Synaptic ribbon dynamics in photoreceptors of mice

The present study deals with structural plasticity of a special type of chemical synapse, the ribbon synapse. Near the presynaptic membrane ribbon synapses contain conspicuous electron-dense synaptic bodies which appear mainly as rod-like profiles under the transmission electron microscope. In addition, club-shaped and spherical profiles may be present, the function of which is unclear. To gain some insight into the significance of the latter structures we studied their presence in rod-type photoreceptor cells of BALB/c mice under different lighting conditions. Quantification revealed that the club-shaped and the spherical profiles showed a clear light/dark dependence. They were virtually absent at night and increased strikingly in number when the animals were exposed to light. When darkness was extended into the morning, the profiles remained low in number. As the rod cells diminish their neurotransmitter release during the light phase, the present findings are interpreted as signs of synapse inactivation.