Figure 10. Steady-state and action potential-evoked BK_i currents are reduced compared to BK_s in SCN neurons.

(a) Representative macroscopic BK current traces from daytime WT neurons in response to a subthreshold voltage protocol (−60 to −30 mV in 5 mV steps). BK currents were identified as BK_i or BK_s from a maximally activating step to +90 mV (as in Fig. 2). (b) Current–voltage relationship showing significantly more activation of BK_s current above −60 mV compared to BK_i. (c) Pre-recorded action potential commands (top) were used to elicit BK currents from BK_i or BK_s neurons. From a holding potential of −150 mV to remove inactivation completely, cells were stepped to the inter-spike potential (−48 mV) with a sequence of three action potential commands as depicted (Peak, 8 mV; t_1/2, 5.5 ms; and AHP/antipeak, −54 mV). Arrow, the subthreshold current level in d was taken just before the second action potential command. (d,e) Average subthreshold BK current density from the inter-spike interval (d) or at the peak of the action potential (e). n values for (a–d): WT: day/BK_i (9), day/BK_s (5). (f) BK current as a function of conditioning potential in SCN neurons. Representative macroscopic BK current traces from β2 KO neurons during the day with 50 μM β2N^ΔFIW or β2N. Neurons were held at −90 mV for 150 ms to remove inactivation, and then stepped to a conditioning potential for 100 ms to allow channels to transition into the inactivated state, followed by a maximally activating step to +90 mV. (g) The peak BK current elicited from the +90 mV step was plotted as a function of the conditioning potential. β2N causes a reduction in current, but no reduction is observed in the absence of inactivation with β2N^ΔFIW. n=5 for each condition. All values are mean±s.e.m. *P<0.05, Bonferroni post hoc.