Control of transmitter release from retinal amacrine cells by Ca2+ influx and efflux

Evanna Gleason, University of California, Davis
Salvador Borges, University of California, Davis
Martin Wilson, University of California, Davis


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 Ca2+ 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 Ca2+ 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. © 1994.