Abstract
L-type calcium channels are abundant in hippocampal pyramidal neurons and are highly clustered at the base of the major dendrites. However, little is known of their function in these neurons. Single-channel recording using a low concentration of permeant ion reveals a long-lasting facilitation of L-type channel activity that is induced by a depolarizing prepulse or a train of action potential waveforms. This facilitation exhibits a slow rise, peaking 0.5-1 sec after the train and decaying over several seconds. We have termed this behavior "delayed facilitation," because of the slow onset. Delayed facilitation results from an increase in opening frequency and the recruitment of longer duration openings. This behavior is observed at all membrane potentials between -20 and -60 mV, with the induction and magnitude of facilitation being insensitive to voltage. beta-Adrenergic receptor activation blocks induction of delayed facilitation but does not significantly affect normal L-type channel activity. Delayed facilitation of L-type calcium channels provides a prolonged source of calcium entry at negative membrane potentials. This behavior may underlie calcium-dependent events that are inhibited by beta-adrenergic receptor activation, such as the slow afterhyperpolarization in hippocampal neurons.