Carp horizontal cells in culture respond selectively to L-glutamate and its agonists

Neuromodulation: Actions of Dopamine, Retinoic Acid, Nitric Oxide, and Other Substances on Retinal Horizontal Cells

Whereas excitation and inhibition of neurons are well understood, it is clear that neuromodulatory influences on neurons and their synapses play a major role in shaping neural activity in the brain. Memory and learning, emotional and other complex behaviors, as well as cognitive disorders have all been related to neuromodulatory mechanisms. A number of neuroactive substances including monoamines such as dopamine and neuropeptides have been shown to act as neuromodulators, but other substances thought to play very different roles in the body and brain act as neuromodulators, such as retinoic acid. We still understand little about how neuromodulatory substances exert their effects, and the present review focuses on how two such substances, dopamine and retinoic acid, exert their effects. The emphasis is on the underlying neuromodulatory mechanisms down to the molecular level that allow the second order bipolar cells and the output neurons of the retina, the ganglion cells, to respond to different environmental (ie lighting) conditions. The modulation described affects a simple circuit in the outer retina, involves several neuroactive substances and is surprisingly complex and not fully understood.

A Life in Vision

I was drawn into research in George Wald's laboratory at Harvard, where as an undergraduate and graduate student, I studied vitamin A deficiency and dark adaptation. A chance observation while an assistant professor at Harvard led to the major research of my career-to understand the functional organization of vertebrate retinas. I started with a retinal circuit analysis of the primate retina with Brian Boycott and intracellular retinal cell recordings in mudpuppies with Frank Werblin. Subsequent pharmacology studies with Berndt Ehinger primarily with fish focused on dopamine and neuromodulation. Using zebrafish, we studied retinal development, neuronal connectivity, and the effects of genetic mutations on retinal structure and function. Now semi-retired, I have returned to primate retinal circuitry, undertaking a connectomic analysis of the human fovea in Jeffrey Lichtman's laboratory.

Quantifying the effect of light activated outer and inner retinal inhibitory pathways on glutamate release from mixed bipolar cells

Inhibition mediated by horizontal and amacrine cells in the outer and inner retina, respectively, are fundamental components of visual processing. Here, our purpose was to determine how these different inhibitory processes affect glutamate release from ON bipolar cells when the retina is stimulated with full-field light of various intensities. Light-evoked membrane potential changes (ΔV_m ) were recorded directly from axon terminals of intact bipolar cells receiving mixed rod and cone inputs (Mbs) in slices of dark-adapted goldfish retina. Inner and outer retinal inhibition to Mbs was blocked with bath applied picrotoxin (PTX) and NBQX, respectively. Then, control and pharmacologically modified light responses were injected into axotomized Mb terminals as command potentials to induce voltage-gated Ca²⁺ influx (Q^Ca ) and consequent glutamate release. Stimulus-evoked glutamate release was quantified by the increase in membrane capacitance (ΔC_m ). Increasing depolarization of Mb terminals upon removal of inner and outer retinal inhibition enhanced the ΔV_m /Q^Ca ratio equally at a given light intensity and inhibition did not alter the overall relation between Q^Ca and ΔC_m . However, relative to control, light responses recorded in the presence of PTX and PTX + NBQX increased ΔC_m unevenly across different stimulus intensities: at dim stimulus intensities predominantly the inner retinal GABAergic inhibition controlled release from Mbs, whereas the inner and outer retinal inhibition affected release equally in response to bright stimuli. Furthermore, our results suggest that non-linear relationship between Q^Ca and glutamate release can influence the efficacy of inner and outer retinal inhibitory pathways to mediate Mb output at different light intensities.

Calcium dynamics and regulation in horizontal cells of the vertebrate retina: lessons from teleosts

Horizontal cells (HCs) are inhibitory interneurons of the vertebrate retina. Unlike typical neurons, HCs are chronically depolarized in the dark, leading to a constant influx of Ca2+ Therefore, mechanisms of Ca2+ homeostasis in HCs must differ from neurons elsewhere in the central nervous system, which undergo excitotoxicity when they are chronically depolarized or stressed with Ca2+ HCs are especially well characterized in teleost fish and have been used to unlock mysteries of the vertebrate retina for over one century. More recently, mammalian models of the retina have been increasingly informative for HC physiology. We draw from both teleost and mammalian models in this review, using a comparative approach to examine what is known about Ca2+ pathways in vertebrate HCs. We begin with a survey of Ca2+-permeable ion channels, exchangers, and pumps and summarize Ca2+ influx and efflux pathways, buffering, and intracellular stores. This includes evidence for Ca2+-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and N-methyl-d-aspartate receptors and for voltage-gated Ca2+ channels. Special attention is given to interactions between ion channels, to differences among species, and in which subtypes of HCs these channels have been found. We then discuss a number of unresolved issues pertaining to Ca2+ dynamics in HCs, including a potential role for Ca2+ in feedback to photoreceptors, the role for Ca2+-induced Ca2+ release, and the properties and functions of Ca2+-based action potentials. This review aims to highlight the unique Ca2+ dynamics in HCs, as these are inextricably tied to retinal function.

Role of dopamine in distal retina

Dopamine is the most abundant catecholamine in the vertebrate retina. Despite the description of retinal dopaminergic cells three decades ago, many aspects of their function in the retina remain unclear. There is no consensus among the authors about the stimulus conditions for dopamine release (darkness, steady or flickering light) as well as about its action upon the various types of retinal cells. Many contradictory results exist concerning the dopamine effect on the gross electrical activity of the retina [reflected in electroretinogram (ERG)] and the receptors involved in its action. This review summarized current knowledge about the types of the dopaminergic neurons and receptors in the retina as well as the effects of dopamine receptor agonists and antagonists on the light responses of photoreceptors, horizontal and bipolar cells in both nonmammalian and mammalian retina. Special focus of interest concerns their effects upon the diffuse ERG as a useful tool for assessment of the overall function of the distal retina. An attempt is made to reveal some differences between the dopamine actions upon the activity of the ON versus OFF channel in the distal retina. The author has included her own results demonstrating such differences.

The effects of excitatory amino acids and their transporters on function and structure of the distal retina in albino rabbits

PURPOSE

To study the physiological and pathological roles of excitatory amino acid transporters in the distal retina of albino rabbits.

METHODS

Albino rabbits were injected intravitreally in one eye with different doses of L- or D-isomers of glutamate or aspartate, with mixtures of L-glutamate and antagonists to glutamate receptors or with inhibitors of glutamate transporters. The other eye was injected with saline, and served as a control. The electroretinogram (ERG) was recorded 4 h and 2 weeks after injection. At the end of the ERG follow-up period, retinas were prepared for light microscopy.

RESULTS

The ERG b-wave was reduced and the a-wave augmented by both isomers of EAAs when tested 4 h after injection. Long-term (2-week) follow-up indicated severe damage to the retina by both isomers of EAAs. Antagonists to glutamate-gated ionic channels failed to protect the rabbit distal retina from permanent damage. Competitive inhibitors of GLAST-1 transporter were highly effective in blocking synaptic transmission in the OPL and in inducing permanent ERG deficit. Selective inhibition of the GLT-1 transporter caused short-term augmentation of the ERG and no permanent ERG deficit.

CONCLUSION

GLAST-1, the glutamate transporter of Müller cells, plays a major role in synaptic transmission within the OPL of the rabbit retina. Over-activation of GLAST-1 seems to induce permanent damage to the distal rabbit retina via yet unidentified mechanism.

A positive feedback synapse from retinal horizontal cells to cone photoreceptors

Cone photoreceptors and horizontal cells (HCs) have a reciprocal synapse that underlies lateral inhibition and establishes the antagonistic center-surround organization of the visual system. Cones transmit to HCs through an excitatory synapse and HCs feed back to cones through an inhibitory synapse. Here we report that HCs also transmit to cone terminals a positive feedback signal that elevates intracellular Ca²⁺ and accelerates neurotransmitter release. Positive and negative feedback are both initiated by AMPA receptors on HCs, but positive feedback appears to be mediated by a change in HC Ca²⁺, whereas negative feedback is mediated by a change in HC membrane potential. Local uncaging of AMPA receptor agonists suggests that positive feedback is spatially constrained to active HC-cone synapses, whereas the negative feedback signal spreads through HCs to affect release from surrounding cones. By locally offsetting the effects of negative feedback, positive feedback may amplify photoreceptor synaptic release without sacrificing HC-mediated contrast enhancement.

Visual images are projected by the lens of the eye onto a sheet of photoreceptor cells in the retina called rods and cones. Like the pixels in a digital camera, each photoreceptor generates an electrical response proportional to the local light intensity. Each photoreceptor then initiates a chemical signal that is transmitted to downstream neurons, ultimately reaching the brain. But unlike the pixels of a digital camera, photoreceptors indirectly inhibit one another through laterally projecting horizontal cells. Horizontal cells integrate signals from many photoreceptors and provide inhibitory feedback. This feedback is thought to underlie “lateral inhibition,” a process that sharpens our perception of contrast and color. Here we report the surprising finding that horizontal cells also provide positive feedback to photoreceptors, utilizing a mechanism distinct from negative feedback. The positive feedback signal is constrained to individual horizontal cell–photoreceptor connections, whereas the negative feedback signal spreads throughout a horizontal cell to affect many surrounding photoreceptors. By locally offsetting negative feedback, positive feedback boosts the photoreceptor signal while preserving contrast enhancement.

Neurotransmitter-specific identification and characterization of neurons in the all-cone retina of Anolis carolinensis II: Glutamate and aspartate

Immunocytochemical and autoradiographic methods were used to identify neurons in the pure cone retina of the lizard (Anolis carolinensis) that are likely to employ glutamate (GLU) or aspartate (ASP) as a neurotransmitter.

GLU immunocytochemistry demonstrated high levels of endogenous GLU in all cone types and numerous bipolar cells. Moderate GLU levels were found in horizontal and ganglion cells. Müller cells and most amacrine cells had very low GLU levels. GLU immunoreactivity (GLU-IR) in the cones was present from the inner segment to the synaptic pedicle. A large spherical cell type with moderate GLU-IR was identified in the proximal inner plexiform layer (IPL). These cells also contain ASP and have been tentatively identified as amacrine cells. Uptake of [3H]-L-GLU labeled all retinal layers. All cone types and Müller cells sequestered [3H]-D-ASP, a substrate specific for the GLU transporter.

Anti-ASP labeling was observed in cones, horizontal cells, amacrine cells, and cells in the ganglion cell layer. ASP immunoreactivity (ASP-IR) in the cones was confined to the inner segment. One ASP-containing pyriform amacrine cell subtype ramifying in IPL sublamina b was identified.

Analysis of GLU-IR, ASP-IR, and GABA-IR on serial sections indicated that there were two distinct populations of horizontal cells in the Anolis retina: one containing GABA-IR, GLU-IR, and ASP-IR; and another type containing only GLU-IR and ASP-IR. Light GLU-IR was frequently found in GABA-containing amacrine cells but ASP-IR was not.

The distinct distributions of GLU and ASP may indicate distinctly different roles for these amino acids. GLU, not ASP, is probably the major neurotransmitter in the cone-biploar-ganglion cell pathway of the Anolis retina. Both GLU and ASP are present in horizontal cells and specific subpopulations of amacrine cells, but it is unclear if GLU or ASP have a neurotransmitter role in these cells.

Localization of metabotropic glutamate receptors in the outer plexiform layer of the goldfish retina

We studied the localization of metabotropic glutamate receptors (mGluRs) in the goldfish outer plexiform layer by light-and electron-microscopical immunohistochemistry. The mGluR1alpha antibody labeled putative ON-type bipolar cell dendrites and horizontal cell processes in both rod spherules and cone triads. Immunolabeling for mGluR2/3 was absent in the rod synaptic complex but was found at horizontal cell dendrites directly opposing the cone synaptic ribbon. The mGluR5 antibody labeled Müller cell processes wrapping rod terminals and horizontal cell somata. The mGluR7 antibody labeled mainly horizontal cell dendrites invaginating rods and cones and some putative bipolar cell dendrites in the cone synaptic complex. The finding of abundant expression of various mGluRs in bipolar and horizontal cell dendrites suggests multiple sites of glutamatergic modulation in the outer retina.

Vanilloid receptor 1 (TRPV1/VR1) co-localizes with fatty acid amide hydrolase (FAAH) in retinal amacrine cells

Fatty acid amide hydrolase (FAAH) is the degradative enzyme for anandamide (AEA), an endogenous ligand for the vanilloid receptor (TRPV1) and cannabinoid receptor 1. As FAAH and TRPV1 are integral membrane proteins, FAAH activity could modulate the availability of AEA for TRPV1 activation. Previous studies in this laboratory reported an extensive endocannabinoid system in goldfish retina. Immunocytochemistry was used to determine the relative distributions of FAAH-immunoreactivity (IR) and TRPV1-IR in goldfish retina. Here, we show the first example in an intact neural system in which TRPV1-IR co-localizes in subpopulations of FAAH-immunoreactive neurons, in this case amacrine cells. These cells are rare and consist of three subtypes: 1. ovoid cell with granular-type dendrites restricted to sublamina a, 2. pyriform cell with smooth processes in sublamina b, and 3. fusiform cell with smooth processes that project to sublaminae a and b. The varied appearances of reaction product in the dendrites suggest different subcellular localization of TRPV1, and hence function of FAAH activity regarding TRPV1 stimulation among the cell types. Ovoid and pyriform amacrine cells, but not fusiform cells, labeled with GAD-IR and constituted subsets of GABAergic amacrine cells. TRPV1 amacrine cells, though rare, are represented in the ON, OFF and ON/OFF pathways of the retina. As TRPV1 stimulation increases intracellular calcium with numerous downstream effects, co-localization of TRPV1 and FAAH suggests an autoregulatory function for anandamide. Due to the rarity of these cells, the three vanilloid amacrine cell types may be involved in global effects rather than feature extraction, for example: sampling of ambient light or maintaining homeostasis.

Transporter mediated GABA release in the retina: Role of excitatory amino acids and dopamine

In general, the release of neurotransmitters in the central nervous system is accomplished by a calcium-dependent process which constitutes a common feature of exocytosis, a conserved mechanism for transmitter release in all species. However, neurotransmitters can also be released by the reversal of their transporters. In the retina, a large portion of GABA is released by this mechanism, which is under the control of neuroactive agents, such as excitatory amino acids and dopamine. In this review, we will focus on the transporter mediated GABA release and the role played by excitatory amino acids and dopamine in this process. First, we will discuss the works that used radiolabeled GABA to study the outflow of the neurotransmitter and then the works that took into consideration the endogenous pool of GABA and the topography of GABAergic circuits influenced by excitatory amino acids and dopamine.

Activation of NMDA receptors linked to modulation of voltage-gated ion channels and functional implications

Catfish (Ictalurus punctatus) cone horizontal cells contain N-methyl-d-aspartate (NMDA) receptors, the function of which has yet to be determined. In the present study, we have examined the effect of NMDA receptor activation on voltage-gated ion channel activity. NMDA receptor activation produced a long-term downregulation of voltage-gated sodium and calcium currents but had no effect on the delayed rectifying potassium current. NMDA's effect was eliminated in the presence of AP-7. To determine whether NMDA receptor activation had functional implications, isolated catfish cone horizontal cells were current clamped to mimic the cell's physiological response. When horizontal cells were depolarized, they elicited a single depolarizing overshoot and maintained a depolarized steady state membrane potential. NMDA reduced the amplitude of the depolarizing overshoot and increased the depolarized steady-state membrane potential. Both effects of NMDA were eliminated in the presence of AP-7. These results support the hypothesis that activation of NMDA receptors in catfish horizontal cells may affect the type of visual information conveyed through the distal retina.

On the interaction between voltage-gated conductances and Ca(2+) regulation mechanisms in retinal horizontal cells

The horizontal cell is a second-order retinal neuron that is depolarized in the dark and responds to light with graded potential changes. In such a nonspiking neuron, not only the voltage-gated ionic conductances but also Ca(2+) regulation mechanisms, e.g., the Na(+)/Ca(2+) exchange and the Ca(2+) pump, are considered to play important roles in generating the voltage responses. To elucidate how these physiological mechanisms interact and contribute to generating the responses of the horizontal cell, physiological experiments and computer simulations were made. Fura-2 fluorescence measurements made on dissociated carp horizontal cells showed that intracellular Ca(2+) concentration ([Ca(2+)]i) was maintained <100 nM in the resting state and increased with an initial transient to settle at a steady level of approximately 600 nM during prolonged applications of L-glutamate (L-glu, 100 microM). A preapplication of caffeine (10 mM) partially suppressed the initial transient of [Ca(2+)]i induced by L-glu but did not affect the L-glu-induced steady [Ca(2+)]i. This suggests that a part of the initial transient can be explained by the Ca(2+)-induced Ca(2+) release from the caffeine-sensitive Ca(2+) store. The Ca(2+) regulation mechanisms and the ionic conductances found in the horizontal cell were described by model equations and incorporated into a hemi-spherical cable model to simulate the isolated horizontal cell. The physiological ranges of parameters of the model equations describing the voltage-gated conductances, the glutamate-gated conductance and the Na(+)/Ca(2+) exchange were estimated by referring to previous experiments. The parameters of the model equation describing the Ca(2+) pump were estimated to reproduce the steady levels of [Ca(2+)]i measured by Fura-2 fluorescence measurements. Using the cable model with these parameters, we have repeated simulations so that the voltage response and [Ca(2+)]i change induced by L-glu applications were reproduced. The simulation study supports the following conclusions. 1) The Ca(2+)-dependent inactivation of the voltage-gated Ca(2+) conductance has a time constant of approximately 2.86 s. 2) The falling phase of the [Ca(2+)]i transient induced by L-glu is partially due to the inactivation of the voltage-gated Ca(2+) conductance. 3) Intracellular Ca(2+) is extruded mainly by the Na(+)/Ca(2+) exchange when [Ca(2+)]i is more than approximately 2 microM and by the Ca(2+) pump when [Ca(2+)]i is less than approximately 1 microM. 4) In the resting state, the Na(+)/Ca(2+) exchange may operate in the reverse mode to induce Ca(2+) influx and the Ca(2+) pump extrudes intracellular Ca(2+) to counteract the influx. The model equations of physiological mechanisms developed in the present study can be used to elucidate the underlying mechanisms of the light-induced response of the horizontal cell in situ.

The open- and closed-loop gain-characteristics of the cone/horizontal cell synapse in goldfish retina

Under constant light-adapted conditions, vision seems to be rather linear. However, the processes underlying the synaptic transmission between cones and second-order neurons (bipolar cells and horizontal cells) are highly nonlinear. In this paper, the gain-characteristics of the transmission from cones to horizontal cells and from horizontal cells to cones are determined with and without negative feedback from horizontal cells to cones. It is shown that 1) the gain-characteristic from cones to horizontal cells is strongly nonlinear without feedback from horizontal cells, 2) the gain-characteristic between cones and horizontal cells becomes linear when feedback is active, and 3) horizontal cells feed back to cones via a linear mechanism. In a quantitative analysis, it will be shown that negative feedback linearizes the synaptic transmission between cones and horizontal cells. The physiological consequences are discussed.

The feedback pathway from horizontal cells to cones. A mini review with a look ahead

The feedback pathway from HCs to cones forms the basis of the surround responses of the bipolar cells and is essential for the spectral opponency of horizontal cells. The nature of this feedback pathway is an issue of debate. Three hypothesis are presented in literature: (1) a GABAA-ergic feedback pathway; (2) a GABA-independent feedback pathway that modulates the Ca-current in cones; and (3) an electrical feedback pathway. In this review the evidence for the various pathways will be discussed. The conclusion is that the available evidence favors the hypothesis that feedback modulates the Ca-current in the cones in a GABA independent way. An alternative role of GABA in the outer plexiform layer is discussed and finally the functional consequences of the negative feedback pathway from horizontal cells to cones are presented.

Divalent cations modulate glutamate receptors in retinal horizontal cells of the perch (Perca fluviatilis)

Divalent cations had two effects on concentration-response relations of glutamate induced membrane currents recorded from retinal horizontal cells. The first effect was a reduction of maximum currents. Barium, magnesium, cobalt, nickel and an increased calcium concentration caused reductions of maximum currents between 14% and 70%. The second effect of divalent cations was related to the dopamine dependent modulation of glutamate receptors in horizontal cells. The dopamine dependent enhancement of glutamate gated currents requires the presence of divalent cations besides calcium in the extracellular solution. Without such divalent cations application of dopamine caused no increase of the maximum currents induced by glutamate, and only a slight shift of the half maximal saturation concentration was observed. Addition of magnesium or barium cations in millimolar concentration was sufficient to completely restore the dopamine dependent modulation.

AMPA-preferring receptors with high Ca2+ permeability mediate dendritic plasticity of retinal horizontal cells

The synaptic complex formed by the cone photoreceptor pedicles and the dendrites of horizontal cells in the teleost retina undergoes structural changes during light adaptation. Numerous spinules are formed by the terminal dendrites, and they are subsequently retracted during dark adaptation. In a retina kept under continuous illumination, the retraction process can be initiated by analogues of the neurotransmitter glutamate acting at AMPA/kainate receptors. On the other hand, the retraction process depends on calcium influx and the subsequent activation of CaMkII. We show here that the retraction of spinules induced by AMPA or kainate is not impaired in the presence of cobalt, making an involvement of voltage-gated calcium channels unlikely. Using calcium imaging techniques with isolated horizontal cells, we demonstrate that AMPA and kainate, but not NMDA, increase [Ca2+]i in the presence of nicardipine, caffeine and thapsigargin. The increase of [Ca2+]i under these conditions depends on [Ca2+]o and on the agonist in a dose-dependent manner, suggesting that the increase of [Ca2+]i is largely due to calcium influx through the agonist-gated channel. Pharmacological studies were performed to determine whether AMPA- and/or kainate-preferring receptors mediate the calcium influx. The AMPA-preferring receptor antagonist LY303070 blocked glutamate- and kainate-evoked increases of [Ca2+]i in a concentration-dependent manner, indicating that kainate-preferring receptors contributed little or nothing to the observed [Ca2+]i increase. This was supported by experiments where cyclothiazide (which blocks the desensitization of AMPA receptors) and concanavalin A (which potentiates responses mediated by kainate receptors) were applied. In all cases, LY303070 blocked the agonist-evoked increase of [Ca2+]i. The presence of AMPA-preferring receptors with high Ca2+ permeability on horizontal cells was also supported by measuring agonist-induced currents using whole-cell recording techniques. Furthermore, LY303070 was able to impair the retraction of spinules during dark adaption in the in vivo situation.

Ca2+ regulation by the Na(+)-Ca2+ exchanger in retinal horizontal cells depolarized by L-glutamate

This study is concerned with regulation of the intracellular Ca2+ concentration ([Ca2+]i) of horizontal cells isolated from cyprinid fish retinae, with the main emphasis on the role of the (Na+)-Ca2+ exchanger. An inward current was blocked by Ca2+ (4 mM) during prolonged (> 1 h) depolarization by L-glutamate (100 microM) in the whole-cell voltage-clamp configuration, suggesting the persistent activation of voltage-gated Ca2+ channels. This (Co2+)-sensitive current was absent when extracellular Na+ was replaced by Li+ to suppress (Na+)-Ca2+ exchange. Measurement of [Ca2+]i using the Fura-2 ratiometric method gave the following results. (1) L-Glutamate (100 microM) caused [Ca2+]i to increase from the resting level of 75.4+/-36.8 nM (mean +/-S.D., n = 11) to the maximum level (2.2+/-1.4 microM, n = 11) within 15 s and then to decrease to a steady level of 0.59+/-0.23 microM (n = 11). (2) Nifedipine (100 microM) lowered the L-glutamate-induced steady [Ca2+]i level, which was still higher than the resting level. (3) L-Glutamate caused [Ca2+]i to increase even after blockading the voltage-gated Ca2+ channels by nifedipine or by clamping the membrane voltage at -55 mV. (4) (Na+)-free superfusate elevated the L-glutamate-induced steady [Ca2+]i level. (5) The time course of the [Ca2+]i decrease from the L-glutamate-induced steady level to the resting level was prolonged in the (Na+)-free superfusate. These results suggest that the (Na+)-Ca2+ exchanger extrudes intracellular Ca2+ to maintain a low [Ca2+]i level by counteracting the continuous Ca2+ influx through the voltage-gated Ca2+ channels and glutamate-gated channels when horizontal cells in situ are tonically depolarized by L-glutamate released from the photoreceptors. The (Na+)-Ca2+ exchange current isolated by a voltage-clamp experiment depends exponentially on the membrane potential.

Effects of nitric oxide on the horizontal cell network and dopamine release in the carp retina

In the teleost retina the intercellular messenger nitric oxide can be synthesized by several cell types including cone photoreceptors and H1 horizontal cells, indicating a modulatory role within the outer plexiform layer, the first stage of the visual information processing. Therefore, the aim of this study was to elucidate the effects of nitric oxide on the physiology of cone horizontal cells in the intact retina. The nitric oxide donor sodium nitroprusside (0.5-2.5 mM) enhanced the light responsiveness of cone horizontal cells and reduced the degree of electrical coupling in the network. Furthermore, the spread of intracellularly injected Lucifer Yellow was restricted. The effects on light responsiveness and electrical coupling were qualitatively mimicked by 8-bromo-cGMP (0.5 mM) and could not be achieved by ferrocyanide (1 mM), the byproduct of nitric oxide liberation from nitroprusside. The effects of NO on the responsiveness of horizontal cells may be due to an action on green- and red-sensitive cones. Nitroprusside (0.1 mM) diminished the K(+)-stimulated release of endogenous dopamine by 50%, whereas the basal dopamine release was not affected, indicating that the effects on electrotonic horizontal cell coupling were not elicited by an NO-induced release of dopamine. With respect to the morphologic plasticity of the cone-horizontal cell synapse the inhibitor of endogenous nitric oxide synthesis L-nitroarginine (0.1 mM) had no influence on the formation or retraction of spinules. These results show that NO affects the responsiveness and coupling of the horizontal cell network in a dopamine-independent way.

Nitric oxide, 2-amino-4-phosphonobutyric acid and light/dark adaptation modulate short-wavelength-sensitive synaptic transmission to retinal horizontal cells

Light-induced changes in the input resistance (Rin) of external, luminosity (i.e. H1) type horizontal cell (HC) perikarya were studied by the bridge-balance method in light-adapted and dark-adapted retinae of carp. Changes in input resistance (delta Rin) induced by short-(460 nm) and long-wavelength (674 nm) flashes, adjusted in intensity to elicit equal-amplitude membrane voltage responses (equal-voltage condition), were measured. In light-adapted retinae, long-wavelength stimuli increased Rin consistently; in contrast, the increase was much less with short-wavelength stimuli. This equal-voltage chromatic delta Rin difference was lost in dark-adapted retinae whereby the delta Rin (an increase) became the same for short- and long-wavelengths. The chromatic delta Rin difference could be recovered by light adaptation or application of sodium nitroprusside to the dark-adapted retinae. Conversely, the equal-voltage chromatic delta Rin difference was eliminated by injection of NG-monomethyl-L-arginine into H1HCs of the light-adapted retinae or by treating the retinae with 2-amino-4-phosphonobutyrate (APB). These results suggest that H1HCs of the carp retina possess distinct postsynaptic mechanisms which mediate short- and long-wavelength signal transmission. Furthermore, it appears that the short-wavelength-sensitive pathway is active only during the light-adapted state of the retina. Taken together, therefore, the short-wavelength transmission to H1HCs probably operates on an APB-sensitive glutamate receptor, with nitric oxide as a light-adaptive messenger.

Iontophoretic study of the action of excitatory amino acids on rod horizontal cells of the dogfish retina

Much interest has been focused on the amino acids, L-glutamate and L-aspartate, as possible neurotransmitters of vertebrate photoreceptors. These amino acids and a number of their analogues were applied iontophoretically to rod horizontal cells on the surface of dark-adapted dogfish retinal slices under visual control. L-glutamate and kainate were found to be of approximately equal potency in depolarizing rod horizontal cells, while L-aspartate was about one tenth as potent. Simultaneous iontophoretic pulses applied to two barrels, each containing either L-glutamate or kainate, produced a larger depolarization than expected for linear summation. Potentiation was most prominent when synaptic transmitter release was reduced by light, demonstrating that these agonists interact with the same postsynaptic receptors as those acted upon by the rod neurotransmitter. Analysis of dose—response curves indicated that at least two molecules of agonist were required to open a cationic channel, presumably the basis for the depolarization. The cells did not become desensitized to long or repeated exposures of the agonists.

Effects of glutamic acid and related agents on horizontal cells in a marine teleost retina

Excitatory amino acids (EAAs) such as glutamic and aspartic acids, considered as the most likely neurotransmitters at the photoreceptor-horizontal cell synapse of teleost retinas, as well as agonists such as kainic acid and several of their antagonists, were applied to isolated and superfused retinas of the teleost Eugerres plumieri. Intracellular recordings from horizontal cells reveal that EAA receptors are of the kainate-quisqualate type. There is competitive inhibition between the agonist and antagonist agents used, and under their combined effect, the synapse under study remains operational, in a functional state, able to modulate the horizontal cell membrane potential upon retinal illumination.

Glutamate immunoreactivity in the cat retina: a quantitative study

Immunocytochemical methods were used to visualize glutamate immunoreactivity in the cat retina and to compare its localization with that of aspartate, GABA, and glycine. The cellular and subcellular distribution of glutamate was analyzed at the light-microscopic level by optical densitometry and at the electron-microscopic level by immunogold quantification. The findings were consistent with the proposed role for glutamate as the neurotransmitter of photoreceptors and bipolar cells as particularly high concentrations of staining were found in synaptic terminals of these cells. Ganglion cells were also consistently stained. Aspartate was totally colocalized with glutamate in neuronal cell bodies but the synaptic levels of aspartate were much lower than for glutamate. In addition to the staining of photoreceptor, bipolar, and ganglion cells, glutamate immunoreactivity was also observed in approximately 60% of the amacrine cells. These cells exhibited colocalization with either GABA or glycine. The elevated levels of Glu in amacrine cells may reflect its role as a transmitter precursor in GABAergic cells and as an energy source for mitochondria in glycinergic cells.

High affinity uptake of glutamate and aspartate in the developing rat retina

Uptake for glutamate and aspartate in both retina and synaptosomes was found to be saturable, temperature sensitive, sodium dependent and reduced by metabolic inhibitors. The P1 and P2 synaptosomal fractions showed high affinity systems for glutamate (3 and 9 microM) and aspartate (6 and 3 microM) respectively. Early after birth, glutamate accumulation was much higher than that of aspartate. It showed a rapid increase reaching the adult values about day 15. Aspartate uptake progressively increases with age up to about day 30. Our findings suggest that glutamate and aspartate may be transmitters at specific cell populations in the rat retina.

Evidence for D2 receptor regulation of dopamine release in the goldfish retina

The possible existence of a dopamine D2 receptor-mediated regulation of dopamine release was investigated in the goldfish retina. Isolated retinas were preloaded with [3H]dopamine and superfused with D2 dopamine receptor agonists or antagonists to determine if there was an effect on [3H]dopamine release. The D2 receptor antagonist sulpiride increased both baseline [3H]-dopamine release and [3H]dopamine release induced by an increase in extracellular potassium concentration. The D2 receptor agonists LY-171555 and RU-24213 did not reduce baseline [3H]dopamine release but completely inhibited [3H]dopamine release induced by an increase in [K+]o. This action of the D2 agonists was blocked by sulpiride. These studies demonstrate the existence of D2 receptor, possibly autoreceptor, regulation of dopamine release in the teleost retina.

Gap-junctional properties of electrically coupled skate horizontal cells in culture

Whole-cell voltage-clamp recordings were used to examine the unusual pharmacological properties of the electrical coupling between rod-driven horizontal cells in skate retina as revealed previously by receptive-field measurements (Qian & Ripps, 1992). The junctional resistance was measured in electrically coupled cell pairs that had been enzymatically isolated and maintained in culture; the typical value was about 19.92 M omega (n = 45), more than an order of magnitude lower than the nonjunctional membrane resistance. These data and the intercellular spread of the fluorescent dye Lucifer Yellow provide a good indication that skate horizontal cells are well coupled. The junctional conductance between cells was not modulated by the neurotransmitters dopamine (200 microM) or GABA (1 mM), nor was it affected by the membrane-permeable analogues of cAMP or cGMP, or the adenylate cyclase activator, forskolin. Although resistant to agents that have been reported to alter horizontal-cell coupling in cone-driven horizontal cells, the junctional conductance between paired horizontal cells of skate was greatly reduced by the application of 20 mM acetate, which is known to effectively reduce intracellular pH. Together with the results obtained in situ on the receptive-field properties of skate horizontal cells, these findings indicate that the gap-junctional properties of rod-driven horizontal cells of the skate are fundamentally different from those of cone-driven horizontal cells in other species. This raises the possibility that there is more than one class of electrical synapse on vertebrate horizontal cells.

Chromaticity of synaptic inputs to H1 horizontal cells in carp retina: analysis by voltage-clamp and spectral adaptation

Cone photoreceptor inputs to H1 horizontal cells (H1 HCs) in carp retina were studied by measuring light-modulated currents (IL) to monochromatic stimuli (460, 533, 688 nm) under a voltage-clamp condition. By using double-barrelled micro-electrodes H1 HCs were voltage-clamped whilst perfusing with dopamine to uncouple the cells. The IL of the H1 HCs driven by each cone input was segregated by selective chromatic adaptation, and differences in the kinetics of the IL of the H1 HCs were revealed. Thus, all together, three types of IL were observed: (1) a 'fast outward' current to the long-wavelength stimulus; (2) a 'slow outward' current to the middle-wavelength stimulus; and (3) a 'delayed inward' current that followed the peak of 'slow outward' current to the short-wavelength stimulus. The reversal potentials of the three currents were estimated to be at least 20 mV more positive than the dark resting potential by extrapolation of the IL-V curve. These observations are consistent with the idea that the H1 HCs receive sign-inverting, conductance decreasing synaptic input(s) from at least one other cone mechanism, in addition to the main conventional EPSP type synaptic input from red-sensitive cones.

Histochemical demonstration of glutamate dehydrogenase and phosphate-activated glutaminase activities in semithin sections of the rat retina

The activities of the glutamate metabolizing enzymes phosphate-activated glutaminase (PAG) and glutamate dehydrogenase (Gldh) are demonstrated in semithin sections of the rat retina. Highest activities of both enzymes are found in the photoreceptor inner segments, PAG additionally in the outer plexiform layer and Gldh in the inner plexiform layer and in mueller glial cells. Although their non randomly distribution makes a role in neurotransmitter metabolism possible, their high activities in inner segments point towards the general problem of the functional interpretation of both molecules.

Ultrastructural and electrophysiological changes associated with K(+)-evoked release of neurotransmitter at the synaptic terminals of skate photoreceptors

Bathing the skate retina in a Ringer solution containing a high concentration (100 mM) of potassium ions depolarized the visual cells, depleted the receptor terminals of synaptic vesicles, and suppressed completely the b-wave of the ERG and the intracellularly recorded response of horizontal cells (the S-potential). The depletion of synaptic vesicles was accompanied by a large increase in the extent of the plasma membrane resulting in distortion of the normal terminal profile, i.e. distension of the basal surface and elaborate infolding of protoplasmic extensions. Morphometric analysis showed that despite the changes in vesicle content and terminal structure, the combined linear extent of the vesicular and plasma membranes was unchanged from control (superfusion with normal Ringer solution); the increase in plasma membrane was equivalent to the observed loss of vesicular membrane. When returned to a normal Ringer solution, the terminals rapidly began to reform, and in about 10 min they were morphologically indistinguishable from receptor terminals seen in control preparations. After 30 min in the normal Ringer solution, the amount of membrane associated with the vesicles and the plasma membrane had reverted to control values, and once again the total membrane estimated morphometrically remained essentially the same. Thus, there is an efficient mechanism at the photoreceptor terminal for the recycling of vesicle membrane following exocytosis. The K(+)-induced depletion of synaptic vesicles was paralleled by a precipitous loss of responsivity in both the b-wave of the ERG and the S-potential of the horizontal cells. However, after 30-min exposure to the high K+ and a return to normal Ringer solution, the recovery of electrophysiological activity followed a much slower time course from that associated with the structural changes; 60 min or longer were required for the potentials to exhibit maximum response amplitudes. It appears that the rate-limiting step in restoring normal synaptic function following massive depletion of vesicular stores is transmitter resynthesis and vesicle loading rather than vesicle recycling.

Coexistence and function of glutamate receptor subtypes in the horizontal cells of the tiger salamander retina

Effects of the major glutamate receptor agonists, kainate (KA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), quisqualate (QA), N-methyl-D-aspartate (NMDA), L-alpha-amino-4-phosphonobutyrate (L-AP4), and trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) on horizontal cells (HCs) were studied in superfused larval tiger salamander retina. 20 microM of KA, AMPA, and QA mimicked the action of 3 mM glutamate in the absence and presence of 1 mM Co2+. 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked the actions of KA and AMPA, but not those of QA and glutamate, indicative of the existence of CNQX-resistant QA receptors in the tiger salamander HCs. Prolonged application of ACPD hyperpolarized the HCs and enhanced the light responses, probably by shifting the resting HC voltage (Er) to a more hyperpolarized position. It is possible that the KA, AMPA, and CNQX-resistant QA receptors are involved in mediating the postsynaptic light responses in HCs, and ACPD receptors are involved in sensitivity adjustment of the HC responses.

Retinal neuromodulation: the role of dopamine

Dopamine exerts multiple effects on retinal horizontal cells. Dopamine, via cyclic AMP and protein kinase A, reduces the light responsiveness of horizontal cells and the electrical coupling between the cells. The gating kinetics of both gap-junctional and glutamate channels are altered as a result of phosphorylation by protein kinase A. Dopamine also causes a reversible retraction of neurites of horizontal cells maintained in culture. Diacylglycerol analogues as well as phorbol esters mimic this effect of dopamine, but not cyclic AMP analogues or Forskolin. The results suggest that dopamine causes neurite retraction by the activation of protein kinase C via diacylglycerol.

Transmitter-gated currents of GABAergic amacrine-like cells in chick retinal cultures

A subpopulation of cells developing in dissociated neuronal cultures prepared from 8-day-old embryonic chick retinae can be identified as putative in vitro counterparts of GABAergic amacrine cells by immunocytochemical and autoradiographic markers and by their electrophysiological responses to transmitter agonists. In the present study, transmitter-gated conductances expressed by these neurons were examined using the whole-cell patch-clamp technique. At negative holding potentials, the excitatory amino acid agonists N-methyl-D-aspartate (NMDA), kainate quisqualate, and glutamate induced inward currents with reversal potentials close to 0 mV in most of the cells selected for recording. NMDA-evoked responses were selectively blocked by the noncompetitive inhibitor MK 801 and by Mg2+ (in a voltage-dependent manner) and were potentiated in the presence of submicromolar concentrations of glycine. Glutamate apparently interacted with both NMDA and non-NMDA type receptors. All cells tested responded to the inhibitory transmitters GABA and glycine. Both inhibitory agonists could be shown to activate chloride conductances. Responses to GABA and glycine were specifically inhibited in the presence of bicuculline and strychnine, respectively. Thus, GABAergic neurons in retinal cultures express at least two different excitatory amino acid receptors--NMDA and non-NMDA--and two different inhibitory amino acid receptors--the GABAA and the glycine receptor. The results demonstrate the ability of the cultured neurons to develop an apparently mature phenotype and contribute to the understanding of the functional properties of GABAergic amacrine cells in the vertebrate retina.

Effects of excitatory amino acids and their antagonists on the light response of luminosity and color-opponent horizontal cells in the turtle (Pseudemys scripta elegans) retina

Both kainic acid (KA) and N-methyl-d-aspartatic acid (NMDA) depolarize luminosity-type horizontal cells (L-type H cells) in normal turtle retina. The presence of both NMDA and non-NMDA receptors for excitatory amino acids (EAAs) on these cells was highlighted by an unusual effect of the noncompetitive NMDA-antagonist, MK-801. In retinas that had been exposed to MK-801, the action of NMDA was irreversibly altered to one of hyperpolarization, while the depolarizing effect of KA was unaltered. The aim of the present study was to further characterize these receptors on L-type H cells and to extend the investigation to color-opponent H cells (C-type H cells). Intracellular recording was used to study the effects of KA, NMDA, MK-801, the competitive NMDA antagonists, 2-amino-5-phosphonopentanoic acid (AP5) and 2-amino-7-phosphonoheptanoic acid (AP7), and the nonspecific EAA antagonist, kynurenic acid (KYN) on the light responses of L-type and C-type H cells in turtle retina. The effects of combinations of these drugs were also studied. In L-type H cells the agonists caused depolarization and loss of light response, KYN caused hyperpolarization and loss of light response, and MK-801, AP5 or AP7 had no direct effect. However, application of NMDA following MK-801, AP5 or AP7, but not KYN, caused hyperpolarization and loss of light response. The depolarizing effect of KA was unaltered by these antagonists. These data confirm the presence of an unusual NMDA receptor on L-type H cells. In the case of red/green C-type H cells, application of KA caused loss of responses to both red and green light, with loss of green responses preceding loss of red responses. NMDA initially removed responses to both red and green light. The most striking effect of NMDA was seen during early washout where the responses to red were reversed (hyperpolarizing). These responses eventually recovered their normal polarity. These results suggest that the depolarizing response of C-type H cells to red light is mediated by L-type H cells, but not via inhibition of the excitatory input from green cones to C-type H cells.

Dopamine induces neurite retraction in retinal horizontal cells via diacylglycerol and protein kinase C

Dopamine causes a significant retraction of neurites of bull-head catfish horizontal cells maintained in culture. The effects of dopamine are blocked by haloperidol and SCH 23390, a D1 antagonist, but not by sulpiride, a D2 antagonist. The dopamine-induced morphological changes were mimicked by SKF 38393, a D1 agonist, but not by quinpirole, a D2 agonist. Kainate also caused process retraction, but other neuroactive substances tested including glutamate, 5-hydroxytryptamine, N-methyl-D-aspartate, gamma-aminobutyric acid, and glycine caused only minor changes in neurite length. Cyclic AMP analogues do not induce neurite retraction in horizontal cells, indicating that this effect of dopamine is not mediated by cyclic AMP. However, a protein kinase C activator (phorbol 12-myristate 13-acetate) and synthetic diacylglycerol analogs (1-oleoyl-2-acetyl-sn-glycerol and dioctanoglycerol) caused marked neurite retraction. Their effects, as well as the dopamine-induced changes, were blocked by staurosporine, a potent protein kinase antagonist. The results suggest that dopamine causes neurite retraction by the activation of protein kinase C via diacylglycerol.

Synaptic and voltage-gated currents in interplexiform cells of the tiger salamander retina

We have correlated the membrane properties and synaptic inputs of interplexiform cells (IPCs) with their morphology using whole-cell patch-clamp and Lucifer yellow staining in retinal slices. Three morphological types were identified: (a) a bistratified IPC with descending processes ramifying in both sublaminas a and b of the inner plexiform layer (IPL), and an ascending process that branched in the outer plexiform layer (OPL) and originated from the soma, (b) another bistratified IPC with descending processes ramifying in both sublaminas a and b, and an ascending process that branched in the OPL and originated directly from IPC processes in the IPL, and (c) a monostratified IPC with a descending process ramifying over large lateral extents within the most distal stratum of the IPL, and sending an ascending process to the OPL with little branching. Similar voltage-gated currents were measured in all three types including: (a) a transient inward sodium current, (b) an outward potassium current, and (c) an L-type calcium current. All cells generated multiple spikes with frequency increasing monotonically with the magnitude of injected current. The IPCs that send their descending processes into both sublaminas of the IPL (bistratified) receive excitatory synaptic inputs at both light ON and OFF that decay with a time constant of approximately 1.3 s. Slowly decaying excitation at both ON and OFF suggests that bistratified IPCs may spike continuously in the presence of a dynamic visual environment.

Dopamine modulates the kinetics of ion channels gated by excitatory amino acids in retinal horizontal cells

Upon exposure to dopamine, cultured teleost retinal horizontal cells become more responsive to the putative photoreceptor neurotransmitter L-glutamate and to its analog kainate. We have recorded unitary and whole-cell currents to determine the mechanism by which dopamine enhances ion channels activated by these agents. In single-channel recordings from cell-attached patches with agonist in the patch pipette, the frequency of 5- to 10-pS unitary events, but not their amplitude, increased by as much as 150% after application of dopamine to the rest of the cell. The duration of channel openings also increased somewhat, by 20-30%. In whole-cell experiments, agonists with and without dopamine were applied to voltage-clamped horizontal cells by slow superfusion. Analysis of whole-cell current variance as a function of mean current indicated that dopamine increased the probability of channel opening for a give agonist concentration without changing the amount of current passed by an individual channel. For kainate, noise analysis additionally demonstrated that dopamine did not alter the number of functional channels. Dopamine also increased a slow spectral component of whole-cell currents elicited by kainate or glutamate, suggesting a change in the open-time kinetics of the channels. This effect was more pronounced for currents induced by glutamate than for those induced by kainate. We conclude that dopamine potentiates the activity of horizontal cell glutamate receptors by altering the kinetics of the ion channel to favor the open state.

Distribution of muscarinic acetylcholine receptors on processes of isolated retinal cells

Binding of propylbenzilylcholine mustard, a muscarinic acetylcholine receptor antagonist, to isolated retinal cells was examined with light microscopic autoradiography. Dissociation of the adult tiger salamander retina yielded identifiable rod, cone, horizontal, bipolar, amacrine/ganglion, and Müller cells. Preservation of fine structure was assessed with conventional electron microscopy. For all cell types, the plasmalemma was intact and free of adhering debris; in addition, presynaptic ribbon complexes were present in photoreceptor and bipolar axon terminals indicating that synaptic structures were retained. Specific binding to cell bodies and processes was analyzed separately by using morphometric and statistical techniques. The highest grain densities occurred on processes of amacrine/ganglion cells and axons and 2 degrees and 3 degrees dendrites of bipolar neurons. Bipolar cells, however, seemed to be a heterogeneous population because there was great variation in the density of binding sites on both their axons and distal dendrites. Intermediate levels of binding were found on bipolar 1 degree dendrites and horizontal cells. No specific binding was detected on Müller cells and most parts of photoreceptors. Comparisons between cells showed that grain densities were similar for bipolar axons and amacrine/ganglion cell processes but bipolar dendrites were richer in binding sites than horizontal cell dendrites. Thus, muscarinic receptors in the salamander retina are located on amacrine/ganglion, bipolar, and horizontal cells and primarily confined to the processes which compose the two synaptic layers. In the inner plexiform layer, muscarinic receptors reside on processes from all three inner retinal neurons: in the outer synaptic layer, receptors are only on second-order cells and are more numerous along bipolar than horizontal cell dendrites.

Catecholamine-sensitive adenylate cyclase in the white perch (Roccus americanus) retina: evidence for beta-adrenergic and dopamine receptors linked to adenylate cyclase

We have examined the catecholamine-sensitive adenylate cyclase in the retina of the white perch (Roccus americanus). Both dopamine and the beta-adrenergic agonist isoproterenol stimulate cyclic AMP accumulation in this retina, but serotonin, an indoleamine, and phenylephrine, an alpha-adrenergic agonist, had no effect. The stimulation of adenylate cyclase by isoproterenol is more potent and effective than that of dopamine. The effects of dopamine and isoproterenol are mediated via independent dopamine and beta-adrenergic receptors. Haloperidol, a dopamine antagonist, blocks the stimulatory effect of dopamine but not of isoproterenol. Conversely, propranolol, a beta-adrenergic antagonist, blocks the stimulatory effect of isoproterenol but not of dopamine. The effects of dopamine and isoproterenol are not additive. In fractions of purified horizontal cells we found evidence for dopamine receptors linked to adenylate cyclase but did not find evidence for the presence of cyclase coupled beta-adrenergic receptors. The cellular location of the beta-adrenergic receptors is unknown. Our findings demonstrate the existence of both beta-adrenergic and dopamine receptors coupled to adenylate cyclase in the white perch retina. However, we did not find either epinephrine or norepinephrine, endogenous ligands of the beta-receptor, to be present in retinal extracts subjected to HPLC.

Effects of CNQX, APB, PDA, and kynurenate on horizontal cells of the tiger salamander retina

Effects of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 2-amino-4-phosphonobutyrate (APB), cis-2,3-piperidine dicarboxylic acid (PDA), and kynurenate (KYN) on the depolarizing actions of glutamate and kainate on horizontal cells (HCs) were studied in the larval tiger salamander retina. APB, PDA, and KYN hyperpolarized the HCs, but they failed to block either the actions of glutamate and kainate, or the HC light responses. APB and PDA did not cause membrane polarizations in either rods or cones, suggesting that the HC hyperpolarizations were not mediated by presynaptic actions of these compounds. CNQX, the newly synthesized non-NMDA (N-Methyl-D-Aspartate) receptor antagonist, blocked the HC light responses and the action of kainate, but not that of glutamate. These results suggest that the synaptic receptors in the tiger salamander HCs are probably non-NMDA although extra-synaptic NMDA receptors may exist in these cells.

Modulation of an electrical synapse between solitary pairs of catfish horizontal cells by dopamine and second messengers

1. Retinas from channel catfish were dissociated and the cells maintained in culture. Horizontal cells that normally receive input from cone photoreceptors were identified. The conductance of the electrical junction formed between a pair of 'cone' horizontal cells was measured by controlling the membrane voltage of each cell with a voltage clamp maintained through either a micropipette or a patch pipette. The two techniques yielded similar results. 2. Transjunctional current was measured while transjunctional voltage was stepped to values between +/- 60 mV. The current (measured 5 ms after a step) was proportional to voltage over the range tested. For steps to voltages greater than +/- 45 mV, the current exhibited a slight time-dependent decline. 3. Dopamine decreased junctional conductance in a dose-dependent fashion. A 50% reduction was obtained with 10 nM-dopamine. The D1 agonist fenoldopam (100 nM) also decreased junctional conductance. The uncoupling produced by either agent was rapid and reversible. 4. The introduction of 100 microM-cyclic AMP into one cell of a pair decreased junctional conductance by, on average, 40%. Forskolin (1-10 microM), an activator of adenylate cyclase, decreased junctional conductance 50-90%. 5. The introduction of 80 microM-cyclic GMP into one cell of a pair decreased junctional conductance by, on average, 40%. Nitroprusside (1-10 microM), an activator of guanylate cyclase, reduced junctional conductance 40-65%. 6. The introduction of a peptide inhibitor specific for the cyclic AMP-dependent protein kinase reversed a decrease in junctional conductance produced by superfusion with either dopamine (1 microM), fenoldopam (100 nM) or forskolin (5-10 microM). 7. Intracellular Ca2+ concentration was measured with the fluorescent indicator Fura-2. The intracellular Ca2+ concentration was increased by activation of a Ca2+ current. Junctional conductance remained constant as the internal Ca2+ concentration changed from 100 to 700 nM. 8. Intracellular pH was measured with the fluorescent indicator bis-carboxyethylcarboxyfluorescein. The application of acetate (2.5 mM) reduced intracellular pH by 0.2-0.3 units and decreased junctional conductance by approximately 50%. A subsequent application of fenoldopam did not alter intracellular pH, but decreased junctional conductance by more than 50%. 9. The sensitivity of the junctional conductance between isolated horizontal cells to dopamine is consistent with dopamine having a direct effect on coupling in intact retina. Dopamine regulates the activity of a cyclic AMP-dependent protein kinase which in turn modulates junctional conductance. Changes in intracellular pH and Ca2+ concentration are not involved in mediating the effect of dopamine on coupling. Cyclic GMP and intracellular pH may participate in regulatory pathways independent of that used by cyclic AMP.

H1 horizontal cells of carp retina have different postsynaptic mechanisms to mediate short- versus long-wavelength visual signals

Vertebrate photoreceptors release neurotransmitter substance(s) tonically in the dark and this release is curtailed by light. Recently, we have become increasingly aware of the possibility that short- and long-wavelength visual signals are mediated differently during the synaptic transmission to second-order retinal neurons. The experiment described here advances this notion further by demonstrating a postsynaptic difference. Treatment of the carp retina by dopamine reduced the gap-junctional coupling of horizontal cells, and we made use of this known effect to measure the input resistance (Rin) of H1-type horizontal cells. Flashes of light increased Rin. This increase, however, was found to be smaller with short wavelengths, even though the comparison was made when voltage responses were equal in amplitude. Often, Rin was even found to decrease at the blue end of spectrum. No single postsynaptic mechanism can account for any equal-voltage Rin difference such as this. The synaptic spectral segregation thus revealed is probably subserved by a dual scheme wherein the transmitter from blue-sensitive cone photoreceptors acts to decrease the membrane conductance of H1 cells whereas the synapses made by red- and green-sensitive cones are of a classical excitatory type.

Glutamate receptors of ganglion cells in the rabbit retina: evidence for glutamate as a bipolar cell transmitter

1. Intracellular and extracellular recordings were obtained from ganglion cells in the rabbit retina. The effects of glutamate analogues and antagonists were studied using a perfusion method for drug application. 2. Kainate (KA) excited all ganglion cells directly and caused a large increase in firing rate. N-Methyl-DL-aspartate (NMDLA) also excited ganglion cells but it was less potent and caused burst firing. 3. Quisqualate (QQ) and (RS)-2-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) excited many ganglion cells and were approximately as potent as KA. Less frequently, QQ and AMPA had inhibitory effects possibly due to polysynaptic action. 4. General glutamate antagonists such as cis-2,3-piperidine dicarboxylic acid (PDA) and kynurenic acid blocked the light input to all ganglion cells. PDA and kynurenic acid blocked the effects of KA and NMDLA, but not carbachol, indicating that they act as glutamate antagonists in the rabbit retina. Kynurenic acid did not block the excitatory action of QQ, even though light responses were abolished. 5. Amacrine cells were depolarized by KA or QQ and less potently by NMDLA. Their light-evoked responses were blocked by PDA. 6. We conclude that the light input to ganglion cells in the rabbit retina is predominantly mediated by KA receptors. This is consistent with the idea that 'on' and 'off' bipolar cells are excitatory and release glutamate.

Bipolar cells in the turtle retina are strongly immunoreactive for glutamate

Strong glutamate immunoreactivity was observed by both light and electron microscopy in bipolar cells of the turtle (Pseudemys scripta elegans) retina after postembedding immunohistochemistry. Virtually all bipolar cells showed strong labeling, on average 18 times that of the Müller (glial) cells. The data suggest that both on- and off-center bipolar cells are glutamatergic. Photoreceptors were also labeled, but with a labeling intensity about half that of the bipolar cells. Other types of retinal neurons showed less immunoreactivity, except for a small population of strongly labeled amacrine cells.

L-glutamate-induced depolarization in solitary photoreceptors: a process that may contribute to the interaction between photoreceptors in situ

L-Glutamate is a leading candidate for the vertebrate photoreceptor transmitter. In addition to the signal transmission to second-order neurons, photoreceptors communicate with each other not only electrically but also chemically. In the present study, by using solitary turtle photoreceptors, we examined the possibility that L-glutamate mediates interreceptor communication. L-Glutamate evoked an inward current in all subtypes of photoreceptors voltage-clamped to the resting potential. The highest glutamate sensitivity was located at the axon terminal. Both stereoisomers of aspartate were effective, whereas kainate, quisqualate, N-methyl-D-aspartate, and D-glutamate were ineffective. The presence of Na+ was essential to response generation; even Li+ could not substitute for Na+. The relation between L-glutamate-induced current and the membrane voltage was strongly inward-rectifying. These results favor the hypothesis that the L-glutamate-induced response is generated by an electrogenic uptake carrier. However, L-glutamate-induced current was always accompanied by an increase in current fluctuations, a phenomenon commonly observed in ion channels but not expected for an uptake carrier. Although the underlying mechanism needs further elucidation, it seems likely that L-glutamate is a transmitter for communication between photoreceptors.

Calcium action potential and its use for measurement of reversal potentials of horizontal cell responses in carp retina

1. In the carp retina perfused with a solution containing high-Ca2+, Ba2+ and some K+-channel blockers, the horizontal cell produced a regenerative Ca2+ action potential when the cell was depolarized by bath application of L-glutamate (Glu) or L-aspartate (Asp). The action potential was triggered also by a transretinal electrical stimulation which evoked an e.p.s.p. in the horizontal cell. In this solution, some cells produced the action potential spontaneously. 2. The action potential had an overshoot of about 20 mV which lasted for several seconds or even minutes. It had a threshold and showed refractoriness. In addition, it was insensitive to tetrodotoxin, but was blocked by Co2+. These observations revealed, in horizontal cells in situ, the presence of a voltage-dependent Ca2+ channel similar to that found in dissociated cells. It is supposed that, in a physiological environment, the Ca2+ channel is prevented from becoming regenerative probably because it is counteracted by K+ channel activities. 3. Simultaneous recordings from two separate horizontal cells showed full synchronization of the Ca2+ action potentials whose amplitudes were identical. The potential uniformity thus formed in the S-space (Naka & Rushton, 1967) enabled us to measure reversal potentials of horizontal cell responses irrespective of the electrical coupling between the cells. 4. During an overshoot of the Ca2+ action potential, an electrically evoked e.p.s.p. as well as a light response appeared with polarities reversed to those elicited at the resting state. Their reversal potentials could be estimated within a very narrow range between -5 and -10 mV. At this range, both Glu- and Asp-induced potentials reversed the polarity, too. 5. These observations suggest that the ionic mechanisms are identical in the three kinds of horizontal cell response: light response, e.p.s.p. and amino acid-induced potentials.

Voltage-dependent conductances of solitary ganglion cells dissociated from the rat retina

1. Ganglion cells were dissociated from the enzyme-treated rat retina, identified with specific fluorescent labels, and maintained in vitro. Electrophysiological properties of solitary retinal ganglion cells were investigated with both conventional intracellular and patch-clamp recordings. Although comparable results were obtained for most measurements some important differences were noted. 2. The input resistance of solitary retinal ganglion cells was considerably higher when measured with 'giga-seal' suction pipettes than with conventional intracellular electrodes. Under current-clamp conditions with both intracellular and patch pipettes, these central mammalian neurones maintained resting potentials of about -60 mV and displayed action potentials followed by an after-hyperpolarization in response to small depolarizations. The membrane currents during this activity, analysed under voltage clamp with patch pipettes, consisted of five components: Na+ current (INa), Ca2+ current (ICa), and currents with properties similar to the delayed outward, the transient (A-type), and the Ca2+-activated K+ currents (IK, IA and IK(Ca), respectively). 3. Ionic substitution, pharmacological agents, and voltage-clamp experiments revealed that the regenerative currents were carried by both Na+ and Ca2+. 100 nM-1 microM-tetradotoxin (TTX) reversibly blocked the fast spikes carried by the presumptive INa, which under voltage-clamp analysis had classical Hodgkin-Huxley-type activation and inactivation. 4. Single-channel recordings of the Na+ current (iNa) permitted comparison of these 'microscopic' events with the 'macroscopic' whole-cell current (INa). The inactivation time constant (tau h) fitted to the averaged single-channel recordings of iNa in outside-out patches was slower than the tau h obtained during whole-cell recordings of INa. 5. In the presence of 1-40 microM-TTX and 20 mM-TEA, slow action potentials appeared in intracellular recordings and were probably mediated by Ca2+. The potentials were abrogated by 3 mM-Co2+ or 200 microM-Cd2+; conversely, increasing the extracellular Ca2+ concentration from 2.5 to 10-25 mM or substitution of 1 mM-Ba2+ for 2.5 mM-Ca2+ enhanced their amplitude. ICa was measured directly in whole-cell recordings with patch pipettes after blocking INa with extracellular 1 microM-TTX and K+ currents with intracellular 120-mM Cs+ and 20 mM-TEA. 6. During whole-cell recordings with patch electrodes, extracellular 20 mM-TEA suppressed IK and, to a lesser extent, IA. Extracellular 5 mM-4-AP or a pre-pulse of the membrane potential to -40 mV prior to stronger depolarization completely blocked IA.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine enhances excitatory amino acid-gated conductances in cultured retinal horizontal cells

In the teleost retina, cone horizontal cells receive extensive innervation from dopaminergic interplexiform cells, and possess dopamine receptors whose activation stimulates adenylate cyclase. Exogenously applied dopamine modifies several aspects of horizontal cell activity in the intact retina, including the responsiveness of these neurons to light and the strength of electrical coupling between them. We have used whole-cell voltage clamp methods to examine whether dopamine can alter the light-responsiveness of horizontal cells by changing their sensitivity to the neurotransmitter released by the photoreceptors. We report that dopamine and cyclic AMP, although having little direct effect on resting membrane conductance, greatly enhance ionic conductances gated by kainate, an agonist of the transmitter released by the photoreceptors, and by L-glutamate, the agent proposed to be the photoreceptor transmitter. Our results provide the first direct evidence for dopaminergic regulation of excitatory amino-acid neurotransmission in the vertebrate nervous system and suggest a possible mechanism to explain the reduction in the responsiveness of horizontal cells observed when retinas are treated with dopamine.