Purpose: Efficient and precise release of glutamate from retinal bipolar cells is ensured by the positioning of L-type Ca2 channels close to release sites at the base of the synaptic ribbon. We investigated whether Ca2 channels at bipolar cell ribbon synapses are fixed in position or capable of moving in the membrane. Methods: We tracked the movements of individual L-type Ca2 channels in bipolar cell terminals after labeling channels with quantum dots (QDs) attached to α2d4 accessory Ca2 channel subunits via intermediary antibodies. Results: We found that individual Ca2 channels moved within a confined domain of 0.13-0.15 μm2 in bipolar cell terminals, similar to ultrastructural estimates of the surface area of the active zone beneath the ribbon. Disruption of actin expanded the confinement domain indicating that cytoskeletal interactions help to confine channels at the synapse, but the relatively large diffusion coefficients of 0.3-0.45 μm2/s suggest that channels are not directly anchored to actin. Unlike photoreceptor synapses, removing membrane cholesterol did not change domain size, indicating that lipid rafts are not required to confine Ca2 channels at bipolar cell ribbon synapses. Conclusions: The ability of Ca2 channels to move within the presynaptic active zone suggests that regulating channel mobility may affect release from bipolar cell terminals.
|Original language||English (US)|
|Number of pages||9|
|State||Published - Jan 7 2013|
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