Low Ba²⁺ and Ca²⁺ induce a sustained high probability of repolarization openings of L-type Ca²⁺ channels in hippocampal neurons: Physiological implications

Openings of single L-type Ca²⁺ channels following repolarization to negative membrane potentials from a depolarizing step (repolarization openings, ROs) have been described previously in brain cell preparations. However, these ROs have been reported to occur only infrequently. Here we report that the frequency of ROs in cell-attached patches of cultured rat hippocampal neurons can be increased dramatically by lowering the pipette Ba²⁺ concentration to 20 mM from the usual 90-110 mM. This increased opening probability can last for hundreds to thousands of milliseconds following repolarization. Current-voltage analyses of open probability show that the depolarization pulse threshold for inducing ROs in 20 mM Ba²⁺ is -10 to 0 mV but that the probability of ROs reaches maximal levels following depolarizing pulses that approach the apparent null (equilibrium) potential for Ba²⁺. Comparable current-voltage curves in 110 mM Ba²⁺ from a more positive holding potential (-50 mV) indicate that membrane surface charge screening accounts for some, but not all, of the effect of lowering the Ba²⁺ concentration. Consequently, current-dependent inactivation or some other ion-dependent mechanism (e.g., ion binding inside the pore) also appears to regulate this potentially major pathway of Ca²⁺ entry. A high probability of ROs also can be induced under relatively physiological conditions (5-ms depolarizing steps, 2-5 mM Ca²⁺ in the pipette). Thus, the high open probability state at negative potentials may underlie the long Ca²⁺ tail currents in hippocampus that were described previously and appears to have major implications for physiological functions (e.g., the slow Ca²⁺-dependent afterhyperpolarization), particularly in brain neurons.