Dendritic morphology of a class of interstitial and normally placed amacrine cells revealed by intracellular Lucifer yellow injection in carp retina

Suppression of electrical synapses between retinal amacrine cells of goldfish by intracellular cyclic-AMP

Retinal amacrine cells of the same class in cyprinid fish are homotypically connected by gap junctions. The permeability of their gap junctions examined by the diffusion of Neurobiotin into neighboring amacrine cells under application of dopamine or cyclic nucleotides to elucidate whether electrical synapses between the cells are regulated by internal messengers. Neurobiotin injected intracellularly into amacrine cells in isolated retinas of goldfish, and passage currents through the electrical synapses investigated by dual whole-patch clamp recordings under similar application of their ligands. Control conditions led us to observe large passage currents between connected cells and adequate transjunctional conductance between the cells (2.02±0.82nS). Experimental results show that high level of intracellular cyclic AMP within examined cells block transfer of Neurobiotin and suppress electrical synapses between the neighboring cells. Transjunctional conductance between examined cells reduced to 0.23nS. However, dopamine, 8-bromo-cyclic AMP or high elevation of intracellular cyclic GMP leaves gap junction channels of the cells permeable to Neurobiotin as in the control level. Under application of dopamine (1.25±0.06nS), 8-bromo-cyclic AMP (1.79±0.51nS) or intracellular cyclic GMP (0.98±0.23nS), the transjunctional conductance also remains as in the control level. These results demonstrate that channel opening of gap junctions between cyprinid retinal amacrine cells is regulated by high level of intracellular cyclic AMP.

Intracellular cyclic-amp suppresses the permeability of gap junctions between retinal amacrine cells

Gap junctions are intercellular channels composed of subunit protein connexin and subserve electrotonic transmission between connected neurons. Retinal amacrine cells, as well as horizontal cells of the same class, are homologously connected by gap junctions. The gap junctions between these neurons extend their receptive fields, and may increase the inhibitory postsynaptic effects in the retina. In the present study, we investigated whether gap junctions between the neurons are modulated by internal messengers. The permeability of gap junctions was examined by the diffusion of intracellularly injected biotinylated tracers, biocytin or Neurobiotin, into neighboring cells since gap junctions are permeable to these molecules freely. 4% Lucifer Yellow and 6% biocytin or Neurobiotin were injected intracellularly into horizontal cells and amacrine cells in isolated retinas of carp and goldfish and Japanese dace following electrophysiological identification. In the control condition, the tracer spread into many neighboring cells from the recorded cells. Superfusion of retinas with dopamine (100 microM) suppressed diffusion of the tracer into the neighboring horizontal cells, but not in the case of amacrine cells. Intracellular injection of cyclic AMP (300 mM) completely blocked diffusion of the tracer into neighboring horizontal cells and amacrine cells. However, superfusion of retinas with 8-bromo-cyclic AMP (2 mM), membrane permeable cyclic AMP analog, permitted the tracer to diffuse into the neighboring horizontal cells or amacrine cells. Intracellular injection of cyclic GMP (300 mM) blocked the diffusion between neighboring horizontal cells, but did not suppress the diffusion between amacrine cells. These results show that the permeability of gap junctions between amacrine cells is regulated by high concentration of intracellular cyclic AMP level, but not for intracellular cyclic GMP or applied dopamine or extracellularly applied low concentrations of intracellular cyclic AMP level. The present study suggests that these laterally oriented inhibitory interneurons, horizontal cells and amacrine cells, express different connexins which may be differentially regulated by intercellular messengers.

Form and Function of on-off Amacrine Cells in the Amphibian Retina

on-off amacrine cells were studied with whole cell recording techniques and intracellular staining methods using intact retina-eyecup preparations of the tiger salamander ( Ambystoma tigrinum) and the mudpuppy ( Necturus maculosus). Morphological characterization of these cells included three-dimensional reconstruction methods based on serial optical sections obtained with a confocal microscope. Some cells had their detailed morphology digitized with a computer-assisted tracing system and converted to compartmental models for computer simulations. The dendrites of on-off amacrine cells have spines and numerous varicosities. Physiological recordings confirmed that on-off amacrine cells generate both large- and small-amplitude impulses attributed, respectively, to somatic and dendritic generation sites. Using a multichannel model for impulse generation, computer simulations were carried out to evaluate how impulses are likely to propagate throughout these structures. We conclude that the on-off amacrine cell is organized with multifocal dendritic impulse generating sites and that both dendritic and somatic impulse activity contribute to the functional repertoire of these interneurons: locally generated dendritic impulses can provide regional activation, while somatic impulse activity results in rapid activation of the entire dendritic tree.

STRUCTURAL AND FUNCTIONAL PROPERTIES OF HOMOLOGOUS ELECTRICAL SYNAPSES BETWEEN RETINAL AMACRINE CELLS

Retinal amacrine cells regulate activities of retinal ganglion cells, the output neurons to higher visual centers, through cellular mechanism of lateral inhibition in the inner plexiform layer (IPL). Electrical properties of gap junction networks between amacrine cells in the IPL were investigated using combined techniques of intracellular recordings, Lucifer yellow and Neurobiotin injection, dual patch-clamp recordings and high voltage electron microscopy in isolated retinas of cyprinid fish. Six types of gap-junctionally connected amacrine cells were classified after their light-evoked responses to light flashes were recorded. Among them, gap junction networks of three types of amacrine cells were studied with structure-function correlation analysis. Cellular morphology of intercellular connections between three homologous cell classes was characterized. The interconnections between laterally extending dendrites in the IPL were localized at dendritic tip terminals. Three types of cells presented the dendrodendritic connections of tip-contact manner in the homologous cell population. High voltage as well as conventional electron microscopy revealed gap junctions between the dendritic tips of Neurobiotin-coupled cells. Receptive field properties of these amacrine cells were examined, displacing a slit of light along the distance from recording sites in the dorsal intermediate region of the retina. Receptive field size, space length constant, response latency and conduction velocity were measured. Spatial and temporal properties of receptive fields were symmetric along horizontally expanding dendrites in the dorsal retina. Simultaneous dual patch-clamp recordings revealed that the lateral gap junction connections between homologous amacrine cells expressed bidirectional electrical synapses passing Na(+) spikes. These results demonstrate that bidirectional electrical transmission in gap junction networks of these amacrine cells is symmetric along the lateral gap junction connections between horizontally extending dendrites. Lateral inhibition regulated by amacrine cells in the IPL appears to be associated with the directional extension of the dendrites and the orientation of dendrodendritic gap junctions.

Slow recovery of goldfish retinal ganglion cells' soma size during regeneration

The goldfish optic nerve regenerates after sectioning. Recently both short-term (30 days) and long-term (4 months) recovery of various goldfish behaviors were observed after optic nerve section. Using intracellular injection of Lucifer Yellow (LY) the morphology of regenerating ganglion cells in goldfish retina after optic nerve section over a 4 month period have been investigated. In normal retinas, most cells (96-98%) were 7-10 microm in soma diameter which increased with increasing distance from the optic disc. Only two or three short, thin processes could be traced with LY. The remaining cells (2-4%) were 13-16 microm in soma diameter and all of the long dendritic trees could be traced with LY. The most conspicuous morphological change observed was cellular hypertrophy, which occurred for 20-90 days after axotomy. Neuronal processes were also hypertrophic in this period. The percentage increase in hypertrophy of the central ganglion cells tended to be slightly higher compared to cells from other regions. These morphological changes peaked at 60 days after axotomy and fully disappeared by 120 days after axotomy. The slow recovery of ganglion cells' soma size may reflect the slow return to the normal number of optic axon terminals in the tectum during regeneration.

Tracer coupling among regenerated amacrine cells in the retina of the goldfish

This study sought to characterize the tracer coupling of regenerated amacrine cells in the retina of the goldfish and assess the integration of regenerated neurons into existing retinal circuits. Regeneration of new neurons from injury-induced progenitors was stimulated by surgically excising a small rectangular piece of retina. Several months after regeneration was complete, intracellular injections of Neurobiotin, a gap junction-permeant tracer, were made into single regenerated amacrine cells or nonregenerated (extant) amacrine cells lying outside the regenerated patch. Two groups of amacrine cells were injected: those that in normal retina are tracer coupled and a single type (the radiate amacrine cell) that is not. The data show that regenerated amacrine cells are tracer coupled to each other and to their homologous counterparts outside the patch of regenerated retina. Regenerated radiate cells possess morphologically abnormal dendrites, but these processes can extend out of regenerated retina into surrounding normal retina. Similarly, the dendrites of extant radiate cells, severed by the original lesion, can regenerate into the patch of regenerated retina. These results indicate that in the goldfish retina the cell-specific junctional circuitry present in normal retina is re-created in the regenerated retina, and suggest that regenerated neurons are functionally integrated into the existing retina.

Double-staining of horizontal and amacrine cells by intracellular injection with lucifer yellow and biocytin in carp retina

Horizontal and amacrine cells in the isolated carp retina were impaled with micropipette electrode, identified by their characteristic light responses, and injected iontophoretically with markers for morphological study. Both Lucifer Yellow CH and biocytin were injected simultaneously. Lucifer Yellow was seen by its own fluorescence while biocytin was visualized by binding with Texas Red-linked or horseradish peroxidase-conjugated avidin. For cone-connected horizontal cells, biocytin-coupled cells were found to be approximately five-times more numerous than Lucifer Yellow-coupled cells. Coupling for both tracers was consistently hampered by intravitreally applied dopamine. In untreated retinas, the injected Lucifer Yellow was restricted within one rod-connected horizontal cell, while biocytin revealed several coupled neighbors. Amacrine cells, labeled by the tracers, were morphologically grouped into eight types, based on our earlier classification. Among them, amacrine cells, belonging to three types (Fnd, Pmb or Pma), were confirmed to be Lucifer Yellow-coupled, and the number of biocytin-coupled cells was more numerous (about 2.5 times) than that of Lucifer Yellow-coupled cells. Most amacrine cells (i.e. Pwd, Fnb and Fna) showed biocytin-coupling with no Lucifer Yellow-coupling. A few classified (i.e. Pwb and Fwa) and unclassified cells did not show any coupling. Since the tracer coupling takes place via gap junctions, the majority of amacrine cells, belonging to certain homologous types, appear to be functionally coupled with each other in the inner plexiform layer. However, dopamine did not influence the range of tracer coupling between amacrine cells in the carp retina under the present experimental conditions.

GABA and glycine in retinal amacrine cells: combined Golgi impregnation and immunocytochemistry

Golgi-impregnated amacrine cells in the all-cone lizard retina (Anolis carolinensis) were characterized on the bases of dendritic and somatic criteria. Four major cell categories, comprising 23 types were identified: three non-stratified, 13 monostratified, five bistratified, and two tristratified types. Four of the cell types comprised two to four subtypes based on stratification of their dendrites within the inner plexiform layer (IPL). Golgi impregnation strongly favoured monostratified amacrine cells with cell bodies at the proximal margin of the inner nuclear layer. The neurotransmitter content of each of the 23 amacrine cell types was examined by combined Golgi-immunocytochemistry after morphological classification. Putative neurotransmitters examined included gamma-aminobutyric acid (GABA), glycine (GLY) and aspartate (ASP). Seventeen cell types showed GABA-immunoreactivity (IR), three cell types showed GLY-IR, and four cell types showed neither GABA-IR nor GLY-IR. No cell types showed ASP-IR. Each cell type had a characteristic neurochemical signature, with the exception of one monostratified cell type that showed three different neurochemical signatures. Postembedding immunocytochemistry on conventionally processed retinas confirmed the localization of glutamic acid decarboxylase, the synthetic enzyme for GABA, to cells similar to several of the GABA-IR Golgi-stained types. Postembedding immunocytochemistry for tyrosine hydroxylase (the synthetic enzyme for catecholamines) and GABA on serial sections demonstrated colocalization of GABA and a catecholamine, probably dopamine, in a bistratified amacrine cell type. We conclude that GABA-IR amacrine cell types are more numerous and morphologically heterogeneous than GLY-IR amacrine cells. The morphological heterogeneity and, with one exception, exclusivity of GABA-IR and GLY-IR amacrine cell types indicate that both neurotransmitters play a variety and different functional roles in the lizard inner retina.

Dendritic morphology of interstitial amacrine cells with monostratified dendrites in different-sized carp retinas

The dendritic morphology of a class of interstitial (IS) amacrine cells in retinas of different-sized carp (body length, 9.1-32.3 cm) was investigated by identifying their fluorescent nuclei pre-loaded with 4,6-diamidino-2-phenylindole (DAPI), followed by iontophoretic injection of Lucifer yellow (LY) in isolated and formaldehyde-fixed flat-mounts under microscopic control. The LY-injected fusiform or pyriform cell bodies were found to locate at the middle of the inner plexiform layer (IPL) or immediately beneath the amacrine cell layer, and their dendrites monostratified in sublamina b of the IPL. The pyriform cells had a short stem from which extended 4-5 stout dendrites, while the fusiform cells extended similar dendrites from the soma. The dendrites of both types of cell were decorated with spines and a few long axon-like processes. The pyriform cells were found more frequently in smaller retinas than in larger retinas, suggesting that the former may migrate proximally during retinal growth. The dendritic field sizes of these IS amacrine cells were wider as the fish became larger, while the dendritic morphology, analyzed by the Sholl's branching model, was very similar in smaller and larger retinas. The results indicate that the IS amacrine cells do not add dendrites, but that their dendritic trees simply expand during retinal growth.

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.

Close tip-to-tip contacts between dendrites of transient amacrine cells in carp retina

In isolated retinas of the carp (Cyprinus carpio) placed receptor side up in a plastic chamber, a subclass of amacrine cells, generating a fast ON-OFF transient response to spot and annular light stimuli, were intracellularly recorded and injected with a fluorescent dye, Lucifer yellow (LY). After brief fixation of the same preparations in aldehyde solution, the retinas were wholemounted vitreous side up in a tissue chamber. Under a fluorescence microscope, one LY-injected cell and several dye-coupled cells were seen; these cells belonged to type Fnd, having a fusiform soma, narrow dendritic field and bistratified dentrites in the inner plexiform layer (IPL). To reveal the interconnections between dendrites, one of such dye-coupled cells was further injected with LY. Close tip-to-tip contacts were predominantly found between dendrites of neighboring type Fnd cells in sublaminae a and b of the IPL, respectively.