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

Evaluation of essential trace and toxic elements in biological samples of normal and night blindness children of age groups 3-7 and 8-12 years

The most common cause of blindness in developing countries is vitamin A deficiency. The World Health Organization estimates 13.8 million children to have some degree of visual loss related to vitamin A deficiency. The causes of night blindness in children are multifactorial, and particular consideration has been given to childhood nutritional deficiency, which is the most common problem found in underdeveloped countries. Such deficiency can result in physiological and pathological processes that in turn influence biological samples composition. Vitamin and mineral deficiency prevents more than two billion people from achieving their full intellectual and physical potential. This study was designed to compare the levels of Zn, Mg, Ca, K, Na, As, Cd, and Pb in scalp hair, blood, and urine of night blindness children age ranged 3-7 and 8-12 years of both genders, comparing them to sex- and age-matched controls. A microwave-assisted wet acid digestion procedure was developed as a sample pretreatment, for the determination of As, Ca, Cd, K, Pb, Mg, Na, and Zn in biological samples of night blindness children. The proposed method was validated by using conventional wet digestion and certified reference samples of hair, blood, and urine. The concentrations of trace and toxic elements were measured by atomic absorption spectrophotometer prior to microwave-assisted acid digestion. The results of this study showed that the mean values of As, Cd, Na, and Pb were significantly higher in scalp hair, blood, and urine samples of male and female night blindness children than in referents (p < 0.001), whereas the concentrations of Zn, Ca, K, and Mg were lower in the scalp hair and blood but higher in the urine samples of night blindness children. These data present guidance to clinicians and other professional investigating deficiency of essential mineral elements in biological samples (scalp hair and blood) of night blindness children.

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.

Light and alcohol evoked electro-oculograms in cystic fibrosis

Cystic fibrosis (CF) is caused by a defect in the cystic fibrosis transmembrane conductance regulator (CFTR) which is a chloride channel. CFTR is expressed in the retinal pigment epithelium (RPE) where it is believed to be important in generating the fast oscillations (FOs) and potentially contributing to the light-electro-oculogram (EOG). The role of CFTR in the alcohol-EOG is unknown. We recruited six individuals with CF (three homozygotes for Delta508 and three heterozygous for Delta508) and recorded the light- and alcohol-EOGs as well as the FOs and compared them to a control group. The results showed that in the CF group the amplitude of the alcohol- and light-EOGs were normal. However, the time to peak of the light- and alcohol-rises were significantly faster than in the control group. We conclude that CFTR is not primarily responsible for the alcohol- or light-rises but is involved in altering the timing of these responses. The FOs showed differences between the homozygotes, heterozygotes and the controls. The amplitudes were significantly higher and the time to the dark troughs were significantly slower in the heterozygote group compared to both controls and the homozygotes. In contrast, the homozygotes did not differ in either amplitude or the timing of the FOs compared to the controls.

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.

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.

A simple and stable d.c.electrode for ocular electrophysiology

We describe the fabrication of a simple silver-silk electrode which permits remarkably stable d.c.recording of the electroretinogram (ERG) and the optic nerve response (ONR). A saline soaked wick of surgical silk, guided into a polyethylene tube connects the tissue to a coil of Ag/AgCl wire placed in a small glass vial, which is filled with 0.9% NaCl. The vial that holds the tube and the wire is closed with a rubber cap allowing easy refilling with NaCl. Examples of the usefulness of the new silver-silk electrode are shown. We applied it in experimental work in the isolated arterially perfused cat eye for d.c.recordings of the ERG and the optic nerve response (ONR), and also in vivo, in anesthetized mice to record c-waves.

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.

Episodic acute hypotonia after Nd:YAG laser capsulotomy--retinal function and choroidal swelling

In a patient with uveitis who had been treated with Nd:YAG laser capsulotomy after cataract surgery, several episodes of acute hypotonia occurred which were associated with changes in clinical tests of the eye and of visual function. Immunosuppressive and immunomodulating treatment appeared to reverse the changes in intraocular pressure and normalise the test results. The significance of these observations is discussed.

Effects of season on electro-oculographic ratio in winter seasonal affective disorder

Low electro-oculographic (EOG) ratios have been reported in patients with seasonal affective disorder (SAD) during the winter. This study evaluated the effects of the changing seasons on EOG ratios in SAD patients. Sixteen outpatients with SAD and 16 age-, sex-, and medication-matched normal volunteers had EOG testing during the winter and again during the summer. There was a significant season x group interaction in EOG ratios, with normal subjects showing higher ratios in winter than in summer--a seasonal variation not observed in SAD patients. SAD patients may have a subsensitivity to environmental light that leads them to experience symptoms during the winter.

Effects of adenosine on chick retinal pigment epithelium: membrane potentials and light-evoked responses

We examined the effects of adenosine, a putative mediator of neuroprotection during cerebral ischemia, on the electrophysiological characteristics of retina-retinal pigment epithelium-choroid preparations obtained from 1-7 day-old chick and maintained in vitro. Our experiments produced the following results. First, superfusion of the retinal surface with adenosine (0.1 mM) increased the trans-tissue potential. The trans-epithelial (but not the trans-retinal) potential was also increased to the same magnitude with a time-course similar to that of the trans-tissue potential. Second, adenosine produced a depolarization of the epithelial basal plasma membrane with a concomitant decrease in its basal membrane resistance. Third, the trans-epithelial (but not the trans-retinal) c-wave in response to a light stimulus was augmented by adenosine. Adenosine reduced the hyperpolarization of the epithelial basal membrane, but had no effect on the extracellular concentration of K+ in the subretinal region. Fourth, the light-peak that was elicited with a 300 s light stimulus was also depressed by adenosine. Fifth, when 4,4'-diisothiocy anostilbene-2,2'-disulfonate (DIDS), a relatively selective inhibitor of Cl- channels, was perfused at 50 microM on the choroidal surface, adenosine-induced increases in the trans-tissue potential and the c-wave were both abolished. These results suggest that adenosine increased the Cl- conductance of the basal plasma membrane of the retinal pigment epithelium and thereby augmented the standing potential as well as the light-elicited membrane potentials of the retinal pigment epithelium, which seems to be involved in the pathophysiology of retinal ischemia.

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.

Sequential recording of photic and nonphotic electro-oculogram responses in patients with extensive extramacular drusen

At present, no clinical electrophysiologic test defines dysfunction of the retinal pigment epithelium. We studied four electrophysiologic responses of the retinal pigment epithelium to compare results from three normal subjects with those from three patients with a diffuse retinal pigment epithelial disorder, extramacular drusen. We recorded the fast oscillation, hyperosmolarity response, acetazolamide response, and light peak by means of a clinical protocol in which these could be elicited consecutively. We found no significant differences between the normal subjects and patients with drusen for any of the four responses. These results suggest that retinal pigment epithelial electrophysiologic function is well maintained despite the widespread physical abnormalities of the retinal pigment epithelium in extramacular drusen. This combined test was well tolerated and may prove useful in characterizing other diseases involving the retinal pigment epithelium.

Electrophysiology of type II mesangiocapillary glomerulonephritis with associated fundus abnormalities

The retinal electrophysiology is reported in four patients with type II mesangiocapillary glomerulonephritis and partial lipodystrophy with associated fundus abnormalities and no visual symptoms. The histological hallmark of the condition is that of widespread electron dense deposits in the renal glomerulus and in the choriocapillaris and Bruch's membrane of the eye. Three of the four patients had the typical fundal appearance of multiple, yellow, drusen-like lesions at the posterior pole of the eye with normal visual acuity. These three patients had abnormally low Arden ratios on electro-oculography with normal electroretinography responses. This is the first clinical model of disease known to be isolated to the choriocapillaris and Bruch's membrane causing an electro-oculographic abnormality without any clinically detectable deficit in visual function.

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.

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.

Dopamine and melatonin interactions in the intact chicken eye. Electrooculographic and biochemical study

Electrophysiological and biochemical techniques were used to investigate the interactions between dopamine (DA) and melatonin (MEL) in the intact chicken eye. Endogenous DA depletion induced by intraocular administration of alpha-methyl-para-tyrosine (alpha-MPT), a selective tyrosine hydroxylase inhibitor, decreases the transepithelial potential (TEP) of the retinal pigment epithelium and reduces the light peak (LP) recorded by an indirect electro-oculographic (EOG) method. An intraocular injection of MEL also reduces the TEP but does not reduce the LP. Retinal MEL is increased after endogenous DA depletion and a tight inverse correlation between DA and MEL contents was found. The present data, together with other findings support the hypothesis (1) that in the intact chicken eye, DA and MEL play respectively a role of light and dark signals on the TEP, and (2) that a balance between these two neurohormones may be responsible for the regulation of RPE events which are dependent on light-dark conditions.

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.

Potassium-evoked responses from the retinal pigment epithelium of the toad Bufo marinus

Changes in the apical and basal membrane potentials and the resultant changes in the transepithelial potential were recorded from the isolated retinal pigment epithelium of the toad Bufo marinus while the potassium concentration superfusing the apical membrane was changed. Lowering apical potassium caused an initial apically-generated hyperpolarization that increased the transepithelial potential which was usually followed by a delayed basally-generated hyperpolarization that decreased the transepithelial potential. Light evoked a similar pattern of apical and basal responses in a preparation of neural retina-retinal pigment epithelium-choroid. The delayed basal hyperpolarization was accompanied by an apparent increase in basal membrane resistance, and was inhibited by adding the anion transport blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid or the metabolic inhibitor dinitrophenol to the solution superfusing the choroidal side of the retinal pigment epithelium (RPE). The results suggest that a change in the chloride equilibrium potential or chloride conductance of the basal membrane mediates the delayed basal response.

Combined photic and nonphotic electro-oculographic responses in the clinical evaluation of the retinal pigment epithelium

In an attempt to simplify the recording technique in electrophysiologic evaluation of the retinal pigment epithelium, we combined the electro-oculographic light rise, hyperosmolarity and acetazolamide responses in a single recording session. Recordings were performed in six normal subjects and in seven patients with diabetic retinopathy or retinitis pigmentosa. In the patients with background diabetic retinopathy, the hyperosmolarity responses were slightly reduced, while the acetazolamide response and the light rise was normal. In the patients with proliferative diabetic retinopathy, the hyperosmolarity response and light rise were remarkably reduced, while the acetazolamide response was normal. In the patients with retinitis pigmentosa, the hyperosmolarity response and light rise were decreased, while the acetazolamide response was normal. Despite a small study population, we concluded that the clinical results from our combined recording protocol were essentially the same as those reported for each response separately. Because this recording technique simplifies electrophysiologic evaluation of the retinal pigment epithelium, it may help clarify the mechanisms or localization of retinochoroidal and pigment epithelial diseases.

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.

EOG and ERG modifications induced in the chicken eye after blockade of catecholamine and 5-hydroxytryptamine biosynthesis

The involvement of catecholaminergic and indoleaminergic systems in the modulation of the standing potential of the eye was tested in chickens by means of an indirect electrooculography method and direct current electroretinogram recordings. D,L alpha-monofluoromethyl dopa (MFMD), 50 and 100 nmol), a highly specific inactivator of aromatic L-amino acid decarboxylase, was injected intravitreally. This treatment is known to induce a selective and irreversible blockade of dopamine and 5-hydroxytryptamine biosynthesis. Five hours after drug injection an important increase in the standing potential appeared. The light peak was delayed by about 5 min but its amplitude was unchanged. The return of the standing potential to basal value during maintained illumination was delayed by 5-10 min. The time course of the dark trough was not modified. The intensity-voltage functions were also studied for the various electroretinogram components, 5 hr after MFMD. The voltage of the b-wave was reduced (by 65% with the highest intensity), while the other components were little affected. Substantial reduction in dopamine and 5-hydroxytryptamine concentrations were found in treated retinas. These data, together with previous results, suggest that the standing potential of the chicken eye may be modulated by a balance between catecholamine and indoleamine systems.

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.

Active ion transport pathways in the bovine retinal pigment epithelium

1. Radioactive tracer flux measurements demonstrate that active ion transport across the isolated bovine retinal pigment epithelium (RPE)-choroid preparation can be maintained for hours after the eye is enucleated and the tissue removed from the eye. 2. It has been shown that 86Rb tracer fluxes can be used to monitor potassium (K+) transport across bull-frog RPE. In bovine RPE, net 86Rb (K+) absorption is zero. Apical barium (Ba2+) elevated active K+ absorption from zero to approximately 0.3 mu equiv cm-2 h-1. This Ba2(+)-induced increase in active K+ absorption was inhibited either by ouabain or bumetanide in the apical bath. 3. In control Ringer solution, buffered with bicarbonate and CO2, the RPE-choroid actively absorbs chloride (Cl-) at a rate of approximately 0.5 mu equiv cm-2 h-1. In contrast, sodium (Na+) is secreted at a rate of approximately 0.5 mu equiv cm-2 h-1. Chloride absorption was inhibited by apical bumetanide, and Na+ secretion was inhibited by apical ouabain. These drugs were only effective when placed in the solution bathing the apical or retinal side of the tissue. 4. Net Cl- absorption requires an exit mechanism at the basolateral membrane. DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) in the basal bath completely inhibited net Cl- absorption in bicarbonate-free Ringer solution. 5. These experiments show that the chloride transport pathway contains at least two components: (1) a bumetanide-sensitive uptake mechanism at the apical membrane; and (2) an efflux mechanism at the basolateral membrane that is blocked by DIDS. 6. Three apical membrane mechanisms were identified that could help modulate [K+]o in the subretinal or extracellular space that separates the distal retina and the RPE apical membrane. They are: (1) an ouabain-sensitive Na(+)-K+ pump; (2) a bumetanide-sensitive mechanism, the putative Na(+)-K(+)-Cl- co-transporter; (3) a barium-sensitive K+ channel that recycles, to the apical bath, most or all of the potassium that is actively taken up by the Na(+)-K+ pump and the co-transporter. 7. These data suggest that light-induced alterations in subretinal potassium that occur in vivo can activate the chloride transport pathway and help modulate RPE intracellular Cl- during transitions between the light and dark.

Metabolic inhibitors reversibly alter the basal membrane potential of the gecko retinal pigment epithelium

The effects of metabolic inhibitors on the apical and basal membrane potentials were studied in the isolated retinal pigment epithelium of the lizard Gekko gekko. Adding dinitrophenol or cyanide or cooling the tissue to 15 degrees C first depolarized the apical membrane and then hyperpolarized the basal membrane. The basal hyperpolarization was accompanied by an apparent increase in basal resistance. These effects were fully reversible. Adding ouabain to inhibit specifically the apical Na(+)-K+ pump irreversibly depolarized the apical membrane but did not produce a basal membrane hyperpolarization. Dinitrophenol, cyanide and azide also reversibly inhibited a basal membrane response that was evoked by changing the apical potassium concentration. Ouabain did not inhibit this potassium-evoked basal response. These results suggest that metabolic inhibitors will be useful tools to study RPE basal membrane function.

Effects of melatonin on the chick retinal pigment epithelium: membrane potentials and light-evoked responses

The indolamine hormone melatonin, synthesized in the retina, is thought to participate in modulating day-night cyclic variations in photoreceptor and retinal pigment epithelium (RPE) function. It has also been shown to alter the electrical activity of the RPE of the mammalian eye (Dawis and Niemeyer, 1985, Soc. Neurosci. 11, 1079:1988, Clin. Vis. Sci. 3, 109-118: Textorius and Nilsson, 1987, Doc. Ophthalmol, 65, 97-111). To determine the origin of such electrical effects studies were performed on in vitro preparations of both chick retina-RPE-choroid and RPE-choroid. In retina-RPE-choroid preparations choroidal perfusion with melatonin (2.10(-6) M) hyperpolarized the basal membrane, increased its apparent resistance, and diminished the amplitude of the c-wave of the electroretinogram (ERG). Retinal perfusion with melatonin (2.10(-6) M) first depolarized the RPE apical membrane and increased its apparent resistance and this gave way to a basal membrane hyperpolarization accompanied by an increase in basal membrane resistance. Both phases were accompanied by c-wave decreases. Experiments in RPE-choroid preparations suggested that the choroidal effect of melatonin was independent of the neural retina, while the retinal effect was more complex and probably included a neural retinal component. Retinal or choroidal melatonin (2.10(-6) M) had little or no effect on the amplitude of the light peak of the DC ERG. These results show that chick RPE membrane potentials and resistances at either the apical or basal membrane can be affected by melatonin directly, or indirectly via effects on other cells.

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.

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.

Biochemical manifestations of diabetes mellitus in microscopic layers of the cornea and retina

Biochemical evidence of glucose toxicity was found in the retinal and corneal layers of diabetic rabbits. It can be reasonably assumed that the observed changes are causally related to the morphological and physiological diabetic pathologies of the retinal and corneal cells. Intracellular glucose is greatly increased, and the polyol pathway activity appears to be enhanced, resulting in an accumulation of intracellular sorbitol, which can be assumed to be oxidized to fructose. Accompanying the alterations of glucose metabolism are disturbances in myoinositol and Na+ handling by the affected structures. The detailed relationship of the observed metabolic effects of hyperglycemia to changes in cellular ion handling and the observed morphological and functional disturbances has yet to be elucidated. The morphologically and functionally discrete populations of RPE and CEN cells, which are readily amenable to experimental manipulation in situ and in cell culture may serve as unique models for systematic examination of the causes and the consequences of diabetes leading to ocular complications in particular and to the complications of other more complex tissues such as nerve and kidney. The present data show that the findings in one population of cells may not be completely reproducible in another as can be seen in the diverse myoinositol responses of the retinal and corneal layers to diabetes mellitus. The diverse responses perhaps reflect unique adaptive capabilities of individual tissues to the diabetic condition. It is a challenge for complications research to fully appreciate diverse responses of various tissues to persistent glucose intoxication and to delineate meticulously the time courses of such heterogeneous responses, which might result in debilitating pathology in certain cases but in a compensated chronic disease state in others. The corneal endothelium and the RPE are relatively resilient structures compared with the mural and endothelial cells of the retinal microvessels which are destroyed by the diabetic condition. Factors and components that protect tissues against the persistent effects of hyperglycemia need to be uncovered. Success in such an endeavor could be of benefit in the management of diabetic complications.

Electrophysiological findings of neuronal ceroid lipofuscinosis in heterozygotes

Nineteen obligate heterozygotes, 8 individuals at risk of being heterozygote, and 10 patients afflicted with four different forms of neuronal ceroid lipofuscinosis were examined electrophysiologically. The group of obligate heterozygotes was compared to age-matched control groups. Statistically significant differences were found between scotopic b-wave amplitudes, P-ERG amplitudes, and EOG light peaks of the obligate carriers of the juvenile type and the control subjects. The photopic L-ERGs and the latencies of the VEPs were mostly within the normal range. The findings represent the first evidence of functional ophthalmological changes in obligate carriers of neuronal ceroid lipofuscinosis and demonstrate that heterozygotes with certain hereditary autosomal recessive diseases may manifest subtle functional signs.

Incomplete congenital stationary night blindness: electroretinogram c-wave and electrooculogram light rise

Three cases of congenital stationary night blindness are reported. In all patients a negative electroretinogram was recorded by single bright flash stimulation and changed to positive on sequential reduction of the stimulus intensities. Oscillatory potentials were recognized. Biphasic dark adaptation curves and moderate elevation of the relative logarithmic final threshold of dark adaptation were also found. Values of critical flicker fusion frequency were reduced to a mild or moderate extent. VEP latency was prolonged beyond the normal range in two cases. In all three the electroretinogram c-wave was extinguished despite a normal electrooculogram light-dark ratio. It is hypothesized that in congenital stationary night blindness some disorder may exist in the apical membrane of the retinal pigment epithelium but not in the basal membrane in view of dissociation between the c-wave response and the light rise in the electrooculogram.

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.

Single-channel recordings from cultured human retinal pigment epithelial cells

We have applied patch-clamp techniques to on-cell and excised-membrane patches from human retinal pigment epithelial cells in tissue culture. Single-channel currents from at least four ion channel types were observed: three or more potassium-selective channels with single-channel slope conductances near 100, 45, and 25 pS as measured in on-cell patches with physiological saline in the pipette, and a relatively nonselective channel with subconductance states, which has a main-state conductance of approximately 300 pS at physiological ion concentrations. The permeability ratios, PK/PNa, measured in excised patches were 21 for the 100-pS channels, 3 for the 25-pS channels, and 0.8 for the 300-pS nonselective channel. The 45-pS channels appeared to be of at least two types, with PK/PNa's of approximately 41 for one type and 3 for the other. The potassium-selective channels were spontaneously active at all potentials examined. The average open time for these channels ranged from a few milliseconds to many tens of milliseconds. No consistent trend relating potassium-selective channel kinetics to membrane potential was apparent, which suggests that channel activity was not regulated by the membrane potential. In contrast to the potassium-selective channels, the activity of the nonselective channel was voltage dependent: the open probability of this channel declined to low values at large positive or negative membrane potentials and was maximal near zero. Single-channel conductances observed at several symmetrical KCl concentrations have been fitted with Michaelis-Menten curves in order to estimate maximum channel conductances and ion-binding constants for the different channel types. The channels we have recorded are probably responsible for the previously observed potassium permeability of the retinal pigment epithelium apical membrane.

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.

Effect of dopamine and haloperidol on the c-wave and light peak of light-induced retinal responses in chick eye

The relation between dopaminergic cells (and centrifugal fibers), the electroretinogram (ERG) c-wave, and the light peak were electrophysiologically investigated by observing the effects of a retrobulbar conduction block and intravitreal injection of either dopamine or haloperidol on these retinal responses. The retrobulbar conduction block (1% lidocaine) caused a decrease in the amplitude of the c-wave and the light peak in newly hatched chicks. Injections (2-20 microliters) containing dopamine (0.1-10 mM) or haloperidol (1.3-13 mM) were given intravitreously while the responses were recorded. Although intravitreous injection of saline for control resulted in no observable change in the responses, dopamine selectively augmented the c-wave of ERGs and the light peak, but not the a-, b-, and d-waves. Haloperidol decreased first the light peak and later the c-wave. The augmentation of the retinal responses by dopamine and their reduction by haloperidol was statistically significant. The estimated threshold concentration of dopamine in the vitreous cavity was 1-3.5 microM. Since in many species the interplexiform cells have been found to contain dopamine, we hypothesize that the modulatory effects on the c-wave and the light peak in this preparation may be due to a centrifugal feed-back loop which includes the interplexiform cells to the horizontal and bipolar cells and the horizontal cells to the cones.

Quantitative analysis of the suppressive effect on the light rise of the hypertonic solution

We previously reported the hyperosmolarity response (a decrease of the ocular standing potential by hyperosmolarity) as a new clinical test of the retinal pigment epithelium (RPE) activity. In the present study a hypertonic solution (Fructmanit, 1.4 X 10(3) m0sm/1) was intravenously injected for 20 min in proportion to a subject's total blood volume (TBV). At the injection speed of 5, 10, and 15% of the subjects' TBV per hour the mean amplitude of the hyperosmolarity response in normal subjects was 19.7, 30.1 and 36.4% respectively. The amplitude of the hyperosmolarity response depends on the logarithm of the dose of the hypertonic solution within the range of the dose tested. We previously found that hyperosmolarity suppresses the light rise. The present study investigated this suppressive effect in a quantitative manner. The light rise (a full-field illumination of 1.2 X 10(3) cdl/m2) was dose-dependently suppressed by Fructmanit. The mean of the light rise to dark trough ratio in normal subjects was 1.81 with no osmotic stress, and 1.64, 1.41 and 1.29 respectively at the injection speeds of 5, 10, and 15%. The suppression of the light rise by hyperosmolarity is compatible with the view that the hyperosmolarity response and the light rise share the basal membrane of the RPE as the main site of their generation.

Experimental diabetes mellitus impairs the function of the retinal pigmented epithelium

The retinal pigmented epithelium (RPE), which influences the composition of the retinal extracellular fluid, is significantly affected in diabetes. Changes in RPE morphology, permeability, and electrophysiology in experimentally diabetic animals have been described. To facilitate the study of diabetes-related changes in RPE metabolism, we applied the techniques of quantitative histochemistry to pure samples of RPE and individual retinal layers from eyes of normal and alloxan-diabetic rabbits. Glucose within the RPE approximated serum levels in both normal and diabetic animals. Other changes in diabetics included increased sorbitol, decreased myo-inositol, elevated total Na, and loss of measurable Na+-K+-ATPase activity within the RPE. The altered ion metabolism was associated with a progressive decrease in the amplitude of the RPE-generated c-wave of the electroretinogram. The deterioration of the c-wave was arrested by treatment of the diabetic animals with either myo-inositol supplementation or with sorbinil, an inhibitor of aldose reduction. Diabetic alterations in the RPE might impair the ability of the tissue to maintain normal transport functions. The subsequently altered composition of the extracellular environment of the retina may play an important role in the pathogenesis of diabetic retinopathy.

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.

Calcium blocks selectively the EOG-light peak

The light peak (LP) is a slow increase in the standing potential of the eye and has been attributed to a depolarization of the basal membrane of the retinal pigment epithelium (RPE). We tested effects of different external calcium ion concentrations ([Ca++]0) on the LP in the arterially perfused cat eye. An increase in Ca++ activity by 3.2 mM depressed the LP by 85-90% of its amplitude under control conditions. In contrast the other components of the light response (c-wave, fast oscillation, and second c-wave) did not change. The small increase in [Ca++]0 had insignificant effects on the amplitude-intensity plots of the a- and b-waves of the electroretinogram. Elevated [Ca++]0 had its effect only during onset of light and not after the LP was once initiated. The effects were reversible. Since there is a very high [Ca++] concentration in the RPE cells and the pigment granulas may release Ca++ during illumination (Hess 1975), our results suggest that this ion plays a role during the initiation of the LP.

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.

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.