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

Type II Opsins in the Eye, the Pineal Complex and the Skin of Xenopus laevis: Using Changes in Skin Pigmentation as a Readout of Visual and Circadian Activity

The eye, the pineal complex and the skin are important photosensitive organs. The African clawed frog, Xenopus laevis, senses light from the environment and adjusts skin color accordingly. For example, light reflected from the surface induces camouflage through background adaptation while light from above produces circadian variation in skin pigmentation. During embryogenesis, background adaptation, and circadian skin variation are segregated responses regulated by the secretion of α-melanocyte-stimulating hormone (α-MSH) and melatonin through the photosensitivity of the eye and pineal complex, respectively. Changes in the color of skin pigmentation have been used as a readout of biochemical and physiological processes since the initial purification of pineal melatonin from pigs, and more recently have been employed to better understand the neuroendocrine circuit that regulates background adaptation. The identification of 37 type II opsin genes in the genome of the allotetraploid X. laevis, combined with analysis of their expression in the eye, pineal complex and skin, is contributing to the elucidation of the role of opsins in the different photosensitive organs, but also brings new questions and challenges. In this review, we analyze new findings regarding the anatomical localization and functions of type II opsins in sensing light. The contribution of X. laevis in revealing the neuroendocrine circuits that regulate background adaptation and circadian light variation through changes in skin pigmentation is discussed. Finally, the presence of opsins in X. laevis skin melanophores is presented and compared with the secretory melanocytes of birds and mammals.

A frog's eye view: Foundational revelations and future promises

From the mid-19th century until the 1980's, frogs and toads provided important research models for many fundamental questions in visual neuroscience. In the present century, they have been largely neglected. Yet they are animals with highly developed vision, a complex retina built on the basic vertebrate plan, an accessible brain, and an experimentally useful behavioural repertoire. They also offer a rich diversity of species and life histories on a reasonably restricted physiological and evolutionary background. We suggest that important insights may be gained from revisiting classical questions in anurans with state-of-the-art methods. At the input to the system, this especially concerns the molecular evolution of visual pigments and photoreceptors, at the output, the relation between retinal signals, brain processing and behavioural decision-making.

Seeing the light to change colour: An evolutionary perspective on the role of melanopsin in neuroendocrine circuits regulating light-mediated skin pigmentation

Melanopsin photopigments, Opn4x and Opn4m, were evolutionary selected to "see the light" in systems that regulate skin colour change. In this review, we analyse the roles of melanopsins, and how critical evolutionary developments, including the requirement for thermoregulation and ultraviolet protection, the emergence of a background adaptation mechanism in land-dwelling amphibian ancestors and the loss of a photosensitive pineal gland in mammals, may have helped sculpt the mechanisms that regulate light-controlled skin pigmentation. These mechanisms include melanopsin in skin pigment cells directly inducing skin darkening for thermoregulation/ultraviolet protection; melanopsin-expressing eye cells controlling neuroendocrine circuits to mediate background adaptation in amphibians in response to surface-reflected light; and pineal gland secretion of melatonin phased to environmental illuminance to regulate circadian and seasonal variation in skin colour, a process initiated by melanopsin-expressing eye cells in mammals, and by as yet unknown non-visual opsins in the pineal gland of non-mammals.

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.

Mesopic state: cellular mechanisms involved in pre- and post-synaptic mixing of rod and cone signals

At least twice daily our retinas move between a light adapted, cone-dominated (photopic) state and a dark-adapted, color-blind and highly light-sensitive rod-dominated (scotopic) state. In between is a rather ill-defined transitional state called the mesopic state in which retinal circuits express both rod and cone signals. The mesopic state is characterized by its dynamic and fluid nature: the rod and cone signals flowing through retinal networks are continually changing. Consequently, in the mesopic state the retinal output to the brain contained in the firing patterns of the ganglion cells consists of information derived from both rod and cone signals. Morphology, physiology, and psychophysics all contributed to an understanding that the two systems are not independent but interact extensively via both pooling and mutual inhibition. This review lays down a rationale for such rod-cone interactions in the vertebrate retinas. It suggests that the important functional role of rod-cone interactions is that they shorten the duration of the mesopic state. As a result, the retina is maintained in either in the (rod-dominated) high sensitivity photon counting mode or in the second mode, which emphasizes temporal transients and spatial resolution (the cone-dominated photopic state). Experimental evidence for pre- and postsynaptic mixing of rod and cone signals in the retina of the clawed frog, Xenopus, is shown together with the preeminent neuromodulatory role of both light and dopamine in controlling interactions between rod and cone signals. Dopamine is shown to be both necessary and sufficient to mediate light adaptation in the amphibian retina.

Symphony of rhythms in the Xenopus laevis retina

The photoreceptor layer in the retina of Xenopus laevis harbors a circadian clock. Many molecular components known to drive the molecular clock in other organisms have been identified in Xenopus, such as XClock, Xper2, and Xcrys, demonstrating phylogenetic conservation. This model system displays a wide array of rhythms, including melatonin release, ERG rhythms, and retinomotor movements, suggesting that the ocular clock is important for proper retinal function. A flow-through culture system allows measurements of retinal rhythms such as melatonin release in vitro over time from a single eyecup. This system is suited for pharmacological perturbations of the clock, and has led to important observations regarding the circadian control of melatonin release, the roles of light and dopamine as entraining agents, and the circadian mechanisms regulating retinomotor movements. The development of a transgenic technique in Xenopus allows precise and reliable molecular perturbations. Since it is possible to follow rhythms in eyecups obtained from adults or tadpoles, the combination of the flow-through culture system and the transgenic technique leads to the fast generation of transgenic tadpoles to monitor the effects of molecular perturbations on the clock.

Transgene expression in Xenopus rods

The photoreceptors of the vertebrate retina express a large number of proteins that are involved in the process of light transduction. These genes appear to be coordinately regulated at the level of transcription, with rod- and cone-specific isoforms (J. Hurley (1992) J. Bioenerg. Biomembr. 24, 219-226). The mechanisms that regulate gene expression in a rod/cone-specific fashion have been difficult to address using traditional approaches and remain unknown. Regulation of the phototransduction proteins is medically important, since mutations in several of them cause retinal degeneration (P. Rosenfeld and T. Dryja (1995) in: Molecular Genetics of Ocular Disease (J.L. Wiggs, Ed.), pp. 99-126, Wiley-Liss Inc.). An experimental system for rapidly producing retinas expressing a desired mutant would greatly facilitate investigations of retinal degeneration. We report here that transgenic frog embryos (K. Kroll and E. Amaya (1996) Development 122, 3173-3183) can be used to study cell-specific expression in the retina. We have used a 5.5 kb 5' upstream fragment from the Xenopus principal rod opsin gene (S. Batni et al. (1996) J. Biol. Chem. 271, 3179-3186) controlling a reporter gene, green fluorescent protein (GFP), to produce numerous independent transgenic Xenopus. We find that this construct drives expression only in the retina and pineal, which is apparent by 4 days post-nuclear injection. These are the first results using transgenic Xenopus for retinal promoter analysis and the potential for the expression in rod photoreceptors of proteins with dominant phenotypes.

Glutamate release by the intact light-responsive photoreceptor layer of the Xenopus retina

In order to study glutamate release from light responsive photoreceptors, we used an eyecup preparation treated with detergent and distilled water, which permitted removal of the inner retina. The remaining 'reduced' retina consists mainly of photoreceptors attached to the pigment epithelium. The viability of the preparation was established by exclusion of trypan blue, light and electron microscopic examination of the photoreceptor layer and by intracellular recordings from rods. The 'reduced' retina was superfused at 1 ml/h and overflow samples were analyzed for their glutamate content by a fluorimetric enzyme assay. We tested the response to dark and light adaptation and to treatment with 100 microM CdCl2. We found a baseline glutamate level in light-adapted preparation which was not affected by cadmium. Dark adaptation induced a 2-fold increase of glutamate release, which was completely blocked by cadmium.

Background contrast modulates kinetics and lateral spread of responses to superimposed stimuli in outer retina

Surround enhancement (sensitization) is a poorly understood form of network adaptation in which the kinetics of the responses of retinal neurons to test stimuli become faster, and absolute sensitivity of the responses increases with increasing level of steady, surrounding light. Surround enhancement has been observed in all classes of retinal neurons in lower vertebrates except cones, in some primate retinal ganglion cells, and in human psychophysical studies. In theory, surround enhancement could be mediated by two broad classes of mechanisms, which are not mutually exclusive: one in which the kinetics of the transduction linking cone voltage to postsynaptic current in second-order neurons is modulated, and another in which the transformation of postsynaptic current to membrane voltage is modulated. We report here that both classes of mechanism play a role in surround enhancement measured in turtle horizontal cells (HCs). We stimulated the retina by modulating sinusoidally the illuminance of a bar placed at various positions in the HC receptive field. The bar was surrounded by either equally luminant or dim, steady light. Interpretation of responses in the context of a model for the cone-HC network led to the conclusion that the speeding up of response kinetics--due to selective increase in response gain at high temporal frequencies--by surround illuminance is almost completely accounted for by the change in the kinetics of the transduction linking cone membrane potential to HC postsynaptic current. However, surround illuminance also had an additional, surprising effect on the transformation between postsynaptic current and voltage: the space constant for signal spread in the HC network for the dim-surround condition was roughly twice as large as that for the bright-surround condition. Thus, increasing surround illuminance had analogous effects in the spatial and temporal domains: it restricted the time course and the spatial spread of signal. Both effects were dependent on the contrast between the mean bar illuminance and that of the surround, rather than on overall light level. When the stimulus with the bright surround was dimmed uniformly by a neutral density filter, the space constant did not increase, and response gain at high temporal frequencies did not decrease. Pharmacological experiments performed with dopamine and various agonists and antagonists indicated that, although exogenous dopamine can influence surround enhancement, endogenous dopamine does not play an important role in surround enhancement. We conclude that contrast in background light modulates the spatiotemporal properties of signal processing in the outer retina, and does so by a non-dopaminergic mechanism.

Identification of cone classes in Xenopus retina by immunocytochemistry and staining with lectins and vital dyes

The aim of the present study was to determine the number of cone classes in the Xenopus retina. We examined the dimensions and staining properties of cones, utilizing two monoclonal antibodies, COS-1 and OS-2, developed by Szel and Rohlich (1985). Living cones also were reacted with the plant lectins peanut agglutinin (PNA) and wheat germ agglutinin (WGA) and with a fluorescent stilbene dye, DIDS, which binds selectively to red-sensitive cones (Kleinschmidt, 1991; Kleinschmidt & Harosi, 1992a,b). Three cone populations were distinguished based on differences in size and staining properties. Eighty-eight percent of all cones were stained strongly by COS-1, PNA, and DIDS, but weakly by OS-2. The group of cones stained by COS-1 had the largest mean dimensions of outer segment length, width, and oil droplet diameter. COS-1 negative cones were divisible into two groups: a subclass of miniature cones (approximately 4% total cones) was stained strongly by OS-2, PNA, and DIDS. The balance, constituting approximately 9% total cones, were of intermediate size, were not stained by PNA and reacted weakly to OS-2 and DIDS. WGA stained all cones. Large, COS-1+ cones appear to be red-sensitive and belong to the class of anion-tunable cone pigments. We suggest that the intermediate size, COS-1 negative cones are blue-sensitive based on the finding that blue-sensitive chromatic horizontal cells connect to them preferentially (Witkovsky et al., work in progress).(ABSTRACT TRUNCATED AT 250 WORDS)

Threshold and chromatic sensitivity changes in fish cone horizontal cells following prolonged darkness

The light-evoked responses of L-type cone horizontal cells in the teleost retina were studied following a prolonged period of complete darkness. Intact, isolated white perch retinas were superfused in complete darkness for more than 90 min, following which horizontal cells were impaled without the aid of any light flashes. Following this prolonged darkness, L-type cone horizontal cell light responses to dim and bright full-field stimuli were slow and small in amplitude and response duration to bright stimuli was considerably longer than stimulus duration. In addition, absolute threshold was 2 log units lower than typical for cone horizontal cells and spectral sensitivity to shorter wavelengths was increased. Following bright light stimulation, light responses became more transient and increased in amplitude, reaching 40-50 mV to bright flashes. Moreover, absolute threshold increased and responses to spectral stimuli were similar to those observed typically for L-type cone horizontal cells after light-sensitization. These results suggest that following prolonged darkness, cone input to cone horizontal cells is reduced and rod input is present.

Effects of submicromolar concentrations of dopamine on photoreceptor to horizontal cell communication

Dopamine has been postulated to act as an intraretinal messenger for light adaptation by biasing retinal circuits to favor cone over rod inputs to second- and third-order neurons. As an experimental test, we studied the effects of dopamine and related ligands on rod to horizontal cell synaptic transfer. Rods and horizontal cells (HC) were recorded from simultaneously. Red and green light-emitting diodes were modulated sinusoidally in counterphase at 1 Hz and their relative intensities adjusted to elicit a rod null. Dark-adapted HC's also showed a null, but exposure to 0.5-1.0 microM dopamine, which corresponds to the endogenous levels, elicited a large imbalance in the HC response while the rod null was maintained. Similar effects were achieved with either a D1 dopamine agonist, SKF 38393 (20 microM) or a D2 dopamine agonist, quinpirole HCl (1 microM). Correspondingly, the frequency range over which the HC responded to red flickering lights increased very substantially. Exposure to a D2, but not a D1 dopamine agonist increased rod flicker, but not as dramatically as in the HC. These data indicate that the synaptic gains of rod and cone transmission to a second order retinal neuron are modifiable by endogenous levels of dopamine. Secondly, the bandpass of rod flicker is altered by dopamine, acting through a D2 dopamine receptor.

The lateral spread of light adaptation in cat horizontal cell responses

To investigate the sites of light adaptation processes in the mammalian distal retina, we studied the lateral spread of adaptation signals in cone-driven cat horizontal (H-) cell responses. The size of the adaptation pool is compared to the receptive field for H-cell responses. H-cell activity was recorded intracellularly in the optically intact, in vivo eye. It is demonstrated that light adaptation as measured in H-cells is not a strictly local process. Background light falling outside a central test region effectively modulates the responses to a small test light, flashed on the receptive field center. The integration area for adaptation signals was quantitatively compared to the H-cell receptive field size by measuring the desensitizing effect of background light on the responses to a small centered test spot, as a function of background spot size. The area-adaptation function is comparable to the area-response function but has a slightly smaller length constant. Light adaptation in H-cell responses, therefore, reveals spread of adaptation over a large distance and is probably mediated through lateral interactions in the H-cell network rather than in the cones.

Ultrastructural and functional connectivity of intracellularly stained neurones in the vertebrate retina: correlative analyses

A variety of intracellular recording and staining techniques has been used to establish structure-function and, in some cases, structure-function-neurochemical correlations in fish, turtle, and cat retinae. Cone photoreceptor-horizontal cell connectivity has been studied extensively in the cyprinid fish retina by intracellular staining with horseradish peroxidase (HRP) and subsequent electron microscopy. The available data suggest that horizontal cell dendrites around the ridge of the synaptic ribbon are postsynaptic, whilst finger-like extensions ("spinules") of lateral dendrites function as inhibitory feedback terminals. An interesting feature of this interaction is its plasticity: the feedback pathway is suppressed in the dark and becomes potentiated by light adaptation of the retina. Intracellular recordings and stainings of ganglion cells in both turtle and cat retinae have been possible. Prelabelling of ganglion cells by retrograde transport of rhodamine from the tectum allows ganglion cells to be stained under visual control, and their synaptic inputs determined by electron microscopy. Such studies have been extended to double labelling by using autoradiography or postembedding immunohistochemistry to identify the neurotransmitter content of the labelled cell and/or the neurotransmitter(s) converging upon it. It is envisaged that further applications of intracellular staining followed by double- or even triple-labelling will continue to enhance greatly our understanding of the functional architecture of the vertebrate retina.

An intensity-dependent biphasic neuron in mudpuppy retina

Intracellular recordings in dark-adapted mudpuppy retinas have revealed a type of infrequently encountered cell with unusual response properties. These cells may be a subclass of horizontal cell since they are encountered at the same depth as horizontal cells and have large receptive fields and response amplitudes. However, they differ from typical horizontal cells in that they are depolarized by low intensity illumination and hyperpolarized by higher intensity illumination at all wavelengths. Both types of responses appear to be driven mainly by 572 nm cones. Both the depolarizing and hyperpolarizing responses were unaffected by APB, indicating that they are not mediated by on-center bipolar cells.

Localization and function of dopamine in the adult vertebrate retina

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)

The influence of short-term adaptation of human rods and cones on cone-mediated grating visibility

1. The influences of short-term visual adaptation of either rods or cones upon cone-mediated grating visibility were compared with their influences upon detection threshold in both the fovea and parafoveal retina. Short-term visual adaptation was induced by 20 deg diameter adapting fields (AFs) generally of 500 ms duration. The AF was either -0.5 log td in illuminance and too dim to influence cones, or 2.5 log td and bright enough to stimulate cones as well as rods. 2. In control experiments, we replicated previous results of Crawford (1947), Baker (1963), and other investigators and determined the influence of these AFs upon the detection threshold of a homogeneous test flash (TF) of 54 min of arc diameter and 10 ms duration. If the AF was 2.5 log td in illuminance and stimuli were presented foveally, TF threshold began to rise several hundred milliseconds before AF onset, was maximal when AF and TF were simultaneous in onset but was less elevated during the remaining presentation of the AF. TF threshold decreased to control value within several seconds after AF offset. These data represent a cone adaptation function since action spectra for both the test and adapting flashes adhere to the spectral sensitivity of the CIE standard luminous efficiency function V lambda. 3. If the AF was -0.5 log td in illuminance and if stimuli were presented parafoveally, the time course of TF detection threshold changes were similar to those described in paragraph 2 above. But these data represent a rod adaptation function since action spectra for both the test and adapting flashes adhered to the spectral sensitivity of the CIE standard luminous efficiency function V' lambda. In the fovea, the -0.5 log td AF had no influence upon the detection threshold of the TF suggesting complete rod-cone independence. 4. The influence of short-term adaptation upon spatial visibility was studied using a vertically oriented, 18 cycle/deg grating which was also 54 min of arc in diameter and 10 ms in duration. We determined the illuminance just necessary to see the bars of the grating (i.e. threshold grating illuminance or TGI) at various time intervals in respect to the onset of an AF. 5. Rod-stimulating (-0.5 log td) AFs, whether 0.5 or 2 s in duration, only influenced TGI after AF onset. TGI gradually decreased (i.e. an increase in sensitivity) during the first 250 ms of AF presentation and then remained stable until AF offset.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycinergic contacts in the outer plexiform layer of the Xenopus laevis retina characterized by antibodies to glycine, GABA and glycine receptors

Electrophysiological experiments have predicted a direct synaptic input from glycinergic interplexiform cells (IPCs) to GABAergic horizontal cells in the Xenopus retina. However, previous ultrastructural studies failed to demonstrate this input. Here, we used three immunocytochemical approaches to investigate this issue. First, double-label postembedding immunocytochemistry with GABA- and glycine-like immunoreactivity (GABA-LI and glycine-LI) was used to study possible interactions of the glycinergic IPC with GABAergic horizontal cells. Processes postsynaptic to glycine-LI IPC terminals in the outer plexiform layer (OPL) fell into two groups, small microtubule-filled processes and larger electron-lucent processes with sparse microtubules and occasional mitochondria. In no case did we find glycine-LI synapses onto GABA-LI cells or processes. Second, pre-embedding immunocytochemistry was used to label GABA-LI cells and processes in the OPL. GABA-LI was sparse in horizontal cell axons and more intense in horizontal cell somas and in small processes. In agreement with our first set of experiments, GABA-LI profiles did not receive input from conventional synapses. Third, we localized glycine-receptor-like immunoreactivity (GlyR-LI) to several types of apparent synapses in the OPL. As expected, it was found at IPC synapses. Unexpectedly, GlyR-LI was also subsynaptic at photoreceptor synapses onto second order neurons, both at ribbon and basal junction type synapses. At least some of the GlyR-LI photoreceptor synapses were from cones. Also, GlyR-LI was apposed to photoreceptors and to unidentified small diameter processes, where no other indication of synaptic input was evident. Because glycine-LI is not found in photoreceptors, we suggest that glycine receptors at photoreceptor synapses are stimulated by glycine that diffuses from other sites, possibly from IPCs. This interpretation is consistent with available physiological studies of glycinergic effects in this retina.

Signal transmission through the dark-adapted retina of the toad (Bufo marinus). Gain, convergence, and signal/noise

Responses to light were recorded from rods, horizontal cells, and ganglion cells in dark-adapted toad eyecups. Sensitivity was defined as response amplitude per isomerization per rod for dim flashes covering the excitatory receptive field centers. Both sensitivity and spatial summation were found to increase by one order of magnitude between rods and horizontal cells, and by two orders of magnitude between rods and ganglion cells. Recordings from two hyperpolarizing bipolar cells showed a 20 times response increase between rods and bipolars. At absolute threshold for ganglion cells (Copenhagen, D.R., K. Donner, and T. Reuter. 1987. J. Physiol. 393:667-680) the dim flashes produce 10-50-microV responses in the rods. The cumulative gain exhibited at each subsequent synaptic transfer from the rods to the ganglion cells serves to boost these small amplitude signals to the level required for initiation of action potentials in the ganglion cells. The convergence of rod signals through increasing spatial summation serves to decrease the variation of responses to dim flashes, thereby increasing the signal-to-noise ratio. Thus, at absolute threshold for ganglion cells, the convergence typically increases the maximal signal-to-noise ratio from 0.6 in rods to 4.6 in ganglion cells.

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.

Suppression of cone-driven responses by rods in the isolated frog retina

The sensitivity of rod- and cone-driven responses was studied in the isolated frog retina during the period of rapid dark adaptation following a conditioning flash which bleached a negligible amount of visual pigment. Following a conditioning flash, cone-driven b-wave responses were first enhanced and then depressed. The time courses of the enhancement and subsequent depression of cone-drive responses varies greatly with the intensity and wavelength of the conditioning flash, but were identical when the conditioning flashes were matched for equal excitation of 502 nm rods. These changes in cone-driven response sensitivity were correlated with the desensitization and recovery of rods following the conditioning flash. When signal transmission from rods to second-order cells was interrupted by the addition of L-glutamate, the conditioning flash did not produce the above-described enhancement and subsequent depression of long-wavelength receptor potential responses. The suppression of cone-driven response therefore appears to be due to a synaptically mediated influence from 502 nm rods which is maximal when the rods are in the dark-adapted state, with little or no contribution from 433 nm rods, and no involvement of the pigment epithelium.

GABA release from Xenopus retina does not correlate with horizontal cell membrane potential

The relationship between horizontal cell membrane potential and the release of GABA was explored in the retina of Xenopus laevis. The intracellularly recorded membrane potential of horizontal cells was monitored while the retina was exposed to different concentrations of depolarizing agents. The dose-response curves obtained revealed a rise from 5 to 95% maximum depolarization in 0.5-1.5 log unit concentration change. The molar concentrations that elicited a 20 mV depolarization were 40 mM (potassium), 0.8 mM (glutamate), 0.8 mM (glycine), 5 microM (kainate) and 1.3 microM (quisqualate). Autoradiography revealed that radiolabel was accumulated almost exclusively by horizontal cells when isolated retinas were incubated in medium containing 1 microM [3H]GABA. Thus, retinal release of radioactivity was used as a measure of [3H]GABA release from horizontal cells. Endogenous GABA released from retinas was measured using high performance liquid chromatography and was taken to reflect both amacrine and horizontal cell GABA pools. The release of both [3H]GABA and endogenous GABA was stimulated by glutamate, kainate and potassium, but not by glycine or quisqualate. Similar dose-response curves for GABA release and for depolarization were obtained in the case of potassium and kainate but not for glutamate. Potassium-evoked release either of endogenous GABA or [3H]GABA was both calcium- and sodium-dependent, whereas kainate- or glutamate-evoked GABA release was sodium-dependent but calcium-independent. The results indicate that depolarization per se is not necessarily associated with transmitter release in Xenopus retinal horizontal cells. It is suggested that the action of a given neurotransmitter upon the efflux of GABA from horizontal cells may depend on the degree to which it modifies the sodium conductance of the horizontal cell.

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.

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.

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.