Intracellular staining of retinal neurones: Applications to studies of functional organization

Calbindin-D28k and calretinin in chicken inner retina during postnatal development and neuroplasticity by dim red light

Members of the family of calcium binding proteins (CBPs) are involved in the buffering of calcium (Ca2+) by regulating how Ca2+ can operate within synapses or more globally in the entire cytoplasm and they are present in a particular arrangement in all types of retinal neurons. Calbindin D28k and calretinin belong to the family of CBPs and they are mainly co-expressed with other CBPs. Calbindin D28k is expressed in doubles cones, bipolar cells and in a subpopulation of amacrine and ganglion neurons. Calretinin is present in horizontal cells as well as in a subpopulation of amacrine and ganglion neurons. Both proteins fill the soma at the inner nuclear layer and the neuronal projections at the inner plexiform layer. Moreover, calbindin D28k and calretinin have been associated with neuronal plasticity in the central nervous system. During pre and early postnatal visual development, the visual system shows high responsiveness to environmental influences. In this work we observed modifications in the pattern of stratification of calbindin immunoreactive neurons, as well as in the total amount of calbindin through the early postnatal development. In order to test whether or not calbindin is involved in retinal plasticity we analyzed phosphorylated p38 MAPK expression, which showed a decrease in p-p38 MAPK, concomitant to the observed decrease of calbindin D28k. Results showed in this study suggest that calbindin is a molecule related with neuroplasticity, and we suggest that calbindin D28k has significant roles in neuroplastic changes in the retina, when retinas are stimulated with different light conditions.

Structure-function correlation in transient amacrine cells of goldfish retina: basic and multifractal analyses of dendritic trees in distinct synaptic layers

Amacrine cells generating light-evoked transient ON-OFF responses were stained by intracellular injection of horseradish peroxidase after determining their input-output (voltage response vs. light intensity) profiles. Ten cells specifically having bistratified dendritic trees were analyzed. The cross-sectional area of the dendrites in each sublamina (a and b) of the inner plexiform layer was initially measured. Although some variability was observed, there was no statistically significant overall difference in the cross-sectional areas of the dendritic trees in sublaminae a and b. Also, the amplitudes of the ON and OFF responses, generated by a midrange criterion stimulus, could not be correlated with the cross-sectional areas or the number of branches of the dendrites in sublaminae b and a, respectively. On the other hand, determination of the generalized fractal spectra revealed that the negative (up to -3) and zero-order fractal dimensions of the dendritic trees in sublamina a were consistently higher than those for sublamina b. Furthermore, there was a positive correlation between response amplitude and some part of the generalized fractal dimension in the respective parts of the dendritic trees. It is concluded that dendritic tree characteristics differ in the two halves of the inner plexiform layer and that these can be related to the cells' light-evoked response amplitudes. Furthermore, generalized fractal analysis appears to be a useful method for correlating structure and function in retinal amacrine cells with complex dendritic trees.

Quantitative analysis of triphasic (H3) horizontal cell-cone connectivity in the cyprinid fish (roach) retina

Although horizontal cells encode chromatic information by means of a variety of spectrally opponent light-evoked response patterns, their synaptic connections with the different spectral classes of cone are not completely understood. In the cyprinid fish retina, where a hierarchical set of interactions between horizontal cells and cone types has been proposed, a particular type of horizontal cell generates light-evoked triphasic (red-hyperpolarizing/green depolarizing/blue-hyperpolarizing) responses. In the present study, we have studied the cone connectivity of these cells by intracellular recording and staining in the roach retina. The horizontal cells were first identified electrophysiologically using spectral stimuli, and then stained intracellularly with horseradish peroxidase. Light microscopy revealed that the cells had consistent H3-like morphologies. At an ultrastructural level, these horizontal cells were deduced to contact selectively blue-sensitive cones. Within the cone pedicles, the majority (approximately 80%) of the contacts were "central" to synaptic ribbons. Some 50% of the "lateral" processes were large and engulfed cone cytoplasm. Spinules were present within the contacted pedicles but not upon the dendrites of the stained horizontal cells, although previous work had suggested that horseradish peroxidase would not interfere with spinule dynamics. The results are discussed in terms of existing modes of horizontal cell-cone connectivity in cyprinid fish retinae.

Intra-vitreal injection of substance P antibodies as an antagonist in the vertebrate (fish) retina

A method is described for using substance P (SP) antibodies as an antagonist in the retina of a cyprinid fish, the roach (Rutilus rutilus). Antibody solution (10 microliters) injected into the vitreous was found by immunohistochemical localization to penetrate the neural retina up to the level of the inner margin of the inner nuclear layer. Thus, the inner plexiform layer, where SP would normally be released, was well infiltrated. Similar penetration patterns were observed 2 or 24 h after injection. The physiological effectiveness of the antibody was demonstrated indirectly by measuring its effect upon the spatial coupling of the horizontal cells. Previous work suggested that SP stimulates dopamine release which normally uncouples the horizontal cell somata but not the syncytium of their axon terminals. In retinae isolated from antibody-injected eyes, the horizontal cell somata (but not axon terminals) were indeed found to be significantly more strongly coupled, consistent with the blockage of SP-induced, presumably tonic, release of dopamine. The results suggest that peptide antisera can be useful as pharmacological tools to investigate electrophysiological effects of neuropeptides in the retina as in other parts of the central nervous system.

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.

Cytochalasin inhibits light-dependent synaptic plasticity of horizontal cells in teleost retina

Previous studies have shown that the horizontal cell-->cone photoreceptor negative feedback synapse in teleost fish retinae is 'plastic', being suppressed in the dark and potentiated by light adaptation. The possible involvement of filamentous actin in ultrastructural and electrophysiological aspects of this plasticity has been investigated using cytochalasins, which inhibit actin turnover, in the cyprinid fish (roach) retinae. Cytochalasin B or D (40 microM) inhibited both the light-dependent formation and maintenance of spinules, and enhancement of the feedback interaction involved in generation of biphasic spectral responses in horizontal cells. The results suggest that actin turnover is essential for both ultrastructural and electrophysiological plasticity of horizontal cell feedback and that spinules could mediate this dynamic interaction.

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)

An interplexiform cell in the goldfish retina: light-evoked response pattern and intracellular staining with horseradish peroxidase

The light-evoked response pattern and morphology of one interplexiform cell were studied in the goldfish retina by intracellular recording and staining. The membrane potential of the cell spontaneously oscillated in the dark. In response to a brief light stimulus, the membrane potential initially gave a slow transient depolarization. During maintained light, the oscillations showed a tendency to be suppressed; the response of the cell to the offset of the stimulus was not so prominent. The perikaryon of the interplexiform cell was positioned at the proximal boundary of the inner nuclear layer. The cell had two broad layers of dendrites; one was diffuse in the inner plexiform layer, the other was more sparse in the outer plexiform layer. The morphological and electrophysiological characteristics of the cell are discussed in relation to dopaminergic interplexiform cells and the light-evoked release pattern of dopamine in the teleost retina.

Dopamine and plasticity of horizontal cell function in the teleost retina: regulation of a spectral mechanism through D1-receptors

The negative feed-back interaction between horizontal cells (HCs) and cones in the cyprinid fish retina is thought to be mediated by horizontal cell spinules. These are "plastic" structures, largely absent from the dark-adapted retina and formed anew during light adaptation. We have previously shown that horizontal cell feed-back is similarly enhanced by light adaptation. The role of the interplexiform cell transmitter dopamine in both processes has been studied in the roach retina. Application of dopamine to dark-adapted retinae induced spinule formation in a dose-dependent way. The effect of dopamine was mimicked by dibutyryl-cAMP and suppressed selectively by D1 receptor antagonists. The effect of light in inducing spinule formation was lost in retinae depleted of endogenous dopamine. However, application of exogenous dopamine to these retinae triggered normal spinule formation. For all pharmacological treatments used, there was a strong correlation between spinule number and degree of feed-back activity in biphasic horizontal cells. Thus, when the spinule content of the cone pedicles was high, biphasic horizontal cell responses exhibited strong depolarizing components and vice versa. It is concluded that light-evoked formation of spinules in HC dendrites involves the action of dopamine upon D1 receptors. Spinules, in turn, are likely to be presynaptic terminals mediating the dynamic negative feed-back effect of horizontal cells upon cones.

Ganglion cells in the goldfish retina: correlation of light-evoked response and morphology

Goldfish retinal ganglion cells were intracellularly stained with horseradish peroxidase after recording their responses to a predetermined set of test stimuli. Depolarizing responses were elicited by cells differing in shapes and sizes of their somata and dendritic fields; these cells were mostly bistratified in the inner plexiform layer (sublamina b and distal sublamina a). Hyperpolarizing responses were generated by cells monostratified in a, and by cells bistratified in a and at the a/b border. Responses that were hyperpolarizing to long wavelengths and involving large superimposed depolarizations for short wavelengths were recorded from cells with somata displaced in the inner nuclear layer. The latter cell group had wide, elliptical dendritic fields (confined to the distal sublamina a) and very fine axons. The ganglion cell types recorded are compared with morphological classification schemes proposed from earlier studies. Possible "structure-function" relations are also discussed.

Voltage clamp study of electrophysiologically-identified horizontal cells in carp retina

Passive membrane properties and electromotive force of light modulated currents of L-, R/G-type and rod-driven horizontal cells were studied by voltage-clamp using double-barrelled micro-electrodes whilst perfusing with 5 microM dopamine to uncouple the gap junctions. Input impedances of horizontal cells in darkness were 31 +/- 1.4 M omega (mean +/- SE, n = 63); the resting potentials were -37 +/- 1.3 mV. Current-voltage relationships had regions of both inward and outward rectification and a region of negative resistance was commonly observed. Reversal potentials of light modulated currents were estimated on average to be -7 +/- 4 mV (n = 14), which is consistent with the involvement of K+ and Na+ and/or Ca2+ gradients. Importantly in R/G cells both depolarizing and hyperpolarizing components of the response had essentially the same reversal potential.

Characterization of a GABAergic population of interstitial amacrine cells in the teleost retina

We used postembedding immunocytochemistry with an antiserum against BSA-conjugated GABA to study the inner plexiform layer of a cyprinid teleost, the roach. In this part of the retina, we observed a distinct banding pattern of GABA-positive material. There was a narrow unstained region separating the distal sublamina a from the proximal sublamina b; each sublamina was further subdivided into four (a) and two (b) sublayers of heavier staining, respectively. Using three-dimensional reconstruction of series of half-micron tangential sections, we were able to characterize a population of interstitial amacrine cells which contained GABA-like immunoreactive material. These cells had elliptical dendritic fields (area: about 0.04 mm2) and conspicuous, thick processes (dia. 4-5 microns). In tangential sections, the dendrites of individual cells appeared to be in close contact, occasionally resulting in difficulties in defining the boundaries of a single dendritic field. Two sub-populations of these cells were observed, one in each sublamina. By comparison with a catalogue of Golgi impregnated amacrine cells and cells microinjected with Lucifer Yellow or HRP, the identity of this type of interstitial amacrine cell is established and its possible physiological properties discussed. Apart from this GABA positive type, a second population of interstitial amacrine cell was observed which did not show positive reaction to the GABA antiserum used.

Variability of light-evoked response pattern and morphological characterization of amacrine cells in goldfish retina

Amacrine cells of the goldfish retina were characterized electrophysiologically and subsequently labelled by intracellular injection of horseradish peroxidase. An attempt was made to broaden the electrophysiological classification of the cells. Light-evoked sustained amacrine cell responses were divided into two subtypes depending on colour opponency. Colour-coded responses (red/depolarizing and green/hyperpolarizing) were found to arise in amacrine cells possessing highly polarized dendritic fields; the dendrites were monostratified in the proximal half (sublamina b) of the inner plexiform layer. Non-colour-opponent sustained responses also arose in monostratified units, but the level of dendritic ramification was in sublamina a or b (hyperpolarizing or depolarizing units, respectively). Transient (ON-OFF) responses were associated mainly with bi- or multi-stratified or diffuse amacrine cells. Some variability was observed in the sizes of the dendritic fields in different sublaminae. There was a tendency for units with brisk components of responses to be narrowly stratified in the inner plexiform layer. Some units possessed "distant" dendrites. Several aspects of structure-function correlation in amacrine cells are discussed.

Haloperidol suppresses light-induced spinule formation and biphasic responses of horizontal cells in fish (roach) retina

In retinae of lower vertebrates, negative feed-back interactions between horizontal cells (second-order neurones), and cone photoreceptors lead to generation of spectrally multi-phasic light-evoked responses (S-potentials) in horizontal cells. Spinules (finger-like extensions of horizontal cell dendrites) have been suggested to mediate these interactions in retinae of teleost fish. We have studied whether prevention of light-dependent spinule formation would indeed affect an S-potential component (the red-sensitive depolarization in H2 horizontal cells), known to depend on such negative feed-back. Haloperidol was used as a dopamine antagonist to suppress light-induced formation of spinules in retinae of the cyprinid fish, the roach. In normal (untreated) retinae, biphasic S-potentials were strongly depolarizing and horizontal cell dendrites possessed abundant spinules. However, following application of haloperidol to the vitreous prior to light adaptation, spinule formation was suppressed, and concomitantly, red-sensitive depolarizing S-potentials remained significantly under-developed. The results are consistent, therefore, with the idea that spinules mediate the negative feed-back interaction between horizontal cells and cones.

Quantitative analysis of cone photoreceptor-horizontal cell connectivity patterns in the retina of a cyprinid fish: electron microscopy of functionally identified and HRP-labelled horizontal cells

Horizontal cells generating photopic luminosity and biphasic/chromaticity-type S-potentials were identified and intracellularly labelled with horseradish peroxidase in the retina of the roach. The synaptic connectivity patterns of the horizontal cell dendrites within pedicles of different spectral types of cone were then quantitatively studied by electron microscopy. Luminosity-type responses were generated by H1-like horizontal cells contacting similar numbers of red- and green-sensitive cones and very few blue-sensitive cones. Most dendritic contacts were lateral to synaptic ribbons. Central contacts with ribbons were made almost exclusively within red-sensitive cone pedicles. Biphasic/chromaticity-type S-potentials were generated by H2-like horizontal cells. The dendrites of the latter contacted green- and blue-sensitive cones, both at central and lateral sites at synaptic ribbons. An attempt was made to correlate cone ribbon connectivity patterns and spectral characteristics of the horizontal cells according to several hypotheses, some proposed in earlier studies.

A horizontal cell selectively contacts blue-sensitive cones in cyprinid fish retina: intracellular staining with horseradish peroxidase

A horizontal cell selectively contacting blue-sensitive cones has been intracellularly stained with horseradish peroxidase in the retina of a cyprinid fish, the roach. The light microscopical morphology of the cell belonged to the H3 category of horizontal cells found in cyprinid fish retinae. In response to spectral stimuli, the cell generated chromaticity-type S-potentials that were hyperpolarizing to blue and depolarizing to yellow-orange. A red-sensitive hyperpolarizing component was absent possibly because of suppression of the negative feedback pathway between luminosity-type (H1) horizontal cells and green-sensitive cones.

Plasticity of cone horizontal cell functioning in cyprinid fish retina: effects of background illumination of moderate intensity

In electrophysiological experiments involving intracellular recording from horizontal cells in the isolated retina of the roach, light adaptation of the retina has been shown to result in potentiation both of (1) the depolarizing component of biphasic chromaticity type S-potentials, and (2) the temporal frequency transfer functions of photopic luminosity type horizontal cells. Under identical light adaptation conditions, the number of spinules on horizontal cell dendrites positioned laterally at cone pedicle ribbon synapses, increase by some threefold. The latter effect occurs equally in pedicles of red- and green-sensitive cones. Thus, horizontal cells are 'plastic' in both structural and electrophysiological respects. Furthermore, since the two electrophysiological parameters studied depend on negative feedback from horizontal cells onto cones, the results suggest that it is the inhibitory synapses that are plastic and that spinules may be sites of the negative feedback interaction. Physiological and behavioural aspects of light-dependent horizontal cell plasticity are also discussed.

Amacrine cells in the retina of a teleost fish, the roach (Rutilus rutilus): a Golgi study on differentiation and layering

We studied 250 amacrine cells in the retina of the tetrachromatic cyprinid Rutilus rutilus (roach) after rapid Golgi impregnation. All cells were recorded in camera lucida drawings from 50 -80 μm sections. For classifications we used independent criteria of presumed functional relevance, most of which could be quantified. These included ‘gross morphological’ features such as size, symmetry and orientation of the dendritic field, pattern of branching and number of ramification points as well as fine structural details like process diameter and the occurrence of spines and varicosities. We also took into account the pattern of radial distribution of dendrites. To obtain information about the subdivision of the inner plexiform layer, we used the relative stratification levels of stratified amacrine cells to plot a frequency distribution diagram showing that the dendrites of these cells are clustered in seven discrete sublayers of unequal width; four sublayers occupy the distal half of the inner plexiform layer (sublamina a) and three sublayers are present in the proximal half (sublamina b). The subdivision was compared with densitometric data of the inner plexiform layer after various staining methods and with previous observations about the location of bipolar terminals and neurochemical bandings. Our findings suggest that this layer is composed of complementary structural components, each of which is subject to a specific layering pattern. On this basis we could distinguish 43 different types of amacrine cell. If individual types occurred in more than one sublayer, they were considered as subtypes; of these, we found 70 different ones. Among the 43 types, 6 were observed only once. In comparison with amacrine cells described in other species, six ‘new’ types were identified. For each individual type, an identity chart was prepared summarizing camera lucida drawings of tangential views at low and high magnification, a semischematical drawing of the radial location of the dendrites, and the most relevant quantitative data. Our observations are discussed in the context of available evidence about light-evoked responses of identified amacrine types in other species, and possible transmitter content. They substantiate a functional concept according to which amacrine cells provide (i) a multicellular aggregate for coupled membrane potential; (ii) unit activity by the action of entire individual cells; and (iii) local microcircuits caused by isolated activation of single dendrites or parts thereof. The great variety of morphological differentiation, and the numerous transmitters found, suggest that within this basic framework individual amacrine types serve highly complex and sophisticated roles in retinal information processing. Our attempt towards a detailed classification and description of amacrine cell types is intended to provide a reference for future intracellular and neurochemical work, to facilitate precise identification.

Micro-electrode measurements and functional aspects of chloride activity in cyprinid fish retina: extracellular activity and intracellular activities of L- and C-type horizontal cells

Extracellular Cl- activity and intracellular Cl- activities of luminosity and biphasic-chromaticity type horizontal cells were measured in freshly isolated, non-superfused roach retinae using double-barrelled Cl- -sensitive micro-electrodes. The extracellular Cl- activity in dark-adapted retinae was found to have a surprisingly wide range (54-143 mM), although in a given preparation it was extremely constant. The mean intracellular Cl- activities of both types of horizontal cell were identical (47 mM), and this value was significantly greater than that required for "passive distribution" i.e. Cl- equilibrium potentials were 11-12 mV more positive than respective membrane resting potentials in the dark. In the presence of 10 microM dopamine, however, the difference between the Cl- equilibrium potential and the membrane resting potential was abolished, consistent with the hypothesis that dopamine increases Cl- conductance, presumably at the interplexiform cell synapse onto horizontal cells. In turn, it is suggested that a functional consequence of this pathway is to modulate the input impedances of the horizontal cells, and hence their sensitivity to light.