Modification of diurnal changes in the ultrastructure of synaptic ribbons of the turtle

Synaptic ribbons of rod cells of the red-eared turtle under light-dark (LD) cycle, continuous dark (DD) and continuous light (LL) were examined by electron microscopy. In the LD cycle, several stick-shaped (plate-shaped, three-dimensionally) ribbons formed multilayered ribbon complexes at 1000 hr, and they turned into aggregates of rounded ribbons at 1900 hr (1 hr after light-off). In DD, stick-shaped ribbons were situated singly near presynaptic membranes. Large ribbon complexes consisting of 7-8 parallel layers, as shown in the LD cycle, were not observed. Aggregates of rounded ribbons, or club-shaped ones could not be observed. In LL, aggregates of irregular ribbon fragments were noted. None of the ribbons in this regimen showed sharp contours. Cyclic LD illumination is necessary to form multilayered ribbon complexes and rounded ribbons. Internal circadian rhythm contributes little part to the diurnal changes in the structure of the synaptic ribbons.