In comparison, slow PIII amplitudes were unaffected after 1 week of hyperglycemia, but were significantly reduced in STZ mice at the 2-week time point. The decrease in amplitude occurred before any identifiable decrease to the a-wave. At the later time point, the a-wave became involved, although the slow PIII reductions were more pronounced. Morphological abnormalities in the RPE, including increased thickness and altered melanosome distribution, were identified in diabetic animals.

Conclusions

Because the c-wave and slow PIII were both reduced, these results demonstrated that diabetes-induced reductions to the c-wave cannot be attributed to an early increase in the Müller glia-derived potassium conductance. Furthermore, because the a-wave, slow PIII and c-wave reductions were not equivalent, and varied in their onset, the reductions cannot reflect the same mechanism, such as a change in membrane resistance. The presence of small changes to RPE architecture indicate that the c-wave reductions present in diabetic mice likely represents a primary change in the RPE induced by hyperglycemia.

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.

A cone-triggered c-wave in the chicken ERG time integration characteristics

The c-waves recorded from the cone-dominant retina of the chicken proved to be cone-triggered. The time course and time integration properties of this retinal potential were compared with those of the rod-triggered c-wave recorded in the frog under scotopic conditions. The results clearly demonstrate that there is no evidence for time integration in cone-triggered c-waves but that rod-triggered c-waves follow the Bunsen-Roscoe law over more than 2 log units. We propose that these time-related differences may be used in other species as a differentiation criterion between cone-triggered and rod-triggered c-waves.

Some problems involved in employing the ERG c-wave in pharmacological experiments: conditioning in pigmented rabbits

The large inter- and intra-individual variations in ERG c-waves have prevented the application of the component in pharmacological experiments on the retinal pigment epithelial function. We examined some of the problems involved in employing the c-wave in pharmacological experiments using rabbit retinas. 1) Under our experimental conditions, the c-wave amplitudes in some rabbits reached equilibrium under intermittent stimuli, while in others they increased with time and did not reach a constant level in 3 h. 2) The c-wave amplitudes of the right and left eyes were similar, indicating that the contralateral eye can be used as a control during local administration of drugs. 3) The c-wave amplitude under 50 lux light had smaller inter-individual variations than that in the dark. This may be a useful indicator of the retinal pigment epithelium function when drugs are administered systemically.

Evidence for both photopic and scotopic characteristics in the c-wave of chicken and frog ERG

The electroretinographic c-wave was studied in two species with duplex retinas, the frog (rod-dominant) and the chicken (highly cone-dominant). In both species a cone driven c-wave was recorded in addition to the classical rod-driven c-wave. An attempt was made to specify the differential characteristics with respect to time-course, time integration, adaptational changes and spectral sensitivity of cone-driven vs rod-driven c-waves. These data confirm the generality of the c-wave generation by both rod and cone systems.

From sea lemons to c-waves

This review of retinal pigment epithelial (RPE) physiology pays tribute to Anthony L. F. Gorman, who introduced the author to the giant neuron of Anisodoris nobilis (the sea lemon) and cellular neurobiology. The RPE is an epithelial monolayer with tight junctions, which controls the environment of the photoreceptor outer segments. The apical and basal membranes have different electrical properties and generate a standing potential across the eye. The RPE helps maintain adhesion between the retina and the wall of the eye. Adhesion is weakened by cyanide, low pH or low calcium, but enhanced by ouabain or acetazolamide. The RPE transports water from the subretinal space toward the choroid. This water movement is inhibited by hypoxia or cyanide but enhanced by ouabain or acetazolamide. The c-wave of the electroretinogram is a composite of a cornea-positive wave produced by hyperpolarization of the apical RPE membrane and a cornea-negative wave produced by the Muller cells, both in response to the fall in extracellular potassium that follows illumination of the photoreceptors. The "light response" of the standing potential is produced by depolarization of the basal membrane of the RPE. These examples illustrate how principles of cellular neurophysiology can be applied to questions of clinical relevance.

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.