A freeze-fracture study of synaptogenesis in the distal retina of larval Xenopus

Kinetics of synaptic transmission at ribbon synapses of rods and cones

The ribbon synapse is a specialized structure that allows photoreceptors to sustain the continuous release of vesicles for hours upon hours and years upon years but also respond rapidly to momentary changes in illumination. Light responses of cones are faster than those of rods and, mirroring this difference, synaptic transmission from cones is also faster than transmission from rods. This review evaluates the various factors that regulate synaptic kinetics and contribute to kinetic differences between rod and cone synapses. Presynaptically, the release of glutamate-laden synaptic vesicles is regulated by properties of the synaptic proteins involved in exocytosis, influx of calcium through calcium channels, calcium release from intracellular stores, diffusion of calcium to the release site, calcium buffering, and extrusion of calcium from the cytoplasm. The rate of vesicle replenishment also limits the ability of the synapse to follow changes in release. Post-synaptic factors include properties of glutamate receptors, dynamics of glutamate diffusion through the cleft, and glutamate uptake by glutamate transporters. Thus, multiple synaptic mechanisms help to shape the responses of second-order horizontal and bipolar cells.

Synaptic transmission at retinal ribbon synapses

The molecular organization of ribbon synapses in photoreceptors and ON bipolar cells is reviewed in relation to the process of neurotransmitter release. The interactions between ribbon synapse-associated proteins, synaptic vesicle fusion machinery and the voltage-gated calcium channels that gate transmitter release at ribbon synapses are discussed in relation to the process of synaptic vesicle exocytosis. We describe structural and mechanistic specializations that permit the ON bipolar cell to release transmitter at a much higher rate than the photoreceptor does, under in vivo conditions. We also consider the modulation of exocytosis at photoreceptor synapses, with an emphasis on the regulation of calcium channels.

The immunocytochemical localization of connexin 36 at rod and cone gap junctions in the guinea pig retina

Connexin 36 (Cx36) is a channel-forming protein found in the membranes of apposed cells, forming the hexameric hemichannels of intercellular gap junction channels. It localizes to certain neurons in various regions of the brain including the retina. We characterized the expression pattern of neuronal Cx36 in the guinea pig retina by immunocytochemistry using specific antisera against Cx36 and green/red cone opsin or recoverin. Strong Cx36 immunoreactivity was visible in the ON sublamina of the inner plexiform layer and in the outer plexiform layer, as punctate labelling patterns. Double-labelling experiments with antibody directed against Cx36 and green/red cone opsin or recoverin showed that strong clustered Cx36 immunoreactivity localized to the axon terminals of cone or close to rod photoreceptors. By electron microscopy, Cx36 immunoreactivity was visible in the gap junctions as well as in the cytoplasmic matrices of both sides of cone photoreceptors. In the gap junctions between cone and rod photoreceptors, Cx36 immunoreactivity was only visible in the cytoplasmic matrices of cone photoreceptors. These results clearly indicate that Cx36 forms homologous gap junctions between neighbouring cone photoreceptors, and forms heterologous gap junctions between cone and rod photoreceptors in guinea pig retina. This focal location of Cx36 at the terminals of the photoreceptor suggests that rod photoreceptors can transmit rod signals to the pedicle of a neighbouring cone photoreceptor via Cx36, and that the cone in turn signals to corresponding ganglion cells via ON and OFF cone bipolar cells.

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.

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.

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.

Fine structure of synaptogenesis in the vertebrate central nervous system

This article reviews studies of the formation of synaptic junctions in the vertebrate central nervous system. It is focused on electron microscopic investigations of synaptogenesis, although insights from other disciplines are interwoven where appropriate, as are findings from developing peripheral and invertebrate nervous systems. The first part of the review is concerned with the morphological maturation of synapses as described from both qualitative and quantitative perspectives. Next, epigenetic influences on synaptogenesis are examined, and later in the article the concept of epigenesis is integrated with that of hierarchy. It is suggested that the formation of synaptic junctions may take place as an ordered progression of epigenetically modulated events wherein each level of cellular affinity becomes subordinate to the one that follows. The ultimate determination of whether a synapse is maintained, modified or dissolved would be made by the changing molecular fabric of its junctional membranes. In closing, a hypothetical model of synaptogenesis is proposed, and an hierarchial order of events is associated with a speculative synaptogenic sequence. Key elements of this hypothesis are 1) epigenetic factors that facilitate generally appropriate interactions between neurites; 2) independent expression of surface specializations that contain sufficient information for establishing threshold recognition between interacting neurites; 3) exchange of molecular information that biases the course of subsequent junctional differentiation and ultimately results in 4) the stabilization of synaptic junctions into functional connectivity patterns.

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.

Ultrastructure of the synaptic ribbons in photoreceptor cells of Rana catesbeiana revealed by freeze-etching and freeze-substitution

The three-dimensional structure of synaptic ribbons in photoreceptor cells of the frog retina was studied with freeze-etching and freeze-substitution methods, combined with a rapid-freezing technique. Although the synaptic ribbon consisted of two electron-dense plaques bisected by an electron-lucent layer in conventional thin sections, such lamellar nature was not so evident in freeze-etched replicas. The cytoplasmic surfaces of the synaptic ribbon presented an extremely regular arrangements of small particles 4-6 nm in diameter. Fine filaments 8-10 nm in diameter and 30-50 nm in length connected synaptic vesicles and the ribbon surface. These connections were mediated by large particles on both ends of the filaments. Approximately 3-5 filaments attached to one synaptic vesicle. Synaptic ribbons were anchored to a characteristic meshwork underlying the presynaptic membrane via another group of similar fine filaments. The meshwork seemed to be an etched replicated image of the presynaptic archiform density observed in thin sections.

Freeze-fracture analysis of synaptogenesis in glomeruli of mouse olfactory bulb

The relationship between intramembranous particle (IMP) aggregates appearing on the extracellular leaflet (E-face) of the postsynaptic membrane and postsynaptic densities was examined by electron microscopy during mouse olfactory bulb development. During prenatal development the IMP aggregates first increased in size and then decreased in size to the adult level, while the length of the postsynaptic densities tended to increase to a plateau. Concomitant with the size change, the shape of the IMP aggregates appeared to change during development from small, round clusters to large, anastomotic aggregations. Some of the IMP aggregates appeared to have a particle-free area in their centre. As development proceeded, the overall IMP density increased. The density of particles measuring 7-11 nm remained unchanged throughout prenatal life and decreased in the adult. These particles may be involved in stabilization of initial contacts and maintenance of mature synapses, rather than representing receptors or ion channels which would be expected to increase during development. The density of particles smaller than 7 nm increased prenatally, decreased at birth, then increased in the adult. These particles may represent two or more different macromolecules, one important in synaptogenesis, the other important in adult synapses.

The distribution of filipin-sterol complexes in photoreceptor synaptic membranes

The polyene antibiotic filipin, which binds to membrane sterols, has been used to investigate the relative distribution of cholesterol at photoreceptor synaptic junctions in the chick retina. Following anesthesia and aldehyde perfusion fixation, the retina is removed and immersed in fixative solution containing the filipin for 36-48 hours. The retinas are then processed for freeze-fracture. Electron microscopy of freeze-fracture replicas demonstrates that the filipin-sterol complexes are not evident between intramembrane particles of the presynaptic and postsynaptic particle arrays that are present at ribbon and basal junctions. In contrast, the synaptic vesicle fusion zone of ribbon junctions contains large numbers of filipin-sterol complexes which are observed merging with the free margins of the presynaptic particle array. There is a scarcity of such complexes, however, around the free margins of basal junction presynaptic particle arrays. These latter sites do not contain a vesicle fusion zone. Particle-poor areas of membrane that surround postsynaptic particles arrays of ribbon and basal junctions also do not contain filipin binding sites. The nonsynaptic membrane of photoreceptor terminals contains large numbers of filipin-sterol complexes, less tightly packed than in the synaptic vesicle fusion zone. Coated vesicle fusion sites in the presynaptic membrane contain groups of intramembrane particles but the filipin-sterol complexes are excluded from these sites. The observations suggest that synaptic membrane domains which interact with cytoskeletal components, such as clathrin and pre- and postsynaptic densities contain less cholesterol than other domains such as the synaptic vesicle fusion zones.

The effects of temperature and light on particles associated with crayfish visual membrane: a freeze-fracture analysis and electrophysiological study

Depending on the pre-experimental treatment, densities as well as sizes of particles associated with the visual membranes in the eyes of Procambarus clarkii varied. The highest mean particle density (5268 +/- 969 microns -2) and the smallest mean particle diameter (5.57 +/- 1.35 nm) were found in crayfish which had been kept in the dark for 10 weeks in aerated fresh water of 10 degrees C. Crayfish kept under a 12 h dark/light regime in water of 10 degrees C or 30 degrees C for three weeks displayed particle densities of 1076 +/- 180 and 2899 +/- 249 microns -2, respectively; particle diameters were of the order of 8 nm. Temperature did not alter the shape or the slope of the V/log I curves, but ERG recordings show that maximum spectral sensitivity was shifted from lambda max = 560 nm in cold water crayfish (10 degrees C) to lambda max = 580 nm in crayfish from the 30 degrees C tank, and that the 10 degrees C curve was somewhat narrower than the 30 degrees C curve. It is suggested that the observed shift was caused by a combination of factors, of which the following may have played key roles: (1) The filter effect of screening pigment granules and other intracellular components such as vesicles, vacuoles, endoplasmic reticulum, and mitochondria, some of which were developed to a considerably greater extent in 30 degrees C material; (2) increased membrane fluidity due to higher temperature as well as the presence of photoproducts in the light, and the 'countermeasures' taken by the visual pigment molecules to stabilize the lipid bilayer, e.g. higher density, possible 12-s-cis linkages etc.; and (3) increased regeneration or synthesis of rhodopsin due to higher metabolic activity of retinula cells at higher temperatures. Temperature-induced changes of visual pigments in a variety of organisms are discussed and evidence for the rhodopsin-aggregate model of crayfish visual pigment is presented. It is concluded that the retinula cytoplasm is involved in restoring depleted stocks of photopigment, and that the biological sense of possessing an increase in red sensitivity during the warm summer months lies in the correlation of light penetration in the natural habitat of P. clarkii and optimal exploitation of the habitat.

Freeze-fracture morphology of the vestibular hair cell--primary afferent synapse in the chick

The intramembrane specializations at vestibular hair cell-primary afferent synapses have been identified and characterized in complementary freeze-fracture replicas from prehatch and hatchling chick cristae and maculae. Hair cell protoplasmic (P) faces at sites where presynaptic bodies are present exhibit small, tightly packed arrays of 9 nm particles. Hair cell external (E) faces have corresponding arrays of pits. Multiple arrays are often observed in contiguity. Opposite the presynaptic bodies, postsynaptic afferent boutons and calyces exhibit a more extensive array of scattered, irregular E-face particles. Corresponding P-fracture faces of afferent boutons and calyces display little topographical specialization opposite these E-face arrays, which are presumed to be the intramembrane correlate of the postsynaptic density. Examination of complementary replicas has allowed identification of the intramembrane synaptic specializations for all membrane faces at the synaptic apposition.

Development of synaptic junctions in cerebellar glomeruli

In the glomerular synapses of developing mouse cerebellar cortex, 2 components of the synaptic junctions assemble independently in the immature granule cell dendrites, and then combine. 'Initial' junctions between mossy fiber axons and immature granule cell dendrites have presynaptic and postsynaptic electron-dense fuzz and a widened synaptic cleft, but lack the aggregate of intramembrane particles associated with the extracellular half of the postsynaptic membrane which characterizes mature synaptic junctions. In the vicinity of 'initial junctions' there are particle aggregates which resemble those at mature synaptic junctions, but which are less densely packed and which are not associated with the other features of a junction. The constituent particles of these aggregates move to the sites of 'initial junctions' to combine with them and form 'immature synaptic junctions'. Many of these immature junctions are larger in area than mature synaptic junctions. The immature junctions accumulate a fairly uniform complement of intramembrane particles, which increase in packing density as the junctions decrease in area and attain smaller, adult size.

Synaptic membrane domains in photoreceptors of chick retina: a thin-section and a freeze-fracture study

In this freeze-fracture study of synaptic terminals of chick photoreceptors, three regions of synaptic terminal plasmalemma can be distinguished on the basis of intramembrane characteristics. The first region is the synaptic vesicle fusion region in which rows of P-face depressions and E-face mounds are observed. In the absence of exocytotic figures this zone is relatively free of P-face particles and E-face pits. Adjacent to this, a second region is seen, rich in P-face particles and complementary E-face pits. This second region waxes and wanes in size during dark and light stimulation (Cooper and McLaughlin, 1982) and may correspond to similar expansions and contractions of synaptic plasmalemma induced by less physiological modes of stimulation, as observed in other synaptic terminals (Ceccarelli et al., 1979b; Model et al., 1975; Boyne and McLeod, 1979). During the waxing period, P-face particles and E-face pits are present in this membrane, and its expansion gives rise to diverticula of the synaptic terminal. During the waning period when the diverticula begin to disappear, aggregates of P-face particles and complementary patches of E-face pits are seen in the diverticular membrane. The third region, the nonsynaptic plasmalemma enclosing the terminal, contains a high density of P-face particles but does not contain E-face pits. Serial sections of vacuoles within the cytoplasm demonstrate that some vacuoles retain connections with this nonsynaptic plasmalemma. Vacuoles that are connected in this way are depleted of intramembrane particles. Such regions appear to represent separate domains within the photoreceptor terminal and are discussed in the context of membrane addition and retrieval.

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.

Synapse formation and modification between distal retinal neurons in larval and juvenile Xenopus

A serial section analysis of photoreceptor synaptic bases was undertaken in the clawed frog Xenopus laevis. The developmental period from tadpole stage 48 through metamorphosis was studied. Horizontal cells contacted rod and cone photoreceptors at ribbon synapses; the number of such contacts per receptor base was constant for rods, but increased for cones as a function of developmental stage. In pre-metamorphic animals bipolar cells contacted receptors only through basal junctions; their number in cone bases increased dramatically during development but was unchanged in rod bases. A densitometric estimation of the cleft width of basal junctions showed that it ranged from 10 to 18 nm, but the junctions could not be divided reliably into the 'wide' and 'narrow' categories reported for other vertebrate species. Near metamorphic climax a new type of ribbon-related bipolar cell junction appeared. Gap junctions between horizontal cells and conventional synapses of horizontal cell onto bipolar cell processes were first seen in mid-larval developmental stages.