Characterization of a Calcium‐dependent Current Generating a Slow Afterdepolarization of CA3 Pyramidal Cells in Rat Hippocampal Slice Cultures

A depolarization‐induced, slowly decaying inward current was examined in slice‐cultured CA3 pyramidal cells by voltage‐clamp techniques and microfluorometric measurements of cytosolic free Ca²⁺ concentration ([Ca²⁺]_i). Action potentials elicited by intracellular injection of short‐lasting (50 – 100 ms) depolarizing current pulses were followed by a slowly decaying afterhyperpolarization (AHP). After switching to voltage‐clamp mode, short‐lasting (50 – 100 ms) depolarizing voltage jumps from –60 mV to between –30 and 0 mV induced a slowly decaying outward aftercurrent (I_AHP) which was depressed by bath application of muscarine (0.5 μM). In the presence of muscarine, the same depolarizations induced a slowly decaying afterdepolarization (ADP) or inward aftercurrent (I_ADP)in voltage‐clamp mode. This current was also induced in the presence of trans(±)‐1‐aminc‐1,3‐cyclopenta‐nedicarboxylic acid (t‐ACPD, 5 μM), an agonist of metabotropic glutamate receptors, but not in the presence of noradrenalin (5 μM), while both of these agonists depressed I_AHP. I_ADP was depressed by reducing the external Ca²⁺ concentration from 3.8 to 0.5 mM, by external Co²⁺ (1 mM) and by external Cd²⁺ (10 – 100 μM). Combined voltage‐clamp recordings and microfluorometric measurements of [Ca²⁺]_i using the Ca²⁺ indicator fura‐2 revealed that the amplitude of I_ADP was correlated with the amplitude of depolarization‐induced Ca²⁺ influx, I_ADP was absent at membrane potentials < –90 mV, and reached maximal amplitudes at ∼–55 mV. Raising the extracellular K⁺ concentration from 2.7 to 13.5 mM increased the amplitude of I_ADP and resulted in a positively directed shift of the apparent reversal potential of I_ADP. When the external Na⁺ concentration was reduced from 157 to 33 or 18 mM the current reversed at more negative potentials and was reduced to 40 and 21%, respectively, of control amplitude. Lowering the external Cl^‐ concentration from 159 to 20 mM did not affect I_ADP. We conclude that I_ADP most likely represents a Ca²⁺‐activated cation current, rather than a Ca²⁺ tail current, or an electrogenic Ca²⁺ extrusion current.