Intraretinal analysis of the a-wave of the electroretinogram (ERG) in dark-adapted intact cat retina

It has often been assumed that the recovery of the a-wave from its trough is caused by the intrusion of the b-wave. This study examined the recovery following the a-wave trough using intraretinal recordings in dark-adapted intact cat retina. Adult cats were anesthetized and paralyzed. The vitreal ERG was recorded between the vitreous humor and a reference electrode near the eye. Intraretinal recordings were made by referencing a microelectrode to the vitreal electrode. Bright flashes of diffuse white light were used to elicit a- and b-waves. Intravitreal injections of 2-amino-4-phosphonobutyrate (APB), cis 2,3-piperidine dicarboxylic acid (PDA), and kynurenic acid (KYN) were used to block the responses of bipolar and horizontal cells. Intravitreal injections of UL-FS 49 or DK-AH 269 were used to block Ih, a hyperpolarization-activated potassium current. Since the microelectrode was referenced to the vitreal electrode, recordings from the inner retina showed only the oscillatory potentials and b-waves. In the inner retina, the potential was flat until the b-wave became measurable, approximately 17 ms from the onset of the flash. The a-wave started to appear as the microelectrode reached the photoreceptors and its amplitude increased with depth until the microelectrode reached the choroid. The a-wave peaked at approximately 8 ms in response to flashes that saturated its amplitude and then began to recover well before any inner retinal responses were apparent. After injections of APB, PDA, and KYN, vitreal and intraretinal recordings showed only the a-wave, which consisted of an increase to peak at approximately 10 ms followed by a recovery to a plateau which was reached at approximately 25 ms. Blockers of Ih reduced the recovery, but did not eliminate it. The a-wave peaks and partially recovers before the b-wave intrudes. Both phases survive blockers of second-order neurons which implies that the photoreceptors generate both the rising and recovery phase of the a-wave. The recovery phase may be due to a current generated by the inner segment of photoreceptors.     

Post-receptoral contributions to the rat scotopic electroretinogram a-wave

The electroretinogram is a widely used objective measure of visual function. The best characterised feature of the full-field dark-adapted flash ERG, is the earliest corneal negativity, the a-wave, which primarily reflects photoreceptoral responses. However, recent studies in humans and primates show that there are post-receptoral contributions to the a-wave. It is not clear if such contributions exist in the rat a-wave. We consider this issue in the rat a-wave, using intravitreal application of pharmacological agents that isolate post-receptoral ON-pathways and OFF-pathways. In anaesthetised adult long Evans rats, we show that the ON-pathway (2-amino-4-phosphonobutyric acid, APB sensitive) makes negligible contribution to the a-wave. In contrast, CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) or PDA (cis-piperidine-2,3-dicarboxylic acid) sensitive mechanisms modify the a-wave in two ways. First, for bright luminous energies, OFF-pathway inhibition (CNQX or PDA) results in a 22% reduction to the early phase of the leading edge of the a-wave up to 14 ms. Second, OFF-pathway inhibition removed a corneal negativity that resides between the a-wave trough and the b-wave onset.     

Test of the paired-flash electroretinographic method in mice lacking b-waves

Previous studies of rod photoreceptors in vivo have employed a paired-flash electroretinographic (ERG) technique to determine rod response properties. To test whether absence versus presence of the ERG b-wave affects the photoreceptor response derived by the paired-flash method, we examined paired-flash-derived responses obtained from nob mice, a mutant strain with a defect in signal transduction between photoreceptors and ON bipolar cells that causes a lack of the b-wave. Normal littermates of the nob mice served as controls. The normalized amplitude-intensity relation of the derived response determined in nob mice at the near-peak time of 86 ms was similar to that determined for the controls. The full time course of the derived rod response was obtained for test flash strengths ranging from 0.11 to 17.38 scotopic cd s m(-2) (sc cd s m(-2)). Time-course data obtained from nob and control mice exhibited significant but generally modest differences. With saturating test flash strengths, half-recovery times for the derived response of nob versus control mice differed by approximately 60 ms or less about the combined (nob and control) average respective values. Time course data also were obtained before versus after intravitreal injection of L-2-amino-4-phosphonobutyrate (APB) (which blocks transmission from photoreceptors to depolarizing bipolar cells) and of cis 2,3-piperidine dicarboxylic acid (PDA) (which blocks transmission to OFF bipolar cells, and to horizontal, amacrine and ganglion cells). Neither APB nor PDA substantially affected derived responses obtained from nob or control mice. The results provide quantitative information on the effect of b-wave removal on the paired-flash-derived response in mouse. They argue against a substantial skewing effect of the b-wave on the paired-flash-derived response obtained in normal mice and are consistent with the notion that, to good approximation, this derived response represents the isolated flash response of the photoreceptors in both nob and normal mice.     

ERG rod a-wave in Oguchi disease

We analyzed the change in the ERG rod a-wave waveform during the course of dark adaptation in two patients with Oguchi disease. Two Japanese patients showed a homozygous arrestin 1147delA mutation. Scotopic flash ERGs were recorded after different periods of dark adaptation. ERG rod a-waves were obtained after subtraction of the cone ERG contribution. The rod a-waves were fitted with a model of the rod receptor signal. The parameters, Rm(p3) (maximum a-wave amplitude) and S (sensitivity) were calculated. Longer periods in the dark produced larger rod a-wave but only to the first flash presented. The amplitude of the response to subsequent flashes was essentially independent of the period of dark adaptation. Rm(p3) increased with advance of dark adaptation. However, S was nearly constant. Our results suggest that the cause of delayed dark adaptation is not to be sought in the activation of phototransduction process or the regeneration of rhodopsin per se but rather in the deactivation process of the phototransduction cascade.     

Analysis of photoreceptor function and inner retinal activity in juvenile X-linked retinoschisis

Thirteen retinoschisis males with genotyped XLRS1 gene mutations were examined by electroretinogram (ERG) techniques to determine photoreceptor involvement and ON-pathway and OFF-pathway sites of dysfunction. Parameters R(max) and logS determined by fitting the mathematical model of the activation phase of phototransduction to the scotopic and photopic a-wave responses, were not significantly different from normal. However, the XLRS photopic a-wave amplitudes were significantly lower than normal across all intensities, consistent with defective signaling in the OFF pathway. Long flash (150 ms) ON-OFF photopic responses showed reduced b-wave amplitude but normal d-wave amplitude, giving a reduced b/d ratio of <1.32 Hz photopic flicker ERG fundamental frequency responses showed reduced amplitude and delayed phase, consistent with abnormal signaling by both the ON- and OFF-pathway components. These results indicate that the XLRS1 protein appears not to affect photoreceptor function directly for most XLRS males, and that ERG signaling abnormalities occur in both the ON- and OFF-pathway components that originate in the proximal retina.     

Flash responses of mouse rod photoreceptors in the isolated retina and corneal electroretinogram: comparison of gain and kinetics

Purpose. To examine the amplification and kinetics of murine rod photoresponses by recording ERG flash responses in vivo and ex vivo from the same retina. We also aimed to evaluate the two available methods for isolating the rod signal from the ERG flash response, that is, pharmacology and paired flash method on the isolated retina. Methods. Dark-adapted ERG responses to full-field flashes of green light were recorded from anesthetized (ketamine/xylazine) C57BL/6N mice. ERG flash responses to homogenous light stimuli arriving from the photoreceptor side were then recorded transretinally from the same retinas, isolated and perfused with Ringer's or Ames' solution at 37°C. The responses were analyzed to determine the a-wave kinetics as well as the estimated flash sensitivity and kinetics of the full rod responses derived with the paired flash protocol. The analysis was complemented with pharmacologic blockade of glutamatergic transmission in the isolated retina. Results. The a-waves were of comparable size, sensitivity and kinetics in vivo and in the isolated retina, but the onset of the b-wave was delayed in the isolated retina. The Lamb-Pugh activation constants determined for the a-waves were similar in both preparations. The kinetics of the derived photoreceptor responses were similar in both conditions, although the responses were consistently slightly slower ex vivo. This was not explicable as a direct effect of ketamine or xylazine on the photoreceptors or as their indirect effect through hyperglycemia, as tested on the isolated retina. Conclusions. Through comparison to the corneal ERG, the transretinal ERG is a valuable tool for assaying the physiologic state of isolated retinal tissue. The rod photoreceptor responses of the intact isolated retina correspond well to those recorded in vivo. The origin of their faster kinetics compared to single cell recordings remains to be determined.     

Retinal pathway origins of the pattern ERG of the mouse

This study investigated contributions from the retinal On and Off pathways, and the spiking and nonspiking activity of neurons in those pathways to the pattern ERG of the mouse. Light-adapted pattern and ganzfeld ERGs were recorded from anesthetized C57BL/6 mice 3-4 months of age. Recordings were made before and after intravitreal injections of PDA (cis-2,3-piperidine-dicarboxylic acid) to block transmission to hyperpolarizing 2nd order and all 3rd order neurons, TTX (tetrodotoxin) to block Na⁺-dependent spiking, APB (2-amino-4-phosphonobutyric acid) to block synapses between photoreceptors and ON-bipolar cells, and APB + TTX and PDA + TTX cocktails. The pattern stimuli consisted of 0.05 cy/deg gratings reversing in contrast at 1 Hz, presented at various contrasts (50-90%) and a rod saturating mean luminance. For flash ERGs, brief green ganzfeld flashes were presented on a rod-suppressing green background. Recordings were made 39-42 days after unilateral optic nerve crush (ONC) in a subset of animals in which ganglion cell degeneration was subsequently confirmed in retinal sections. Pattern ERGs were similar in waveform for all contrasts, with a positive wave (P1) peak for 90% contrast around 60 ms on average and maximum trough for a negative wave (N2) around 132 ms after each contrast reversal; amplitudes were greatest for 90% contrast which became the standard stimulus. ONC eliminated or nearly eliminated the pattern ERG but did not affect the major waves of the flash ERG. PDA and TTX both delayed P1 and N2 waves of the pattern ERG, and reduced their amplitudes, with effects of PDA on N2 greater than those of TTX. In the flash ERG, PDA reduced a-wave amplitudes, removed OPs but hardly affected b-wave amplitudes. In contrast, TTX reduced b-wave amplitudes substantially, as previously observed in rat. APB removed P1 of the pattern ERG, but left a negative wave of similar timing and amplitude to N2. In the flash ERG, APB removed the b-wave, producing a negative ERG. Addition of TTX to the APB injection removed most of N2 of the pattern ERG, while other waves of the pattern and flash ERG resembled those after APB alone. Addition of TTX to the PDA injection had little effect on the pattern ERG beyond that of PDA alone, but it prolonged the b-wave of the flash ERG. In conclusion, this study confirmed that a selective lesion of ganglion cells will practically eliminate the pattern ERG. The study also showed that P1 of the mouse pattern ERG is dominated by contributions, mainly spiking, from ON pathway neurons, whereas N2 reflects substantial spiking activity from the OFF pathway as well as nonspiking contributions from both pathways.     

Contribution of rod, on-bipolar, and horizontal cell light responses to the ERG of dogfish retina.

Simultaneous extracellular ERG and intracellular recordings from horizontal and ON-bipolar cells were obtained from the dark-adapted retina of the dogfish. The light intensity-peak response relation (IR) and time course of on-bipolar cell responses closely resembled that of the ERG b-wave, but only at low light intensities [<10 rhodopsin molecules bleached per rod (Rh*)]. Block of on-bipolar cell responses with 50 microM 2-amino-4-phosphonobutyrate (APB) abolished the b-wave and unmasked a vitreal-negative wave. Subtraction from the control ERG resulted in the isolation of a vitreal-positive ERG with an IR which matched that of on-bipolar cells over the full range of light intensities. The D.C. component of the ERG arises as a result of sustained depolarization of on-bipolar cells in response to long (>0.5 s) dim light stimuli, or following bright light flashes. The IR of horizontal cells and the vitreal-negative wave unmasked by APB could be matched by scaling at low light intensities (<5 Rh*). However, horizontal cell responses saturated at about 30 Rh*, while the vitreal-negative wave continued to increase in amplitude. The time course of horizontal cell membrane current with dim flashes could be matched to the rising phase of the vitreal-negative wave, assuming that the delay in generating the voltage response in horizontal cells is due to their long (100 ms) membrane time constant. Blocking post-photoreceptor activity resulted in a much smaller vitreal-negative wave than that unmasked by APB alone. We conclude that the b-wave arises from on-bipolar cell depolarization, while the leading edge of the a-wave is a composite of the change in extracellular voltage drop across the rod layer and a component (proximal PIII) reflecting a decrease in extracellular K+ as horizontal cell synaptic channels close with light.     

Impact of aging and age-related maculopathy on activation of the a-wave of the rod-mediated electroretinogram

PURPOSE: To examine the impact of aging and age-related maculopathy (ARM) on the activation of phototransduction in rod photoreceptors by measuring the a-wave of the flash, full-field electroretinogram (ERG). METHODS: Enrollees consisted of older adults (> or = 60 years of age) in normal retinal health (n = 41) and those with early (n = 39) or late ARM (n = 7), in whom disease presence and severity were defined based on grading of stereoscopic color fundus photographs according to the Wisconsin Age-Related Maculopathy grading system. Young adults (ages 16-30 years; n = 27) were enrolled for comparison purposes. Previously established procedures were used to estimate the ERG response to two families of flash intensities. By computer subtraction of responses, the isolated rod response was identified. Each participant's ensemble rod responses were fit with the following equation to describe the response (R) as function of flash intensity (I), and time (t): R(I,t) = [1 - exp[-I x S x (t - td)2]] x RmP3, where S is sensitivity, td is the delay before onset of the a-wave, and Rm(P3) is the maximum amplitude. RESULTS: In analyses of older adults, there was no impact of early ARM presence or severity on log S, Rm(P3), or td after adjustment for age and intraocular lens presence. Differences between young and old normal subjects in log S, RmP3, and td disappeared when analyses were limited to older adults with intraocular lenses. CONCLUSIONS: When the light absorption of the aged lens is taken into account and reliable definitions of normal retinal aging and ARM are used, the activation of the a-wave as measured by the rod-mediated full-field ERG is not affected by early ARM, nor is it impacted by normal retinal aging.     

A comparison of three techniques to estimate the human dark-adapted cone electroretinogram

The dark-adapted cone electroretinogram (ERG) is difficult to isolate because of unwanted rod intrusion. We compare dark-adapted cone estimates derived using three techniques. The first uses the cone response on a moderate rod saturating background to estimate the dark-adapted cone response. The second uses red and blue flashes to tease apart cone and rod responses (red-minus-blue technique, [Investigative Ophthalmology and Visual Science 31 (1990) 2283]). The third uses a bright flash to temporarily saturate rods, followed by a test flash that generates a putative cone-only response (2-flash technique [Investigative Ophthalmology and Visual Science 36 (1995) 1603]). By subtracting the cone estimates from 'mixed' ERG responses in the dark, rod isolated responses can be derived. The rod phototransduction parameters, derived using a computational model, are similar using the light-adapted and 2-flash cone estimates, but differ using the red-minus-blue estimates. The 2-flash cone estimate gives a cone waveform similar to the dark-adapted response of a patient with Oguchi stationary night blindness (a patient with no rod ERG responses and normal cone ERG responses). The growth of the cone response during light adaptation to steady backgrounds causes significant differences between the light-adapted and 2-flash cone waveforms at times beyond the first few milliseconds.     

Assessing abnormal rod photoreceptor activity with the a-wave of the electroretinogram: Applications and methods

The impact of a disease on phototransduction can be assessed by fitting the leading edge of the rod a-wave to high-energy flashes with a quantitative expression. Two parameters of rod receptor activity are obtained, S (sensitivity) and Rm (maximum response). In this study, the meaning of these parameters and examples of conditions that change them were examined. In addition, a new protocol was developed for obtaining these parameters. A set of three to five white flashes were first presented in the dark and then on an adapting field (30 cd/m²). Subtracting the light-adapted responses from the dark-adapted responses yielded isolated rod a-wave responses. A clinical protocol was developed based on a single white flash energy. It is possible to determine whether a disease is producing a change in S and/or Rm with this single flash energy without the use of any equations.     

Utility in clinical practice of standard vs. high-intensity ERG a-waves

Purpose: Standard ERG a-waves represent contributions from both photoreceptor and inner retinal cells, while the leading edge of the high-intensity a-wave is produced only by photoreceptors. This has raised questions about the value of the a-wave as an indicator of photoreceptor disease, and has led to suggestions for standardizing higher-intensity stimuli. Our objective was to compare the behavior of standard and high-intensity a-waves in clinical practice. Methods: Standard ISCEV (International Society for Clinical Electrophysiology of Vision) a-waves and high-intensity a-wave responses were recorded under scotopic and photopic conditions from normal subjects and from patients with photoreceptor dystrophies and other diseases. Results: The standard scotopic a-wave amplitude followed the high-intensity a-wave closely among patients with different diagnoses, and the results did not change significantly when cone a-waves were subtracted to isolate rod signals. The only exception was one patient with the enhanced S cone syndrome (ESCS) whose dark-adapted responses were cone-driven. Initial peak times clustered in a small range for both standard and high-intensity responses, and were not very sensitive to disease. Conclusion: High-intensity a-waves can show photoreceptor characteristics directly, and may help analyze some rare disorders. However, in our study the amplitude of conventional scotopic a-waves mirrored that of the high-intensity responses quite closely over a wide range of patients. This suggests that for practical purposes even if it is not perfect, the standard ERG is an excellent indicator of photoreceptor disease.     

mGluR5对大鼠视网膜对光反应的调控

目的探索代谢型谷氨酸受体5（mGluR5）对大鼠暗适应视网膜电图（ERG）不同成分的贡献。方法实验研究。出生后30日龄大鼠（RCS-rdy+-p+）12只,暗适应大于12 h后,分别行视网膜下腔注射mGluR5激动剂CHPG[（R,S）-2-chloro-5-hydroxyphenylglycine] 200 µmol（6只,CHPG组）和抑制剂MPEP[2-methyl-6-（phenylethynyl）-pyridine] 200 µmol（6只,MPEP组）,对侧眼注射同等体积PBS作为对照。注射后2 d采用RETI-scan系统记录其系列刺激光强度下（-4.5、-2.5、-0.5、-0.02、0.5 1 logcd×m×s-2）的暗适应ERG。导出数据,采用配对t检验进行双眼间数据比较。结果CHPG组,a、b波幅值在不同光强下均显著降低（除最低光强）（-4.5＜t＜-2.3,P＜0.05）,进一步分析得出a、b波最大幅值（Rmax和Vmax）均明显降低（-4.5＜t＜-2.3,P＜0.05）,但其敏感度均未出现明显变化,且b/a未见明显变化。MPEP组,a、b波幅值在不同光强下均显著升高（除最低光强）（-3＜t＜-2,P＜0.05）,进一步分析得出a、b波Rmax和Vmax均明显升高（-3＜t＜-2,P＜0.05）,但其敏感度均未出现明显变化,且b/a未见明显变化。2组的振荡电位（OPs）与相应的对照眼比较未见明显差别。结论mGluR5激动后,ERG a、b波幅值明显降低,mGluR5主要调控外层视网膜的对光反应。     

Rod and cone photoreceptor function in patients with cone dystrophy

PURPOSE: To determine the extent of rod and cone photoreceptor dysfunction in patients with cone dystrophy using psychophysical and electrophysiological tests. METHODS: Ten patients with cone dystrophy participated. Rod and cone system psychophysical thresholds were measured as a function of retinal eccentricity. Bright-flash full-field electroretinograms were obtained under dark-adapted (rod-mediated) and light-adapted (cone-mediated) conditions. The a-wave data were fitted with a model based on photopigment transduction to obtain values for log Rmax (maximum response) and log S (sensitivity). b-Wave parameters were also examined by fitting a nonlinear, saturating function (the Naka-Rushton equation) to the rod-mediated responses. Oscillatory potentials were measured to the cone-mediated high-intensity flashes. RESULTS: On average, the rod-mediated psychophysical thresholds were elevated by 0.5 log unit. These threshold elevations did not differ significantly with retinal eccentricity. In contrast, cone-mediated psychophysical thresholds were elevated up to 3.0 log units. Threshold elevation was greatest in the central retinal locations. For rod-mediated conditions, the a-wave Rmax parameter was significantly reduced in three patients; the a-wave log S parameter was within normal limits. The rod-mediated b-wave Rmax parameter was reduced in six patients; log k was abnormal in one patient. For cone-mediated conditions, the a-wave Rmax parameter was reduced in six patients and the a-wave log S parameter was reduced in two patients. The cone system oscillatory potentials were abnormal in nine patients. CONCLUSIONS: Patients with cone dystrophy show different patterns of psychophysical rod versus cone system sensitivity losses with retinal eccentricity. The full-field electrophysiological data indicate that most of the patients had abnormal cone photoreceptor function. Some patients also showed rod photoreceptor abnormalities. The rod system changes were smaller than the cone system changes.     

Dark adaptation of rod photoreceptors in normal subjects, and in patients with Stargardt disease and an ABCA4 mutation

PURPOSE: Psychophysical and electroretinographic (ERG) studies indicate that patients with Stargardt disease exhibit abnormally slow rod dark adaptation after illumination that bleaches a substantial fraction of rhodopsin. However, relatively little information is available concerning rod recovery in this disease after weaker adapting (i.e., conditioning) light. With the use of a paired-flash ERG method, properties of the derived rod response to a low-bleach (<1%) but rod-saturating conditioning flash were investigated in seven normal subjects and in five Stargardt patients with identified sequence variations in the ABCA4 gene. METHODS: In the first of two experiments, the interval between a fixed conditioning flash (67 or 670 scotopic cd s m(-2)) and a bright probe flash of fixed strength was varied to determine the falling-phase kinetics of the derived rod response to the conditioning flash. In the second, the instantaneous amplitude-intensity function for the rod response at an intermediate stage of recovery from the conditioning flash was determined by presenting a test flash of various strengths at a fixed time after the conditioning flash, and a probe flash at 200 ms after the test flash. RESULTS: The maximum peak amplitude of the dark-adapted, rod-mediated a-wave determined in Stargardt patients (211 +/- 87 microV) was on average lower than that determined in normal subjects (325 +/- 91 microV; P = 0.06). The derived rod response to the 670 scotopic cd s m(-2) conditioning flash determined in normal subjects and Stargardt patients exhibited a biphasic recovery, and the kinetics of the early stage of this recovery were similar in the two subject groups. For both normal subjects and patients, normalized amplitude-intensity functions describing the dark-adapted derived rod response exhibited half-saturation at approximately 1.5 log scotopic troland second. In both groups, the normalized amplitude-intensity function determined at approximately 2 seconds after the 67 scotopic cd s m(-2) conditioning flash and at approximately 9 seconds after the 670 scotopic cd s m(-2) conditioning flash exhibited an average desensitization (i.e., an increase of test flash strength at half-saturation) of approximately 0.5 to 0.6 log unit relative to that determined under dark-adapted conditions. CONCLUSIONS: The results indicate that, despite a reduction in the average dark-adapted maximum a-wave amplitude in the Stargardt/ABCA4 patients, the early-stage recovery kinetics of the derived rod response to a low-bleaching conditioning flash as well as the lingering rod desensitization produced by such a flash are similar to those determined in normal subjects.     

The development of scotopic sensitivity

PURPOSE. Test the hypothesis that the developmental increases in rod photoreceptor sensitivity and rod-mediated visual sensitivity at 10 degrees, 20 degrees , and 30 degrees eccentric are concurrent. It is known that maturation of the parafoveal (10 degrees eccentric) rod outer segments and visual sensitivity is delayed compared to that at 30 degrees eccentric. METHODS: Rod isolated electroretinographic (ERG) responses to full-field stimuli were obtained from dark-adapted subjects (n = 71), ranging in age from early infancy through middle age. Rod photoreceptor sensitivity was calculated by fitting a model of the activation of phototransduction to the a-wave response. Rod driven b-wave sensitivity was calculated from stimulus-response functions. A logistic growth model was used to summarize the developmental increases in sensitivity of the rod photoreceptors and the b-wave. Previously reported dark-adapted, rod-mediated visual sensitivities at 10 degrees , 20 degrees, and 30 degrees eccentric, obtained using preferential looking procedures, were reanalyzed using the logistic growth model. RESULTS: The logistic growth model accounted for 57% to 85% of the variance of each sensitivity parameter with age in normal subjects. The shape of the growth curve and the age at which sensitivity reaches 50% of the adult value is similar (10.0-13.5 weeks) for the rods, the b-wave, and peripheral visual sensitivity, but is significantly older, 19.5 weeks, for rod-mediated parafoveal visual sensitivity. CONCLUSIONS: Rod photoreceptor sensitivity and peripheral, rod-mediated visual sensitivity develop concurrently. A parsimonious explanation is that rod photoreceptor sensitivity determines dark-adapted, rod-mediated visual sensitivity during development.     

Electrophysiologic assessment of photoreceptor function in patients with primary open-angle glaucoma

PURPOSE: Electroretinograms to high-intensity flashes were obtained to determine the extent of rod and cone photoreceptor and postreceptor dysfunction in patients with primary open-angle glaucoma. METHODS: Full-field flash electroretinograms were obtained using brief high-intensity flashes. Dark-adapted (rod-dominated) and light-adapted (cone-dominated) electroretinogram responses were recorded to a "white" light as a function of flash intensity. The a-wave data were fitted with a model based on photopigment transduction to obtain values for the parameters of log Rmax (the maximum response) and log S (sensitivity). Oscillatory potentials were measured to the cone-dominated high-intensity flashes. Standard clinical 30 Hz flicker electroretinogram responses were recorded using a Grass photostimulator. RESULTS: Analysis of rod and cone a-wave data showed that log Rmax and log S values were within the normal range in nearly all of the patients. For some patients, oscillatory potentials were delayed beyond the normal range. CONCLUSION: Our results provide little evidence for widespread photoreceptor abnormalities in primary open-angle glaucoma.     

Retinal degeneration in the nervous mutant mouse. III. Electrophysiological studies of the visual pathway

The nervous (nr) mutation induces a progressive and severe degeneration of cerebellar Purkinje cells and retinal photoreceptors that is virtually complete within the first few months of life. Previous studies of the retina in nervous (nr/nr) mice have focused primarily on the structural abnormalities seen at the level of the photoreceptor cell bodies and outer segments. Here, we have carried out a series of functional studies of the visual pathway in nervous mice and have quantified the status of the inner retinal cell and plexiform layers. Affected animals were obtained by mating nr/+ heterozygotes and screening the offspring for the ataxia characteristic of nervous animals; phenotypically normal littermates (i.e. nr/+ or +/+) were used as controls. As described previously, there is a substantial loss of photoreceptors cells in the nervous retina and a marked shortening of the inner and outer segments. These changes are accompanied by a more modest decline in the thickness of the inner plexiform and inner nuclear layers. These anatomic abnormalities were accompanied by reproducible changes in visual function, as measured with the electroretinogram (ERG) and visual evoked potential (VEP). The dark-adapted ERGs of nervous and control mice had similar waveforms, although the nervous responses were substantially smaller in amplitude. The reductions in the amplitude of the ERG a-wave corresponded to the loss of photoreceptor cells and shortened outer segments seen histologically. Nevertheless, the kinetics of the leading edge of the a-wave did not differ between nervous and control mice, indicating that the rod outer segments of nervous mice continue to respond to light in a normal fashion. The amplitudes of cone ERGs were also reduced in nervous mice, although the extent of this reduction in any given animal was always less than that for rod-mediated ERG components. Overall, this result is consistent with cone involvement occurring only as a secondary effect of rod photoreceptor degeneration. The peak latencies of VEPs of nervous mice were slower than those of control littermates. These functional abnormalities correspond well to the structural changes induced by the nervous mutation, which does not appear to prevent visual signals from being transmitted centrally, beyond the limitations imposed by the degenerative process.     

Encapsulated cell-based intraocular delivery of ciliary neurotrophic factor in normal rabbit: dose-dependent effects on ERG and retinal histology

PURPOSE: ERG and histologic changes were investigated in normal rabbits after intravitreal implantation of encapsulated cell technology (ECT) devices releasing ciliary neurotrophic factor (CNTF). METHODS: Fifteen adult New Zealand White albino rabbits had ECT devices secreting CNTF at 22, 5, or 0 ng/d implanted in the superior temporal quadrant of the left eye. The low dose has been shown to produce substantial rescue of photoreceptors in the rcd1 canine model of retinal degeneration. Right eyes were untreated. Ganzfeld dark- and light-adapted ERGs and clinical observations were performed at 5, 15, and 25 days after implantation. Rod a-waves and rod and cone b-waves and outer nuclear layer (ONL) morphology were evaluated at 25 days. RESULTS: Clinical examination showed minimal changes in a few CNTF-treated eyes, including vitreous membranes and engorgement of iris vessels at day 25. Retinas appeared normal. CNTF did not significantly affect the rod a- or b-waves, although the b-wave amplitude tended to be larger in CNTF-treated retinas at low flash intensities. The cone b-wave amplitude was significantly reduced in high-dose eyes at some flash intensities. The ONL area in high-dose eyes was significantly greater because of increased thickness than in fellow retinas. ONL cell size was significantly increased, and staining density decreased in CNTF-treated retinas. CONCLUSIONS: CNTF, given by intravitreal ECT device at doses that protect photoreceptors in a canine model of retinal degeneration (5 ng/d), did not adversely affect either rod or cone ERG function of normal rabbit retina. The cone ERG was more sensitive to suppression being reduced, at low flash intensities, by 22 ng/d. Dose-related changes in the ONL and photoreceptor cell nuclei did not represent a toxic effect, because they were not associated with deficits in the rod ERG over a broad range of intensities.     

Differential changes in retina function with normal aging in humans

We evaluated the full field electroretinogram (ERG) to assess age-related changes in retina function in humans. ERG recordings were performed on healthy subjects with normal fundus appearance, lack of cataract and 20/20 acuity, aged 20–39 years (n = 27; mean age 25 ± 5, standard deviation), 40–59 years (n = 20; mean 53 ± 5), and 60–82 years (n = 18; mean 69 ± 5). Multiple ERG tests were applied, including light and dark-adapted stimulus-response function, dark adaptation and dynamic of recovery from a single bright flash under dark-adapted conditions. Changes in ERG properties were found in the oldest age group when compared with the two younger age groups. (1) The photopic hill effect was less pronounced. (2) Both photopic a-wave and b-wave amplitudes and implicit times were increased at high stimulus strengths. (3) Dark adaptation time was delayed for pure rod and L/M cone-driven responses, respectively. (4) Dark-adapted a-wave but not b-wave amplitudes were reduced, yielding higher B/A ratios. (5) Dark-adapted a- and b-waves implicit times were prolonged: there was a direct proportional correlation between minimal a-wave implicit times and age. (6) The dynamic of dark current recovery from a bright flash, under dark-adapted conditions, was transiently faster at intervals between 0.9 and 2 s. These results denote that aging of the healthy retina is accompanied by specific functional changes, which must be taken into account to optimally diagnose potential pathologies.