Abstract
Cultured retinal amacrine cells show quantal GABAergic synaptic transmission. Voltage clamping pre- and postsynaptic cells of an isolated pair has allowed us to examine the entry and removal of Ca 2+ at synaptic terminals. Brief presynaptic Ca 2+ currents elicit an initial postsynaptic current that probably reflects the roughly synchronous exocytosis of docked vesicles. Prolonged Ca 2+ currents elicit an additional second phase of release whose time course can greatly exceed that of the presynaptic voltage step. The time course of this second phase reflects a sustained increase in cytosolic Ca 2+ and is matched closely by the activity of the presynaptic NaCa exchanger, as revealed by an exchange current. Eliminating the activity of the exchanger by removal of external Na + prolongs this second phase of transmission greatly. Because transmitter release at these synapses outlasts Ca + channel opening, Na-Ca exchange plays a significant role in shaping transmission.
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