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Afterhyperpolarization Regulates Firing Rate in Neurons of the Suprachiasmatic Nucleus

Cluster I neurons of the suprachiasmatic nucleus (SCN), which are thought to be pacemakers supporting circadian activity, fire spontaneous action potentials that are followed by a monophasic afterhyperpolarization (AHP). Using a brain slice preparation, we have found that the AHP has a shorter duration in cells firing at higher frequency, consistent with circadian modulation of the AHP. The AHP is supported by at least three subtypes of KCa channels, including apamin-sensitive channels, iberiotoxin-sensitive channels, and channels that are insensitive to both of these antagonists. The latter KCa channel subtype is involved in rate-dependent regulation of the AHP. Voltage-clamped, whole-cell Ca2+ channel currents recorded from SCN neurons were dissected pharmacologically, revealing all of the major high-voltage activated subtypes: L-, N-, P/Q-, and R-type Ca2+channel currents. Application of Ca2+ channel antagonists to spontaneously firing neurons indicated that predominantly L- and R-type currents trigger the AHP. Our findings suggest that apamin- and iberiotoxin-insensitive KCachannels are subject to diurnal modulation by the circadian clock and that this modulation either directly or indirectly leads to the expression of a circadian rhythm in spiking frequency.