BK channel activation by L-type Ca channels Ca1.2 and Ca1.3 during the subthreshold phase of an action potential.

Mammalian circadian (24 h) rhythms are timed by the pattern of spontaneous action potential firing in the suprachiasmatic nucleus (SCN). This oscillation in firing is produced through circadian regulation of several membrane currents, including large-conductance Ca- and voltage-activated K (BK) and L-type Ca channel (LTCC) currents. During the day steady-state BK currents depend mostly on LTCCs for activation, whereas at night they depend predominantly on ryanodine receptors (RyRs). However, the contribution of these Ca channels to BK channel activation during action potential firing has not been thoroughly investigated. In this study, we used a pharmacological approach to determine that both LTCCs and RyRs contribute to the baseline membrane potential of SCN action potential waveforms, as well as action potential-evoked BK current, during the day and night, respectively. Since the baseline membrane potential is a major determinant of circadian firing rate, we focused on the LTCCs contributing to low voltage activation of BK channels during the subthreshold phase. For these experiments, two LTCC subtypes found in SCN (Ca1.2 and Ca1.3) were coexpressed with BK channels in heterologous cells, where their differential contributions could be separately measured. Ca1.3 channels produced currents that were shifted to more hyperpolarized potentials compared with Ca1.2, resulting in increased subthreshold Ca and BK currents during an action potential command. These results show that although multiple Ca sources in SCN can contribute to the activation of BK current during an action potential, specific BK-Ca1.3 partnerships may optimize the subthreshold BK current activation that is critical for firing rate regulation. BK K channels are important regulators of firing. Although Ca channels are required for their activation in excitable cells, it is not well understood how BK channels activate using these Ca sources during an action potential. This study demonstrates the differences in BK current activated by Ca1.2 and Ca1.3 channels in clock neurons and heterologous cells. The results define how specific ion channel partnerships can be engaged during distinct phases of the action potential.