Intraretinal recordings of slow electrical responses to steady illumination in monkey: isolation of receptor responses and the origin of the light peak

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

Retinal pigment epithelial function: a role for CFTR?

In the vertebrate eye, the photoreceptor outer segments and the apical membrane of the retinal pigment epithelium (RPE) are separated by a small extracellular (subretinal) space whose volume and chemical composition varies in the light and dark. Light onset triggers relatively fast (ms) retinal responses and much slower voltage and resistance changes (s to min) at the apical and basolateral membranes of the RPE. Two of these slow RPE responses, the fast oscillation (FO) and the light peak, are measured clinically as part of the electrooculogram (EOG). Both EOG responses are mediated in part by apical and basolateral membranes proteins that form a pathway for the movement of salt and osmotically obliged fluid across the RPE, from retina to choroid. This transport pathway serves to control the volume and chemical composition of the subretinal and choroidal extracellular spaces. In human fetal RPE, we have identified one of these proteins, the cystic fibrosis transmembrane conductance regulator (CFTR) by RT-PCR, immunolocalization, and electrophysiological techniques. Evidence is presented to suggest that the FO component of the EOG is mediated directly or indirectly by CFTR.

The delayed basolateral membrane hyperpolarization of the bovine retinal pigment epithelium: mechanism of generation

1. Conventional and ion-selective double-barrelled microelectrodes were used in an in vitro preparation of bovine retinal pigment epithelium (RPE)-choroid to measure the changes in membrane voltage, resistance and intracellular Cl- activity (aCli) produced by small, physiological changes in extracellular potassium concentration ([K+]o). These apical [K+]o changes approximate those produced in the extracellular (subretinal) space between the photoreceptors and the RPE following transitions between light and dark. 2. Changing apical [K+]o from 5 to 2 mM in vitro elicited membrane voltage responses with three distinct phases. The first phase was generated by an apical membrane hyperpolarization, followed by a (delayed) basolateral membrane hyperpolarization (DBMH); the third phase was an apical membrane depolarization. The present experiments focus on the membrane and cellular mechanisms that generate phase 2 of the response, the DBMH. 3. The DBMH was abolished in the presence of apical bumetanide (100 microM); this response was completely restored after bumetanide removal. 4. Reducing apical [K+]o, adding apical bumetanide (500 mM), or removing apical Cl- decreased aCli by 25 +/- 6 (n = 8), 28 +/- 1 (n = 2) and 26 +/- 5 mM (n = 3), respectively; adding 100 microM apical bumetanide decreased aCli by 12 +/- 2 mM (n = 3). Adding apical bumetanide or removing apical bath Cl- hyperpolarized the basolateral membrane and decreased the apparent basolateral membrane conductance (GB). 5. DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) blocked the RPE basolateral membrane Cl- conductance and inhibited the DBMH and the basolateral membrane hyperpolarization produced by apical bumetanide addition or by removal of apical Cl-o. The present results show that the DBMH is caused by delta[K]o-induced inhibition of the apical membrane Na(+)-K(+)-2Cl- cotransporter; the subsequent decrease in aCli generated a hyperpolarization at the basolateral membrane Cl- channel.

Effects of phototherapy on electrooculographic ratio in winter seasonal affective disorder

Low electrooculographic (EOG) ratios have been reported in patients with seasonal affective disorder (SAD). This study was undertaken to replicate these results and to consider the effects of light therapy on the EOG in SAD patients. Sixteen outpatients with SAD and 16 age-, sex-, and medication-matched control subjects had EOG testing before and after 1 week of light therapy during the winter. The EOG ratios in the SAD patients were only marginally lower than those in the normal control subjects. These differences persisted after light therapy. Although the slightly decreased EOG ratios in SAD patients might have resulted from an artifact of test variability, drowsiness, or other confounding factors, the difference between patients and control subjects raises the possibility of retinal abnormality in SAD.

Acute blockade of dopamine receptors with haloperidol: a retinal model to study impairments of dopaminergic transmission

A pulse of dopamine produces a transient dose-correlated increase in the transepithelial potential (TEP) of the chicken eye, mimicking the light-induced response, the light peak (LP). Acute blockade of retinal dopaminergic transmission with haloperidol, a mixed antagonist, produced a dose-correlated TEP voltage decrease which was rapidly reversed by intravitreal injection of dopamine. The LP recorded thereafter was strongly reduced. These data confirm the hypothesis that dopamine released by light from amacrine cells triggers light-induced changes in the TEP of the intact chicken eye, and that these potentials could well provide an electrophysiological tool to evaluate retinal dopaminergic deficiency.

The electro-oculogram in central retinal vein occlusion

As part of a prospective masked study, the electro-oculogram (EOG) was recorded from 28 patients within 48 days of developing central retinal vein occlusion (CRVO). The EOG light peak/dark trough ratio (Lp/Dt) x 100 was significantly lower in the affected than in the unaffected eyes of patients (p < 0.001), and abnormally low in absolute terms in 20 patients (71%). All unaffected fellow eyes had a normal EOG ratio. The mean Lp amplitude of affected eyes was significantly smaller than that of unaffected eyes (p < 0.001), whereas the differences in mean Dt amplitudes between affected and unaffected eyes were not statistically significant. The Lp amplitude in the affected eye was 48% or less of that in the unaffected eye in the eight patients (29%) who developed rubeosis iridis during the 9 month follow-up, and in six others. No patient whose Lp amplitude in the affected eye was greater than 48% of that in the unaffected eye, developed rubeosis. It is concluded that the Lp amplitude is abnormal in patients with acute CRVO. The degree of this abnormality bears a relation to the development of rubeosis, which might prove a useful indicator of whether to institute or withhold panretinal photocoagulation.

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.

Is dopamine involved in the generation of the light peak in the intact chicken eye?

The implication for dopamine (DA) in the modulation of the standing potential (SP) and the light peak (LP) was tested in intact chickens using an indirect EOG method. After an intravenous or intravitreal injection of DA, a transient, dose-dependent increase in the SP was observed. The LP, recorded after an intravenous injection, was preserved. But after an intravitreal injection, the LP was strongly reduced or even abolished depending on the dose of DA, whereas the photoreceptor response was unchanged. The data supports the hypothesis that the light peak, which is generated by a neural retina-pigment epithelium interaction, could be triggered by dopamine released at light onset from the inner retinal layers.

Effects of intravitreal and intravenous administrations of dopamine on the standing potential and the light peak in the intact chicken eye

We studied the modifications of the standing potential (SP) of the eye and of the light peak (LP) after exposure to dopamine, a neurotransmitter released at light by the inner retina and known to affect electrical properties of the retinal pigment epithelium. Intravenous or intravitreal injections of dopamine (DA) were performed on intact chickens. "Choroidal" application (through an intravenous injection) induced a transient increase of the SP and the LP was preserved. On the other hand, "apical" applications of DA (through an intraocular injection) also increased the SP but considerably depressed the LP. These results are in agreement with the hypothesis that the light-induced release of dopamine from the neuroretina may be responsible for the LP generation in the intact chicken eye.

Electrooculographic study in the chicken after treatment with neurotoxin 6-hydroxydopamine

The implication of dopamine in the modulation of the standing potential of the eye was tested in the chicken by an indirect electrooculogram (EOG) method. After a single rapid systemic injection of dopamine, a transient dose-dependent increase in the EOG voltage was observed. EOG recordings during light and dark adaptation were performed after retinal dopamine depletion was induced by intraocular injections of the neurotoxin 6-hydroxydopamine (6-OHDA). The eyes were injected on two successive days with a mixture of 6-OHDA (50 micrograms), pargyline (a monoamine oxidase inhibitor), and ascorbate added as an antioxidant. Following this treatment EOG recordings were performed 1, 4, and 8 days after the second injection. The electrophysiological changes appeared most spectacular on the fourth day: an important increase in the EOG basal values as well as of the amplitude of the light peak and of the dark trough were observed. Substantial reduction in retinal concentration of dopamine was found in treated retinas. These unexpected electrophysiological data offer additional evidence for the involvement of a catecholamine in the generation of the light peak and the dark trough of the EOG.

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.

Origin of the fast oscillation in the electroretinogram of the macaque

To investigate the origin of the fast oscillation, a phenomenon in the electroretinogram evoked with stimulus frequencies of about 8 mHz (a period time of about 2 min), we recorded responses from retina and pigment epithelium in the macaque. Micropipettes were placed in the subretinal space and in the vitreous close to the retina; the reference electrode was in the orbit behind the eye. Thus, simultaneous recordings were obtained of the trans-epithelial, the trans-retinal and the trans-tissue (vitreal) potential. At 10 mHz the trans-retinal and the trans-epithelial responses are of about equal magnitude but of opposite phase, resulting in a small and rather variable vitreal potential. The origin of the fast oscillation evoked with repetitive stimuli lies in subtle differences between retinal and pigment epithelial potentials, in which a pigment epithelial event plays an important role. For single stimuli lasting 60 s again the trans-epithelial and trans-retinal responses were of equal magnitude and opposite polarity. The epithelial responses were found to return more quickly towards the baseline than the retinal responses. In vitreal recordings this causes a trough between the c-wave and the light peak which is referred to as the "trough" fast oscillation. Most of the "trough" fast oscillation is caused by a pigment epithelial event. In view of the complexity of the fast oscillation evoked with repetitive stimuli it might be difficult to relate pathology to specific neuro-epithelial structures.

The standing potential of the human eye reflects differences between upper and lower retinal areas

In 12 healthy subjects the "light peak" of the electrooculogram was measured following localized stimulation of various retinal locations. Significant differences in "light peak" amplitudes were found between central and peripheral stimulation, and at 10 deg eccentricity the "light peak" amplitudes were significantly larger following upper retinal stimulation than those elicited by lower retinal stimuli. In addition, the "light peak" amplitude produced by upper or lower retinal stimulation behaved differently when test light intensity increased. The upper retinal areas showed consistently a higher sensitivity to light intensity changes than the lower retinal areas. The "light peak" of the EOG is believed to index the rate of retinal metabolism elicited by light stimuli. Our findings show that upper retinal areas display a higher level of light-induced activity reflecting the interaction between the photoreceptors and the retinal pigment epithelium than lower retinal areas. The results are interpreted as a superiority of the upper over the lower retina and are related to other electrophysiological and functional differences between upper and lower retinal areas of man.

Interactions between the retinal pigment epithelium and the neural retina

The retinal pigment epithelium (RPE) interacts with the photoreceptors, which it faces across the subretinal space. In these interactions the RPE acts as three types of cell - epithelium, macrophage, and glia. This review briefly describes selected interactions between the RPE and photoreceptors in ion and water transport, Vitamin A transport, phagocytosis of shed portions of outer segments, ensheathment of photoreceptors outer segments, and electrical responses. The electrical interactions can be recorded at the cornea in the c-wave, fast oscillation, and light peak of the DC electroretinogram (DC-ERG) and electrooculogram (EOG). Each response reflects photoreceptor-RPE interactions in a distinct way. The three responses taken together provide perhaps the best opportunity to learn how pathophysiological conditions alter the interactions between the RPE and photoreceptors.

The electroretinogram, standing potential, and light peak of the perfused cat eye during acid-base changes

DC recordings of light-evoked responses were made in the isolated, arterially perfused cat eye during four acid-base changes designed to alter intracellular pH (pHi) without appreciably altering extracellular pH (pH0). Two acid-base changes were designed to decrease pHi: substitution of high pCO2, high [HCO3-] perfusate for control perfusate and injection of NaHCO3 solution (pH 7.4) into the control perfusate. The initial effects of these two changes were similar: standing potential decreased, the b-wave amplitude decreased, and the c-wave amplitude increased. Subsequent effects, which included rebounds, were complex. The two other acid-base changes were designed to increase pHi: substitution of low pCO2, low [HCO3-] perfusate for the control perfusate and injection of NH4Cl solution into the control perfusate. The initial effects of these two changes were similar; the effects were opposite to those described above for acid-base changes (i) and (ii). The effects of all four acid-base changes were reversible. From these and previously published findings on the effects of pH0, we conclude that during acid-base changes, the initial change in the standing potential varies directly with pHi/pH0, the initial change in b-wave amplitude varies directly with pHi, and the initial change in c-wave amplitude varies inversely with pHi. We also studied the effects of the four acid-base changes on the light peak, a slow voltage response to light generated by the retinal pigment epithelium. Under acid-base changes (i), (ii), and (iii) the light peak was severely depressed. Injection of 2 mM NH4Cl, acid-base change (iv), had little effect on the light peak; however, injection of 5-10 mM NH4Cl did depress the light peak. These results may be interpreted in several ways, for example, the light peak may be sensitive to changes in [HCO-3]0 or to pHi. In any case, we conclude that pH0 is a relatively minor factor influencing the amplitude of the light peak.

Delayed basal hyperpolarization of cat retinal pigment epithelium and its relation to the fast oscillation of the DC electroretinogram

Previous work has shown that the cat retinal pigment epithelium (RPE) is the source of two potential changes that follow the absorption of light by photoreceptors: a hyperpolarization of the apical membrane, peaking in 2-4 s, which leads to the RPE component of the electroretinogram (ERG) c-wave, and a depolarization of the basal membrane, peaking in 5 min, which leads to the light peak. This paper describes a new basal membrane response of intermediate time course, called the delayed basal hyperpolarization. Isolation of this response from other RPE potentials showed that with maintained illumination the hyperpolarization begins approximately 2 s after light onset, peaks in 20 s, and slowly ends as the membrane repolarizes over the next 60 s. The delayed basal hyperpolarization is very small for stimuli less than 4 s in duration and grows with duration, becoming approximately 15% as large as the preceding apical hyperpolarization with stimuli longer than 20 s. Extracellularly, this response contributes to the transepithelial potential (TEP) across the RPE. In response to light the TEP first rises to a peak, the c-wave, as the apical membrane hyperpolarizes. For stimuli longer than approximately 4 s, the decline of the TEP from the peak of the c-wave results partly from the recovery of apical membrane potential and partly from the delayed basal hyperpolarization. For long periods of illumination (300 s) the delayed basal hyperpolarization leads to a trough in the TEP between the c-wave and light peak. This trough is largely responsible for a corresponding trough in vitreal recordings, which has been called the "fast oscillation." The term "fast oscillation" has also been used to denote the sequence of potential changes resulting from repeated stimuli approximately 1 min in duration. In addition to the delayed basal hyperpolarization, such responses also contain a basal off-response, a delayed depolarization.

Light adaptation of primate cones: an analysis based on extracellular data

Extracellular photo-cone responses were isolated in the intact rhesus monkey eye by fractional recording across the outersegment layer in the fovea with a bipolar microelectrode. The cone response vs intensity function was determined in the presence of adapting backgrounds up to 10(6) td. Increment and decrement responses, as well as the response to the steady backgrounds were recorded. All steady state and transient response data could be described with the equation V/Vm = In/(In + sigma n) with n = 0.74 and V and I representing total response and total incident light intensity. This invariant response function is shifted both along the intensity and the response axis with increasing background intensity. The decrease in sensitivity, corresponding to these shifts, could be attributed to cellular adaptation (sigma-adaptation), pigment bleaching and response compression. An analysis of the threshold vs intensity function shows how each of these mechanisms contributes to produce Weber behaviour.

Origin and sensitivity of the light peak in the intact cat eye

1. The light peak is a large light-induced change in the DC potential across the eye (standing potential) that reaches its maximum in 5-13 min in mammals. The light peak of the intact cat eye was studied in order to define its cellular origin and stimulus-response characteristics. Direct-coupled recordings were made with a vitreal electrode and also with intraretinal and intracellular micro-electrodes. Light peaks were generally evoked with 300 sec periods of diffuse white illumination.2. Micro-electrode recordings made in the subretinal space just outside the apical membrane of the retinal pigment epithelium (r.p.e.) showed that the light peak was a change in trans-epithelial potential. No component was generated in the neural retina.3. Intracellular recordings from r.p.e. cells showed that the change in trans-epithelial potential resulted from a depolarization of the basal membrane (facing the choroid). This depolarization came after the hyperpolarization of the apical membrane that gave rise to the r.p.e. component of the c-wave of the e.r.g.4. The light peak amplitude at a constant retinal illumination was nearly linear with stimulus duration over the range 15-180 sec, and saturated at about 300 sec. The time-to-peak remained nearly constant at about 300 sec over this range. Large light peaks could be evoked with flashes as short as 10 sec if the retinal illumination was several log units above rod saturation.5. When stimulus duration was held constant at 300 sec, light peak amplitude was graded with illumination over a wide range, from 3 log units below to 2 log units above rod saturation. The threshold of the light peak was below that of the e.r.g. and only about 1.5-2.5 log units above the absolute threshold of the most sensitive ganglion cells. The increase of light peak amplitude above rod saturation was not due primarily to cones.6. The trans-epithelial light peak had an unusual dependence on stimulus area, being at least twice as large in response to diffuse light as it was in response to a large spot (10 deg diameter) of the same retinal illumination.7. These findings indicate that the light peak represents a normal physiological interaction between the retina and the r.p.e. They also suggest that the interaction involves a change in the concentration of a diffusible substance in the retina, which then either enters the r.p.e. itself, or triggers an internal messenger to cause the basal depolarization.

Origin of the light peak: in vitro study of Gekko gekko

1. The light peak is a large, light-evoked increase in standing potential recorded in mammals, birds and reptiles. We have studied the cellular origin of the light peak in an in vitro preparation of neural retina-pigment epithelium (r.p.e)-choroid from the lizard, Gekko gekko. The tissue was mounted between two separate bathing solutions; the trans-tissue potential was recorded retinal-side positive; micro-electrodes were introduced to measure the trans-epithelial potential (t.e.p.) and to record intracellularly from the r.p.e.2. A 10 min stimulus of diffuse white light evoked an increase in trans-tissue potential that reached maximum amplitude, the light peak, about 15 min after stimulus onset. Since the light peak is present in vitro, it must originate in either the neural retina or the r.p.e.3. A micro-electrode was positioned in the subretinal space and the trans-retinal potential and t.e.p. were measured simultaneously. A 10 min stimulus produced an increase in t.e.p. equal in magnitude and time course to the trans-tissue light peak; no potential was present across the retina. The light peak is therefore generated solely across the r.p.e.4. Intracellular r.p.e. recordings were made to determine whether the light peak was generated at the apical or basal membrane or across the paracellular shunt. A 10 min stimulus first caused a hyperpolarization of both membranes with a time course similar to the r.p.e. c-wave followed by a depolarization of both membranes with the time course of the light peak. We conclude that whereas the r.p.e. c-wave results from a hyperpolarization of the apical membrane, the light peak is generated by a depolarization of the basal membrane of the r.p.e.5. Changes in tissue resistance, R(t), and the ratio of apical to basal membrane resistances, a, were monitored during the light peak by passing current across the tissue and measuring the appropriate current-induced voltages. R(t) decreased and a increased with the time course of the light peak. Assuming that the paracellular shunt resistance is constant, we conclude that the light peak is accompanied by an increase in basal membrane conductance.6. This and the following paper present the first direct demonstration of an interaction between the neural retina and the basal membrane of the r.p.e. The light peak, initiated by absorption of light by photoreceptors, results in a depolarization and conductance increase of the basal membrane.

Fractional recording and component analysis of primate LERG: separation of photoreceptor and other retinal potentials

The local electroretinogram, as obtained with a single micro electrode in the rhesus monkey fovea is known to be the sum of a number of components. This paper shows how the receptor component may be isolated from the LERG by using a bipolar micro electrode that records the potential across the outer segments of the foveal cones. A component analysis of the single electrode LERG, performed at a number of electrode depths in the retina, yields four components: a receptor-, b-, dc- and slow component. The latter has not been reported previously in the monkey and it is proposed that this is the slow PIII of the primate retina. The amplitude-depth profiles of the four components show why the receptor component may be isolated by fractional recording. The recording area of a bipolar electrode is 10 times smaller than that of a monopolar electrode. Contribution of rods to the responses is often present in the dark adapted foveal LERG but is absent in fractional records from the central fovea. The infusion of sodium aspartate in the eye abolishes the b-component, but does not affect the slow component. Aspartate produces, however, such further complexities that the method seems not suitable for isolation of the receptor potential in the intact primate eye.