PLASTICITY OF RETINAL RIBBON SYNAPSES

Ribbon synapses differ from conventional chemical synapses in that the y contain, within the cloud of synaptic vesicles (SV's), a specialized synaptic body, most often termed synaptic ribbon (SR). This body assu mes 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 pr esent in afferent synapses of photoreceptors, bipolar nerve cells, and hair cells of both the organ of Corti and the vestibular organ. Synap tic 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 fu nction of SR's is enigmatic. The prevailing concept is that SR's funct ion as conveyor belts to channel SV's to the presynaptic membrane for neurotransmitter release by means of exocytosis. The present article r eviews the evidence that speaks for a plasticity of these organelles i n the retina and the third eye, as reflected in changes in number, siz e, 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 pinea l SR numbers increase at night and decrease during the day. In teleost cones, SR's are in their prime during daytime and decrease or disappe ar at night, when transmitter release is enhanced. In addition to nume rical 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 subt le and not always consistent. They may reflect changes in function or may represent signs of degradation. To distinguish between the two, mo re detailed studies under selected experimental conditions are require d. Probably the strongest evidence for SR plasticity in the mammalian retina is that in hibernating squirrels SR's leave the synaptic site a nd accumulate in areas as far as 5 mu m from the synapse. Changes in s hape include the occurrence of club-shaped SR's and round SR's or syna ptic spheres (SS's). SS's may represent a special type of synaptic bod y, yet belonging to the family of SR's, or may be related to the catab olism of SR's. SR number is regulated by Ca2+ in teleost cones, wherea s in the mammalian pineal gland cGMP is involved. An interesting bioch emical feature of ribbon synapses is that they lack synapsins. The pre sently reviewed results suggest to us that SR's do not primarily funct ion as conveyor belts, but are devices to immobilize SV's in inactive ribbon synapses. (C) 1996 Wiley-Liss, Inc.