Transient synaptic ribbons in the mammalian retina at unusual sites

In the vertebrate retina the presence of synaptic ribbons (SRs) is well documented in two sites only, viz., in photoreceptor axon terminals in the outer plexiform layer and in bipolar cell axons in the inner plexiform layer. The present paper reports the presence of non-photoreceptor SRs in the outer plexiform layer of cattle and mouse, where they were seen in small numbers in thin cell processes near cone pedicles of light-adapted animals. They were never seen near rod spherules. Quantitative data obtained in mice killed at different time-points revealed that the SRs under consideration increased in number during day time and were absent during the dark phase. Moreover, under high light intensity of 10000 lux they were more frequent in number compared to 100-lux-exposed animals. It is concluded that the cell processes revealing the temporary presence of SRs are processes of flat bipolar cells which may provide a feedback to cones during the light phase.