The horizontal cells

Zinc modulation of hemi-gap-junction channel currents in retinal horizontal cells

Hemi-gap-junction (HGJ) channels of retinal horizontal cells (HCs) function as transmembrane ion channels that are modulated by voltage and calcium. As an endogenous retinal neuromodulator, zinc, which is coreleased with glutamate at photoreceptor synapses, plays an important role in shaping visual signals by acting on postsynaptic HCs in vivo. To understand more fully the regulation and function of HC HGJ channels, we examined the effect of Zn(2+) on HGJ channel currents in bass retinal HCs. Hemichannel currents elicited by depolarization in Ca(2+)-free medium and in 1 mM Ca(2+) medium were significantly inhibited by extracellular Zn(2+). The inhibition by Zn(2+) of hemichannel currents was dose dependent with a half-maximum inhibitory concentration of 37 microM. Compared with other divalent cations, Zn(2+) exhibited higher inhibitory potency, with the order being Zn(2+) > Cd(2+) approximately Co(2+) > Ca(2+) > Ba(2+) > Mg(2+). Zn(2+) and Ca(2+) were found to modulate HGJ channels independently in additivity experiments. Modification of histidine residues with N-bromosuccinimide suppressed the inhibitory action of Zn(2+), whereas modification of cysteine residues had no significant effect on Zn(2+) inhibition. Taken together, these results suggest that zinc acts on HGJ channels in a calcium-independent way and that histidine residues on the extracellular domain of HGJ channels mediate the inhibitory action of zinc.

Receptive fields of retinal bipolar cells are mediated by heterogeneous synaptic circuitry

Center-surround antagonistic receptive field (CSARF) organization is the basic synaptic circuit that serves as elementary building blocks for spatial information processing in the visual system. Cells with such receptive fields converge into higher-order visual neurons to form more complex receptive fields. Retinal bipolar cells (BCs) are the first neurons along the visual pathway that exhibit CSARF organization. BCs have been classified according to their response polarities and rod/cone inputs, and they project signals to target cells at different sublaminae of the inner plexiform layer. On the other hand, CSARFs of various types of BCs have been assumed be organized the same way. Here we examined center and surround responses of over 250 salamander BCs, and demonstrated that different types of BCs exhibit different patterns of dye coupling, receptive field center size, surround response strength, and conductance changes associated with center and surround responses. We show that BC receptive field center sizes varied with the degree of BC-BC coupling, and that surround responses of different BCs are mediated by different combinations of five lateral synaptic pathways mediated by the horizontal cells and amacrine cells. The finding of heterogeneous receptive field circuitry fundamentally challenges the common assumption that CSARFs of different subtypes of visual neurons are mediated by the same synaptic pathways. BCs carrying different visual signals use different synaptic circuits to process spatial information, allowing shape and contrast computation be differentially modulated by various lighting and adaptation conditions.

Interaction between rod and cone inputs in mixed-input bipolar cells in goldfish retina

One class of goldfish bipolar cells, the mixed-input bipolar cell, contacts both rods and cones. Although the morphology of the different mixed-input bipolar cell subtypes has been described, insight into the interaction between rods and cones at the bipolar cell level is scarce. The aim of this study was to characterize this interaction in the different physiological types of mixed-input bipolar cells. We found mixed-input bipolar cells that depolarized, hyperpolarized, or showed a combination of the two types of response after center stimulation. The relative contributions of rod and cone inputs varied strongly in these cell populations. Depolarizing mixed-input bipolar cells are rod-dominated, having the highest sensitivity and the smallest dynamic range. Hyperpolarizing mixed-input bipolar cells, on the other hand, have a more balanced rod-cone input ratio. This extends their dynamic range and decreases their sensitivity. Finally, opponent mixed-input bipolar cells seem to be mostly cone-dominated, although some rod input is present. The antagonistic photoreceptor inputs form a push-pull system that makes these mixed-input bipolar cells very sensitive to changes in light intensity. Our finding that spectral tuning changes with light intensity conflicts with the idea that the separate non-opponent and opponent channels are related to coding of brightness and color, respectively. The organization of mixed-input bipolar cells into various classes with different dynamic ranges and absolute sensitivities might be a strategy to transmit information about all visual aspects most efficiently, given the sustained nature of bipolar cell responses and their limited voltage range.

Electrical coupling, receptive fields, and relative rod/cone inputs of horizontal cells in the tiger salamander retina

Light responses, dendritic/axonal morphology, receptive field diameters, patterns of dye coupling, and relative rod/cone inputs of various types of horizontal cells (HCs) were studied using intracellular recording and Lucifer yellow/neurobiotin dye injection methods in the flatmount tiger salamander retina. Three physiologically and morphologically distinct types of HC entities were identified. 1) The A-type HCs are somas that do not bear axons, with average (+/-SE) soma diameters of 20.01 +/- 0.59 microm, relatively sparse and thick dendrites, and they resemble the A-type HC in mammals. The average receptive field diameter of these cells is 529.6 +/- 10.87 microm and they receive inputs predominantly from cones. 2) The B-type HCs are broad-field somas that bear thin and long axons, with average soma diameters of 17.67 +/- 0.38 microm, thinner dendrites of higher density, and they resemble the B-type HC in mammals. The average receptive field diameter of these cells is 1,633.55 +/- 37.34 microm and they receive mixed inputs from rods and cones. 3) The B-type HC axon terminals are broad-field, coarse axon terminal processes and they resemble the B-type HC axon terminal in rabbits. The average receptive field diameter of these axon terminals is 1,291.67 +/- 24.02 microm and they receive mixed inputs from rods and cones. All these types of HC are dye-coupled with adjacent HCs of the same type. Additionally, B-type HCs and axon terminals are dye-coupled with subpopulations of bipolar cells whose axon terminals ramify in the proximal half of the inner plexiform layer, raising the possibility that these HCs may send feedforward antagonistic surround responses to depolarizing bipolar cells through electrical synapses.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

Uncoupling of horizontal cells alters the receptive fields of retinal bipolar cells

Effects of uncoupling of horizontal cells by 1-octanol, a non-specific gap junction uncoupling agent, on the receptive field organization of cone-dominant bipolar cells were investigated in isolated, superfused carp retina, using intracellular recording techniques. At 1 mM, 1-octanol increased responses of cone driven horizontal cells to light spots, but decreased those to light annuli, indicating a reduction of the receptive field size of these cells by uncoupling. Furthermore, 1-octanol eliminated the surround response of OFF type bipolar cells and increased their center response. Similar effects of 1-octanol on the center and surround responses were observed in ON type bipolar cells. These results suggest that uncoupling of horizontal cells can significantly alter the receptive field organization of retinal bipolar cells.

Mechanism linking NMDA receptor activation to modulation of voltage-gated sodium current in distal retina

In this study, we investigated the mechanism that links activation of N-methyl-D-aspartate (NMDA) receptors to inhibition of voltage-gated sodium channels in isolated catfish cone horizontal cells. NMDA channels were activated in voltage-clamped cells incubated in low-calcium saline or dialyzed with the calcium chelator BAPTA to determine that calcium influx through NMDA channels is required for sodium channel modulation. To determine whether calcium influx through NMDA channels triggers calcium-induced calcium release (CICR), cells were loaded with the calcium-sensitive dye calcium green 2 and changes in relative fluorescence were measured in response to NMDA. Responses were compared with measurements obtained when caffeine depleted stores. Voltage-clamp studies demonstrated that CICR modulated sodium channels in a manner similar to that of NMDA. Blocking NMDA receptors with AP-7, blocking CICR with ruthenium red, depleting stores with caffeine, or dialyzing cells with calmodulin antagonists W-5 or peptide 290-309 all prevented sodium channel modulation. These results support the hypothesis that NMDA modulation of voltage-gated sodium channels in horizontal cells requires CICR and activation of a calmodulin-dependent signaling pathway.

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.

Dopaminergic Modulation of Transient Neurite Outgrowth from Horizontal Cells of the Fish Retina is not Mediated by cAMP

Horizontal cell dendrites invaginating the cone pedicles in the fish retina exhibit a marked light dependent plasticity in the morphology of their synaptic connections. Upon light adaptation of the retina, numerous spinules are formed which disappear during dark adaptation. This process is paralleled by a strengthening and weakening, respectively, of the horizontal cell's inhibitory output. The formation of spinules during light adaptation requires dopaminergic activity as it does not occur in dopamine-depleted retinas, but can be partially induced in depleted retinas by the exogenous administration of dopamine. Although horizontal cells do have D1 receptors the action of dopamine is not coupled to a stimulation of cAMP. An increase of intracellular cAMP either by injection of a cAMP analogue or by metabolic interference does not result in any spinule formation. The data suggest that dopamine must act through a cAMP independent intracellular mechanism.

I4AA-Sensitive chloride current contributes to the center light responses of bipolar cells in the tiger salamander retina

Light-evoked currents in depolarizing and hyperpolarizing bipolar cells (DBCs and HBCs) were recorded under voltage-clamp conditions in living retinal slices of the larval tiger salamander. Responses to illumination at the center of the DBCs' and HBCs' receptive fields were mediated by two postsynaptic currents: DeltaI(C), a glutamate-gated cation current with a reversal potential near 0 mV, and DeltaI(Cl), a chloride current with a reversal potential near -60 mV. In DBCs DeltaI(C) was suppressed by L-2-amino-4-phosphonobutyric acid (L-AP4), and in HBCs it was suppressed by 6,7-dinitroquinoxaline-2,3-dione (DNQX). In both DBCs and HBCs DeltaI(Cl) was suppressed by imidazole-4-acetic acid (I4AA), a GABA receptor agonist and GABA(C) receptor antagonist. In all DBCs and HBCs examined, 10 microM I4AA eliminated DeltaI(Cl) and the light-evoked current became predominately mediated by DeltaI(C). The addition of 20 microM L-AP4 to the DBCs or 50 microM DNQX to HBCs completely abolished DeltaI(C). Focal application of glutamate at the inner plexiform layer elicited chloride currents in bipolar cells by depolarizing amacrine cells that release GABA at synapses on bipolar cell axon terminals, and such glutamate-induced chloride currents in DBCs and HBCs could be reversibly blocked by 10 microM I4AA. These experiments suggest that the light-evoked, I4AA-sensitive chloride currents (DeltaI(Cl)) in DBCs and HBCs are mediated by narrow field GABAergic amacrine cells that activate GABA(C) receptors on bipolar cell axon terminals. Picrotoxin (200 microM) or (1,2,5,6-tetrahydropyridine-4yl) methyphosphinic acid (TPMPA) (2 other GABA(C) receptor antagonists) did not block (but enhanced and broadened) the light-evoked DeltaI(Cl), although they decreased the chloride current induced by puff application of GABA or glutamate. The light response of narrow field amacrine cells were not affected by I4AA, but were substantially enhanced and broadened by picrotoxin. These results suggest that there are at least two types of GABA(C) receptors in bipolar cells: one exhibits stronger I4AA sensitivity than the other, but both can be partially blocked by picrotoxin. The GABA receptors in narrow field amacrine cells are I4AA insensitive and picrotoxin sensitive. The light-evoked DeltaI(Cl) in bipolar cells are mediated by the more strongly I4AA-sensitive GABA(C) receptors. Picrotoxin, although acting as a partial GABA(C) receptor antagonist in bipolar cells, does not suppress DeltaI(Cl) because its presynaptic effects on amacrine cell light responses override its antagonistic postsynaptic actions.

Effects of GABA on horizontal cells in the tiger salamander retina

Application of gamma-aminobutyric acid (GABA) slows down the horizontal cell response time course (HCRRT) and induces membrane depolarization in horizontal cells (HCs) after synaptic inputs are blocked by Co2+. We present evidence that suggests both effects are probably mediated by GABAA receptors which open chloride channels in the HC membrane. In any given concentration of GABA, ranged from 0 to 100 microM, the HC membrane potential (VHC) in saturating light and in the presence of 100 microM Co2+ are identical. This result suggests that GABA in both light and 100 microM Co2+ opens the same amount of chloride channels (same gCl) so that VHC determined by chloride and leak conductances has the same value. Higher concentrations of Co2+ (> 300 microM) not only blocks synaptic transmission from photoreceptors to HCs, but also acts as an antagonist that suppresses the GABA-mediated depolarization in HCs.

Sustained and transient calcium currents in horizontal cells of the white bass retina

Calcium currents were recorded from cultured horizontal cells (HCs) isolated from adult white bass retinas, using the whole-cell patch-clamp technique. Ca2+ currents were enhanced using 10 mM extracellular Ca2+, while Na+ and K+ currents were pharmacologically suppressed. Two components of the Ca2+ current, one transient, the other sustained, were found. The large transient component of the Ca2+ current, which has not been seen before in HCs, is similar, but not identical, to the T-type Ca2+ current described previously in a variety of preparations. The sustained component of the Ca2+ current is similar, but not identical, to the L-type current described in other preparations. FTX, a factor isolated from the venom of the funnel-web spider, Agelenopsis aperta, preferentially and irreversibly blocks the sustained component of the Ca2+ current at very dilute concentrations. The sustained component of the Ca2+ current inactivates slowly, over the course of 15-60 s, in some HCs. This inactivation of the sustained Ca2+ current, when present, is primarily voltage dependent rather than Ca2+ dependent.

Feedforward lateral inhibition in retinal bipolar cells: input-output relation of the horizontal cell-depolarizing bipolar cell synapse

Lateral inhibition is the ubiquitous strategy used by visual neurons for spatial resolution throughout the animal kingdom. It has been a puzzle whether lateral inputs in retinal bipolar cells are mediated by the horizontal cell (HC)-cone feedback synapse, by the HC-bipolar cell feedforward synapse, or by both. By blocking the central inputs of the depolarizing bipolar cells (DBCs) with L-2-amino-4-phosphonobutyrate, we were able to eliminate the contribution of the feedback synapse and to demonstrate the postsynaptic light response in DBCs mediated by the HC-DBC feedforward synapse. The HC-DBC feedforward synapse contributes roughly one-third of the surround response whereas the HC-cone-DBC feedback synapse probably contributes the rest.

Gap junction morphology of retinal horizontal cells is sensitive to pH alterations in vitro

Isolated goldfish retinae were incubated in NaHCO3-reduced solutions, a treatment known to lower intracellular pH and to decrease gap-junction-mediated coupling between cells. The morphology of the gap junctions of horizontal cells examined by means of freeze-fracture replicas and ultrathin sections displays alterations after such treatment. The gap-junctional particles aggregate into dense clusters or crystalline arrays, whereas controls (pH 7.5) display a loose arrangement of particles. Incubation in NaHCO3-reduced solution leads to the appearance, in ultrathin sections, of prominent, electron-dense material beneath the gap-junctional membranes. Both effects, the increasing density of particles and the appearance of electron-dense material, are reversible. The application of dopamine, which uncouples horizontal cells, and its antagonist haloperidol produce less clear-cut effects on particle density in vitro.

Localization of cyclic adenosine monophosphate in the teleost retina: effects of dopamine and prolonged darkness

Localization of cyclic adenosine monophosphate (cAMP) in the white perch retina was carried out with immunohistochemical and autoradiographic methods. Following exposure to dopamine or prolonged darkness, cAMP staining was observed by immunohistochemistry in the distal part of the inner nuclear layer, i.e. in the horizontal cells. After exposure to dopamine, increased levels of cAMP were also observed by autoradiography in many horizontal cells. Finally, increased levels of cAMP staining were observed immunohistochemically following incubation with dopamine in all types of cone-related horizontal cells that had been isolated and maintained in culture.

Retinal horizontal cell gap junctional conductance is modulated by dopamine through a cyclic AMP-dependent protein kinase

The action of many neuromodulators is mediated by intracellular second messengers such as cyclic AMP. In the retina, exogenously applied dopamine alters the conductance of gap junctions between cultured horizontal cells and this effect is mediated by cyclic AMP. However, it is not known how cyclic AMP modulates horizontal cell gap junction function. Here I report that cyclic AMP works by way of a cyclic AMP-dependent protein kinase. Cyclic AMP-dependent protein kinase injected into coupled horizontal cells from white bass (Roccus chrysops) rapidly and reversibly uncoupled the cells, mimicking the actions of dopamine. The threshold for the effect was between 0.06 and 0.03 microM. Injection of Walsh inhibitor of protein kinase [Walsh, D. A., Ashby, C. D., Gonzalez, C., Calkins, D., Fischer, E. H. & Krebs, E. G. (1971) J. Biol. Chem. 246, 1977-1985] blocked the effect of dopamine. Thus, the action of dopamine is to raise intracellular levels of cyclic AMP, which then activates a cyclic AMP-dependent protein kinase. Although not tested, it is likely that the cyclic AMP-dependent protein kinase phosphorylates a protein, possibly a gap junction protein, to alter conductance.

Effects of gamma-aminobutyric acid on cones and bipolar cells of the tiger salamander retina

The gamma-aminobutyric acid (GABA) system in the tiger salamander retina was studied using autoradiographic and electrophysiological techniques. A high-affinity uptake mechanism for GABA has been localized in about 60% of the horizontal cells and about 30% of the amacrine cells. Effects of exogeneously applied GABA on the membrane conductance of cones and hyperpolarizing bipolar cells (HBC) were examined using the two electrode current-clamp technique in the living retinal slices. In both cell types, 1 mM of GABA caused a conductance increase. In perfused eyecups, 2 mM of GABA selectivity abolished the surround response of the HBC and left the center response unchanged. These results are consistent with the notion that a population of horizontal cells and a population of amacrine cells in the salamander retina may use GABA as their neurotransmitter.

Responsiveness and receptive field size of carp horizontal cells are reduced by prolonged darkness and dopamine

In the fish retina the interplexiform cells contain dopamine and provide a centrifugal pathway from the inner plexiform layer to horizontal cells of the outer plexiform layer. Dopamine application reduced the responsiveness and receptive field size of cone horizontal cells, as did a prolonged period of complete darkness. Other results suggest that the interplexiform cells may release dopamine after a prolonged period in the dark. The interplexiform-horizontal cell system may modify the strength of the antagonistic surrounds of retinal neurons as a function of time in the dark.

Dopamine decreases conductance of the electrical junctions between cultured retinal horizontal cells

Horizontal cells from the white perch were isolated by enzymatic treatment and trituration of the retina and were maintained in culture for 1-5 days. Overlapping pairs of horizontal cells were identified, and the two cells were recorded from simultaneously, using whole-cell patch clamp techniques. Electrical coupling between cells was determined by passing current pulses into one cell, the driver cell, while (i) recording voltage changes in the other, follower cell, or (ii) measuring current flow into the follower cell. Most cell pairs of the same morphological type were coupled electrically, with coupling coefficients often greater than 0.9. Junctional resistance was typically found to be between 20 and 60 M omega and junctional conductance was between 150 and 500 nS. After application of 1-microliter pulses of dopamine (200 microM) to coupled pairs of cells, the coupling coefficient fell to approximately equal to 0.1, junctional resistance increased to 300-700 M omega, and junctional conductance decreased to 15-30 nS. Recovery of coupling took, for most cell pairs tested, 8-15 min after dopamine application. The exogenous application of 8-bromo-cyclic AMP (0.5-1 mM) also caused uncoupling of horizontal cell pairs; however, neither isoprenaline nor L-glutamate altered coupling significantly.

Monoclonal antibodies distinguish subtypes of retinal horizontal cells

Sixteen hybridomas have been identified that secrete antibodies specific to horizontal cells in the carp retina. The hybridomas have been classified into three groups based on their antibody staining patterns: group I, staining associated with all horizontal cells; group II, staining associated with the most abundant subtype of horizontal cell (CH1); and group III, staining associated with other subtypes of horizontal cells. Most of the hybridomas fall in group II; some of these antibodies stain the entire horizontal cell, but others are specific only to the cell perikarya and do not stain axonal processes. Our results suggest that there are surface molecules specific (i) to all retinal horizontal cells, (ii) to individual subtypes of horizontal cells, and (iii) to portions of horizontal cells. Furthermore, a group II antibody, which recognizes a 48- to 50-kDa membrane protein, has been found to provide a substrate selective for horizontal cell growth. Horizontal cells plated on coverslips coated with this antibody remain healthy in culture and extend long and elaborate processes for at least 3 weeks.

Electrical coupling between horizontal cell bodies in the tiger salamander retina

We studied lateral interactions between horizontal cells in the tiger salamander retina in order to determine the underlying mode of synaptic transmission. Pairs of cells, visually identified by inspection of the in vitro retinal slices, were impaled with two separate electrodes and the transmission between them was examined by injecting current into one cell while recording the resulting voltage response from the other soma. It was found that coupling between horizontal cells remained intact and even increased somewhat during the light response and in the presence of cobalt or acetylcholine. It was also observed that a fluorescent dye injected into one cell would often migrate and stain neighbouring horizontal cells. These results suggest that synaptic transmission between horizontal cell bodies is mediated mainly by electrotonic conduction. The increase in apparent coupling observed under certain experimental conditions can be explained by the increase in input resistance of each horizontal cell.

Ionic currents of solitary horizontal cells isolated from goldfish retina

Solitary horizontal cells, dissociated from papain-treated goldfish retinas, produce action potentials and show a non-linear current-voltage relationship. Underlying ion-conductance mechanisms were analysed by a single-micro-electrode voltage-clamp technique. Pharmacological and ion-substitution experiments revealed that ionic currents could be separated into at least four voltage-dependent currents: a Ca current and three types of K currents. The Ca current was activated by membrane depolarization beyond -45 mV, reached a maximal value near 0 mV, and became smaller at more positive potentials. By extrapolation, the reversal potential was estimated to be approximately +50 mV. The Ca current was inactivated by accumulation of intracellular Ca ions but not by membrane depolarization. Co ions (4mM) blocked this current. The first type of K current showed anomalous (inward-going) rectification near the resting potential (congruent to -60 mV). Hyperpolarization from the resting level produced a large, almost steady inward current, while depolarization evoked only a small, steady outward current. The current-voltage relationship revealed a shallow negative resistance region at membrane potentials beyond -50 mV. The current was blocked by Cs (10 mM) or Ba (1 mM) ions. The second type of K current (the transient outward current) was activated by membrane depolarization beyond -25 mV. The peak amplitude increased almost exponentially as the membrane was depolarized. During steady depolarization this current decayed exponentially (time constant congruent to 500 ms at +20 mV). The current was inactivated by conditioning depolarization (greater than 10 s) beyond -30 mV and blocked by 4-aminopyridine (10 mM). The third type of K current was the maintained outward current which was activated by membrane depolarization beyond -20 mV, increased to a steady level in a few hundred milliseconds, and showed little inactivation. The amplitude increased as the membrane was depolarized. The current was blocked by tetraethylammonium ions (20 mM). A Ca-mediated K current was not detected. Action potentials and the non-linear current-voltage relationship of solitary horizontal cells can be explained qualitatively by the combination of the four ionic currents.

Sodium and calcium currents measured in isolated catfish horizontal cells under voltage clamp

A low resistance suction microelectrode was used to record intracellularly from enzymatically dissociated horizontal cells obtained from catfish retina (Ictalurus punctatus). This microelectrode was connected to a voltage clamp circuit and the transmembrane currents were recorded during depolarizing and hyperpolarizing clamp pulses. Two fast transient inward currents were recorded, one sensitive to tetrodotoxin and one insensitive to this drug. A slower and persistent tetrodotoxin insensitive inward current was also recorded. The results indicate that separate membrane conductances exist for both sodium and calcium ions. We suggest that the action potential recorded from isolated horizontal cells is generated by both sodium and calcium currents.

Discrete nonlinear reduction model for horizontal cell response in the carp retina

In this paper we present a new model for the spatial behavior of the external horizontal cells in the carp retina which elucidates more precisely the mechanisms of their intercellular connections. The area and intensity effects of a brief light stimulus on the L-type response were recorded and analysed. We discovered clear nonlinearities in the behavior of the response which could not be explained by the simple summative model presently accepted. Proposed here is a new model, the spatial reduction mechanism, which explains the experimental evidences including these nonlinearities. We also present the results of a computer simulation using our new model in parallel with the actual results obtained experimentally. The physiological implications of these findings are discussed.

Convergence of signals from red-sensitive and green-sensitive cones onto L-type external horizontal cells of the goldfish retina

The types of photoreceptors converging onto L-type external horizontal cells (LEHCs) were studied intracellularly in live, immobilized goldfish under dark-adapted conditions, and interactions between inputs from these photoreceptors were demonstrated. Our analysis of the color-dependence of response waveform, and the spectral sensitivity of LEHCs during various phases of their responses, suggests that LEHCs receive inputs not only from red-sensitive cones, but also from green-sensitive cones. Furthermore, inputs from red- and green-sensitive cones were found to interact so as to enhance LEHC responses. By presenting two successive flashes of different colors, the response enhancement was demonstrated only when green flashes preceded red flashes, indicating that responses to the input from green-sensitive cones affected the input from red-sensitive cones. A hypothetical model which incorporates reciprocal connections between green-sensitive cones and LEHCs is proposed, and shown to be consistent with most of the phenomena described in this paper, including the response enhancement.

Biochemical and biophysical studies of isolated horizontal cells from the teleost retina

Neurons in the vertebrate retina are interconnected by a complex network of chemical and electrical synapses which make it difficult to study the intrinsic biochemical and biophysical properties of individual cells. During the past decade, enzymatic methods have been developed for dissociating adult retinas into viable and readily identifiable single cells. Using this preparation, we and other investigators have studied the physiological and biochemical properties of several classes of isolated retinal cells. In particular, we have shown that a type of retinal interneuron, the GABA-ergic horizontal cell of teleost retinas, may be an excellent model system for examining the intrinsic membrane properties as well as the cellular mechanisms regulating GABA uptake, synthesis and release from an identified CNS neuron.

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

Horizontal cells were enzymatically isolated from the carp retina and maintained in culture for 2-7 days. Cultured horizontal cells typically had resting membrane potentials of -50 to -70 mV and input resistances of 100-150 m omega. The cells were treated with a number of neurotransmitter agents and their analogues. Significant responses were evoked only by 3,4-dihydroxyphenylalanine (dopamine), L-glutamate, and certain glutamate analogues. The responses to dopamine were inconsistent; most often, the membrane hyperpolarized and input resistances increased. However, highly characteristic responses to L-glutamate and its analogues, quisqualate and kainate, were observed in virtually all of the cells tested. The responses consisted of an initial graded depolarization accompanied by a resistance increase, followed in most cases by a prolonged (1- to 2-min) regenerative depolarization. The regenerative component of the response appears to be Ca2+ dependent, while the underlying graded potential may be due to a decrease in K+ conductance of the membrane.

A model for the temporal organization of X- and Y-type receptive fields in the primate retina

A model is proposed for the temporal characteristics of X- and Y-type responses of ganglion cells in the primate retina. The main suggestions of the model are: (I) The X-type temporal response is determined primarily by the delay between center and surround contributions. (II) The Y-type response is generated in the inner plexiform layer by a derivative-like operation on the bipolar cell's input, followed by a rectification in the convergence of these inputs onto the Y-ganglion-cell. (III) The derivative-like operation is obtained by recurrent inhibition in the dyad synaptic structure. The X- and Y-type responses predicted by the model, for a variety of stimuli, were examined and compared with available electrophysiological recordings. Finally, certain predictions derived from the model are discussed.

Pikeperch horizontal cells identified by intracellular staining

Micropipettes filled with Procion yellow dye were used to record from and to stain pikeperch horizontal cells intracellularly. Three major types were found: a distal layer (H1) of relatively small cells which were luminosity or L-type; a second and more proximal layer of larger L-type cells (H2); and a third and yet more proximal layer of very stellate chromatic or C-type cells (H3). A few anucleate processes which displayed slow-potentials were found in the proximal area of the inner nuclear layer. Cells of each of the three layers were shown to be cone related by both anatomical and physiological methods. L-type cells were further categorized by the area over which each exhibited spatial summation and the relative sensitivity of each to red and green lights. Receptive field sizes of H2's were found to range from less than 2 mm to greater than 5 mm in diameter, whereas those of the few H1's tested were all less than 2 mm. Results from spectral screening tests indicate that most H1 and H2 cells are maximally sensitive to orange light, whereas the H3 cells hyperpolarize maximally to green and depolarize maximally to red. A small percentage of sampled C-cells displayed an additional depolarization to violet.

Goldfish retina: a correlate between cone activity and morphology of the horizontal cell in clone pedicules

In the cone pedicules, the digitations of horizontal cell process lateral to the synaptic ribbon disappear after dark adaptation. This disappearance is correlated with the loss of color opponency and cone function shown in ganglion cell recordings in isolated retinas. Cone function and color-opponent responses are restored by reapplying background light.

Receptor contacts of horizontal cells in the retina of the domestic cat

The terminal aggregations of A- and B-type horizontal cells, stained by the Golgi-Colonnier method, have been analysed. The pattern of the aggregations is regular and is shown to be in register with the cone mosaic. Both tyes of horizontal cell are in contact with at least 80% of the cones above their dendritic fields. Therefore, the different horizontal cell classes cannot be selective for a special kind of cone but must have at least 60% of the cone input in common. Each A-type horizontal cell makes contacts with between 120 and 170 cones, and each B-type horizontal cell with 60-90 cones. An individual A-type horizontal cell occupies an average of 20% of the lateral elements of the triads in a cone pedicle, but an individual B-type cell fills only some 13%. Each and every cone is connected with several of both types of horizontal cell. An estimation of the number of rods converging onto a single axon terminal system showed that it could be as many as 3000.

Regenerative amacrine cell depolarization and formation of on-off ganglion cell response

1. Recordings from amacrine and ganglion cells in the mudpuppy retina suggest mechanisms whereby the relatively slow, sustained light responses measured in bipolar cells are converted to rapid, brief, transient activity in the on-off ganglion cells. 2. Double-barrel electrodes were used to control the membrane potential under voltage clamp. The clamp revealed synaptic currents, but eliminated the otherwise obvious spike activity elicited by steps of illumination in both amacrine and ganglion cells, suggesting that the spikes are initiated near the somata. 3. The synaptic current in the on-off ganglion cells was biphasic: a brief inward (depolarizing) membrane current preceded a transient outward (hyperpolarizing) membrane current by about 20 msec. Each component could be isolated by polarizing the membrane to a level near the reversal potential for the other. Each was apparently due to a transient conductance increase of sawtooth shape with a 40 msec time to peak and a decay longer than 400 msec. 4. Synaptic membrane current in amacrine cells was monophasic and inward (depolarizing) of similar sawtooth shape at all potential levels. It was apparently mediated by a conductance increase to ions with a reversal potential more positive than the dark level. 5. When amacrine cells were depolarized in the dark under voltage clamp, a large transient inward membrane current with threshold within 4 mV of the dark level was generated. This regenerative event is capable of boosting a small, 4 mV e.p.s.p. to more than 30 mV in a few milliseconds, thereby generating the leading edge of a rapid sawtooth response. 6. The results suggest that the rapid transient on-off activity in ganglion cells is mediated by opposing sawtooth shaped synaptic currents with different latencies. It is inferred that each of these antagonistic imputs is generated by a regenerative depolarization in amacrine cells which then form synaptic inputs to the ganglion cells.

Receptive field organization of ganglion cells in the frog retina: contributions from cones, green rods and red rods

1. The impulse discharge of ganglion cells was recorded with extracellular micro-electrodes in the excised and opened eye of the common frog, Rana temporaria. 2. When a single unit was isolated, the cell type was first determined according to the Maturana, Lettvin, McCulloch & Pitts (1960) classification with the aid of varying moving and stationary stimuli. 3. Class 4 cells respond only to a decrease of light when cones are stimulated but respond to an increase of light when green rods are stimulated. A distinct class of deviating class 4 cells was found that give a brief high frequency burst at 'off' from their small excitatory receptive fields (ERF); unlike typical class 4 cells they possess a purely inhibitory surrounding field (IRF).4. The contributions from the cones and the green and red rods were isolated by measuring the thresholds of the discharges with on-off stimuli of varying wave-lengths against strong yellow backgrounds, or against a very weak background or no background at all. The spatial distribution of the contributions to the ERF was determined by mapping threshold profiles, and additional information about ERF and IRF was obtained from area-threshold curves. 5. The cone-mediated ERFs were found to be 0-06-0-50 mm wide (1-5-12 degrees of visual field), which agrees well with the sizes of the dendritic trees of the ganglion cells. The green rod-mediated ERFs can be 0-5-1-5 mm wide and have less distinct boundaries than the cone-mediated. The green rod-mediated ERF of an individual ganglion cell is always larger than the cone-mediated ERF of the same cell. The red rod-mediated ERFs seem to be somewhat larger than the cone-mediated but smaller than the green rod-mediated. 6. The green rods contribute only to the on thresholds of class 1, 2 and 4 cells, but both to on and off in typical class 3 cells, while the cones contribute to on and off in classes 1-3 and only to off in class 4.7. When the red rods begin to contribute during dark adaptation they seem to enter the cone but not the green rod channels. 8. All three receptor types contribute to the IRF surrounding the ERF of classes 1, 2, 3 and deviating class 4 cells. Normal class 4 cells have no IRF. 9. The organization of the receptive fields is discussed in relation to the anatomy and electrophysiology of the cell types transmitting the signals from the receptors to the ganglion cells.

Colour receptors, and their synaptic connexions, in the retina of a cyprinid fish

Morphologically speaking, there are five kinds of cone cells in the retina of the rudd ( Scardinius erythrophthalmus ). But two of them, the principal elements of the double cones and the free principal cones, are probably functionally equivalent, while another, sparse, population of small ( oblique ) cones (which disappear in older fish), is unlikely to make a significant contribution to visual spectral sensitivity. Thus, principal and accessory cones (usually paired with one another), and single cones seem to be the three receptors which underlie the fish’s trichromacy. Photographic densitometry of individual cone cells was used to provide evidence that accessory cones contain a green-absorbing photopigment and the single cones a blue one. Other arguments are given in support of those identifications, and they also strongly suggest that principal cones contain the red-absorbing pigment. Golgi-impregnated bipolar cells were examined electron-microscopically to determine the specific patterns of synaptic connexion they make with these different, anatomically identifiable, colour cones and with the retinal rods. Three principal arrangements were distinguished (see figure 69, page 190). (1) Rod bipolar cells comprise two distinct morphological types, both of which connect exclusively to principal (red) cones as well as to the rods within the outlines of their dendritic fields. (2) Selective cone bipolar cells, more delicate neurons with considerably wider dendritic fields, connect (according to type) to one or other of the different colour cone populations. Examples analysed were specific for the accessory (green) or for the single (blue) cones; no bipolar cells were found connected only to red cones. (3) Mixed cone bipolars have the smallest dendritic fields, and connect to combinations of cones (for example, red and green, or green and blue, but not red and blue). They also have synaptic input (usually relatively sparse) from the rods. Cells were encountered connecting to all three cone types, but they were only partially analysed, and are not described at length. The light microscopic morphology of these bipolar cell types consistently reflects the detailed pattern of connexion each makes with the different receptor populations (just as the morphology of the cones reflects the spectral properties of their photopigment). But while their synaptic connectivity is generally highly specific for cone type, they do occasionally make anomalous connexions with the ‘wrong’ receptors. There is a high degree of divergence (page 85) at the receptor-bipolar synapses, and the different kinds of cones each characteristically connect to different numbers of bipolar cells. Principal (red) cones, which are the most numerous, individually connect to more bipolars than cones of other types, whose characteristic synaptic divergence is likewise related to the frequency with which they occur in the retina. However, rods, which are much more numerous than cones, do not conform with this generalization. The selectivity with which the synaptic terminals of the different cones are connected together by their invaginating basal processes was also examined. These processes link neighbouring synaptic terminals of differently coloured cones: specifically, principal (red) cone basal processes invaginate accessory (green) cone pedicles, and vice versa. Single (blue) cone basal processes connect only to accessory cone pedicles, but that synaptic relation is not reciprocated. These synapses between the cones have important bearing upon interpretation of the bipolar cell connectivity patterns. In their light, the interaction between colour channels which the convergence of different cones onto the mixed cone bipolar dendrites mediates, seems to re-iterate a process already undertaken more peripherally. Likewise, whereas the anatomy of the selective cone bipolars appears designed to convey activity from the individual cone populations, the responses of the receptors they sample must already be influenced by activity in other colour channels.

The effects of maintained light stimulation on S-potentials recorded from the retina of a teleost fish

1. S-potential responses to transient and maintained light stimuli have been recorded from units in the mixed rod-cone retina of a teleost fish species Eugerres plumieri. 2. Four spectral classes of S-potential were observed, three cone- and one rod-type. The cone-type responses were subdivided into two L-type (referred to as L1 and L2), and a C-type response. Two classes of transient depolarization response were also recorded from those retinal levels associated with the S-potential responses and these are attributed, tentatively, to rod and cone bipolar activity. 3. L2-type S-potentials do not yield constant hyperpolarization during maintained light stimulation, the time course of the response potential, V, being given approximately by (see article) where Vt is the response potential at time t sec following the onset of stimulation, Vo being the initial response potential. In contrast, both hyperpolarizing and depolarizing components of the C-type response were maintained under conditions of steady illumination. 4. Under maintained light stimulation at saturation illumination level, the rod S-potentials escape from hyperpolarization in a manner similar to that previously observed for the skate (Dowling & Ripps, 1971). 5. L2-type responses to transient test stimuli of illumination level I, superimposed on a steady background field of illumination level I', are in some respects consistent with Alpern, Rushton & Torii's (1970) empirical formula (see article) with K, I one-half and ID constants. However, for the present data, the value of I one-half is dependent on I'. 6. The significance of Ricco's law for S-potential responses is discussed in relation to these findings.