CONTINUOUS AND TRANSIENT VESICLE CYCLING AT A RIBBON SYNAPSE

Optical methods were used to study the Ca2+ dependence of vesicle cycl ing in bipolar cells isolated from goldfish retinas. Uniformly raising the Ca2+ concentration to between 0.8 and 20 mu M produced a continuo us vesicle cycle of balanced exocytosis and endocytosis with a maximum rate equivalent to the turnover of the entire surface membrane of a t erminal every 2 min (or similar to 900 vesicles sec(-1)). Increasing t he Ca2+ concentration above 20 mu M inhibited continuous vesicle cycli ng. In contrast, influx of Ca2+ through voltage-gated channels produce d a transient burst of exocytosis that increased the surface area of a terminal by a maximum of 12% (equivalent to the addition of 13,000 ve sicles). Endocytosis was delayed until after Ca2+ influx stopped and t he average Ca2+ concentration in the terminal declined. Hence, a singl e terminal has mechanisms for both continuous and transient vesicle cy cling.