Modulation of connexon densities in gap junctions of horizontal cell perikarya and axon terminals in fish retina: effects of light/dark cycles, interruption of the optic nerve and application of dopamine.

In the fish retina, connexon densities of gap junctions in the outer horizontal cells are modulated in response to different light or dark adaptation times and wavelengths. We have examined whether the connexon density is a suitable parameter of gap junction coupling under in situ conditions. Short-term light adaptation evoked low connexon densities, regardless of whether white or red light was used. Short-term dark adaptation evoked high connexon densities; this was more pronounced in the axon terminal than in perikaryal gap junctions. Under a 12 h red light/12 h dark cycle, a significant difference in connexon densities between the light and the dark period could be established in the gap junctions of the perikarya and axon terminals. Under a white light/dark cycle, only the gap junctions of axon terminals showed a significant difference. Crushing of the optic nerve resulted in an increase in connexon densities; this was more pronounced in axon terminals than in perikarya. Dopamine injected into the right eye of white-light-adapted animals had no effect. However, dopamine prevented the effect of optic-nerve crushing on connexon density. The reaction of axon-terminal gap junctions to different conditions thus resembles that of perikaryal gap junctions, but is more intense. Axon terminals are therefore thought to play an important role in the adaptation process.

Endogenous dopamine and cyclic events in the fish retina, II: Correlation of retinomotor movement, spinule formation, and connexon density of gap junctions with dopamine activity during light/dark cycles.

In the fish retina, retinomotor movement, spinule formation, and alteration of connexon density within gap junctions occur in response to changes in ambient light conditions. All of these morphological parameters can also be influenced by the application of dopamine. This study examines whether the morphological alterations of these structures are correlated with the activity of endogenous dopamine during an entrained 12-h light/12-h dark cycle and after 1-h sort-term adaptation periods. The two measured parameters of retinomotor movement, cone inner segment length and pigment dispersion, were well-correlated with endogenous cyclic dopamine activity. However, retinomotor movement was initiated already at the end of the entrained dark period, before the onset of light and before the onset of dopamine turnover. Furthermore, a 1-h dark-adaptation period in the middle of the light phase reduced dopamine activity but did not affect retinomotor movement. At the switch from light to dark and after a 1-h light period at midnight retinomotor movement correlated exactly with dopamine turnover and illumination conditions. The formation of spinules was correlated with dopaminergic activity during all phases of the light/dark cycle and during short-term adaptation periods. Spinules were expressed in the light when dopamine activity was high and they were retracted when dopamine activity was reduced during darkness. Connexon density of horizontal cell gap junctions showed a weaker correlation with the endogenous dopamine turnover. In this case, a high activity of endogenous dopamine was paralleled by a high density of connexons. Our results suggest that endogenous dopamine is involved in the cyclic regulation of the observed morphological alterations and that dopamine is part of the light signal for these mechanisms.

Light-dependent dynamics of gap junctions between horizontal cells in the retina of the crucian carp.

The dynamics of gap junctions between outer horizontal cells or their axon terminals in the retina of the crucian carp were investigated during light and dark adaptation by use of ultrathin-section and freeze-fracture electron microscopy. Light adaptation was induced by red light, while dark adaptation took place under ambient dark conditions. The two principal findings were: (1) The density of connexons within an observed gap junction is high in dark-adapted retina, and low in light-adapted retina. This, respectively, may reflect the coupled and uncoupled state of the gap junction. (2) The size of individual gap junctions is larger in light- than in dark-adapted retinae. Whereas the overall area occupied by gap junctions is reduced with dark adaptation, the percentage of small and very small gap junctions increases dramatically. A lateral shift of connexons in the gap junctional membrane is strongly suggested by these reversible processes of densification and dispersion. Two additional possibilities of gap junction modulation are discussed: (1) the de novo formation of very small gap junctions outside the large ones in the first few minutes of dark adaptation, and (2) the rearrangement of a portion of the very large gap junctions. The idea that the cytoskeleton is involved in such modulatory processes is corroborated by thin-section observations.

Gap junctions between horizontal cells in the cyprinid fish alter rapidly their structure during light and dark adaptation.

The dynamics of the structure of gap junctions between outer horizontal cells (HCs) and between their axonal terminals in the retina of the goldfish during light and dark adaptation is described by means of quantitative freeze-fracture replica examination. The light adaptation was performed in red light. In dark-adapted retinae the gap junctional connexons are arranged much more dense than in light-adapted retinae. The rearrangement during the first minutes of light adaptation proceeds faster than during the first minutes of dark adaptation. Since dark adaptation is accompanied by surround enhancement and presumably by coupling of HCs it is concluded that densification of HC gap junctions may correlate with coupling, and scattering of HC gap junctions with uncoupling of this type of electrotonic synapse.

Dynamics of gap junctions between horizontal cells in the goldfish retina.

Experimental alterations of gap junctions between outer horizontal cells have been demonstrated in freeze-fracture replicas of goldfish retina. The alterations consisted predominantly of an increase of connexon densities and a decrease in the variability of the arrangement of connexons. They were observed i. in dark adapted retinae, ii. in animals with crushed optic nerves, iii. in picrotoxin- and bicuculline-treated animals. Since experiment i. is characterized by a depolarization of the horizontal cell, and experiment iii. was shown by others to result in uncoupling of horizontal cells, we conclude that the functional connectivity of horizontal cells might be correlated with the structure of gap junctions. An interesting detail is the differentiated reaction of axonal and perikaryal gap junctions on dark adaptation or blindness: whereas normally the axonal gap junctions are less densely packed, they increase their connexon density in darkness or blindness much more than the perikaryal gap junctions.

Lack of orthogonal particle assemblies and presence of tight junctions in astrocytes of the goldfish (Carassius auratus). A freeze-fracture study.

The cytoplasmic membranes of astrocytes in the optic nerve of the goldfish were investigated by means of freeze-fracture techniques. Astrocytes of normal and regenerating optic nerves did not differ in the fine structure of plasma membranes. Emphasis is placed on the following results: Astrocytic membranes of fish do not reveal the orthogonal particle assemblies that are believed to be generally characteristic for astrocytes in the white matter. Astrocytes reveal extensive membrane areas occupied by tight junctions, which to date have not been described as characteristic astrocytic structures. These junctions are astro-astrocytic and are frequently intercalated by gap junctions. Desmosomes are another characteristic type of astro-astrocytic junction. By means of freeze-fracture replicas it can be demonstrated that they occur in more or less close association with tight and gap junctions. Caveolae are also seen in the astrocytic membranes of fish: their density and distribution show distinct variations. Caveolae occur at the interface between astrocytes and the interstitial space, between astrocytes and myelin sheaths, and in astrocytic processes. It is suggested that the differences between the astrocytic membranes of fish and mammals reflect different physiological functions. They are discussed in relation to the problem of neuronal-glial interrelationships and the behavior of astrocytes during fiber regeneration in the CNS.

Morphological study on the regeneration of the retina in the rainbow trout after ouabain-induced damage: evidence for dedifferentiation of photoreceptors.

The retina of the rainbow trout is capable of marginal regeneration after ouabain-induced degeneration (intraocular injection of 5 microliters 10(-4) M ouabain). In the central area where the pigment epithelium proliferates to a multicellular layer, the neural retina does not regenerate up to 182 days after injection of ouabain. The regeneration process begins in the marginal growth zone with an increase in the mitotic rate; the growth zone itself is not damaged after ouabain administration. The proliferate differentiates with time into a newly layered retina; this portion of the retina is called the paramarginal zone, i.e., the "first" regenerated zone. The paramarginal zone is arranged concentrically to the retinal margin. Cells surviving ouabain administration, located outside, although close to the margin and occurring mostly in the outer nuclear layer, reveal signs of dedifferentiation: loss of the outer segment, amalgamation of the presynaptic terminal with the perikaryal cytoplasm, alteration of cell shape, and mitotic activity. The area in which these dedifferentiation processes are observed is found adjacent and concentric to the paramarginal zone; it is thinner than the latter and incompletely structured ("second" regenerated zone). The third zone adjoins the second zone and is characterized by folds, which were described previously as "rosettes". Extracellular microtubule-like structures, which are found between the horizontal cells in the normal retina of the rainbow trout, regenerate only sparsely in the paramarginal zone, whereas they are lacking in the incompletely regenerated zones.

Induction of paracrystalline arrays by vincristine in the synaptic formations of the teleost retina.

After injection of 10 microliter 10(-3) M vincristine into the vitreous body of the eye of the rainbow trout (Salmo gairdneri, Richardson), paracrystalline arrays in cell bodies, cell processes and presynaptic formations of the retina cells were observed. The structures resemble the paracrystalline lattice identified by several authors as microtubular protein. The paracrystals in both microtubule-rich cell processes and in synaptic areas, which show only a few or no microtubules, appear to be equivalent. The synaptic paracrystals are suggested to arise from both soluble tubulin and synaptic vesicles, indicating a functional role of tubulin in synaptic transmission.

Microtubules and extracellular microtubule-like structures in the retina of the rainbow trout.

In the retina of the rainbow trout (Salmo gairdneri, Richardson) two types of microtubular structures are demonstrated. Besides the normal type of microtubules (about 200 A in diameter), occurring in all cell types of the retina, a second type is described which is termed microtubule-like structure (MLS) because of its extracellular localization. These MLS have a diameter of about 250 A under the same preparative conditions in which the normal microtubules appear 180-200 A thick. The interspace between the tubules is smaller than between the microtubules. Specific MLS to membrane associations exist, which are analyzed by serial sectioning and tilting procedures. It is suggested that the MLS have their origin at small membranous extrusions of the plasmalemma. These extrusions could contain nucleation sites for the MLS-formation within the extracellular space. It remains unknown which cell type produces the MLS proteins and which factors are responsible for the aggregation of the subunits to intact MLS.