Calcium influx selects the fast mode of endocytosis in the synaptic terminal of retinal bipolar cells

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Abstract
To investigate the regulation of endocytosis by Ca 2+ , we have made capacitance measurements in the synaptic terminal of depolarizing bipolar cells from the retina of goldfish. After a brief depolarization, all of the excess membrane was retrieved rapidly (τ ≈1 s). But when the rise in free [Ca 2+ ] was reduced by the introduction of Ca 2+ buffers [1,2-bis(2-aminophenoxy)ethane- N,N,N ′ ,N ′-tetraacetate (BAPTA) or EGTA], a large fraction of the membrane was retrieved by a second, slower mechanism (τ ≥ 10 s). The block of fast endocytosis by EGTA could be overcome by increasing the amplitude of the Ca 2+ current, demonstrating that Ca 2+ influx was the trigger for fast endocytosis. These manipulations of the Ca 2+ signal altered the relative proportions of fast and slow endocytosis but did not modulate the rate constants of these processes. A brief stimulus that triggered fast endocytosis did not generate a significant rise in the spatially averaged [Ca 2+ ], indicating that Ca 2+ regulated endocytosis through an action close to the active zone. The slow mode of retrieval occurred at the resting [Ca 2+ ]. These results demonstrate that Ca 2+ influx couples fast endocytosis and exocytosis at this synapse.

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