Figure 6. Both terfenadine and endogenous ACh reduce a current responsible for late-phase spike frequency adaptation.

(A) demonstration of method used to define kinetics of currents responsible for SFA. From a standard pair of cumulative spike count plots (left panel, responses from a computer simulation with and without a linearly-increasing K⁺ current), a difference cumulative spike count function is computed (middle panel). Finally, a series of slope estimates, representing the instantaneous rate of SFA, is generated at different times during the step response. Slopes at the ‘Early’ and the ‘Late’ phases of the response are quantified in D. (B) Estimates of the kinetics of SFA modulated by the same three K⁺ channel blockers assayed in Figure 5, revealing the declining functional effect of SK current (left panel) and the increasing functional effect of Terf-sensitive ERG current during the same type of 2 s duration depolarizing step responses. (C) Rate of SFA increases during step responses when endogenous ACh is released. (D) Summary of the initial and final rates of SFA. Blockade of SK channels with NS8593 decreased the rate of SFA: ***p=0.000249, T = 7.714, while both Terf and ChR2-mediated ACh release increase the rate of SFA: * Terf: p=0.0432, T = 2.692; * ChR2 p=0.0471, T = 2.192; XE991 n.s., p=0.923, T = 0.102; all paired t-tests. Experiments using Terf and XE991 were conducted using an internal solution with strong Ca²⁺ buffering (10 mM BAPTA) to attenuate SK-mediated effect. (E) Schematic diagram of the kinetics of the main K⁺ currents hypothesized to underlie SFA under control conditions (left: SK, blue; M, green; ERG, red) and following mAChR activation (right) with attenuated ERG and reduced late-phase SFA. The spiking response during the early phase of the step (left dashed box) was insensitive to mAChR modulation and was likely mediated by M or SK current. In contrast, neuronal excitability was selectively increased in the late phase (right dashed box), likely by mAChR-mediated attenuation of ERG. Intracellular responses duplicated from Figure 1B.

Figure 6—figure supplement 1. Rate of spike frequency adaptation was increased following ERG blockade with Terf in experiments with weak Ca²⁺ buffering.

Parallel experiment protocol used in Figure 6D but with 0.2 mM EGTA internal solutions (and Rin compensated). *p=0.0296, T = 2.838, paired t-test.