Size variations in synaptic terminals among different types of photoreceptors and across the zebrafish retina.

Photoreceptor synaptic terminals are responsible for transmitting visual information to downstream neurons. In vertebrate retinas, photoreceptor synaptic terminals are of different sizes and structures. The molecular mechanisms that underlie photoreceptor synaptic development are not clearly understood. Here, we have systematically examined the size variations in the synaptic terminals of cone and rod photoreceptors in the adult zebrafish retina. We reveal that the average cone pedicle sizes expand in the order of UV, blue, green, and red cones, echoing the increasing maximally sensitive wavelengths of the opsins expressed in the corresponding cone types. In addition, rod spherules are smaller than all cone pedicles. The terminals of each photoreceptor type also display distinct regional variations across the retina and between males and females. These findings establish the basis for using the zebrafish retina to study the molecular mechanisms that regulate the sizes and structures of photoreceptor terminals for proper visual functions.