The formation of photoreceptor synapses in the retina of larval Xenopus

Development and maintenance of vision's first synapse

Synapses in the outer retina are the first information relay points in vision. Here, photoreceptors form synapses onto two types of interneurons, bipolar cells and horizontal cells. Because outer retina synapses are particularly large and highly ordered, they have been a useful system for the discovery of mechanisms underlying synapse specificity and maintenance. Understanding these processes is critical to efforts aimed at restoring visual function through repairing or replacing neurons and promoting their connectivity. We review outer retina neuron synapse architecture, neural migration modes, and the cellular and molecular pathways that play key roles in the development and maintenance of these connections. We further discuss how these mechanisms may impact connectivity in the retina.

Ultrastructure of the distal retina of the adult zebrafish, Danio rerio

The organization, morphological characteristics, and synaptic structure of photoreceptors in the adult zebrafish retina were studied using light and electron microscopy. Adult photoreceptors show a typical ordered tier arrangement with rods easily distinguished from cones based on outer segment (OS) morphology. Both rods and cones contain mitochondria within the inner segments (IS), including the large, electron-dense megamitochondria previously described (Kim et al.) Four major ultrastructural differences were observed between zebrafish rods and cones: (1) the membranes of cone lamellar disks showed a wider variety of relationships to the plasma membrane than those of rods, (2) cone pedicles typically had multiple synaptic ribbons, while rod spherules had 1-2 ribbons, (3) synaptic ribbons in rod spherules were ∼2 times longer than ribbons in cone pedicles, and (4) rod spherules had a more electron-dense cytoplasm than cone pedicles. Examination of photoreceptor terminals identified four synaptic relationships at cone pedicles: (1) invaginating contacts postsynaptic to cone ribbons forming dyad, triad, and quadrad synapses, (2) presumed gap junctions connecting adjacent postsynaptic processes invaginating into cone terminals, (3) basal junctions away from synaptic ribbons, and (4) gap junctions between adjacent photoreceptor terminals. More vitread and slightly farther removed from photoreceptor terminals, extracellular microtubule-like structures were identified in association with presumed horizontal cell processes in the OPL. These findings, the first to document the ultrastructure of the distal retina in adult zebrafish, indicate that zebrafish photoreceptors have many characteristics similar to other species, further supporting the use of zebrafish as a model for the vertebrate visual system.

Expression of vesicular glutamate transporter 1 in the mouse retina reveals temporal ordering in development of rod vs. cone and ON vs. OFF circuits

Glutamatergic transmission is crucial to the segregation of ON and OFF pathways in the developing retina. The temporal sequence of maturation of vesicular glutamatergic transmission in rod and cone photoreceptor and ON and OFF bipolar cell terminals is currently unknown. Vesicular glutamate transporters (VGLUTs) that load glutamate into synaptic vesicles are necessary for vesicular glutamatergic transmission. To understand better the formation and maturation of glutamatergic transmission in the rod vs. cone and ON vs. OFF pathways of the retina, we examined the developmental expression of VGLUT1 and VGLUT2 immunocytochemically in the mouse retina. Photoreceptor and bipolar cell terminals showed only VGLUT1-immunoreactivity (-IR); no VGLUT2-IR was present in any synapses of the developing or adult retina. VGLUT1-IR was first detected in cone photoreceptor terminals at postnatal day 2 (P2), several days before initiation of ribbon synapse formation at P4-P5. Rod terminals showed VGLUT1-IR by P8, when they invade the outer plexiform layer (OPL) and initiate synaptogenesis. Developing OFF bipolar cell terminals showed VGLUT1-IR around P8, 2-3 days after bipolar terminals were first identified in the inner plexiform layer (IPL) by labeling for the photoreceptor and bipolar cell terminal marker, synaptic vesicle protein 2B. Although terminals of ON bipolar cells were present in the IPL by P6-P8, most did not show VGLUT1-IR until P8-P10 and increased dramatically from P12. These data suggest a hierarchical development of glutamatergic transmission in which cone circuits form prior to rod circuits in both the OPL and IPL, and OFF circuits form prior to ON circuits in the IPL.

Differential synaptic organization of GABAergic bipolar cells and non-GABAergic (glutamatergic) bipolar cells in the tiger salamander retina

The synaptic organizations of gamma-aminobutyric acid-immunoreactive (GABA-IR, GABAergic) and non-GABA-IR (non-IR, glutamatergic) bipolar cells in salamander retina were compared by postembedding immunoelectron microscopy. A total of 238 presynaptic bipolar cell synapses were studied; 61 were GABA-IR and 177 were non-IR. Both groups were similar in that (1). they made asymmetrical ribbon synapses as well as asymmetrical non-ribbon synapses; (2). they made ribbon synapses at dyads, triads, and monads; and (3). the vast majority of ribbon synapses ( approximately 90%) were with dyads. The differences were that synapses of GABA-IR bipolar cells had a higher proportion of (1). direct contact with ganglion cells, (2). non-ribbon synapses, (3). output to GABA-IR amacrine cells, and (4). output in sublamina a. Overall, the output of GABA-IR ribbons was equally split between amacrine and ganglion cell processes, whereas for non-IR ribbons, it was approximately 2:1 in favor of amacrine cells. The ribbon:non-ribbon synapse ratio was approximately 1.2:1 (33:28) for GABA-IR but approximately 2:1 (118:59) for non-IR terminals. Thus, GABA-IR bipolar cells made more direct contacts with ganglion cells and used a higher proportion of non-ribbon synapses. GABA-IR dyads were more likely to contact GABA-IR amacrine profiles (52% vs. 38%). Finally, GABA-IR ribbon synapses were more common in sublamina a than sublamina b (2:1), whereas non-IR synapses were equally present in sublaminas a and b. This differential targeting of ganglion cells and amacrine cells in the OFF vs. ON layers indicates a difference in the role of bipolar cells in the generation of receptive field properties, depending on whether or not they use GABA as well as glutamate for their transmitter.

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.

Synapse formation is arrested in retinal photoreceptors of the zebrafish nrc mutant

We describe the effects of a recessive mutation on visual behavior, the electroretinogram (ERG), and photoreceptor structure in zebrafish. At 6 d post-fertilization (dpf), no optokinetic reflex could be elicited in no optokinetic response c (nrc) mutant animals under any test condition. The animals exhibited ERG responses at 5-7 dpf that were markedly abnormal and could be categorized into two groups. The first showed an initial negative a-wave followed by a delayed positive b-wave of small amplitude. Often a second ERG-like response was recorded after the initial b-wave. The second group showed only a large negative a-wave; an initial b-wave was not evident. In most recordings additional oscillatory waves varying in number, amplitude, and time course were observed. Multiple responses at the cessation of long-duration flashes were also observed. Light and electron microscopy revealed that the cone photoreceptor pedicles of nrc fish were highly abnormal. Although the appropriate number of synaptic ribbons formed in these terminals, they "floated" in the terminal, unassociated with postsynaptic processes or arciform densities. The few processes invaginating the nrc pedicles resembled those of horizontal cells. Invaginating bipolar cell processes were rare, but basal contacts were observed on pedicle surfaces. The severity of the mutation did not change between 6 and 8 dpf, showing that there is neither a delay in development nor a degeneration of the terminals; rather, nrc pedicle development appears arrested. Bipolar cell terminals in the inner plexiform layer made normal ribbon synapses; thus, the mutation appears to affect only the outer retina.

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.

Morphologic and neurochemical target selectivity of regenerating adult photoreceptors in vitro

Regenerating adult central nervous system (CNS) neurons must re-establish synaptic circuits in an environment very different from that present during development. However, the complexity of CNS circuitry has made it extremely difficult to assess the selectivity and mechanisms of synaptic regeneration at the cellular level in vivo. The synaptic preferences of adult photoreceptors were examined by using a defined cell culture system known to support regenerative process growth, presynaptic varicosity formation, and establishment of functional synapses. Immunolabeling for synaptic vesicle protein 2 and ultrastructural analysis demonstrated that cell-cell contacts made by photoreceptors were synaptic in nature. Target selectivity was determined by quantitative analysis of contacts onto normal and novel target cell types in cultures in which opportunities to contact all retinal cell types were present. Target cells were identified by morphology and immunolabeling for the amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), and glycine. Regenerating photoreceptors showed a strong preference for novel multipolar cell targets (amacrine and ganglion cells) over normal photoreceptor, horizontal, and bipolar cell targets. Additionally, photoreceptors were selective for targets containing the transmitter GABA. These results indicate first, that the normal synaptic partners for photoreceptors are not intrinsically the optimal targets for regenerative synapse formation, and second, that GABA may modulate synaptic targeting by adult photoreceptors.

Plasticity of retinal ribbon synapses

Ribbon synapses differ from conventional chemical synapses in that they contain, within the cloud of synaptic vesicles (SV's), a specialized synaptic body, most often termed synaptic ribbon (SR). This body assumes various forms. Reconstructions reveal that what appear as rod- or ribbon-like profiles in sections are in fact rectangular or horseshoe-shaped plates. Moreover, spherical, T-shaped, table-shaped, and highly pleomorphic bodies may be present. In mammals, ribbon synapses are present in afferent synapses of photoreceptors, bipolar nerve cells, and hair cells of both the organ of Corti and the vestibular organ. Synaptic ribbons (SR's) are also found in the intrinsic cells of the third eye, the pineal gland, and in the lateral line system. The precise function of SR's is enigmatic. The prevailing concept is that SR's function as conveyor belts to channel SV's to the presynaptic membrane for neurotransmitter release by means of exocytosis. The present article reviews the evidence that speaks for a plasticity of these organelles in the retina and the third eye, as reflected in changes in number, size, shape, location, and grouping pattern. SR plasticity is especially pronounced in the mammalian and submammalian pineal gland and in cones and bipolar cells of teleost fishes. Here, SR number and size wax and wane according to the environmental lighting conditions. In the pineal SR numbers increase at night and decrease during the day. In teleost cones, SR's are in their prime during daytime and decrease or disappear at night, when transmitter release is enhanced. In addition to numerical changes, SR's may also exhibit changes in size, shape, grouping pattern, and location. In the mammalian retina of adults, in contrast to the developing retina, the reported signs of SR plasticity are subtle and not always consistent. They may reflect changes in function or may represent signs of degradation. To distinguish between the-two, more detailed studies under selected experimental conditions are required. Probably the strongest evidence for SR plasticity in the mammalian retina is that in hibernating squirrels SR's leave the synaptic site and accumulate in areas as far as 5 microns from the synapse. Changes in shape include the occurrence of club-shaped SR's and round SR's or synaptic spheres (SS's). SS's may represent a special type of synaptic body, yet belonging to the family of SR's, or may be related to the catabolism of SR's. SR number is regulated by Ca2+ in teleost cones, whereas in the mammalian pineal gland cGMP is involved. An interesting biochemical feature of ribbon synapses is that they lack synapsins. The presently reviewed results suggest to us that SR's do not primarily function as conveyor belts, but are devices to immobilize SV's in inactive ribbon synapses.

Expression of carnosine-like immunoreactivity during retinal development in the clawed frog (Xenopus laevis)

The development of neurons immunoreactive to carnosine (beta-alanyl-L-histidine) was studied in the retina of Xenopus laevis during the premetamorphic period. Carnosine-like immunoreactivity was detected in photoreceptors from stage 39/40 (according to Nieuwkoop and Faber [Normal Tables of Xenopus laevis (Daudin), Elsevier, Amsterdam, 1956]) and in bipolar cells and their processes in the inner plexiform layer from stage 44/45. At all the developmental stages studied, neuroepithelial cells at the ciliary margin were completely unstained, suggesting that carnosine is only present in postmitotic retinal neurons. This study demonstrates a correlation between the times of appearance of carnosine-like immunoreactivity during retinal development and the onset of visual function.

Membrane skeleton protein 4.1 in developing Xenopus: expression in postmitotic cells of the retina

Membrane skeleton protein 4.1 plays a key role in modulating the interactions of spectrin, actin, and integral membrane proteins in erythroid and nonerythroid cells. We have investigated its structure and expression during embryonic development of Xenopus laevis. An analysis of the complete 2758-nucleotide sequence and predicted translation of 801 amino acids (85.5 kDa) of X. laevis oocyte protein 4.1 reveals that, within overlapping regions, oocyte protein 4.1 is 74% identical to a composite amino acid sequence of human erythroid and lymphoid protein 4.1 and has an identity similar to that of amino acid motifs variably expressed in either human erythroid or lymphoid protein 4.1 S1 nuclease protection analysis demonstrates the presence of a single species of protein 4.1 transcript in embryos. Antibodies produced against X. laevis protein 4.1 fusion protein recognize two bands of 180 and 115 kDa on Western blots of X. laevis embryos and retina and, using immunocytochemical techniques, label the developing retina most intensely. In vitro transcription of a cDNA construct fully encoding X. laevis protein 4.1 yields a synthetic mRNA which, when translated in vitro, produces a polypeptide that comigrates on SDS-polyacrylamide gels with the 115-kDa form of embryos and retina. Protein 4.1 is found exclusively in photoreceptors following the terminal mitosis of retinal neurons. When retinal synaptogenesis is complete, protein 4.1 is also expressed in the inner retina. In adult amphibian retinas, protein 4.1 is detected in photoreceptors, bipolar cells, and ganglion cell axons. As these cell types have previously been shown to express spectrin, actin, and ankyrin, it is likely that the membrane skeleton of erythrocytes and retinal cells share functional similarities.

Prenatal development of "synaptic" ribbons in the guinea pig pineal gland

Pineal "synaptic" ribbons are a heterogeneous population of organelles. "Synaptic" ribbons (SR) sensu stricto, "synaptic" spherules (SS), and intermediate forms (IMF) are present. Their function and origin are unknown, and a knowledge of their prenatal development is lacking. Thus the pineal glands of prenatal, neonatal, and adult guinea pigs were prepared for electron microscopy. "Synaptic" ribbons were studied morphologically and quantitatively. The three categories of "synaptic" ribbons reported in adult pineal glands were also present in prenatal pineal glands. Their structural features, distribution, grouping, and composition patterns are similar to those in adults. "Synaptic" ribbons were first detected in pinealocytes of the distal region of a 42-day postcoitus (PC) pineal gland and were comparable with those in adults. They increased in number with age and reached a peak at 63 days PC, followed by a steep decline at 66 and 67 days PC. By day 69 PC, the numbers increased again and showed a dramatic increase after birth. Several true ribbon synapses were seen at day 63 PC between pinealocyte cell processes or between pinealocyte cell process and pinealocyte cell body. Since true ribbon synapses have not been found in adult guinea pig pinealocytes, their synaptic nature could have been lost during development. No precursors for the "synaptic" ribbons were found. The endoplasmic reticulum cisternae may be the origin for the ribbon vesicles because of their close association with the "synaptic" ribbons.

Serotoninergic neurons in the retina of Xenopus laevis: selective staining, identification, development, and content

Uptake of 3H-serotonin followed by autoradiography, and uptake of the serotonin analog 5,7-dihydroxytryptamine (5,7-DHT), with subsequent staining, were each used to define a unique set of neurons in the retina of the African clawed frog, Xenopus laevis. Both techniques demonstrated the same population of neurons, on the basis of perikaryal size, shape, and position within the retina. Two classes of amacrine cells accumulated 5,7-DHT at the proximal (vitread) margin of the inner nuclear layer; the two classes were distinguished by the size of their perikarya. Two similar populations of cells, observed in the ganglion cell layer with lower frequency, may represent "displaced" counterparts of these two amacrine cell types. A class of bipolar cells whose perikarya were located in middle-to-distal regions of the inner nuclear layer also accumulated 5,7-DHT and 3H-serotonin. Processes of these cells contributed to a dense plexus of fine fibers that appeared evenly distributed throughout the inner plexiform layer. 3H-Serotonin-accumulating cells first appeared in the developing retina at stage 35/36, a time immediately after retinal stratification but before elaboration of either plexiform layer. Electron microscopic analysis permitted an identification of 3H-serotonin-accumulating terminals in the inner plexiform layer. Serotonin-labeled terminals containing conventional contacts, suggestive of amacrine cells, were presynaptic to unidentified processes and postsynaptic to bipolar cells. Labeled terminals containing ribbon contacts, indicative of bipolar cells, were postsynaptic to amacrine cells. The amount of serotonin contained in isolated retinas was 15 pmol/mg protein as measured by HPLC with electrochemical detection. We attempted to stimulate the release of accumulated 3H-serotonin from mature retinas by increasing the K+-concentration in the bathing medium. Although preloaded glycine is readily released from 14C-glycine-accumulating neurons, from the same retinas there was no calcium-dependent, K+-stimulated release of 3H-serotonin. This finding suggests that serotonin and glycine are processed differently by retinal neurons, the consequence of which results in differing responses to 40 mM K+.

Choline acetyltransferase and cholinesterases in the developing Xenopus retina

To understand the developmental regulation of acetylcholine (ACh) synthesis in the Xenopus retina, the properties of choline acetyltransferase (CAT) and cholinesterase (ChE), as well as histochemical localization of ChE in the retina, were studied during development. CAT activity first became detectable in the developing eyecup at stages 35/36. This was followed by a rapid, 50-fold rise in specific activity between stages 35/36 and 44. Since this rapid rise coincided with an almost identical increase in total ACh synthesis in whole retinae found in previous studies, it is suggested that this increase was sufficient to account for the rapid increase in total ACh synthesis. Moreover, it also correlated with increased rates of synaptogenesis in both the inner and the outer plexiform layers. Total ChE was resolved into specific and nonspecific ChE by the use of tetraisopropylpyrophosphoramide. Total ChE activities first became detectable at stages 35/36. Specific ChE [acetylcholinesterase (AChE)] increased from 50% at stage 39 to 95% of total ChE activities at stage 66. Again, the most rapid increase in both total ChE and AChE activities occurred between stages 35/36 and 44. Histochemical studies showed that AChE was localized predominantly in the two plexiform layers, with the inner plexiform layer more heavily stained at all stages. Moreover, a stratified staining pattern, clearly discerned in the inner plexiform layer, also correlated with synaptogenesis during this early period of retinal development.

The development of photoreceptors in the zebrafish, Brachydanio rerio. I. Structure

Morphological development of photoreceptors in the central retina of the zebrafish, Brachydanio rerio, was studied by using light and electron microscopic techniques. Outer segments (OS) first appeared at 2.5 days postfertilization (d2.5). On d3, synaptic elaborations were seen. By d8, two OS types were present and were identified as cones. The first indication of rod formation was also evidenced at this time, when vitreally positioned nuclei were observed and rodlike cells were infrequently detected in electron micrographs. At d12, the full complement of zebrafish photoreceptors, rods and four cone types, was identified. From this time on cells grew until adult dimensions were reached at d24. These structural observations are consistent with results of functional studies which utilized physiological and behavioral techniques.

Synaptogenesis in the first optic neuropile of the fly's visual system

The developmental transformation of the chief afferent class of fly photoreceptor synapse has been examined from serial electron micrographs of animals fixed at 74, 81, 94 and 100% pupal development (100% pupal development being defined by the time of normal adult eclosion). Animals were selected both by their age and the conformity of their eye coloration to standards for each stage. Two animals were analysed from each stage, one in greater detail than the other. For the first, the exact coordinates of the cartridges (the synaptic columns of the first optic neuropile) from which the analyses were made were mapped and selected to be within the same region of the eye field at all stages. From all animals a portion of one or two cartridges was analysed from series of up to 100 sections and the synapse populations (greater than 80) were analysed for their fine structure and postsynaptic composition. Adult synapses are confirmed as tetrads, with two of the four postsynaptic elements invariably from two monopolar interneurons L1 and L2, one from each. The two others are usually from alpha processes of the same amacrine cell. Synapses appear during the last half of pupal development, with no obvious asynchrony of ultrastructural maturation and in parallel with those of at least one of the other synaptic classes present (which were otherwise not studied). Many adult features of synaptic ultrastructure emerge late, only by 94% pupal development. These include adult numbers of synaptic vesicles, the complete form of the presynaptic ribbon with platform and the postsynaptic cisternae of L1/L2. Prior to 94% the synapses are smaller with postsynaptic elements having a less regular geometry and with postsynaptic densities which are subsequently lost (alpha processes) or replaced by cisternae (L1/L2). At the presynaptic sites of the younger animals (74%, 81%) dyads and triads of postsynaptic elements coexist with tetrads, those of older animals having, on average, more postsynaptic processes per synapse. It is suggested that individual synapses assemble piecemeal, element by element.

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

Synapse formation between photoreceptor, bipolar and horizontal cells of the larval Xenopus retina was studied by the freeze-fracture technique. Photoreceptors and horizontal cells were joined by ribbon synapses; photoreceptor and bipolar cells by basal junctions. Gap junctions were found between photoreceptors and between horizontal cells. Horizontal cell dendrites invaginated receptor bases before the plasma membrane of either cell showed zones of intramembrane (IMP) particle accumulation. Subsequently the receptor cell began to form a synaptic ridge where P-face IMPs aggregated at a protrusion of the surface membrane. The length of the ridge and the density of its IMPs increased between larval stages 40 and 56. Cross-fractured views of receptor cytoplasm at different larval stages showed that synaptic ribbons and synaptic vesicles developed in conjunction with the ridge. Plasmalemmal deformations suggesting sites of vesicle fusion or uptake were noted adjacent to the apex of the ridge. Horizontal cell dendritic membrane first accumulated P-face IMPs at several small regions; subsequently the IMPs became aligned over a broad membrane area. Both rod- and cone-related horizontal cell dendrites also manifested a loose patch of E-face IMPs which subsequently was transformed into a linear array. Basal junctions were characterized by a P-face IMP aggregate in the photoreceptor membrane and an E-face IMP aggregate in the bipolar cell membrane. Basal junctions appeared suddenly in a mature configuration at larval stage 42.

Direct electrical connections between transient amacrine cells in the catfish retina

Transient amacrine cells were identified by their intracellularly recorded response to flashes of light. These cells typically respond with a transient depolarization, often followed by a steady-state response during the stimulus. When two electrodes were placed in different transient amacrine cells, current of either polarity passed through one electrode produced a steady-state voltage change that was recorded by the electrode in the nearby cell. Following identification of the physiological type, transient amacrine cells were injected with horseradish peroxidase and the tissue was processed for light and electron microscopy. Both conventional chemical synaptic junctions and gap junctions were found to connect amacrine cells.

Gap junctions in the outer plexiform layer of te chick retina: thin section and freeze-fracture studies

Previous studies have established that gap junctions between presumptive retinal neurons of the chick retina disappear during the course of embryogenesis. The present study examines the 2-3-week-old chick retina to determine if gap junctions are present in the outer plexiform layer of the more mature animal as would be in accordance with evidence from morphological and physiological studies on a variety of other vertebrates. Thin section and freeze-fracture techniques are used in a complementary manner to demonstrate that gap junctions are present between horizontal cell processes in the distal regions of the outer plexiform layer. These junctions appear to be between axon terminals and between spines that project from axon terminals to rods and double cones. Gap junctions are also observed between photoreceptors. They are seen on the synaptic terminals of all classes of cones and are located between the cone synaptic terminals and cone basal processes. Gap junctions are also seen between unidentified photoreceptor basal processes within the neuropil of both distal and proximal parts of the outer plexiform layer. Gap junctions are also present between cone synaptic terminals and deeply invaginated, vesicle-containing processes the origin of which remains to be determined.

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.

The emergence, localization, and maturation of neurotransmitter systems during development of the retina in Xenopus laevis. III. Dopamine

The uptake, synthesis, and release of dopamine was studied in retinas of Xenopus laevis. In the tadpole and adult retina, 3H-dopamine is accumulated by cells located in the inner nuclear layer. Retinas preloaded with 3H-dopamine release this compound in response to high K+ concentrations in the medium. This release is probably Ca++-dependent as it is inhibited by Co++ in the medium. Adult retinas are also capable of synthesizing 3H-dopamine from 3H-tyrosine. The appearance and maturation of these dopaminergic properties were followed during retinal development. Our data indicate that synthesis of dopamine can first be detected as early as stage 35/36 whereas uptake of dopamine first occurs at stage 43. K+-stimulated release of preloaded 3H-dopamine from putative dopaminergic neurons is, however, not evident until stage 46. These results show that similar to the development of GABA-ergic and glycinergic properties, the uptake, synthesis, and release mechanisms for dopamine emerge at different stages during retinal differentiation in Xenopus Laevis.

The emergence, localization and maturation of neurotransmitter systems during development of the retina in Xenopus laevis. I. Gamma aminobutyric acid

The high-affinity uptake, biosynthesis and release of GABA have been studied in the retina of Xenopus laevis. In the mature retina, [3H]-GABA is accumulated predominantly by horizontal cells. A second population of cells located in the inner nuclear layer (possibly a type of amacrine cell) also showed a specific GABA uptake. In addition, this retina contains significant activities of L-glutamic acid decarboxylase and also releases [3H]-GABA in response to increasing K+ concentrations in the medium. We have followed the appearance and maturation of these GABA-ergic properties during embryonic development of this retina. Our results indicate that these properties emerge in a precise temporal pattern during retinal differentiation: the specific neuronal uptake of GABA precedes GABA synthesis which is followed by K+-stimulated GABA release.

Morphological changes in afferent vestibular hair cell synapses during the postnatal development of the cat

In the course of postnatal development in the cat, there is a decrease of about 93% in the total number of synaptic bodies (synaptic balls and synaptic bars) in type I hair cells. In type II hair cells, there is no change in the number of synaptic balls. Simultaneously, the length of specialized neuroepithelial contact increases by approximately 300% during type I hair cell maturation. Only the synaptic bars displaying a polylamellar ultrastructure persist in the type I hair cells of the adult animal. It is suggested that the afferent vestibular synapses of the type I hair cell are transformed during ontogeny.