Intracellular responses of the Müller (glial) cells of mudpuppy retina: their relation to b-wave of the electroretinogram

Cellular communication in the circadian clock, the suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus functions as the circadian clock in the mammalian brain. Communication between the cells of the suprachiasmatic nucleus is likely to be responsible for the generation and accuracy of this biological clock. Communication between many cells of the brain is mediated by action potentials that pass down the axon and cause release of neurotransmitters at the neuronal synaptic junction. Additional mechanisms of cellular communication appear to operate in the suprachiasmatic nucleus. Several lines of evidence point to multiple modes of cellular communication: these include the continuing operation of the clock after Na(+)-mediated action potentials have been blocked, the orchestrated metabolic rhythms of suprachiasmatic nucleus cells prior to synaptogenesis, the entrainment of fetal to maternal rhythms, and the rapid recovery of function after suprachiasmatic nucleus transplants into arrhythmic rodents. Possible alternative means of intercellular communication in the suprachiasmatic nucleus are examined, including calcium spikes in presynaptic dendrites, ephaptic interaction, paracrine communication, glial mediation, and gap junctions. This paper identifies and examines some of the unanswered questions related to intercellular communication of suprachiasmatic nucleus cells.

Recent developments in certain X-linked genetic eye disorders

Over the past few years, genetic diseases of the ocular system have become very active and fast-growing research areas in the vision field. The rapid development of the recombinant DNA techniques together with somatic cell genetics, during the last two decades has fueled this progress. As a result, many genetic disease genes have been localized in the human chromosome and several of them have been isolated and characterized. These and other studies have profoundly enriched our basic understanding of genetic eye disorders. Although gene replacement therapy, prenatal diagnosis and carrier detection have not been extensively tried for genetic eye diseases, such attempts will now be feasible. Molecular analyses made it clear that there are many challenging problems that need attention. This report highlights some of these initial developments, particularly on the X-linked major genetic eye diseases. In order to help the beginners and general audience, a brief description of the clinical pathology and the molecular probes used to locate the genetic defects of certain disorders are presented. Disorders are arranged according to their linkage from telomere to telomere on the chromosome to give a coherent structure. It is hoped that this information is useful and of general interest for the beginners, established investigators and ophthalmologists.

Self-sustained spreading depressions in the chicken retina and short-term neuronal-glial interactions within the gray matter neuropil

The chicken retina is an accessible piece of intact gray matter in which a self-sustained form of the 'Spreading Depression' (SD) wave can be easily elicited and recorded for many hours with double barrel ion-sensitive electrodes in the extracellular space. The blockade of glial (Müller) cell potassium channels with barium chloride added to the perfusing Ringer depressed both the negative potential shift typical of SDs and the velocity of spread. Moreover, there was separation of the extracellular increase of potassium and the drop in the extracellular potential: the peak of the potassium wave was increased, as well as its duration whereas the potential wave could be depressed to zero or even inverted to positive. By contrast the transient extracellular calcium drop could not be separated from the extracellular potential wave but appeared related to it: no transient calcium drop was observed when the negative potential was completely depressed or inverted. Both, the amplitude of the extracellular potential and extracellular calcium activity appeared to be important factors controlling the velocity of spread.

Red-green flicker photometry and nonlinearities in the flicker electroretinogram

We investigated whether responses from different cone classes are combined before or after the nonlinearity that is responsible for generating nonlinear response components of the flicker electroretinogram (ERG). We measured the nonlinear response of the retina while systematically varying the modulation in the long-wavelength-sensitive and middle-wavelength-sensitive cones by changing the proportions of flickering 633- and 543-nm lights that compose a sum-of-sinusoids temporal waveform. We found that at high temporal frequencies the ERG responses are best accounted for by a model in which the principal retinal nonlinearity is located before the convergence of signals from the two cone classes. At low temporal frequencies the ERG signal is dominated by cone-antagonistic responses. At frequencies of 30 Hz and higher, the flicker ERG and psychophysical flicker photometry give similar estimates of the relative proportions of long- and middle-wavelength-sensitive cones. The ERG photometric null is frequency dependent, whereas the psychophysically determined ratio is much less sensitive to changes in frequency.

Dystrophin expression in the human retina is required for normal function as defined by electroretinography

We have studied retinal function by electroretinography in five Becker and six Duchenne muscular dystrophy patients. All had abnormal electroretinograms with a markedly reduced amplitude for the b-wave in the dark-adapted state. Using three antisera raised to different domains of dystrophin, we identified dystrophin in the outer plexiform layer of human retina. The retinal dystrophin is present in multiple isoforms as the result of alternative splicing. The localization of dystrophin to the outer plexiform layer coincident with the abnormal b-wave suggests that dystrophin is required for normal retinal electrophysiology.

Temporal frequency dependent adaptation at the level of the outer retina in humans

The focal electroretinogram (FERG) was used to examine temporal frequency tuning at the outer retinal level in humans by measuring temporal modulation thresholds. Changes in FERG thresholds as a function of ambient light level were compared to temporal modulation thresholds obtained psychophysically using the same stimuli. At lower temporal frequencies, both FERG and psychophysical thresholds changed sensitivity proportional to the mean illuminance level. At higher illuminance levels, both threshold measures were relatively independent of illuminance. The comparison of the FERG to the behavioral data suggest that most of the adaptation-dependent changes in temporal sensitivity in humans occur at the level of the photoreceptor complex.

Foveal photopigment kinetics--abnormality: an early sign in myotonic dystrophy?

Twelve subjects with minimal expression of the myotonic dystrophy (MyD) gene were investigated by retinal densitometry, a technique which has been used to study the properties of photopigments in the living eye and to detect photoreceptor abnormalities. Other investigations included slit-lamp examination, funduscopy, raleigh matches with the anomaloscope, tonometry, and neurological examination, including electroretinography (ERG) and pattern visual evoked potentials recording. Foveal densitometry demonstrated reduced values of the macular photopigment density difference with normal photopigment kinetics in early phases of the disease, even in asymptomatic individuals. The densitometric values correlated with decreased amplitudes of the photopic ERG a-wave. These findings may be explained by loss or dysfunction of the outer segments of foveal receptors. It is yet unknown whether or not these changes are secondary to other observed neuroretinal abnormalities in MyD. The most likely explanation might be an abnormality of the Na, Ca:K exchanger at the level of the outer segments of the photoreceptors whether or not in combination with a dysfunction of voltage generation systems, involving both photoreceptors and retinal pigment epithelium.

Spatial buffering of extracellular potassium by Müller (glial) cells in the toad retina

We examined the role of Müller (glial) cells in buffering light-evoked changes in extracellular K+ concentration, [K+]o, in the isolated retina of the toad, Bufo marinus. We found evidence for two opposing Müller cell current loops that are generated by a light-evoked increase in [K+]o in the inner plexiform layer. These current loops, which are involved in the generation of the M-wave of the electroretinogram (ERG), prevent the accumulation of K+ in the inner plexiform layer by transporting K+ both to vitreous and to distal retina. In addition, under dark-adapted conditions, we found evidence for a Müller cell current loop that is generated by a light-evoked decrease in [K+]o in the receptor layer. This current loop, which is involved in the generation of the slow PIII component of the ERG, helps to buffer the light-evoked decrease in [K+]o throughout distal retina by transporting K+ from vitreous. The spatial buffering fluxes of K+ can be abolished by blocking Müller cell K+ conductance with 200 microM Ba2+. The separate contributions of the M-wave and slow PIII currents to Müller cell spatial buffering were isolated by various pharmacological treatments that were designed to enhance or suppress light-evoked activity in specific retinal neurons. Our results show that Müller cell K+ currents not only buffer light-evoked increases in [K+]o, but also buffer light-evoked decreases in [K+]o, and thereby diminish any deleterious effects upon neuronal function that could arise in response to large changes in [K+]o in the plexiform layers. Moreover, our results emphasize that spatial buffering currents generate many components of the electroretinogram.

Retinal ischemia induced by occlusion of both common carotid arteries in rats as demonstrated by electroretinography

In 2 models of reduced cerebral blood flow-permanent occlusion of the vertebral arteries plus transient occlusion of the common carotid arteries (4VO) and transient clamping of the common carotid arteries (BCCA)-the acute effects on the electrical function of the retina were monitored by recording the photopic electroretinogram. During both 4VO and BCCA the amplitude of the b-wave was reduced. Within 30 min of reperfusion after 4VO and after BCCA the b-wave had fully recovered. In contrast, the a-wave was not affected by either treatment. The data suggest that occlusion of common carotid arteries leads to retinal ischemia and might represent a useful model of amaurosis fugax.

Study of retinal dystrophy in RCS rats: a comparison of Mg-ATP dependent light scattering activity and ERG b-wave

A comparative study using the techniques of ERG to measure the b-wave and light scattering relaxation spectrophotometry (LSRS) to determine the dynamic behavior Mg-ATP dependent processes in the rod photoreceptors of pigmented control and dystrophic RCS rats has been carried out. LSRS results, based exclusively on photoreceptor rod outer segment dynamics, suggest a progressive failure in the dark and light-induced Mg-ATP dependent processes as a function of age. The dark signal amplitude in the dystrophic rats decreases to about 50% of the control by 5 weeks post-natal; the light-induced signal has decreased by 30% in the same period. The ERG b-wave results indicate that the differences in the amplitude and the time required to attain the peak amplitude become increasingly pronounced between the control and dystrophic groups of rats again as a function of age. By 10 weeks of age, the intensity of light required to obtain a b-wave with a amplitude of 100 microM is 10(3) greater in the dystrophic RCS rats. Similarly, the time to achieve this peak increases in the dystrophs with age. These results indicate that the retinal dystrophy in the RCS rat affects the activity of the rod photoreceptor cells.

Effect of sulphur containing amino acids on [3H]-acetylcholine release from amacrine cells of the rabbit retina

1. The effects of the sulphur containing amino acids, homocysteic acid, homocysteine sulphinic acid, cysteic acid and cysteine sulphinic acid on the release of [3H]-acetylcholine ([3H]-ACh) from the cholinergic amacrine cells of the rabbit retina were examined. 2. All the compounds stimulated the spontaneous resting release and abolished the light-evoked release of [3H]-ACh. Except for homocysteine sulphinic acid these actions occurred at concentrations that did not affect the erg b-wave amplitude, indicating a site of action at the inner retina. 3. N-methyl-D-aspartate (in Mg(2+)-containing medium) clearly blocked the effects of homocysteic acid and homocysteine sulphinic acid on the resting release of [3H]-ACh but had no effect on the actions of cysteic acid and cysteine sulphinic acid. 4. Since N-methyl-D-aspartate is an antagonist of the light-evoked endogenous bipolar cell transmitter released onto cholinergic cells, these results are consistent with the suggestion that homocysteic acid or homocysteine sulphinic acid may be a transmitter released from this subpopulation of bipolar cells. 5. The present experiments indicate the existence of excitatory amino acids that have closer pharmacological properties to a bipolar cell transmitter than glutamate but it remains to be seen whether homocysteic acid or homocysteine sulphinic acid occur in these particular bipolar cells.

A computational model of the amplitude and implicit time of the b-wave of the human ERG

To improve the usefulness of the ERG in identifying the sites and mechanisms of adaptation, development, and disease processes, a theoretical framework based upon Granit's analysis of the ERG was evaluated. The framework assumes that the ERG is the sum of two potentials, one, P3, generated by the receptors and the other, P2, generated by the cells of the INL. Hood and Birch (1990a, b) demonstrated that the leading edge of the a-wave can be quantitatively described by a model used to describe the response from single rod receptors. This model provides P3(t), a theoretical receptor response as a function of time, for any given flash intensity. The ERGs from normal observers and patients with retinal diseases were analyzed in this framework, first by deriving P2 by computer subtracting the predicted P3(t) responses. This analysis was successful and a computational model of the ERG was then derived. The model of P2(t) was constructed with linear filters and a static nonlinearity and using P3(t) as the input. The ERG for any given flash intensity is then P3(t) + P2(t). The model describes (1) the change both in implicit times and in trough-to-peak b-wave amplitudes with flash intensity for the normal, dark-adapted observers; and (2) the changes in b-wave implicit times and amplitudes for three patients with retinal diseases. Among the implications drawn from these analyses were as follows: (1) The fits of the Naka-Rushton equation to trough-to-peak b-wave amplitudes must be interpreted with great care. (2) When the INL is affected by retinal disease, the b-wave may be a very poor reflection of INL activity. (3) The implicit time of the b-wave can provide a measure of receptor sensitivity.

Characterization of retinal and hippocampal L-AP4 receptors using conformationally constrained AP4 analogues

In the past, the absence of useful 2-amino-4-phosphonobutanoic acid (AP4) analogues has hampered the pharmacological study and comparison of different systems which are sensitive to L-AP4. Several conformationally constrained AP4 analogues have now been synthesized: (E)- and (Z)-1-amino-3-phosphonocyclopentanecarboxylic acid [(E)- and (Z)-cyclopentyl AP4], and (E)- and (Z)-1-amino-3-phosphonocyclohexanecarboxylic acid [(E)- and (Z)-cyclohexyl AP4], and the recently synthesized cyclopropyl analogues (E)- and (Z)-2-amino-2,3-methano-4-phosphonobutanoic acid [(E)- and (Z)-cyclopropyl AP4]. Therefore, we have examined and report here the pharmacology of two retinal and two hippocampal L-AP4 sensitive systems using these analogues. In addition, the pharmacology of two kainic acid/alpha-amino-3-hydroxy-5-methylisoxazole-4- propionic acid (KAIN/AMPA) pathways and one N-methyl-D-aspartate (NMDA) hippocampal pathway was examined. We found that the rank order potency of the L-AP4 sensitive systems were similar though not identical. The KAIN/AMPA and NMDA systems had a quite different rank order of potencies than the L-AP4 systems. These data suggest that the L-AP4 receptors in these different systems are structurally similar to each other and differ from both KAIN/AMPA and NMDA receptors.

The rod bipolar cell of the mammalian retina

Three approaches to study the function of mammalian rod bipolar cells are described. Extracellular recordings from the intact cat eye under light- and dark-adapted conditions showed that in dark-adapted retina all light responses can be blocked by 2-amino-4-phosphonobutyrate (APB). Immunocytochemical staining with an antibody against protein kinase C (PKC) labeled rod bipolar cells in all mammalian retinae tested. When rat retinae were dissociated, PKC immunoreactivity was also found in isolated bipolar cells and could be used for their identification as rod bipolars. Patch-clamp recordings were performed from such dissociated rod bipolar cells and their responses to APB were measured. APB closed a nonselective cation channel in the cell membrane. The actions of GABA and glycine were also tested and both opened chloride channels in dissociated rod bipolar cells. These results suggest that rod bipolar cells are depolarized by a light stimulus and that GABA as well as glycine modulate their light responses.

DL-2-amino-4-phosphonobutyric acid does not eliminate "ON" responses in the visual system of goldfish

DL-2-Amino-4-phosphonobutyric acid (APB) suppresses activity in retinal ON pathways. It is generally assumed that loss of the ON pathway would result in loss of ON responses in the visual system. We tested this assumption by recording activity from the optic nerves of intact goldfish (Carassius auratus) before and after intraocular injection of APB. Whole-nerve responses to increments and decrements of light were compared to electroretinogram responses and to tectal evoked potentials. APB severely reduced the amplitude of the electroretinogram b-wave but left ON and OFF responses from the optic nerve and tectum intact, although decreased in sensitivity. We conclude that APB does not completely eliminate ON responses in the visual system, at least in goldfish. The selectivity and effectiveness of APB must be evaluated in other species before this agent can be relied upon as a useful tool in understanding the roles of ON and OFF pathways in visual function.

A deconvolution technique for improved estimation of rapid changes in ion concentration recorded with ion-selective microelectrodes

In biological preparations, measurements of rapid, stimulus-evoked changes in ion concentration by ion-selective microelectrodes can be distorted by the limited bandwidth of these sensors. Techniques were developed to reconstruct the actual change in ion concentration using deconvolution of the electrode's output signal and the electrode's transfer function. In the vertebrate retina, a knowledge of the actual time course of a light-evoked increase in extracellular K+ concentration was used to provide a rigorous test of a hypothesis regarding the electrical origin of a clinically important component of the electroretinogram.

"Sunbursts" in the inner plexiform layer: a spectacular feature of Müller cells in the retina of the cat

A previously unrecognised structure in the cat retina is described. Seen in Golgi-impregnated, wholemounted retinas, each such structure comprises processes radiating across the inner plexiform layer from a dense, vellate core. The processes are numerous, and largely unbranched, and give the impression of rays radiating from a point source; the structure is therefore termed a "sunburst." Evidence is presented from Golgi-impregnated retinas, and from retinas labelled with monoclonal antibodies to Müller cells, that the core of each sunburst is the inner process of a Müller cell. The sunbursts are numerous and overlap extensively, so that when neighbouring sunbursts are impregnated, they are seen to form a dense mat of processes extending across the IPL. It is suggested that each Müller cell forms a sunburst and that sunbursts form a major glial component of the neuropil of the inner plexiform layer.

The retinal degeneration slow (rds) gene product is a photoreceptor disc membrane-associated glycoprotein

Mice homozygous for the retinal degeneration slow (rds) mutation exhibit abnormal development of photoreceptor cells, followed by their slow degeneration. We have recently cloned the rds gene and determined the structure of the wild-type rds mRNA. Here we show that the gene is expressed exclusively in photoreceptor cells. We demonstrate that it encodes a 39 kd membrane-associated glycoprotein that is restricted to photoreceptor outer segments. By electron microscopy, we show that the rds protein is distributed uniformly within outer segment discs. The developmental appearance of the rds protein coincides with outer segment disc formation. We propose that the rds protein functions as an adhesion molecule for stabilization of the outer segment discs.

A DC electroretinography method for the recording of human a-, b- and c-waves

In the clinical ERG the c-wave is not usually recorded due to methodological problems. Because of the potential importance of the c-wave recording in assessing the function of the pigment epithelium in several retinal diseases, we describe a DC ERG method which is convenient for the patient and suitable also for clinical practice. The light stimuli are provided by a Ganzfeld stimulator and the potentials are recorded with a disposable corneal wick electrode. The method allows the recording of the c-wave from co-operative subjects as well as to study the a- and b-wave properties.

Electroretinographic effects induced in humans by psychopharmacologic agents

We studied the effects of single doses of different dopamine agonists and antagonists on the electroretinogram of a group of healthy volunteers. The results demonstrated significant b-wave amplitude changes after drug administration, suggesting that electroretinograms can be employed to evaluate the effects on retinal dopaminergic activity induced by psychotropic drugs and that the study of the electroretinographic effects of psychopharmacologic agents can provide new insights into the relationship between retinal dopaminergic mechanisms and the electroretinogram b-wave origin.

Evidence for retinal pathology following interruption of neural regulation of choroidal blood flow: Müller cells express GFAP following lesions of the nucleus of Edinger-Westphal in pigeons

Choroidal blood flow in pigeons is regulated by the medial part of the nucleus of Edinger-Westphal (EW) via the ipsilateral ciliary ganglion. Interruption of this circuit by unilateral lesions of EW results in pathological modifications in the morphology of retinal photoreceptors in the ipsilateral eye in pigeons housed under 12hr light (400 lux)/12hr dark conditions. In the present study, we examined the effects of unilateral EW lesions on glial fibrillary acidic protein (GFAP) expression by retinal Müller cells in pigeons housed under the same lighting conditions. Since Müller cells in the retina of land vertebrates express increased GFAP during conditions of retinal pathology or stress (e.g. inflammation or hypoxia), this study would enable us to further evaluate the effects of disruption in the neural regulation of choroidal blood flow on the retina. We found that following EW lesions, retinal Müller cells expressed GFAP, with the precise intracellular location of the GFAP dependent on the amount of time elapsed following the lesion. One week after the EW lesions, GFAP labelling was restricted to the Müller cell endfeet in the nerve fiber layer and ganglion cell layer. By two-three weeks, the labelling had extended outward (or sclerad) into the portions of the Müller cells spanning the inner plexiform layer. Finally, by six weeks post-lesion, the entire extent of the Müller cell from the nerve fiber layer to the outer limiting membrane contained GFAP. No GFAP immunoreactivity in Müller cells was observed in the eyes contralateral to the EW lesions or in eyes in which the pupil had been fixed and dilated by lesions of the pretectal region. Our results suggest that the retina is in a state of physiological stress following interruption of the neural regulation of choroidal blood flow by EW lesions. Although the precise mechanisms by which altered choroidal blood flow regulation affects Müller cell GFAP production require elucidation, the results nonetheless highlight the importance of intact neural regulation of choroidal blood flow for retinal health.

K(+)-evoked Müller cell depolarization generates b-wave of electroretinogram in toad retina

We tested the hypothesis that a light-evoked increase in [K+]o produces a depolarization of the Müller cell membrane, which in turn generates the electroretinogram b-wave current. Using Bufo marinus isolated retinas and K(+)-selective microelectrodes, we recorded two distinct light-evoked increases in extracellular K+ concentration: one in the inner plexiform layer and the other near the outer plexiform layer; the "distal" K+ increase was found over only 10-microns depth and had a maximum amplitude of 0.3 mM. We also recorded the electroretinogram and the light-evoked responses of rods and Müller cells. After correction for the response time of the K(+)-selective microelectrode, the waveforms of all three of these responses were almost exactly as predicted by an earlier computer simulation of the K+/Müller cell hypothesis of the b-wave by Newman and Odette [Newman, E.A. & Odette, L.L. (1984) J. Neurophysiol. 51, 164-182]. The distal K+ increase and the b-wave varied in a similar manner as a function of stimulus irradiance. Superfusion with 0.2 mM Ba2+ attenuated both the Müller cell depolarization and the b-wave by approximately 65% but had no significant effect upon the distal K+ increase. Because Ba2+ reduces K+ conductance of Müller cells, these results are very strong support of the K+/Müller cell hypothesis of the origin of the electroretinogram b-wave; the light-evoked increase in extracellular potassium concentration still is present during superfusion with Ba2+, but the K(+)-evoked Müller cell depolarization and the b-wave are decreased in amplitude because Müller cell K+ conductance is reduced.

Ion-selective microelectrodes suitable for recording rapid changes in extracellular ion concentration

A method for fabricating double-barrel, ion-selective microelectrodes with fine tips (0.5-1.5 microns) and rapid response times is described. When made into K(+)-selective microelectrodes, the electrodes respond to changes in [K+]o with a time constant of 70-95 ms. The electrical response of these electrodes to common-mode voltages can be made to have a time constant of less than 2 ms, which minimizes electrical artifacts from field potentials. The application of these microelectrodes to the measurement of rapid, transient changes in retinal [K+]o is presented.

Müller cells in adult rabbit retinae: morphology, distribution and implications for function and development

We describe the morphology and distribution of Müller cells in wholemounts of rabbit retinae labelled with either monoclonal antibodies (anti-Vimentin, 3H3, 4D6, and 4H11), or intracellular horseradish peroxidase. Several new features of Müller cell organization are noted. First, Müller cells appear to compose a single morphological class and their morphology varies systematically with retinal thickness. Second, in contrast to other retinal glia, Müller cells have a neuronlike distribution, with a peak density of 10,700-15,000 cells per mm2 at the visual streak and a minimum density of 4,400-6,000 per mm2 at both the superior and inferior retinal edges. There are 4.2 +/- 0.5 x 10(6) Müller cells per retina. Third, unlike in other species, rabbit Müller cells do not contact blood vessels, suggesting that they do not participate in the transfer of metabolites or in the blood:retinal barrier. Fourth, each Müller cell has a vitread endfoot about 20-40 microns in diameter composed of numerous fimbriae. The fimbriae from a single Müller cell generally contact several axon fascicles in the nerve fibre layer, and at each point along its length each fascicle is enclosed by the overlapping fimbriae from several Müller cells. Fifth, in the inner and outer plexiform layers, numerous filamentous branchlets extend 20 microns or more from the radial trunk, interweaving with branchlets from nearby Müller cells to form dense and continuous strata. In the ganglion cell layer and outer nuclear layer, Müller cell processes completely wrap neuronal somata, whereas in the inner nuclear layer they partially wrap somata. We discuss the functional and developmental implications of these observations.

Ontogenesis of the electroretinogram in a precocial mammal, the guinea pig (Cavia porcellus)

1. Ontogenesis of the electroretinogram, the mass electrical response of the retina to flash light stimuli, was studied in the guinea pig (Cavia porcellus), a precocial species with visual function at birth. 2. a-Wave components, b-wave, oscillatory potentials, slow PIII, and c-wave responses to flash stimuli developed between 55 and 64 days of gestation (full term is 68-69 days). 3. a-Waves attributable to photoreceptor functions were fully mature at 60 days. 4. ERG development lagged behind the reported critical milestones in retinal development; its prenatal onset indicates that no history of light entrainment is required for initiation of a mature ERG response.

A late receptor dystrophy in the Labrador retriever

Dark adapted single flash and light adapted 30 Hz photopic flicker ERGs were recorded from a litter of seven Labrador Retrievers bred from parents affected with generalized progressive rod-cone dystrophy. After an initial increase of b-wave amplitudes from 5 weeks to 4 months similar to the controls, the b-wave amplitudes of the litter were significantly decreased (P less than 0.005) at 7 months. At 21 months the b-wave amplitudes were very low, although some response to 30 Hz photopic flicker was still left. The ERG changes indicated a late onset progressive rod-cone dystrophy which developed after maturation of the retina. The development of this photoreceptor dystrophy has not previously been described.

Temporal relationship between ERG components and geniculate unit activity in rabbit

The purpose of this study was to examine if there is an ERG component (alpha-wave, b-wave, oscillatory potentials) which can be used to account for the intraretinal processing time. To address this issue, ERGs and LGN unit responses were recorded simultaneously from anesthetized and paralyzed rabbits. A gradual decrease in the intensity of the light stimulus yields a progressive increase in the latency of the LGN unit response. A similar, highly correlated (r = 0.91 +/- 013), latency shift was also noted for one oscillatory potential (OP2). In comparison, correlation coefficients of r = 0.63 +/- 0.27 and r = 0.70 +/- 0.29 were obtained for the alpha- and b-wave respectively. Furthermore, in 77.6% of the cells examined, OP2 preceded LGN unit activity while the b-wave preceded LGN activity in only 17.8% of the cases. Our results suggest that, of all the retinal potentials considered, the peak time of OP2 better reflects the primary visual processes and the intraretinal time taken for visual processing.

X-linked congenital retinoschisis

The natural history and electrophysiological findings of 52 patients with X-linked congenital retinoschisis with a follow-up of up to 26 years are described. The mean visual acuity was reduced to 0.24 +/- 0.2 and remained unchanged in most patients during this time. If visual loss occurred, it usually happened in the first decennium. The complications were retinal detachments in 11% and vitreous hemorrhages in 4% of the eyes. In general, the vitreous hemorrhages resolved spontaneously. Retinal detachments were treated successfully with conventional buckling procedures. Redetachments occurred in about 40%. Prophylactic laser coagulation was of no use because it was complicated by detachment in 43% of our series. The electro-oculogram was usually normal. In addition to the known electrorentinographic findings of normal a-wave and reduced b-wave amplitudes, we found prolonged b-wave latencies and implicit times, as well as a reduced 30 Hz flicker response.

Extracellular pH in the isolated retina of the toad in darkness and during illumination

1. Extracellular pH (pHo) was measured in the isolated retina preparation of the toad, Bufo marinus, using H(+)-selective microelectrodes. During superfusion with phosphate-buffered solution (pH 7.8), which had a low buffering capacity, pHo in the inner retina was 7.0-7.2 and there was a pHo gradient throughout the distal retina and into the bathing solution. 2. The retinal acidity appears to be due in part to the combined reactions of glycolysis and ATP hydrolysis, since anoxia greatly increased the pHo gradient, while superfusion with either glucose-free pyruvate solution or strophanthidin decreased this gradient. 3. Maintained illumination evoked both an acidification in the proximal retina and an alkalinization in the distal retina. Blocking synaptic transmission to second-order neurones (1.0 mM-aspartate) decreased the acidification but had little effect on the alkalinization, consistent with the notion that the alkalinization is of receptoral origin, while the acidification is of post-receptoral origin. 4. Retinal neurones extrude a significant amount of acid via Na(+)-H+ exchange, since 2.0 mM-amiloride, a blocker of Na(+)-H+ exchange, caused a sustained alkalinization in darkness and decreased the light-evoked changes in pHo, while 1.0 mM-4-acetamido-4'-isothiocyanatostilbene-2.2'-disulphonic acid (SITS), a blocker of Cl(-)-HCO3- exchange, produced a much smaller alkalinization. 5. Switching to a bicarbonate-buffered solution having a 75 times greater buffering capacity than the phosphate-buffered solution caused retinal pHo to become less acidic and significantly decreased the amplitude of the light-evoked pHo changes. 6. Addition of 2.0 mM-acetazolamide, a carbonic anhydrase inhibitor, to the bicarbonate-buffered solution increased both the pHo gradient and the light-evoked changes in pHo. These data are consistent with the idea that carbonic anhydrase, which is concentrated in Müller (glial) cells and to a lesser extent in horizontal cells, increases the effectiveness of the bicarbonate buffer system. 7. Switching from bicarbonate-buffered to phosphate-buffered solutions attenuated the b-wave of the electroretinogram, most likely by acidifying pHo. Overall, our results emphasize the importance of the bicarbonate buffer system in buffering pHo during periods of variable acid extrusion in light and in darkness.

Monitoring of patients taking canthaxanthin and carotene: an electroretinographic and ophthalmological survey

1. Patients who have taken canthaxanthin and beta-carotene to avert phototoxicity have been monitored by electroretinographic testing. 2. Patients took the compounds only during the summer months, but were monitored for more than 1 year to determine if seasonal changes in the ERG were visible. 3. The characteristic refractile retinal crystals reduced during the winter. 4. The scotopic b-wave amplitude increased during the winter, whether evoked by red or blue flashes. 5. No other ERG parameter altered. 6. Changes noted in 3 and 4 above are reversible. 7. A dose/ERG-amplitude response relationship was established, but no correlation between blood level or total cumulative dose and b-wave amplitude could be found. 8. It is suggested that the Müller cells concentrate canthaxanthin and this is the mechanism which affects the ERG.

Effects of 2-amino-4-phosphonobutyric acid (APB) and glycine on the oscillatory potentials of the rat electroretinogram

Oscillatory potentials of the electroretinogram were monitored in dark-adapted rats following intravitreal injection of 2-amino-4-phosphonobutyric acid (APB), a glutamate analog that preferentially blocks the light response of depolarizing bipolar cells, or glycine, a known endogenous inhibitory neurotransmitter that suppresses the light response of cells in the inner retina postsynaptic to glycinergic neurons. Oscillatory potentials were abolished in conjunction with the b-wave with APB and selectively reduced or eliminated by glycine: neither agent attenuated the a-wave. The results are compatible with the idea that light-induced depolarizing bipolar cell, hyperpolarizing bipolar cell, and glycinergic amacrine cell responses are all necessary for the generation of oscillatory potentials in the rat. The results also suggest that hyperpolarizing bipolar cells do not contribute to b-wave generation.

Potassium conductance block by barium in amphibian Müller cells

The effect of barium on Müller cell K+ conductance was evaluated in the tiger salamander using enzymatically dissociated cells and cells in situ (retinal slice and isolated retina). Barium effects were similar in both cases. In dissociated cells, 50 microM Ba2+ depolarized cells 14.7 mV and raised cell input resistance from a control value of 16.0 to 133 M omega. For cells in situ, 50 microM Ba2+ depolarized cells 6.9 mV and raised cell resistance from 12.5 to 50.4 M omega. At corresponding Ba2+ concentrations, the resistance of cells in situ was somewhat lower than was the resistance of dissociated cells, a phenomenon that may be due to the small degree of electrical coupling present between Müller cells in situ. There was a similar positive correlation between the magnitude of Ba2+-induced depolarization and input resistance in both dissociated cells and in situ cells. The magnitude of depolarizations generated by localized K+ ejections onto Müller cells was reduced substantially by Ba2+. These observations indicate that Ba2+ is an effective K+ channel blocker in Müller cells in situ as well as in enzymatically dissociated 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.

Tyrosine phosphorylated proteins accumulate in junctional regions of the developing chick neural retina

Antibodies specific for protein phosphotyrosyl residues were used to localize sites of action of tyrosine-specific protein kinases in developing chick neural retina by immunoperoxidase staining. Phosphotyrosine-modified proteins became prominent in growth cone- and process-rich regions of embryonic retina during neuronal differentiation. Maximal levels accumulated in the synaptic layers and limiting membranes of the adult retina, where numerous junctional complexes reside. Two major phosphotyrosine-modified proteins in adult retina (80, 42 kDal) increased markedly during maturation. In contrast, the synaptic layers of optic tectum and other brain regions exhibited low protein phosphotyrosine levels. These results suggest a specific role for protein tyrosine phosphorylation in the retina at sites of synapses and other intercellular junctions.

Abnormal pattern electroretinogram in Alzheimer's disease: evidence for retinal ganglion cell degeneration?

We recorded pattern-reversal electroretinograms, flash electroretinograms, pattern-reversal visual evoked potentials, and flash visual evoked potentials in 6 patients with clinically diagnosed Alzheimer's disease and 6 age- and sex-matched control subjects. The mean amplitude of the pattern-reversal electroretinogram in the Alzheimer patients was significantly less than that of the control group (p = 0.004). This anomaly of the pattern-reversal electroretinogram may be a reflection of documented axonal depletion within the optic nerve and the degeneration of retinal ganglion cells seen in Alzheimer's disease. We found Alzheimer patients to have normal pattern-reversal visual evoked potentials and flash electroretinograms, but a delayed second positive component of the flash visual evoked potential. This combination of findings may be of diagnostic import in Alzheimer's disease.

L-homocysteic acid--a possible bipolar cell transmitter in the rabbit retina

The identity of the retinal bipolar cell transmitter(s) is unknown although there is much indirect evidence that suggests it may be glutamate or a related compound. Some bipolar cells synapse onto cholinergic amacrine cells and in the rabbit retina acetylcholine (ACh) release is increased by light flashes and by the excitatory amino acids glutamate, aspartate and homocysteic acid (HCA). In the retina, the amino acid agonist N-methyl-D-aspartate (NMDA) is unusual in that it sometimes acts as an antagonist, and in the present experiments it blocked the light-evoked release of ACh by acting as an antagonist of the bipolar cell transmitter. However, NMDA did not block the actions of glutamate or aspartate on amacrine cell ACh release, a result that argues against either of these amino acids being the bipolar cell transmitter. On the other hand, the HCA evoked release of ACh was clearly antagonised by NMDA suggesting that HCA may be the bipolar cell transmitter released onto cholinergic amacrine cells. This suggestion is supported by the finding that the rabbit retina possesses HCA at a concentration of 0.8 nmol/g wet wt.

Losses of temporal modulation sensitivity in retinal degenerations

Sensitivity losses in patients with retinitis pigmentosa (RP) have been attributed to a decrease in photopigment density, to a reduction in the number of photoreceptors, and also to a change in temporal response properties of the receptors. The sensitivity losses in patients with macular degeneration have also been attributed to a loss of photoreceptors. To test these explanations for sensitivity loss we obtained electrophysiological and psychophysical temporal modulation transfer functions (MTFs) on normal subjects in response to varying stimulus luminances and retinal loci. These stimulus manipulations did not duplicate the changes observed in the temporal MTFs of patients. The temporal response properties of the receptors were tested electrophysiologically by manipulating stimulus presentation interval. The results provided evidence for sensitivity losses in RP patients being due to alterations in the temporal response properties of the receptors.

Electrophysiologic testing techniques for children

This practical article for clinical electrophysiologists discusses the evaluation of infant and child patients' visual systems using electroretinographic (ERG), electrooculographic (EOG), and visually evoked potential (VEP) techniques. These techniques not only help to secure specific diagnoses, but by systematic assessment of function along the visual pathways can also localize dysfunction underlying visual deficits of pediatric patients. Among children, development as well as disease can affect electrophysiological parameters. Therefore diagnosis of normal or abnormal depends critically on an adequate description of normal responses for age. Procedures that the authors have found feasible, reliable, and valid are summarized. Standardization of pediatric testing appears to be an important next step. The power of ERG, EOG and VEP recordings to demonstrate the neurophysiological basis for pediatric visual impairment is predicted to stimulate further research in this area.

Sensitivity of ERG components from dark-adapted goldfish retinas treated with APB

The electroretinogram (ERG) of the dark-adapted goldfish was examined before and after intravitreal injection of DL-2-amino-4-phosphonobutyric acid (APB). APB abolished the b-wave and decreased absolute sensitivity of the remaining waveform, which was composed of a vitreal-negative component followed by a vitreal-positive component. The sensitivity, time course and amplitude of these components differed from ERGs obtained from animals treated with sodium aspartate. Spectral sensitivity of both post-APB components closely resembled that of the normal dark-adapted b-wave. The results suggest that APB does not act selectively on any particular class of photoreceptors or photoreceptor pathways in the dark-adapted goldfish retina.

B-wave of the electroretinogram. A reflection of ON bipolar cell activity

Light-evoked intraretinal field potentials (electroretinogram, ERG) have been measured simultaneously with extracellular potassium fluxes in the amphibian retina. The application of highly selective pharmacologic agents permitted us to functionally isolate various classes of retinal neurons. It was found that: (a) application of APB (2-amino-4-phosphonobutyrate), which has previously been shown to selectively abolish the light responsiveness of ON bipolar cells, causes a concomitant loss of the ERG b-wave and ON potassium flux. (b) Conversely, PDA (cis 2,3-piperidine-dicarboxylic acid) or KYN (kynurenic acid), which have been reported to suppress the light responses of OFF bipolar, horizontal, and third-order retinal neurons, causes a loss of the ERG d-wave as well as OFF potassium fluxes. The b-wave and ON potassium fluxes, however, remain undiminished. (c) NMA (N-methyl-DL-aspartate) or GLY (glycine), which have been reported to suppress the responses of third-order neurons, do not diminish the b- or d-waves, nor the potassium fluxes at ON or OFF. This leads to the conclusion that the b-wave of the ERG is a result of the light-evoked depolarization of the ON bipolar neurons. This experimental approach has resulted in two further conclusions: (a) that the d-wave is an expression of OFF bipolar and/or horizontal cell depolarization at the termination of illumination and (b) that light-induced increases in extracellular potassium concentration in both the inner (proximal) and outer (distal) retina are the result of ON bipolar cell depolarization.

Fast activity and oscillatory potential of carp retina in the frequency domain

There are two kinds of fast activity in the ERG: fast retinal potentials (FRP), an irregular series of spiky wavelets and oscillatory potentials (OP), a rhythmic sequence of events. Corneal ERG from nine intact young carps, evoked by extended pulses of diffuse white light under mesopic adaptation, displayed two different groups of wavelets related to ON and OFF, respectively. Stimulation and recording conditions were established to permit separate Fourier analysis of both groups of wavelets. Power distributions of normalized ON spectra showed both a wide dispersion and a high inter-subject variability. All normalized OFF spectra showed, instead, components within a narrow band from 52 to 56 Hz, most of them maximum relative power peaks. It is concluded that FRP originating in highly labile sources dominate ON fast activity, while the predominant OFF fast activity are OP originating in a stable discrete source.

The electroretinogram in multiple sclerosis and demyelinating optic neuritis

The electroretinogram (ERG) to flashes of white light presented under photopic conditions and the pattern reversal visual evoked potentials (PR-VEPs) from both eyes were recorded from 14 patients with multiple sclerosis (MS) with monocular demyelinating optic neuritis (DON) and from 11 patients soon after presenting with monocular demyelinating optic neuritis alone. Fifteen and 10 normal subjects, matched for age and sex, were used as controls for each group of patients respectively. In the DON group of patients and controls the flicker following ERG (FF-ERG) to white flashes of light at 40 Hz was also recorded. Skin electrodes and averaging procedures were used for all the recordings. The PR-VEP elicited with stimulation of the affected eye was absent or abnormally delayed, and the amplitude of the 'b' wave of ERG of the affected eye was diminished in all patients. The 'b' wave latency, however, was similar in both affected and non-affected eyes and the controls. There was no difference in 'a' wave amplitude and latency between eyes of patients and normal subjects. The FF-ERG in 8 out of 10 patients with satisfactory recordings was diminished in the affected eye. These results provide neurophysiological evidence that retinal damage is not due to loss of myelin but is an early feature of demyelinating optic neuritis. This damage preferentially affects the retinal elements associated with the generation of the 'b' wave of the ERG, probably the glial cells of Müller.

Current source density analysis of linear and non-linear components of the primate electroretinogram

1. We have used the method of current source density analysis to locate the generators of harmonic electroretinogram (ERG) responses to contrast-modulated pattern and uniform-field stimuli in the primate retina. 2. Sinusoidal steady-state analysis was used, with a stimulus temporal frequency of 8 Hz. Fundamental and second-harmonic response components were measured for the uniform-field response. The second harmonic of the average of contrast-reversal pattern responses obtained at a series of spatial phases was also determined in the same experiments. In addition, retinal tissue resistance was measured. All of these measurements were obtained at a series of equally spaced depths in the retina. 3. Retinal resistivity was not observed to vary systematically with depth. In addition, any plausible undetected inhomogeneities of resistivity with depth were found to slightly affect the relative magnitudes of estimated current sources and sinks, but to have little effect on their localization. 4. In a given penetration, the phase lag of each harmonic component was relatively constant with depth in most cases; however the magnitude of this phase lag sometimes varied in different penetrations. To compare data from different penetrations, the constant phase lag for each harmonic was estimated, and the response data phase-shifted so as to bring all data into a standard (cosine) phase. 5. The resulting current source density analyses were found to be quite consistent in overall form for different penetrations and in different animals. These data were averaged to obtain a final estimate of the depth profiles for generators of different ERG components. 6. The uniform-field fundamental response was found to have a predominant source-sink pair in the distal half of the retina (receptor layer to outer plexiform layer). The pattern (second-harmonic) response generators had a quite different depth profile, consisting mainly of a source-sink pair in the proximal 20% of the retina (encompassing the nerve fibre layer to the middle of the inner plexiform layer). The uniform-field second-harmonic response showed a current source density (CSD) depth profile with multiple sources/sinks, as if it contained contributions from the other two.

A quantitative analysis of glial cell coupling in the retina of the axolotl (Ambystoma mexicanum)

The strength of gap junctional coupling of radial glial cells (Müller cells) in the isolated axolotl retina was assessed by monitoring the spread of dye between cells, and by injecting current into one cell and recording the voltage response in surrounding cells. Dye injected into one Müller cell spread to surrounding Müller cells, and could be detected up to 130 micron away, i.e. over 4 times the mean Müller cell spacing of 30 micron. Injecting 1 nA of current into a Müller cell evoked responses of 7 mV in that cell, 1 mV in next neighbour cells, and 0.2 mV in cells at 60 micron distance. Analysis of these data indicates an electrical space constant for the Müller cell network of 15 micron, and predicts that isolated cells should have a resistance of 11.4 M omega. Müller cells isolated by papain dissociation of the retina were found, by whole-cell patch-clamping, to have a mean resistance of 12.4 M omega. These results on lateral coupling are combined with data showing that over 90% of the Müller cell potassium conductance is in the vitreal endfoot of these cells to provide a fairly complete electrical description of the radial glial cell network in the retina. Gap junctional coupling of Müller cells increases by 60% the 'spatial buffering' that these glial cells can carry out to reduce localized rises in extracellular potassium concentration. The location of the majority of the Müller cell potassium conductance in the cell endfoot ensures that laterally buffered K+ is deposited in the vitreous, rather than depolarizing surrounding retinal neurones.