S-Potentials in the Skate Retina : Intracellular recordings during light and dark adaptation

The S-potentials recorded intracellularly from the all-rod retina of the skate probably arise from the large horizontal cells situated directly below the layer of receptors. These cells hyperpolarize in response to light, irrespective of stimulus wavelength, and the responses in photopic as well as scotopic conditions were found to be subserved by a single photopigment with λ_max = 500 nm. The process of adaptation was studied by recording simultaneously the threshold responses and membrane potentials of S-units during both light and dark adaptation. The findings indicate that the sensitivity of S-units, whether measured upon steady background fields or in the course of dark adaptation, exhibits changes similar to those demonstrated previously for the ERG b-wave and ganglion cell discharge. However, the membrane potential level of the S-unit and its sensitivity to photic stimulation varied independently for all the adapting conditions tested. It appears, therefore, that visual adaptation in the skate retina occurs before the S-unit is reached, i.e., at the receptors themselves.     

Quantum Relations of the Rat Electroretinogram

The rat retina is uniform and contains almost exclusively rods. Therefore the rat eye, when uniformly illuminated, produces a gross electroretinogram (ERG) which is simply related to the activity of the individual retinal sources of the ERG. Characteristics of ERG's are shown on an intensity scale of the average number of quanta absorbed per rod per stimulus flash obtained by direct accurate measurement of all quantities involved. An independent check on the accuracy of these measurements is applied to pigment-bleaching data reported by Dowling (1963). When ERG characteristics are placed on this scale it is found that: (a) The b-wave can usually be observed when fewer than one out of two hundred rods absorbs a quantum, the threshold being determined by the noise of the preparation. (b) Near threshold the b-wave amplitude is proportional to intensity. (c) The a-wave appears when there are more than two to four absorptions per rod per flash. (d) The b-wave latency decreases with intensity, and the amplitude becomes proportional to the logarithm of intensity when fewer than one out of ten rods absorbs a quantum. This implies that the b-wave sources must combine excitation from more than one rod (probably more than seven). Therefore the b-wave cannot arise from independent rods or rod-bipolar synapses, but probably reflects activity of entire inner nuclear layer cells.     

Influence of picrotoxin on the retinal sensitivity at different levels of background illumination

The intensity-response function of the b-wave of the ERG in the retina of the turtle (Emys orbicularis) was investigated at two different background illuminations--0.01 and 10 lx, before and after blockade of the GABA-ergic transmission by 0.4 mmol/l picrotoxin (PT). A shift of the curve to the left along the intensity axis at both backgrounds was observed after PT treatment. The region where the b-wave amplitude was linear function of the logarithm of the stimulus intensity was also considerably narrowed. The registered experimental data fitted in well with the Naka-Rushton equation. The participation of GABA-ergic neurones in the processes of light adaptation and gain control in the retina is discussed.     

Slow PIII component of the carp electroretinogram

The slow PIII component of the electroretinogram (ERG) was studied in the isolated, aspartate-treated carp retina. Although the latter is richly populated with cones, slow PIII appeared to reflect almost exclusively the activity of rods; e.g. the spectral sensitivity of the potential paralleled closely the rod pigment curve, its operating range (i.e. the V-log I curve) was limited to 3 log units above absolute threshold, and raising background intensities to photopic levels produced saturation of the increment threshold function without evidence of a cone-mediated segment. Only after bleaching away a significant fraction of the porphyropsin was it possible to unmask a small photopic contribution to slow PIII, as evidenced by a displacement in the action spectrum to longer wavelengths. The spatial distribution of the slow PIII voltage within the retina (Faber, D.S. 1969. Ph.D. Thesis. State University of New York. Buffalo, N.Y.; Witkovsky, P.J. Nelson, and H. Ripps. 1973. J. Gen Physiol. 61:401) and its ability to survive aspartate treatment indicate that this potential arises in the Muller (glial) fiber. Additional support for this conclusion is provided by the slow rise time (several seconds) and long temporal integration (up to 40s) of the response. In many respects the properties of slow PIII resemble those of the c-wave, a pigment epithelial response also subserved by rod activity. On the other hand, the receptoral (fast PIII) and the b-wave components of the ERG behave quite differently. Unlike slow PIII, response saturation could not be induced, since both potentials are subserved by cones when the stimulus conditions exceed the limits of the scotopic range. Receptors appear to govern light adaptation at photopic background levels; both fast PIII and b-wave manifest identical incremental threshold values over this range of intensities. However, under scotopic conditions, the sensitivity of the b-wave is affected by luminous backgrounds too weak to alter fast PIII threshold, indicating a postreceptoral stage of adaptation.     

Electrical activity of the retina in different axial lengths of the eye in humans

In subjects with normal vision, we studied the relationship between the axial length of the eye and the amplitude-temporal characteristics of different types of ganzfield ERG. The amplitude of the ERG had the highest variability in the middle of the range of the axial eye lengths. The variability decreased at both ends of the range. With increasing axial length, the amplitude of the b-wave of all types of ERG decreased, and the latency decreased concurrently. The b-wave reflects the activity of the retinal ON-neurones depolarizing in response to the light stimulus. We assume that the decrease in the amplitude of b-wave may be related to the decreased number of photoreceptors and of neurons in the following retinal levels and/or increased inhibition in proximal retina, as well as an increase in relative activity of retinal OFF-neurones hyperpolarizing in response to the light stimulus.     

Comparative study of the rod and cone contributions to the generation of b-wave ERG and tectal evoked potential in the dark-adapted carp

Amplitudes and peak latencies as functions of wave length and monochromatic light intensity were investigated for b-wave ERG and tectal evoked potentials (EP) in the dark-adapted carp (Cyprinus carpio L). It was found, that independently of light intensity b-wave action spectra had one maximum in the medium wave band, corresponding to rod sensitivity area. For tectal EP, similar action spectra with maximum in the middle-wave were seen at low light intensity only. The b-wave amplitude growth was significant for the whole band of light intensities, and these changes were accompanied with a slight decrease in peak latency (to 50-100 ms). Tectal EP amplitude increased when low-intensity light was changed for medium intensity light and did not considerably increase to brighter light stimuli. However, tectal EP time latency significantly decreased (to 100-200 ms) during light intensity increasing. This differences show that retinal rod system, which in responsible for ERG b-wave in darkness, is not a key factor in the generation of tectal EP.     

鳜鱼视觉特性及其对捕食习性适应的研究Ⅰ．视网膜电图光谱敏感性和适应特性

采用电生理方法研究了夜行性凶猛鱼类鳜鱼视网膜电图的一般特性、光谱敏感性和适应特性。顷鱼的视网膜电图不显示典型的混合型视网膜特征。明视和暗视视网膜电图的光谱敏感曲线形状基本相同,峰值都在５３０ｎｍ处,没有出现Ｐｕｒｋｉｎｊｅ氏位移。明适应曲线仅出现下降型变化,暗适应过程异常缓慢。鳜鱼的视网膜仅存在单一的光感受系统,即暗视系统,不可能形成色觉。但级鱼视网膜具有很高的光敏感性,适于弱光视觉。     

Oscillatory potentials in the electroretinogram.

This paper presents an analog circuit model of the electroretinogram b-wave and subsequent oscillations postulated to result from the depolarization of the retinal glial cells produced by potassium. The system is described by a second-order equation which oscillates under conditions of light adaptation. With dark adaptation the oscillations undergo attenuation. Light falling on the retina ultimately liberates potassium ions and this ionic current is considered to be the system input. A first approximation is an exponentially decaying current which varies depending upon light background, flash intensity and various drug manipulations. This model is a reasonable fit to the electroretinogram over considerable light background levels and is suggested as a useful tool in the study of various pharmacological effects on retinal electrophysiology.     

蛙离体视网膜中杆系统对锥系统的抑制作用

在蛙离体视网膜标本上研究了由条件刺激(F_c)引起的快速暗适应(RDA)过程中b 波的变化。当F_c超过一定强度时,以501nm 闪光为测试刺激(F_(t(501)))时b波阈值单调下降,先为锥相后为杆相,两者过渡处呈现杆-锥转折;但以654nm闪光为测试刺激时(F_(t(654))),阈值先略有下降,继而上升,之后再下降。F_(t(654))应阈值在RDA 过程中的这种升高是反映575锤系统的活动受到502杆系统抑制的结果,433杆系统对此没有明显的贡献,主要依据是:(1)F_(t(654))反应阈值的上升在时间上与暗适应曲线的杆相的出现一致;(2)在RDA 过程的锥相,反应的光谱敏感性与锥色素吸收光谱一致,而在F_(t(654))反应阈值上升到最大值时,反应的光谱敏感性与502杆色素的吸收光谱非常接近;(3)对502杆系统作用等效的F_(c(439))、F_(c(501))和F_(c(616))作用后,F_(t(654))的敏感度变化在整个RDA 过程中基本相同。     

The d-wave in fish and the state of light adaptation

Comparative electroretinographic studies of the d-wave evoked with long duration photo stimuli in dark- and light-adapted fish species (three marine and three freshwater) were performed. At the end of prolonged photo-stimulation in scotopic conditions a negative d-wave appears in electroretinograms of dogfish shark, eel and goldfish diminishing and eventually changing with intensity of photo-stimulation, while in rudd it only increases. Dark-adapted electroretinograms of two percids (perch and painted comber) exhibit a positive d-wave that approaches the b-wave amplitude under bright photopic conditions. Judging from the d-wave, only the rod pathway is active in dark-adapted dogfish shark, eel, and goldfish. Under the light adaptation, cone pathways are active in eel and goldfish, whereas the positive response to the end of light stimuli in dogfish shark could be explained by independent ON and OFF pathways from outer to inner retina via bipolar cells. In the case of two percids, dark adaptation has no influence on cone pathways. The d-wave of rudd behaves like cone-driven d-waves but in opposite sign. The data thus show that the d-wave form, amplitude and sign depend on interconnection of ON and OFF pathways as determined by the state of adaptation and/or type of photoreceptor.     

Visual Adaptation in the Retina of the Skate

The electroretinogram (ERG) and single-unit ganglion cell activity were recorded from the eyecup of the skate (Raja erinacea and R. oscellata), and the adaptation properties of both types of response compared with in situ rhodopsin measurements obtained by fundus reflectometry. Under all conditions tested, the b-wave of the ERG and the ganglion cell discharge showed identical adaptation properties. For example, after flash adaptation that bleached 80% of the rhodopsin, neither ganglion cell nor b-wave activity could be elicited for 10–15 min. Following this unresponsive period, thresholds fell rapidly; by 20 min after the flash, sensitivity was within 3 log units of the dark-adapted level. Further recovery of threshold was slow, requiring an additional 70–90 min to reach absolute threshold. Measurements of rhodopsin levels showed a close correlation with the slow recovery of threshold that occurred between 20 and 120 min of dark adaptation; there is a linear relation between rhodopsin concentration and log threshold. Other experiments dealt with the initial unresponsive period induced by light adaptation. The duration of this unresponsive period depended on the brightness of the adapting field; with bright backgrounds, suppression of retinal activity lasted 20–25 min, but sensitivity subsequently returned and thresholds fell to a steady-state value. At all background levels tested, increment thresholds were linearly related to background luminance.     

The translocation of signaling molecules in dark adapting mammalian rod photoreceptor cells is dependent on the cytoskeleton

In vertebrate rod photoreceptor cells, arrestin and the visual G-protein transducin move between the inner segment and outer segment in response to changes in light. This stimulus dependent translocation of signalling molecules is assumed to participate in long term light adaptation of photoreceptors. So far the cellular basis for the transport mechanisms underlying these intracellular movements remains largely elusive. Here we investigated the dependency of these movements on actin filaments and the microtubule cytoskeleton of photoreceptor cells. Co-cultures of mouse retina and retinal pigment epithelium were incubated with drugs stabilizing and destabilizing the cytoskeleton. The actin and microtubule cytoskeleton and the light dependent distribution of signaling molecules were subsequently analyzed by light and electron microscopy. The application of cytoskeletal drugs differentially affected the cytoskeleton in photoreceptor compartments. During dark adaptation the depolymerization of microtubules as well as actin filaments disrupted the translocation of arrestin and transducin in rod photoreceptor cells. During light adaptation only the delivery of arrestin within the outer segment was impaired after destabilization of microtubules. Movements of transducin and arrestin required intact cytoskeletal elements in dark adapting cells. However, diffusion might be sufficient for the fast molecular movements observed as cells adapt to light. These findings indicate that different molecular translocation mechanisms are responsible for the dark and light associated translocations of arrestin and transducin in rod photoreceptor cells.     

Retinal mechanisms of visual adaptation in the skate

Electrical potentials were recorded from different levels within the skate retina. Comparing the adaptive properties of the various responses revealed that the isolated receptor potential and the S-potential always exhibited similar changes in sensitivity, and that the b-wave and ganglion-cell thresholds acted in concert. However, the two sets of responses behaved differently under certain conditions. For example, a dimly iluminated background that had no measurable effect on the senitivities of either of the distal responses, raised significantly the thresholds of both the b-wave and the ganglion cell responses. In addition, the rate of recovery during the early, "neural" phase of dark adaptation was significantly faster for the receptor and S-potentials than for the b-wave or ganglion cell discharge. These results indicate that there is an adaptive ("network") mechanism in the retina which can influence significantly b-wave and gaglion cell activity and which behaves independently of the receptors and horizontal cells. We conclude that visual adaptation in the skate retina is regulated by a combination of receptoral and network mechanisms.     

The proximal negative response and visual adaptation in the skate retina

The proximal negative response (PNR), a complex extracellular potential derived mainly from amacrine cell activity, was studied in the all-rod retina of the skate. Tetrodotoxin (10(-6) mg/ml) did not affect either the waveform or the latency of the response, indicating that the PNR reflects the graded, nonregenerative components of the amacrine cell potential. As regards its adaptive properties, the PNR exhibited both the extreme sensitivity to weak background light and the slow time course of light and dark adaptation that are characteristic of other responses from the proximal retina. Thus, the PNR, like the b-wave and ganglion cell discharge, appears to reflect adaptive processes located within the neural network of the inner retina.     

The DC-Recorded Dog Electroretinogram in Ketamine -Medetomidine Anaesthesia

A new selective alpha 2-adre-noreceptor agonist, medetomidine hydrochloride was combined with low dosage ketamine hydrochloride and vecuronium bromide for d.c. (direct current) recordings of fast electroretinographic (ERG) components in nine ophthalmoscopically healthy dark adapted dogs. The dogs were tracheally intubated and manually ventilated. They were given full field single flash stimuli of different intensities starting with near b-wave threshold blue light (tests 1-3), followed by white light (tests 4-6) and 30 Hz photopic flicker (test 7). The a- and b-wave amplitudes and flicker responses were measured from the base line. The latencies were measured from the stimulus moment to the highest point of the different waves. Statistical analysis of results gave individual differencies which had a good constancy. This showed that the dogs had an individual ERG profile according to the standardized method. The latencies varied very little as expected, but the amplitudes differed individually and showed a good constancy as seen by reproducibility tests made nine to ten days later on three of the dogs’ ipsilateral eyes. The combination of drugs used in this study was considered suitable for short term (10-12 minutes) stable d.c.–ERG recordings in dogs as the rod and cone responses had higher amplitudes when compared to an identical examination made with other anaesthetic combinations on the same dogs. Involuntary eye movements and other involuntary muscular activity caused by ketamine in dogs were negligible when using medetomidine premedication and was completely absent when using vecuronium. The anaesthetic method described can be recommended for ambulatory ERG recordings in dogs because of the above mentioned advantages.     

Dark adaptation of the photoreceptors in the isolated frog retina during induced lipid peroxidation

The influence of lipid peroxidation (LPO) in isolated frog retina on dark adaptation of photoreceptors was studied. Stimulus-response functions, late receptor potential (LRP) as function of the stimulus light intensity were measured before bleach and in a steady state after dark adaptation. It was shown that accumulation of LPO products influenced dark adaptation in photoreceptors. Based on the displacements of the stimulus-response curves and experimental measurement data on the rate of LRP collapse after retina treatment with strophanthin it is concluded that the most probable mechanism of such an influence lies in a change of photoreceptor plasma membrane permeability.     

Detection of light intensity in the frog retina: evidence from dark and light adaptation

The frog retina was stimulated with light flashes homogeneous in space but not time. The time heterogeneity of stimulation was created by abrupt change of a referent stimulus for a stimulus with different luminance. Such changes form a time pattern, as well as sharp borders of luminance between the neighbor areas of the visual field form a spatial pattern. The electroretinogram recorded in response to presentation of a triad of stimuli: the onset of a short flash of homogeneous light after long dark (or light) adaptation of a retina, brief sequence of the referent and test light flashes varied in luminance, and the offset, with returning to the initial level of adaptation. It was shown that responses of the retina under conditions of time heterogeneity of stimulation could be divided in two types as well as under conditions of spatial heterogeneity. Such a dual change in amplitude confirms our earlier hypothesis on the existence of two mechanisms of luminance coding in the frog retina. The first mechanism encodes power characteristics of light, it forms the information on the absolute level of the environmental luminance. Its activity is connected basically with receptors and cells of the external plexiform layer of the frog's retina. It is responsible for the b-wave of the electroretinogram. The other mechanism associated with RERG is based on a vector code of stimuli. This mechanism forms the information on spatial and time differentiation of the light flow in the visual field and is connected basically with cells of the internal plexiform layer. The results suggest that the frog retina has the individual mechanism for time pattern detection, distinguishing it from the homogeneous light flow in a similar way as in case of spatial light pattern detection. It is possible that the first mechanism is responsible for the detection of any new stimulus in general, irrespective of its specificity, whereas the second mechanism serves for the measurement of suprathreshold differences between stimuli.     

Adaptation properties of the ERG in the grasshopper,Romalea microptera

The grasshopper ERG displays a rapid recovery of responsivity following the onset of a background light. Although observed earlier in skate and frog, this phenomenon has not previously been seen in an invertebrate. Furthermore, a period of hyperresponsivity exists in early dark adaptation and resembles that found in skate and frog. Thus, recovery in the light and hyperresponsivity in the dark seem to be corollaries of each other. Finally, spectral sensitivity of the ERG is determined and two peaks are found: one at 510 nm and the other at 360 nm. The former appears to be a rhodopsin-mediated sensitivity but the latter does not and they are not clearly separated by chromatic adaptation.     

The Rat Electroretinogram : II. Bloch's law and the latency mechanism of the b-wave

Electroretinograms were obtained from the all-rod eye of the rat with uniform illumination of the entire retina and stimulus flashes of less than 3 msec. duration. Bloch's law of temporal summation was verified for the b-wave latency by varying the time between two equal intensity flashes and observing that no change occurred in the latency when measured from the midpoint of the two flashes. The results of this and other experiments are described in terms of a simple but general model of the latency-determining mechanism. It is shown that this latency mechanism acts as if it depends on a linear additive process; and also that a hypothetical excitatory substance which triggers activity in the sources of the b-wave must accumulate rapidly in time after the flash, approximately as t⁸. The rate at which this substance accumulates is accurately represented by the diffusion equation for more than 4 to 6 log units in the flash intensity. This suggests that the rate-determining step in the latency mechanism may be diffusion-limited.     

Oxygen distribution and consumption in the cat retina during normoxia and hypoxemia

Oxygen tension (PO2) was measured with microelectrodes within the retina of anesthetized cats during normoxia and hypoxemia (i.e., systemic hypoxia), and photoreceptor oxygen consumption was determined by fitting PO2 measurements to a model of steady-state oxygen diffusion and consumption. Choroidal PO2 fell linearly during hypoxemia, about 0.64 mmHg/mmHg decrease in arterial PO2 (PaO2). The choroidal circulation provided approximately 91% of the photoreceptors' oxygen supply under dark-adapted conditions during both normoxia and hypoxemia. In light adaptation the choroid supplied all of the oxygen during normoxia, but at PaO2's less than 60 mmHg the retinal circulation supplied approximately 10% of the oxygen. In the dark-adapted retina the decrease in choroidal PO2 caused a large decrease in photoreceptor oxygen consumption, from approximately 5.1 ml O2/100 g.min during normoxia to 2.6 ml O2/100 g.min at a PaO2 of 50 mmHg. When the retina was adapted to a rod saturating background, normoxic oxygen consumption was approximately 33% of the dark-adapted value, and hypoxemia caused almost no change in oxygen consumption. This difference in metabolic effects of hypoxemia in light and dark explains why the standing potential of the eye and retinal extracellular potassium concentration were previously found to be more affected by hypoxemia in darkness. Frequency histograms of intraretinal PO2 were used to characterize the oxygenation of the vascularized inner half of the retina, where the oxygen distribution is heterogeneous and simple diffusion models cannot be used. Inner retinal PO2 during normoxia was relatively low: 18 +/- 12 mmHg (mean and SD; n = 8,328 values from 36 profiles) in dark adaptation, and significantly lower, 13 +/- 6 mmHg (n = 4,349 values from 19 profiles) in light adaptation. Even in the dark-adapted retina, 30% of the values were less than 10 mmHg. The mean PO2 in the inner (i.e., proximal) half of the retina was well regulated during hypoxemia. In dark adaptation it was significantly reduced only at PaO2's less than 45 mmHg, and it was reduced less at these PaO2's in light adaptation.