Ultrastructural changes of photoreceptor synaptic ribbons in relation to time of day and illumination

Electron microscopic sections through rod and cone ribbon synapses reveal mainly rodlike synaptic ribbon profiles, but a few unusual spherical and club-shaped profiles also occur. To elucidate the meaning of the latter two forms, the authors have investigated these ribbon synapses at different times during the 24-hour cycle and under various lighting conditions.

The various types of ribbon profiles were counted, and their sizes were measured by means of transmission electron microscopy in retinas of male BALB/c mice exposed to 12 hours light (lights on at 6 AM) and 12 hours dark (LD 12:12), continuous light, or continuous darkness for 4 days.

A 24-hour study of mice exposed to LD 12:12 showed that spherical and club-shaped profile numbers ranged from 0% to 29%, depending on the time of day. They reached a maximum at 3 hours after light onset, followed by a gradual decrease to approach zero at night and reappearing after light onset the next morning. After 4 days of continuous light, the spherical profiles were significantly decreased in number (examined at 9 AM). After continuous darkness, the spherical and club-shaped profiles were significantly reduced in number. Administration of 4 hours of light after 92 hours of continuous darkness restored the number of spherical and club-shaped profiles to normal values. The rodlike ribbon profiles were found to be longer in darkness than in light. In rod terminals containing spherical profiles, the rodlike ribbon profiles were shorter than in terminals without spherical profiles. CONCLUSIONS. The club-shaped and the spherical profiles were related to the turnover of the synaptic ribbons. Soon after light exposure in the morning, the synaptic ribbons formed distal swellings, giving rise to club-shaped profiles and a decrease in length. The swellings appeared to bud off, thus forming spherical synaptic bodies. This article discusses whether these changes are signs of degradation of spent ribbons, or whether they play a physiological role related to the inactivation of the ribbon synapses after light exposure.