Morphology of horizontal cells in the frog retina

Structure and function of photoreceptor and second-order cell mosaics in the retina of Xenopus

The structure, physiology, synaptology, and neurochemistry of photoreceptors and second-order (horizontal and bipolar) cells of Xenopus laevis retina is reviewed. Rods represent 53% of the photoreceptors; the majority (97%) are green light-sensitive. Cones belong to large long-wavelength-sensitive (86%), large short-wavelength-sensitive (10%), and miniature ultraviolet wavelength-sensitive (4%) groups. Photoreceptors release glutamate tonically in darkness, hyperpolarize upon light stimulation and their transmitter release decreases. Photoreceptors form ribbon synapses with second-order cells where postsynaptic elements are organized into triads. Their overall adaptational status is regulated by ambient light conditions and set by the extracellular dopamine concentration. The activity of photoreceptors is under circadian control and is independent of the central body clock. Bipolar cell density is about 6000 cells/mm2 They receive mixed inputs from rods and cones. Some bipolar cell types violate the rule of ON-OFF segregation, giving off terminal branches in both sublayers of the inner plexiform layer. The majority of them contain glutamate, a small fraction is GABA-positive and accumulates serotonin. Luminosity-type horizontal cells are more frequent (approximately 1,000 cells/mm2) than chromaticity cells (approximately 450 cells/mm2). The dendritic field size of the latter type was threefold bigger than that of the former. Luminosity cells contact all photoreceptor types, whereas chromatic cells receive their inputs from the short-wavelength-sensitive cones and rods. Luminosity cells are involved in generating depolarizing responses in chromatic horizontal cells by red light stimulation which form multiple synapses with blue-light-sensitive cones. Calculations indicate that convergence ratios in Xenopus are similar to those in central retinal regions of mammals, predicting comparable spatial resolution.

Photoreceptor classes and transmission at the photoreceptor synapse in the retina of the clawed frog, Xenopus laevis

The photoreceptor population in Xenopus consists of a green-sensitive rod (lambda(max) = 523 nm), a blue-sensitive rod (lambda(max) = 445 nm) and three classes of cone. The largest cone is red-sensitive (lambda(max) = 611 nm). The intermediate cone is presumed to be blue-sensitive based on physiological criteria, whereas the miniature cone may be UV-sensitive. Horizontal cells (HC) are of two sorts: axon-bearing and axonless. The axon-bearing HC is of the luminosity type and probably contacts all types of photoreceptor. The axonless HC is of the chromaticity type and contacts only intermediate (blue) cones and at least one type of rod. During development dendrites of HCs and bipolar neurons penetrate photoreceptor bases. A progressive maturation of HC and bipolar synapses with rods and cones occurs between tadpoles stages 37/8 and 46. Neighboring rods and cones are joined by gap junctions. During this same period, the outer segments are laid down and photopigments synthesized. A linear relation was found between the quantum capturing ability of the rod and its absolute threshold. Mature rods of the Xenopus retina release glutamate in a calcium-dependent manner. Glutamate release was found to be a linear function of calcium influx through L-type calcium channels. Both types of HC possess ionotropic glutamate receptors of the AMPA subtype.

Morphology and synaptic connections of HRP-filled, axon-bearing horizontal cells in the Xenopus retina

Axon-bearing horizontal cells of the Xenopus retina were studied by intracellular injection of HRP following physiological characterization. The profile of the cell viewed in whole mount consisted of a round or oval perikaryon about 50 microns in diameter and an axon about 1 mm long which lacked a prominent terminal expansion. The axonal diameter was 0.5-1.0 microns in its proximal third but 2-4 microns in its distal portion. Along its course the axon emitted 25-40 branchlets each 0.2 micron in diameter, up to 10 micron long and terminating in a cluster of two to six synaptic knobs. Electron microscopic examination revealed that both perikaryal dendrites and axon branchlets ended in both rod and cone synaptic bases; cone contacts outnumbered rod contacts by two- to threefold. We were unable to document synapses of presumed interplexiform cells onto identified horizontal cells. Horizontal cell axons are joined in their distal portions by numerous, small (0.2 micron long) gap junctions. Other gap junctions were noted between horizontal cell processes within the synaptic endings of photoreceptors. An hypothesis is advanced whereby the cluster of axon branchlet synaptic knobs permits dynamic interaction of rod and cone synaptic inputs to the horizontal cell.

Gamma-aminobutyric acid- and glutamic acid decarboxylase-immunoreactive neurons in the retina of different vertebrates

The localization of gamma-aminobutyric acid (GABA)- and L-glutamate 1 carboxy-lyase (GAD)-immunoreactive neurons was compared in the skate, frog, pigeon, chicken, rabbit, and man. Horizontal cells show both GABA and GAD immunoreactivity in the skate, frog, and bird. Certain amacrine cells show GABA and GAD immunoreactivity in all species. The distribution of GABA- and GAD-immunoreactive cell bodies and cell processes was very similar, if not identical, in the skate and man. In the other species, cell populations with GAD immunoreactivity also showed GABA immunoreactivity. However, in the bird, frog, and rabbit, the GABA-immunoreactive amacrine cells were at least twice as numerous as the GAD-immunoreactive cells. In birds, the distributions of the GAD and GABA immunoreactivities were different in the sublayers of the inner plexiform layer. The reason for the difference is currently unknown. GABA-immunoreactive bipolar-like cells were seen in the frog.

The horizontal cells in the retina of the owl, Tyto alba, and owlet, Carinae noctua

One type of short-axon horizontal cell (HC) and one type of axonless HC are described in the retina of Carinae noctua, a crepuscular bird and Tyto alba, a pure nocturnal bird. The axonless HCs in both birds are similar to type II axonless HC found in diurnal birds. The short-axon HC of Carinae noctua is similar to the same cell type found in diurnal birds; the only difference being the axon length. The short-axon HC of the Tyto alba retina is morphologically different from the same cell type found in the retina of diurnal birds and Carinae noctua and very similar to the short-axon HC described in the mammalian retina and the primate retina in particular. The striking similarity between the short-axon HCs found in the retina of owl and primates may be related to the same rod-cone ratio found in these retinas.

The cellular origin of an unusual type of S-potential: an intracellular horseradish peroxidase study in a cyprinid fish retina

L2-type S-potentials are mainly blue/green-sensitive hyperpolarizing responses with a red-sensitive depolarizing component which is either weak or absent. They were first described in the retina of the roach, a cyprinid fish, by Djamgoz (1978, 1984) and Djamgoz & Ruddock (1978, 1979a). The cellular origin of these responses has been determined and characterized by intracellular recording, horseradish peroxidase staining, and light and electron microscopy. They were found to arise in horizontal cells with H2-like morphologies on average (Stell & Lightfoot, 1975). The dendrites of these cells contacted green- and blue-sensitive cone pedicles within which both lateral and central contacts were made at ribbon synapses. The laterally-positioned dendrites had incompletely formed spinules associated with them. A number of similarities between these units and the biphasic, chromaticity (Cb)-type S-potentials have been outlined and it is suggested that L2 units are essentially Cb-units with a weak depolarizing component. In turn, it is suggested that the depolarizing component is reduced as a consequence of the relatively dark-adapted states of the retinae. It is concluded that the negative feed-back pathway that subserves the generation of depolarizing (Cb-type) S-potentials is weak or absent in dark-adapted retinae and that spinules may be the site of this feed-back interaction.

Multiaxonal horizontal cells in the retina of the tree shrew, Tupaia glis

The retinas of most vertebrates contain two or more morphologically distinct types of horizontal cell, and usually one of these types lacks an axon. Among mammals, in which two types are observed, primates are exceptional in that both types of horizontal cell have axons. It then seemed of interest to study the horizontal cells of tree shrews (Tupaia glis), insectivores thought to be closely related to primates. Golgi impregnations of whole, flat-preparations revealed two types of horizontal cell. Uniaxonal cells have a compact dendritic organization with clusters of terminals, and a single thin axon with short collaterals and a few terminals, located along its length. Multiaxonal cells have a relatively large dendritic tree, and arising from the tips of about four to eight dendrites of an individual cell are thin axonlike processes which terminate as profusely branched telodendritic arborizations. This identification of the multiaxonal horizontal cells in Tupaia retina is the first time any vertebrate horizontal cell has been found to possess more than a single axon. A comparison of horizontal cells in tree shrew, monkey, cat, and squirrel retinas shows a remarkable morphological diversity within this class of mammalian retinal neuron.

A bifurcate axon of horizontal cells in the carp retina

In flatmounts of the carp retina, among 368 cone-connected horizontal cells, which were singly marked by intracellular Lucifer yellow under various experimental conditions, 15 cells (4.1%) were found to possess a bifurcate axon and two terminals. The lengths of single and bifurcate axonal processes (an axon plus its terminal) are all comparable, ranging from 400 to 600 micron.

Uptake of 3H-glycine in the outer plexiform layer of the retina of the toad, Bufo marinus

The uptake of 3H-glycine in the retina of the toad, Bufo marinus, was investigated by light and electron microscopical autoradiography. Uptake of 3H-glycine was very prominent in large cell bodies in the inner nuclear layer as well as in discrete clusters in both the outer plexiform layer (OPL) and the inner plexiform layer. This pattern in similar to that described for 3H-glycine-accumulating putative interplexiform cells in goldfish, frog, and Xenopus retinas. Electron microscopical autoradiography of the OPL revealed large, grain-containing varicosities which had electron-lucent cytoplasm and contained both small, agranular and large, dense-core vesicles. The varicosities made extensive en passant and spine synapses in the OPL. Definitive identification of their postsynaptic targets was not achieved. However, autoradiographic analysis with 3H-GABA uptake as well as electrophysiological evidence suggests that axons but not cell bodies or dendrites of 3H-GABA-accumulating horizontal cells (H1 cells) are postsynaptic targets of the varicosities. The presence of dense-core vesicles in the varicosities suggested co-occurrence of glycine and a biogenic amine or neuropeptide. The indirect immunofluorescence technique was used to determine whether any such substances were present in the OPL of the toad retina. However, no specific labeling was found in the OPL for any of 19 substances tested. The extensive synaptic output provided by glycine-accumulating varicosities in the toad OPL may indicate an important role of glycine in the synaptic function of the distal toad retina. We suggest that these varicosities derive from a presumably glycinergic interplexiform cell.

Regular orientation of horizontal cells in the river lamprey retina

In isolated retinas of the river lamprey (Lampetra japonica), two types (fast and slow) of light-induced responses (S-potentials) were recorded from two distinct classes of axon-bearing horizontal cells. After the spectral responses of recording cells were examined, a fluorescent dye Lucifer yellow CH (LY) was ionophoretically injected into individual cells. Such a cellular marking was made for 15-25 points per retina successively in time and in space. Then the retinas were processed as flatmounts for fluorescence microscopic observation. Horizontal cells marked with LY were correlated with the recordings and mapped in the retinal field. Both classes of horizontal cells were found to be regularly arranged in space around the optic disc; the long axes of somata and axonal processes are oriented in parallel with the latitude line of the eyeball through the optic disc.

The actions of gamma-aminobutyric acid, glycine and their antagonists upon horizontal cells of the Xenopus retina

We examined the effects of gamma-aminobutyric acid (GABA) and glycine and their respective antagonists, picrotoxin and strychnine, upon the membrane potential and light-evoked responses of the type H1 horizontal cell of the Xenopus retina. This horizontal cell receives mixed input from rod and cone receptors. Under control conditions the mean membrane potential was -37.8 +/- 9.7 mV. Addition of 5 mM-GABA to the superfusate hyperpolarized the cell by 4.0 +/- 2.6 mV within 3-5 min; addition of 0.5 mM-picrotoxin depolarized the cell by 4.3 +/- 2.1 mV. Prolonged (greater than 15 min) exposures to the drugs elicited more pronounced changes in membrane potential. GABA and picrotoxin affected primarily the cone-dependent input to the H1 horizontal cell. Under dark-adapted conditions, response wave forms were essentially unaltered by the drugs, but when the horizontal cell was moderately or fully light adapted, GABA reduced and picrotoxin enhanced the cone-dependent component of its response to light. Long-term (greater than 15 min) exposures to GABA and picrotoxin elicited changes in response kinetics usually associated with dark and light adaptation, respectively. Glycine, at bath concentrations of 0.6 mM or greater, depolarized horizontal cells by 21 mV on average and reduced or abolished their light response. This action did not occur in the presence of 0.1 mM-strychnine. When all light-evoked activity was blocked by 20-40 mM-magnesium, the depolarizing action of glycine still occurred. Thus, glycine appears to act directly upon the horizontal cell membrane. Neither GABA nor glycine, nor their respective antagonists, affected the spatial extent of the horizontal cell receptive field.

Intracellular recording from identified photoreceptors and horizontal cells of the Xenopus retina

Intracellular recordings were made from rods, cones and horizontal cells of the Xenopus retina. The cells under study were identified by injection of the fluorescent dye, Lucifer yellow. Rod spectral sensitivity peaked near 524 nm, that of cones near 612 nm whereas horizontal cells reflected input from both these classes of photoreceptors. No intracellular recordings were made from blue-sensitive rods (lambda max = 445 nm) nor did this rod appear to provide an input to the horizontal cell. Under dark-adapted conditions, horizontal cells had a slow waveform, a Vmax less than or equal to 18 mV and were driven by 524 nm rods only. When light-adapted, horizontal cell responses were fast, Vmax was 30-40 mV and the responses reflected only 612 nm cone input. In the mesopic state rod and cone inputs to the horizontal cell interacted non-linearly: weak green backgrounds greatly enhanced the response to a superimposed red flash compared to the red flash response on a dark field. The length constant of the horizontal cell exceeded its dendritic arbor by 2-15 fold. All of the stained horizontal cells, however, possessed a long slender axon without a terminal but which emitted periodic short branches that appeared to contact receptors.

Rod and cone inputs to bipolar and horizontal cells of the Xenopus retina

This report summarizes some recent studies of the Xenopus retina in which intracellular recordings were made from photoreceptors, horizontal and bipolar cells. The studied cells were identified by injection of Lucifer yellow. Rod spectral sensitivity functions conformed to the density spectrum of a 524 nm pigment, those of cones to that of a 612 nm pigment. Horizontal cell responses reflected both these classes of photoreceptor input. Rod input evoked a slow waveform, with Vmax less than or equal to 18 mV, cone input a faster waveform with Vmax = 30-40 mV. In the mesopic state the horizontal response reflected both waveforms. Rod and cone inputs to the horizontal cells appeared not to act independently, in that a steady weak green background greatly enhanced the response to a superimposed red flash, but not the reverse. A third photoreceptor type (blue-sensitive rod, Y lambda max = 445 nm) provided input to a chromatic bipolar cell which was hyperpolarized by blue light and depolarized by red light. Such chromatic bipolars had broad areas of spatial integration and lacked center-surround organization.

Blue-sensitive rod input to bipolar and ganglion cells of the Xenopus retina

Intracellular recordings were obtained from chromatic and non-chromatic bipolar cells, identified by Lucifer yellow injection in the Xenopus retina. The chromatic cells, which lacked center-surround organization, were short wavelength hyperpolarizing (lambda max 445 nm) and long wavelength depolarizing. Under photopic conditions the depolarizing component was driven by 612 nm cones, but under mesopic conditions it appeared that 524 nm rods also constituted an input to the response. The non-chromatic bipolars encountered were of the off-center (hyperpolarizing) variety, with an active antagonistic surround, and peak spectral sensitivity in the red portion of the spectrum. Extracellular recordings were obtained from color-coded ganglion cells classified as type 1 or 2 in frog retina by Maturana et al. (1966) [J. gen. Physiol. 43, 129-175] and Bäckström and Reuter (1975) [J. Physiol. 246, 79-107]. The spectral sensitivity of the long latency "on" component was matched by the density spectrum of the 445 nm rod. This response component lacked center-surround organization and showed a relatively broad area of spatial integration. In contrast, a short latency component had a spectral sensitivity matched by the 612 nm cone pigment under photopic conditions, was either "on" or "off" center, showed center-surround organization and had a relatively small area of spatial integration. We speculate that in Xenopus retina, both chromatic and non-chromatic bipolar cells provide synaptic input to the class 1,2 ganglion cell.