Effects of NALCN-Encoded Na⁺ Leak Currents on the Repetitive Firing Properties of SCN Neurons Depend on K⁺-Driven...

Neurons in the suprachiasmatic nucleus (SCN) generate circadian changes in the rates of spontaneous action potential firing that regulate and synchronize daily rhythms in physiology and behavior. Considerable evidence suggests that daily rhythms in the repetitive firing rates (higher during the day than at night) of SCN neurons are mediated by changes in subthreshold potassium (K⁺) conductance(s). An alternative "bicycle" model for circadian regulation of membrane excitability in clock neurons, however, suggests that an increase in NALCN-encoded sodium (Na⁺) leak conductance underlies daytime increases in firing rates. The experiments reported here explored the role of Na⁺ leak currents in regulating daytime and nighttime repetitive firing rates in identified adult male and female mouse SCN neurons: vasoactive intestinal peptide-expressing (VIP⁺), neuromedin S-expressing (NMS⁺) and gastrin-releasing peptide-expressing (GRP⁺) cells. Whole-cell recordings from VIP⁺, NMS⁺, and GRP⁺ neurons in acute SCN slices revealed that Na⁺ leak current amplitudes/densities are similar during the day and at night, but have a larger impact on membrane potentials in daytime neurons. Additional experiments, using an in vivo conditional knockout approach, demonstrated that NALCN-encoded Na⁺ currents selectively regulate daytime repetitive firing rates of adult SCN neurons. Dynamic clamp-mediated manipulation revealed that the effects of NALCN-encoded Na⁺ currents on the repetitive firing rates of SCN neurons depend on K⁺ current-driven changes in input resistances. Together, these findings demonstrate that NALCN-encoded Na⁺ leak channels contribute to regulating daily rhythms in the excitability of SCN neurons by a mechanism that depends on K⁺ current-mediated rhythmic changes in intrinsic membrane properties.SIGNIFICANCE STATEMENT Elucidating the ionic mechanisms responsible for generating daily rhythms in the rates of spontaneous action potential firing of neurons in the suprachiasmatic nucleus (SCN), the master circadian pacemaker in mammals, is an important step toward understanding how the molecular clock controls circadian rhythms in physiology and behavior. While numerous studies have focused on identifying subthreshold K⁺ channel(s) that mediate day-night changes in the firing rates of SCN neurons, a role for Na⁺ leak currents has also been suggested. The results of the experiments presented here demonstrate that NALCN-encoded Na⁺ leak currents differentially modulate daily rhythms in the daytime/nighttime repetitive firing rates of SCN neurons as a consequence of rhythmic changes in subthreshold K⁺ currents.