Figure 4.

Circadian regulation of suprachiasmatic nucleus (SCN) neuronal excitability. A schematic that summarizes the daily changes in excitability of SCN neurons as a “bicycle model.” Each neuron has the intrinsic capacity to generate daily oscillations in electrical activity. During the day or “up-state,” a Na⁺ leak conductance (g_NALCN) increases while the overall K⁺ conductance (g_K) decreases, thereby resulting in an increase in input resistance (R_IN) and a more depolarized membrane potential (V_m) and higher firing rates. At night, the neurons enter a “down-state,” with lower R_IN, hyperpolarized V_m, elevated g_K, and lower g_NALCN and firing rates. In this way, the excitatory drive of Na-currents during the day is opposed by elevated K-currents at night. (Redrawn from data in Flourakis et al. 2015.)