Synaptic ribbons in the pineal system of normal and light deprived golden hamsters.

Synaptic ribbons were studied in the pineal system of the golden hamster (Mesocricetus auratus) under various conditions of light deprivation. Five weeks of constant darkness led to an increase in the number of synaptic ribbons in both the superficial and the deep part of the organ, and after ten weeks a further increase was observed. In animals returned to normal lighting conditions for one week, after ten weeks of constant darkness, a marked decrease was recorded and five weeks after termination of light deprivation the number of synaptic ribbons was slightly below the normal level. In addition to long rod-shaped ribbons, atypically formed aggregations of electron dense material appear in the synaptic ribbon fields of light-deprived hamsters; dense core vesicles were observed in the vicinity of the ribbons with particular frequency. It is assumed that the topographical relationship between dense core vesicles and synaptic ribbons is of functional significance in relation to the regulation of pineal antigonadal activity.