BETA-ADRENERGIC STIMULATION SELECTIVELY INHIBITS LONG-LASTING L-TYPE CALCIUM-CHANNEL FACILITATION IN HIPPOCAMPAL PYRAMIDAL NEURONS

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 rec ording using a low concentration of permeant ion reveals a long-lastin g facilitation of L-type channel activity that is induced by a depolar izing prepulse or a train of action potential waveforms. This facilita tion exhibits a slow rise, peaking 0.5-1 sec after the train and decay ing over several seconds. We have termed this behavior ''delayed facil itation,'' because of the slow onset. Delayed facilitation results fro m an increase in opening frequency and the recruitment of longer durat ion openings. This behavior is observed at all membrane potentials bet ween -20 and -60 mV, with the induction and magnitude of facilitation being insensitive to voltage. beta-Adrenergic receptor activation bloc ks induction of delayed facilitation but does not significantly affect normal L-type channel activity. Delayed facilitation of L-type calciu m channels provides a prolonged source of calcium entry at negative me mbrane potentials, This behavior may underlie calcium-dependent events that are inhibited by beta-adrenergic receptor activation, such as th e slow afterhyperpolarization in hippocampal neurons.