Calcium-dependent inactivation of calcium current in synaptic terminals of retinal bipolar neurons

Giant synaptic terminals (approximately 10 micrometer diameter) of bipolar neurons from goldfish retina were used to directly investigate calcium-dependent inactivation of presynaptic calcium current. During sustained depolarization, calcium current was initially constant for a period lasting up to several hundred milliseconds and then it declined exponentially. The duration of the initial delay was shorter and the rate of inactivation was faster with larger calcium current. The fastest time constant of inactivation (in the range of 2-5 sec) was observed under weak calcium buffering conditions. Inactivation was attenuated when external Ca2+ was replaced with Ba2+ and when terminals were dialyzed with high concentrations of internal BAPTA. Elevation of intracellular calcium concentration ([Ca2+]i) by application of the calcium ionophore ionomycin or by dialysis with pipette solutions containing buffered elevated [Ca2+] produced inactivation of calcium current. The rate of recovery from inactivation was not determined by the recovery of [Ca2+]i to baseline after a stimulus. The results demonstrate that the presynaptic calcium current in bipolar neurons is inactivated by elevated [Ca2+]i, but the inactivation is approximately 100-fold slower than previously described calcium-dependent inactivation in other types of cells.