Effects of light and dark upon photoreceptor synapses in the retina of Xenopus laevis

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.

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.

Diurnal changes in synaptic ribbons of rod cells of the turtle

Diurnal morphological changes in synaptic ribbons of the rod cells of the turtle were revealed by electron microscopy with serial ultrathin sections. Freshwater turtles (Pseudemys) were maintained under the light-dark cycle with lights on from 0600 to 1800 hr. Retinas around the posterior pole of the eyeball were fixed in 2.5% glutaraldehyde and 1% osmium tetroxide. In total, 124 rod cells and several hundred cone cells were observed. At 0000 (midnight), ribbons are situated close to and perpendicular to the presynaptic membrane. They show single stick-shaped profiles on thin section. From midnight toward noon, the stick-shaped ribbons grow into large multilayered ribbon complexes composed of several sticks arranged parallel to each other. Then, the ribbon complexes begin to disintegrate into irregular fragments from noon toward night. At 1900, aggregates of rounded or club-shaped ribbons are predominant.

Dense core vesicles during photoreceptor development

The presence of dense core vesicles in the terminal expansions of photoreceptors in development is described in the chick embryo retina, from the 16th to the 18th day of incubation.

Light-related changes in electron-dense material in photoreceptor synaptic clefts of the frog, Rana catesbeiana

Lumps of electron-dense material were observed in synaptic clefts associated with all types of photoreceptors, in the vicinity of the synaptic ribbons, in the retinae of dark-adapted frogs. Frogs were reared under a cyclic illumination (light on at 8:00; light off at 20:00) and then exposed to one of two courses of dark adaptation: one started from 11:00 in the morning, and the other started from 20:00 in the evening. The synaptic clefts of red rods became wider at some places where spherical or polygonal lumps of dense material were accumulated. The frequency and sectional area of the lumps increased faster for the first hour in the regime starting from 20:00 than in the regime starting from 11:00, then they reached the similar saturation levels of about 0.6 (per ribbon) and 1.6 to 1.8 X 10(4) (nm2) in both the regimes. In green-rod synapses, plate-shaped lumps of dense material were present in synaptic clefts and interspaces between the processes of second-order neurons. In cone synapses at the end of about 1 h darkness, the frequency and area of the lumps reached maximum values of about 0.12 (per ribbon) and 9 X 10(3) (nm2) in the regime starting from 11:00 and, about 0.08 (per ribbon) and 4 X 10(3) (nm2) in the regime starting from 20:00. On exposure to light, the dense material abruptly disappeared from all types of photoreceptor synaptic clefts. Large dense-core vesicles, occasionally observed in light-adapted rod photoreceptor terminals, seem to participate in exocytosis of the dense material. The number of dense-core vesicles per synaptic ribbon in a terminal was about 0.55 at the end of 3 h light in the morning and about 1.28 at the end of 12 h light in the evening. The increased number of dense-core vesicles during the daytime may contribute to the faster accumulation of dense material in the synaptic clefts. Although the chemical identification or the functional significance of the electron-dense material remains unknown, it is interesting that this material showed a rise and fall in response to darkness and illumination. Also the fact that this material is clearly visible will be helpful for future analysis of frog photoreceptor synapses.

Diurnal variations in the photoreceptor synaptic terminals of the newt retina

Newt photoreceptor synaptic terminals undergo a variety of morphological changes over a 24-hr (LD 12:12) cycle. During the day, dense-cored synaptic vesicles were found to increase in number and accumulate near the synaptic lamellae; during the dark phase, the dense-cored vesicles decreased in number, while large clear vesicles and profiles of smooth endoplasmic reticulum increased in frequency. The most marked change in photoreceptor synaptic terminal morphology occurred after 10 hr of darkness, at 0730 hr. At this time, photoreceptor synaptic terminal cross-sectional area was found to increase dramatically. Morphometric analysis showed that the number of synaptic vesicles in these terminals remained constant throughout the day, as did the perimeter of photoreceptor terminal profiles. The observed increase in area of synaptic terminals at 0730 hr was found to be due to a decrease in the folding of the terminal plasma membrane. Qualitative observations showed endocytosis to be occurring at a rapid rate at this time as well; and since the number of synaptic vesicles and terminal perimeter did not change, exocytosis of synaptic vesicles was assumed to be occurring at an equally rapid rate. These findings support an extension to the hypothesis of Monaghan and Osborne (1975), suggesting that photoreceptor synaptic vesicles become "supercharged" with transmitter substance in the light.

Identification of sulfur groups in the outer plexiform layer of the newt retina

The staining of newt photoreceptor synaptic vesicles with the zinc iodide-osmium (ZIO) method can be abolished by prior incubation in amphibian Ringer's containing 0.1 M-N-ethyl maleimide which specifically blocks -SH groups. The existence of high concentrations of sulfur groups in the outer plexiform layer of the newt retina has been further demonstrated with mercury orange and a newly synthesized fluorescent thiol probe, DACM. The reason why such high concentrations of -SH groups are present within photoreceptor synaptic vesicles is unclear, however, it is possible that ZIO stains the photoreceptor neurotransmitter itself.

Light-dependent plasticity of the morphology of horizontal cell terminals in cone pedicles of fish retinas

Horizontal cell terminals lateral to the synaptic ribbons in goldfish cone pedicles give rise to 0.3 micron long, finger-like extensions in the light-adapted state. These structures, called spinules, disappear almost completely after dark adaptation. The ultrastructure of the horizontal cell terminals is characterized by the presence of occasional microtubules, microfilaments and sparse irregular vesicles; in the dark, large multivesicular bodies can also be found. Two types of membrane densities are described in the horizontal cell terminals, one of which is typically located at the tip of the spinules. Their positive reaction to E-PTA makes it probable that the spinules are synaptic structures. Reconstruction of serial sections shows about 12 spinules per terminal in the light but only two in the dark. Formation and disappearance of the spinules takes about 60 min and involves a transitional stage in the form of a spherical structure. Spinules can be found in five other teleost species, with a darkness-induced reduction in number, but not in the horizontal cell terminals of the mudpuppy, turtle and mammals.

Ultrastructural changes in the anterior horn synapses of rat spinal cord under different locomotor conditions

Ultrastructural changes in axodendritic synapses of the spinal cord ventral horn are studied in rats subjected to different locomotor conditions: immobilization, control and physical loading. Enhanced motor activity results in a reduction of the number of synaptic vesicles in the axon terminal, as well as flattening and diminution of their size. Other changes correlated with the different motor regimes are likewise established in the number of terminal ending's mitochondria, in "active zones", in subjunctional dense bodies, and in the bouton area. Data are treated by means of variation analysis. The significance of differences is established by "T"-criterion.