Ca 2+-dependent C − current at the presynaptic terminals of goldfish retinal bipolar cells

Abstract

In ON-type bipolar cells dissociated from the goldfish retina, a slowly declining inward current (I tail) was observed after the termination of depolarizing; voltage step commands, during which a Ca 2+ current was elicited. The properties of I tail were investigated under the whole-cell voltage clamp. Introduction of the membrane permeant Ca 2+ chelator, BAPTA/AM, into the cell suppressed I tail, indicating that I tail, was activated by the increase of intracellular free Ca 2+ concentration ([Ca 2+] i). The major component of I tail was identified as the Ca 2+-dependent Cl − current (I Cl(Ca)), since the reversal potential of I tail was almost identical to the Cl − equilibrium potential at various extracellular Cl − concentrations ([Cl −] o). The contribution of the Na +/Ca 2+ exchanger current to I tail was very small. I Cl(Ca) was partially suppressed by 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulfonic acid (SITS) when it was locally applied to the axon terminal but not to the cell body region, suggesting that Ca 2+-dependent Cl − channels were localized to the axon terminal. The relationship between the peak amplitude of I Cl(Ca) and the amount of charge carried by the Ca 2+ current was almost linear at levels less than ca. 50 pC, but became saturated at a higher Ca 2+ charge.