Aspartate-induced dissociation of proximal from distal retinal activity in the mudpuppy

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Synchronous period-doubling in flicker vision of salamander and man

Periodic flashes of light have long served to probe the temporal properties of the visual system. Here we show that during rapid flicker of high contrast and intensity the eye reports to the brain only every other flash of light. In this regime, retinal ganglion cells of the salamander fire spikes on alternating flashes. Neurons across the entire retina are locked to the same flashes. The effect depends sharply on contrast and flash frequency. It results from a period-doubling bifurcation in retinal processing, and a simple model of nonlinear feedback reproduces the phenomenon. Pharmacological studies indicate that the critical feedback interactions require only cone photoreceptors and bipolar cells. Analogous period-doubling is observed in the human visual system. Under bright full-field flicker, the electroretinogram (ERG) shows a regime of period-doubling between 30 and 70 Hz. In visual evoked potentials from the occiput, the subharmonic component is even stronger. By analyzing the accompanying perceptual effects, we find that retinal period-doubling begins in the periphery of the visual field, and that it is the cause of a long mysterious illusory flicker pattern.

Current-source density analysis of the electroretinogram of the frog: methodological issues and origin of components

The technique of current-source density (CSD) analysis for extracellular potentials is reviewed, along with some methodological features that are important for performing CSD analysis of the electroretinogram. In addition, three formulas for computing CSD's are examined on model circuits of resistors and current generators. Finally, CSD results from frog retina that bear on the origins of the b, d, and M waves, along with slow PIII, are presented. It is concluded that the b and d waves are generated primarily and directly by bipolar cells, whereas the M wave and the slow PIII are generated by Müller (glial) cells through the K+ spatial buffer mechanism.

Effects of quisqualic acid on the corneal and intraretinal direct-current electroretinogram and on the standing potential of the rabbit eye

Quisqualic acid, an excitatory amino acid agonist, has been shown to stimulate inositol phosphate production in the rabbit retina. Inositol trisphosphate serves as a second messenger and increases intracellular calcium. We investigated the influence of quisqualic acid on the direct-current electroretinogram and on the standing potential of the rabbit eye. After unilateral vitrectomy, the corneal direct-current electroretinogram and the standing potential were recorded from both eyes of albino rabbits during simultaneous unilateral intravitreal perfusion with quisqualic acid alternating with control solution. The contralateral eye was used as a control. Intravitreal perfusion with 100-microM and 200-microM quisqualic acid elevated the standing potential significantly. This elevation was accompanied by a significant increase in c-wave amplitude and a significant decrease in b-wave amplitude. Quisqualic acid at 200-microM concentration decreased the a-wave amplitude also. In vivo intraretinal recordings showed that intravitreal perfusion with quisqualic acid at 200-microM concentration significantly increased the retinal pigment epithelial component of the c-wave. We conclude that quisqualic acid influences the direct-current electroretinogram and the standing potential apparently through its action on the retinal pigment epithelium. A possible mode of action is increased production of inositol trisphosphate, followed by an increase in intracellular release of calcium ions and an increase in basal chloride conductance. The decrease in a- and b-wave amplitudes indicates direct effects of quisqualic acid also on the neural retina.

Correlation of dynamic responses in the ON bipolar neuron and the b-wave of the electroretinogram

2-Amino-4-phosphonobutyrate (APB) is known to selectively suppress the light response of ON bipolar cells in the vertebrate retina, and reduce the b-wave of the electroretinogram (ERG) as a consequence. Using slow drug application, the progressive effect of APB was used to compare the relative response amplitudes of the b-wave and the ON bipolar cell. Simultaneous ERG recordings and ON bipolar intracellular recordings were performed in the amphibian retina. The results indicate that there is a strong positive correlation between these two waveforms. This supports the possibility that the b-wave of the ERG is the direct result of ON bipolar cell activity.

Comparison between the slow cornea-negative PIII component of the ERG and potassium changes in the isolated rabbit retina

Light-induced extracellular potassium changes were measured in the isolated rabbit retina superfused by a plasma saline mixture and compared with the electroretinogram. When the transmission to second-order neurons was blocked by aspartate and glutamate or by Mg2+, the electroretinogram consisted of the receptor potential and the cornea-negative slow PIII. Since the onset of PIII could then be seen to precede the decrease in extracellular potassium concentration ([K+]0) around photoreceptors, the [K+]0 decrease could not be the cause of the onset of slow PIII. A possible source for the initial phase of slow PIII could be the electrogenic Na+/bicarbonate transporter mechanism of glial cells. Slow PIII depended highly on the extracellular sodium concentration, and it was larger in solutions buffered with bicarbonate than with HEPES, while the [K+]0 decrease around receptors was unchanged.

Comparison of pharmacological agents (aspartate vs. aminophosphonobutyric plus kynurenic acids) to block synaptic transmission from retinal photoreceptors in frog

The combination of aminophosphonobutyric plus kynurenic acids (APB/Kyn) was compared to aspartate with respect to its ability to block synaptic transmission from photoreceptors. Like aspartate, APB/Kyn blocks photoreceptor synaptic transmission, as monitored by the b- and d-waves of the electroretinogram, by the proximal negative response and M-wave of the proximal retina, and by the light-evoked increase in extracellular K+ concentration in the inner plexiform layer. Unlike aspartate, APB/Kyn has relatively minor effects on retinal resistance, light-evoked changes in K+ and Ca2+ concentrations in the subretinal space, light-evoked changes in subretinal space volume, resting extracellular concentrations of K+ and Ca2+ in the proximal and distal retina, and the c-wave. Effects of APB/Kyn are generally more reversible than effects of Asp. A disadvantage of APB/Kyn is that the a-wave usually becomes smaller and slower. Overall, APB/Kyn disrupts the retina less than aspartate. Therefore, in some situations in which blockade of photoreceptor synaptic transmission is desired, the use of APB/Kyn may be preferable to that of aspartate.

Light and dark induced variations of the c-wave voltage of the chicken eye after treatment with sodium aspartate

Light and dark-induced variations of the ERG c-wave voltage were recorded in control chickens and after intravitreal injection of Na aspartate, a treatment whose main effect is to functionally disconnect the pigment epithelium-photoreceptor complex from second order neurons. After aspartate, the fast light rise which characterizes this preparation is no longer observed; it is substituted for by a potential variation of much slower time course and of lower magnitude. The data totally confirm previous findings obtained through an indirect EOG technique and suggest the participation of inner retinal layers in the generation of the light peak in the chicken eye.

Responses of isolated white perch horizontal cells to changes in the concentration of photoreceptor transmitter agonists

Current and voltage responses elicited by increasing or decreasing the concentration of L-glutamate or its analog kainate around isolated cone horizontal cells were measured with patch pipettes using the whole cell recording configuration. Application of these photoreceptor transmitter agonists induced inward currents in voltage-clamp experiments (for negative holding potentials) and depolarizing responses in current-clamp experiments. Continuous exposure to either drug produced inward currents which were maintained for as long as superfusion with the drugs continued. Reducing the concentration of the agonists by pressure ejection of pulses of drug-free Ringer's solution onto the cells completely turned off the drug-induced currents. Under current-clamp conditions, pulses of control Ringer's elicited hyperpolarizing responses of large amplitude (40-80 mV). The data demonstrate the ability to simulate in vitro the horizontal cell's photoresponses and thus support the use of cultured cells as a model system for studying horizontal cell physiology and pharmacology.

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.

Excitatory amino acids and rod photoreceptor disc shedding: analysis using specific agonists

L-Glutamate and L-aspartate stimulate photoreceptor disc shedding. In order to evaluate the possible involvement of a receptor, we examined the effects of specific excitatory amino acid agonists on rod photoreceptor disc shedding and neural retinal toxicity. Using eyecups from both Xenopus laevis and Rana pipiens, we found that kainate, quisqualate, and N-methyl-D-aspartate (NMDA) were all neurotoxic, but that kainate caused a more extensive inner retinal lesion. Kainate also caused disc shedding at concentrations as low as 10 microM; dihydrokainate, a structural analogue, was at least 100-fold less potent. In contrast, quisqualate induced disc shedding only at concentrations above 5.0 mM, and NMDA had no effect on disc shedding at any concentration examined. Our results suggest that excitatory amino acids act via a receptor of the kainate type to effect disc shedding. The mechanism in the retina or photoreceptor-pigment epithelial complex by which an excitatory amino acid receptor system influences disc shedding remains to be identified.

Effects of kynurenate and other excitatory amino acid antagonists as blockers of light- and kainate-induced retinal rod photoreceptor disc shedding

Photoreceptor disc shedding in the retina involves detachment of discs from distal outer segments and phagocytosis of those discs by adjacent pigment epithelial cells. The disc-shedding process balances the continuous renewal of photosensitive membrane. In amphibians, rod disc shedding normally is light-stimulated. However, excitatory amino acids such as kainate stimulate disc shedding independent of a dark-light transition. Excitatory amino acid-induced disc shedding is accompanied by toxic changes within the retina. To evaluate the possible role of an endogenous excitatory amino acid in the regulation of light-evoked disc shedding, we examined the effects of excitatory amino acid antagonists on kainate-induced and light-evoked disc shedding and on retinal toxicity. Using eyecups from Rana pipiens, we found that kynurenate, D-O-phosphoserine, and cis-2,3-piperidine dicarboxylic acid (cis-PDA) all block both the neurotoxic and disc-shedding effects of kainate. Kynurenate and D-O-phosphoserine, but not cis-PDA, also block light-evoked disc shedding. Our analysis suggests that kynurenate blocks the mechanism by which light "triggers" disc shedding rather than directly inhibiting disc detachment and phagocytosis. The observation that cis-PDA antagonizes the effects of kainate, but not light, suggests that the receptor mediating the kainate effect on disc shedding may not be involved in the normal initiation of the response by light. In contrast, our data on kynurenate suggest that it antagonizes an endogenous mechanism involved in the normal control of disc shedding. Thus, analysis of the differences between cis-PDA and kynurenate as antagonists in the retina may yield important insight into the mechanism by which light initiates disc shedding.

Membrane turnover in rod photoreceptors: ensheathment and phagocytosis of outer segment distal tips by pseudopodia of the retinal pigment epithelium

We have documented the ultrastructural changes that occur within the photoreceptor outer segment and the retinal pigment epithelium (RPE) during photosensitive membrane turnover. We employed an in vitro eyecup preparation from Xenopus laevis in which a large shedding event was induced by adding the excitatory amino acid L-aspartate (Greenberger & Besharse 1985; J. comp. Neurol. 239, 361-372). We found that during L-aspartate-induced shedding the RPE cells formed, on their apical domains, previously undescribed processes that were directly involved in disc phagocytosis. These processes are structurally similar to processes formed by macrophages during phagocytosis and are accordingly referred to as pseudopodia. Pseudopodia were distinguishable from the apical villous process normally extended from the RPE in that they were closely applied to the surface of the outer segment, had a cytoplasmic matrix of low electron density that was devoid of most cellular organelles and were enriched in thin (7 nm diameter) filaments. Filament size, specific pseudopodial staining with the actin-specific probe rhodamine phalloidin and inhibition of pseudopod formation by cytochalasin D suggested that the thin filaments were composed of actin. Pseudopodial formation also occurs during a normal light-initiated shedding event. However, the low frequency of shedding, the asynchrony of the individual shedding events and the transient appearance of the pseudopodia prevented a full appreciation of their role during normal disc shedding. Associated with massive shedding and pseudopodial formation, there was an increased adherence between retina and RPE. During L-aspartate treatment, the apical portions of the RPE cells partitioned with the distal outer segment during retinal isolation. This effect was directly related to the development of pseudopodia and may reflect alteration of surface features of the rod outer segment (ROS)-RPE interface related to phagocytosis. Our observations show that transiently forming pseudopodia are the organelles of phagocytosis and that they may play a role in disc detachment as well.

Excitatory amino acids have different effects on horizontal cells in eyecup and isolated retina

Horizontal cells in the mudpuppy eyecup responded to continuous superfusion with L-glutamate, L-aspartate, kainate and quisqualate with a transient depolarization and reduction of the light evoked responses. However, in isolated retina preparations, in which these substances were applied to the photoreceptor side of the retina, the effects were sustained as long as the agonists were present. These results suggest that the transient action of these agonists in eyecup preparations was due to the rapid development of an intraretinal diffusion barrier, and are consistent with the hypothesis that photoreceptors release an excitatory amino acid transmitter.

Electroretinogram (ERG) sensitivity and phagosome frequency in the normal pigmented rat

Normal adult pigmented rats, born and raised in cyclic light for 25- to 27 days and then placed in darkness for up to 24 hr, showed an inverse relation between electroretinogram (ERG) sensitivity and phagosome frequency in the pigment epithelium over the course of a day. Linear regression revealed that a two-fold increase in the frequency of large phagosomes was associated with approximately a one-third decrease in ERG sensitivity. The observed 37-40% decline in ERG sensitivity 1.5 hr after expected light onset was significantly greater than what would be expected from the measured 11% shortening of rod outer segments at that time of day. Possible explanations for this disparity are considered.

Light-evoked and kainic-acid-induced disc shedding by rod photoreceptors: differential sensitivity to extracellular calcium

In order to study the light and Ca2+ dependence of disc shedding by rod photoreceptors, we have used eyecups prepared from adult Rana pipiens frogs that had been kept in constant light for 4 days. Disc shedding was initiated by a treatment involving 1 hour of darkness followed by exposure to light or by treatment with kainic acid. Maximal L-evoked disc shedding occurred quickly (within 30-60 minutes) after light onset and could be triggered by brief (15 minutes) exposure to light. L-evoked disc shedding was completely blocked by omission of Ca2+ from culture medium or by treatment with 3mM Co2+ or 12 mM Mg2+ in the presence of Ca2+ (2 mM). The response was also blocked by the organic Ca2+ antagonist nifedipine. Experiments designed to distinguish between Ca2+ dependence of the dark- or light-dependent processes necessary for shedding suggest that voltage-sensitive channels mediate a Ca2+-dependent process involved in light-triggering. Kainic acid caused a dose-dependent stimulation of disc shedding under lighting conditions (continuous culture in light or darkness) that did not normally result in a significant response in the absence of the drug. Disc shedding induced by kainic acid was similar in time course and magnitude to that induced by light. However, kainic-acid-induced disc shedding was not inhibited by medium Ca2+ reduction or by the presence of Co2+. The latter observation suggests that kainic acid activates disc shedding directly, by-passing the Ca2+-dependent process involved in the L-evoked response. The Ca2+-dependent process may involve release of an effector of disc shedding that is mimicked by kainic acid.

The effects of continuous superfusion of L-aspartate and L-glutamate on horizontal cells of the turtle retina

We have studied the effects of prolonged superfusion with L-aspartate and L-glutamate on the membrane potential and photoresponses recorded in luminosity type horizontal cells in the turtle retina using an everted eyecup preparation. These acidic amino acids produce effects which are a function of the past history of the impaled cell. Initial prolonged superfusions with 30 mM of either drug has no pronounced effect on the membrane potential and photoresponses of horizontal cells. Subsequent superfusions with either agent eventually produce depolarizations with reductions in the amplitudes of the light evoked responses. These effects, however, are transient; the horizontal cell rehyperpolarizes and the light evoked response grows with time. In a retina which had been stored at 4 degrees centrigrade for 20 hours, an initial superfusion with L-aspartate solution produced an immediate depolarization of the horizontal cell and complete suppression of the light evoked response for as long as the amino acid was present. The data are consistent with the existence of powerful amino acid uptake mechanisms operating at a number of sites within the inner and outer retina but also raise questions about the role of acidic amino acids in the outer plexiform layer of the turtle retina.

Stimulation of photoreceptor disc shedding and pigment epithelial phagocytosis by glutamate, aspartate, and other amino acids

It has been reported that aspartate and glutamate selectively impair the structure (Olney, '82) and function (e.g., Furakawa and Hanawa, '55) of second- and third-order retinal neurons while leaving the photoreceptor unaffected. Either amino acid may mimic the endogenous photoreceptor neurotransmitter (Ehinger, '82). We report here that excitatory amino acids also induce massive rod photoreceptor disc shedding in eyecups of Xenopus laevis maintained in vitro. Disc shedding is the process whereby photoreceptors eliminate effete discs. It involves interaction between the distal outer segment and pigment epithelium. Millimolar L-aspartate and L-glutamate, as well as micromolar kainic acid, a glutamate analog, stimulate disc shedding three- to fivefold higher than normal light-evoked shedding levels and result in extensive inner retinal damage. Fifty-millimolar KCl, 1.0 microM ouabain, and replacement of sodium with choline also stimulate disc shedding and alter retinal structure. Extensive neurotoxicity appears unrelated to disc shedding since other amino acids having no significant or marginal effects on retinal structure also stimulate shedding. While the site and mechanism of action of these effectors, and in particular the excitatory amino acids, is now undefined, the data show that amino acids thought to act directly and specifically on inner retinal neurons can also markedly alter photoreceptor and pigment epithelial metabolism.

Extracellular K+ activity changes related to electroretinogram components. II. Rabbit (E-type) retinas

Electroretinogram (ERG) and extracellular potassium activity (K+o) measurements were carried out in isolated superfused rabbit eyecup preparations under control conditions and during the application of pharmacological agents that selectively modify the light-responsive retinal network. Light-evoked K+o changes in the rabbit (E-type) retina resemble those previously described in amphibian (I-type) retinas. Different components of the light-evoked K+o changes can be distinguished on the bases of retinal depth, V vs. log I properties, and their responses to pharmacological agents. We find two separable sources of light-evoked increases in extracellular K+: a proximal source and a distal source. The properties of the distal light-evoked K+o increase are consistent with the hypothesis that it initiates a K+-mediated current through Müller cells that is detected as the primary voltage of the electroretinographic b-wave. These experiments also support previous studies indicating that both the corneal-positive component of c-wave and the corneal-negative slow PIII potential result from K+-mediated influences on, respectively, the retinal pigment epithelium and Müller cells.

Extracellular K+ activity changes related to electroretinogram components. I. Amphibian (I-type) retinas

Electroretinographic (ERG) and extracellular potassium activity measurements were carried out in superfused eyecup preparations of several amphibians. Light-evoked changes in extracellular K+ activity were characterized on the bases of depth profile analysis and latency measurements and through the application of pharmacological agents that have selective actions on the retinal network. Three different extracellular potassium modulations evoked at light onset were identified and characterized according to their phenomenological and pharmacological properties. These modulations include two separable sources of light-evoked increases in extracellular K+: (a) a proximal source that is largely post-bipolar in origin, and (b) a distal source that is primarily or exclusively of depolarizing bipolar cell origin. The pharmacological properties of the distal extracellular potassium increase closely parallel those of the b-wave. A distal light-evoked decrease in extracellular potassium appears to be associated with the slow PIII potential, based on a combination of simultaneous intracellular Müller cell recordings and extracellular ERG and potassium activity measurements before and during pharmacological isolation of the photoreceptor responses. The extracellular potassium activity increases are discussed with respect to the Müller cell theory of b-wave generation.