Modulation of a sustained calcium current by intracellular pH in horizontal cells of fish retina

A sustained high voltage-activated (HVA), nifedipine- and cadmium- sensitive calcium current and a sustained calcium action potential (AP) were recorded from horizontal cells isolated from catfish retina. pH indicator dyes showed that superfusion with NH4Cl alkalinized these cells and that washout of NH4Cl or superfusion with Na-acetate acidified them. HVA current was slightly enhanced during superfusion of NH4Cl but was suppressed upon NH4Cl washout or application of Na- acetate. When 25 mM HEPES was added to the patch pipette to increase intracellular pH buffering, the effects of NH4Cl and Na-acetate on HVA current were reduced. These results indicated that intracellular acidification reduces HVA calcium current and alkalinization increases it. Sustained APs, recorded with high resistance, small diameter microelectrodes, were blocked by cobalt and cadmium and their magnitude varied with extracellular calcium concentration. These results provide confirmatory evidence that the HVA current is a major component of the AP and indicate that the AP can be used as a measure of how the HVA current can be modified in intact, undialyzed cells. The duration of APs was increased by superfusion with NH4Cl and reduced by washout of NH4Cl or superfusion with Na-acetate. The Na-acetate and NH4Cl washout- dependent shortening of the APs was observed in the presence of intracellular BAPTA, a calcium chelator, IBMX, a phosphodiesterase inhibitor, and in Na-free or TEA-enriched saline. These findings provide supportive evidence that intracellular acidification may directly suppress the HVA calcium current in intact cells. Intracellular pH changes would thereby be expected to modulate not only the resting membrane potential of these cells in darkness, but calcium- dependent release of neurotransmitter from these cells as well. Furthermore, this acidification-dependent suppression of calcium current could serve a protective role by reducing calcium entry during retinal ischemia, which is usually thought to be accompanied by intracellular acidosis.     

Membrane potential clamping in horizontal cells of the fish retina

The membrane potential of horizontal cells of the retina was clamped by uniform polarization of the layer of these cells by a current passed through extracellular electrodes. The volt-ampere characteristic curve of the synaptic membrane of the horizontal cells in some cases had segments with negative slope. With a sharp change in the level of voltage clamping the time taken for the resistance of the membrane to change was under 20 msec. Comparison of responses to photic stimulation recorded with and without voltage clamping showed that participation of the nonsynaptic membrane in the generation of responses to photic stimulation can affect their shape substantially.Institute of Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 9, No. 4, pp. 402–407, July–August, 1977.     

Some horizontal cells of the bovine retina receive input synapses in the inner plexiform layer

Bovine retinae were stained immunocytochemically with antibodies against the calcium-binding protein, calbindin. Horizontal cells in the outer plexiform layer were heavily labelled. The processes of most horizontal cells were confined to the level of the outer plexiform layer, and the tips of their dendrites were positioned as the lateral elements of the cone triads, viz. the usual mammalian arrangement. However, some of the horizontal cells had additional thick processes descending to branch within the inner plexiform layer, where they were postsynaptic at bipolar cell dyads and where they also received input from amacrine cells. No output synapses of horizontal cells were observed in the inner plexiform layer.     

No evidence of UV cone input to mono- and biphasic horizontal cells in the goldfish retina

Many animal species make use of ultraviolet (UV) light in a number of behaviors, such as feeding and mating. The goldfish (Carassius auratus) is among those with a UV photoreceptor and pronounced UV sensitivity. Little is known, however, about the retinal processing of this input. We addressed this issue by recording intracellularly from second-order neurons in the adult goldfish retina. In order to test whether cone-driven horizontal cells (HCs) receive UV cone inputs, we performed chromatic adaptation experiments with mono- and biphasic HCs. We found no functional evidence of a projection from the UV-sensitive cones to these neurons in adult animals. This suggests that goldfish UV receptors may contact preferentially triphasic HCs, which is at odds with the hypothesis that all cones contact all cone-driven HC types. However, we did find evidence of direct M-cone input to monophasic HCs, favoring the idea that cone–HC contacts are more promiscuous than originally proposed. Together, our results suggest that either UV cones have a more restricted set of post-synaptic partners than the other three cone types, or that the UV input to mono- and biphasic HCs is not very pronounced in adult animals.     

Investigation of the nature of electrical responses of horizontal cells of the fish retina

On the basis of the syncytial structure of the layer of horizontal cells of the fish retina, a method is developed which effectively shifts the membrane potential of cells by means of an electrical current. It is shown that the response of L-type horizontal cells to light and electrical stimulation of the retina is reversed when the membrane of the horizontal cells is depolarized by a direct current. The equilibrium potential of the cells was near the zero level. Consequently, the depolarization response of the horizontal cells to disconnection of the light and to electrical stimulation of the retina is an excitatory postsynaptic potential, whereas hyperpolarization of the horizontal cells to light is a decrease of this potential. It is shown that the membrane of fish horizontal cells have pronounced nonlinear properties: in the case of strong depolarization and especially in the case of hyperpolarization its impedance drops markedly. The latter probably occurs due to an increase of the permeability of the nonsynaptic membrane of the horizintal cells for K⁺. This can also explain the decrease of membrane impedance during the hyperpolarization response of the horizontal cells to bright light. The available data indicate the presence of regenerative properties of the membrane of horizontal cells.Institute of Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 89–98, January–February, 1971.     

Displaced horizontal cells in the chick retina

The retina of the chick contains retinal cells of a morphology very similar to that of the horizontal cells, but the perikarya, axons, and axon terminals lie in the inner plexiform layer. The discovery of this neuronal ectopia appears to support the idea that some horizontal and amacrine cells derive from a common, freely migrating cell.     

Ion exchange in the horizontal cells of the retina

Experimental data indicate that the membrane potential of L-type horizontal cells of the retina to bright light (i.e., when synaptic inputs are completely closed) is close to the potassium equilibrium potential. From this observation the intracellular concentration of K⁺ and Na⁺ was estimated. The latter was found to be relatively high (tens of millimoles/liter), i.e., comparable with the intracellular K⁺ concentration. This result, coupled with data on closeness of the equilibrium potential of the photic response to zero, is evidence that besides sodium conductance, the potassium conductance of the subsynaptic membrane also participates in generation of the photic response by these cells. The steady-state sodium and potassium synaptic currents was shown to be relatively small and to vary only a little over the whole working range of potentials (from –72 to –16 mV), due to the nonlinear properties of the nonsynaptic cell membrane.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 14, No. 1, pp. 3–10, January–February, 1982.     

Interaction between horizontal cells of the turtle retina

Interaction between horizontal cells of the turtle retina was studied by two microelectrodes (polarizing and recording), inserted into different cells at different distances apart. The presence of a direct electrical connection was demonstrated between the L cells of the same type (I, with large, and II, with small receptive fields). Its magnitude depends on the conditions of illumination and the level of the membrane potential, possibly because of the properties of the subsynaptic and nonsynaptic membranes of the horizontal cells. No direct electrical connection exists between L cells of different types. However, hyperpolarization of the type I cells through the microelectrode or by stimulation with a circle of light evoked depolarization in the type II cells. This indirect connection between the horizontal cells, also dependent on the conditions of illumination, can probably be explained by feedback to these cells from the photoreceptors. Polarization of L cells of both types had no effect on horizontal cells of color type.     

Dynamics of chromaticity horizontal cells in the freshwater turtle retina

A model of the cone-horizontal cell circuit is presented based on morphological evidence recently found in the Reeves' turtle: a luminosity horizontal cell (LHC) that receives inputs from red-, green-, and blue-sensitive cones in the ratio of 15:3:1, a triphasic horizontal cell (THC) that receives inputs from one class of red-sensitive and from blue-sensitive cones in the ratio of 2:1; and a biphasic chromaticity horizontal cell (BHC) that receives inputs from green-sensitive cones as well as from a special class of red-sensitive (i.e. the broad spectrum) and from blue-sensitive cones in the ratio of 3:2:1. A study of the simulated impulse responses strongly suggests that the basic response patterns of the BHC and THC can be readily explained by a simple wiring diagram consisting of direct hyper-polarizing inputs from the appropriate cones and a depolarizing input from the LHC which acts as a voltage inverter. A negative feedback circuit from the LHC to the cone pedicles is included and its negative feedback gain increases as the mean illuminance level (Io) increases. The negative feedback circuit, which promotes adaptation in the cones to changing Io's, is not necessary for opponent polarization in the BHC or THC, but does explain variabilities of impulse responses.     

Color opponency in cone-driven horizontal cells in carp retina. Aspecific pathways between cones and horizontal cells

The spectral and dynamic properties of cone-driven horizontal cells in carp retina were evaluated with silent substitution stimuli and/or saturating background illumination. The aim of this study was to describe the wiring underlying the spectral sensitivity of these cells. We will present electrophysiological data that indicate that all cone-driven horizontal cell types receive input from all spectral cone types, and we will present evidence that all cone-driven horizontal cell types feedback to all spectral cone types. These two findings are the basis for a model for the spectral and dynamic behavior of all cone-driven horizontal cells in carp retina. The model can account for the spectral as well as the dynamic behavior of the horizontal cells. It will be shown that the strength of the feedforward and feedback pathways between a horizontal cell and a particular spectral cone type are roughly proportional. This model is in sharp contrast to the Stell model, where the spectral behavior of the three horizontal cell types is explained by a cascade of feedforward and feedback pathways between cones and horizontal cells. The Stell model accounts for the spectral but not for the dynamic behavior of the horizontal cells.     

Motility of cultured fish epidermal cells in the presence and absence of direct current electric fields

The motile behavior and cytoskeletal structures of fish epidermal cells (keratocytes) in the presence and absence of direct current (DC) electric fields were examined. These cells spontaneously show highly directional locomotion in culture, migrating at rates of up to 1 micron/s. When DC electric fields between 0.5 and 15 V/cm are applied, single epidermal cells as well as cell clusters and cell sheets migrate towards the cathode. Cell clusters and sheets break apart into single migratory cells in the upper range of these field strengths. Cell shape and morphology are unaltered when the keratocytes are guided by an electric field. Neither the spontaneous locomotion nor the electrically guided motility were found to be microtubule dependent. 1 mM La3+, 10 mM Co2+, 50 microM verapamil, and 50 microM nitrendipine (calcium channel antagonists) reversibly inhibited lamellipod formation and cell locomotion in both spontaneously migrating and electrically guided cells. Ciba-Geigy Product 28392, which stimulates the opening of calcium channels, and is a competitive inhibitor of nitrendipine, has no effect on the locomotion of keratocytes. Cell motility was also unaffected by hyperpolarizing and depolarizing (low and high K+) media. It is argued that while a tissue cell may accommodate changes in resting membrane potential without becoming more or less motile, the cell may not be able to counterbalance the effects of depolarization and hyperpolarization simultaneously. In this context, a gradient of membrane potential, which is induced by an external DC electric field, will serve as a persistent stimulus for cell locomotion.     

Receptive field properties of rod-driven horizontal cells in the skate retina

The large receptive fields of retinal horizontal cells result primarily from extensive intercellular coupling via gap (electrical) junctions; thus, the extent of the receptive field provides an index of the degree to which the cells are electrically coupled. For rod-driven horizontal cells in the dark-adapted skate retina, a space constant of 1.18 +/- 0.15 mm (SD) was obtained from measurements with a moving slit stimulus, and a comparable value (1.43 +/- 0.55 mm) was obtained with variation in spot diameter. These values, and the extensive spread of a fluorescent dye (Lucifer Yellow) from the site of injection to neighboring cells, indicate that the horizontal cells of the all-rod retina of skate are well coupled electrically. Neither the receptive field properties nor the gap-junctional features of skate horizontal cells were influenced by the adaptive state of the retina: (a) the receptive field organization was unaffected by light adaptation, (b) similar dye coupling was seen in both dark- and light-adapted retinae, and (c) no significant differences were found in the gap-junctional particle densities measured in dark- and light-adapted retinas, i.e., 3,184 +/- 286/microns 2 (n = 8) and 3,073 +/- 494/microns 2 (n = 11), respectively. Moreover, the receptive fields of skate horizontal cells were not altered by either dopamine, glycine, GABA, or the GABAA receptor antagonists bicuculline and picrotoxin. We conclude that the rod-driven horizontal cells of the skate retina are tightly coupled to one another, and that the coupling is not affected by photic and pharmacological conditions that are known to modulate intercellular coupling between cone-driven horizontal cells in other species.     

Endogenous control of spinule formation in horizontal cells of the teleost retina

Summary The processes of horizontal cells invaginating teleost cone pedicles are studded with small finger-like projections which are present only in the light-adapted state. The aim of this study was to investigate whether the formation and degradation of these so-called spinules, which are thought to function as feed-back synapses onto the cones, is endogenously controlled.Three types of experiment were carried out involving fish entrained to a 12 h light/dark cycle: 1) The number of spinules was determined in goldfish at various times during exposure to either constant darkness (36 h) or constant light (57 h). 2) The time course of spinule formation and degradation in goldfish was investigated following exposure to light or darkness at various times during the light/dark cycle. 3) The time course of flash-induced spinule formation in tench following dark adaptation at noon was compared to that following dark adaptation at midnight.The results of these experiments show that spinule formation and degradation are partially under endogenous control but that they need light for full expression. This endogenous rhythm is reflected in the time courses of spinule formation and breakdown during different phases of the light/dark cycle.