Two components of transmitter release in retinal bipolar cells: exocytosis and mobilization of synaptic vesicles.

Ca2+-transmitter release coupling was examined using bipolar cells with large presynaptic terminals dissociated from the goldfish retina. Presynaptic Ca2+ current (I(Ca)) was recorded under the whole-cell voltage clamp. Release of excitatory amino acid transmitter was simultaneously monitored as the current through N-methyl-D-asperate (NMDA) receptors of reporter cells or as the membrane capacitance (C(m)) change associated with exocytosis. When I(Ca) was activated by a long depolarizing pulse, a double-peaked transmitter-induced current (I(tr)) was elicited in reporter cells. The rapid component of I(tr) was evoked immediately after the onset of depolarization, and was affected only slightly by intracellularly applied Ca2+ chelators. The delayed slow component of I(tr) was elicited during depolarization once a fixed amount of Ca2+ was accumulated in presynaptic terminals, and its appearance was suppressed or retarded by Ca2+ chelators. Two components of transmitter release were also recognized by monitoring C(m) changes elicited by the activation of I(Ca). These results suggest that bipolar cells have at least two pools of synaptic vesicles; a small, immediately releasable pool and a large releasable pool. The rapid and the delayed slow components of transmitter release may reflect exocytosis and mobilization of synaptic vesicles, respectively.