Small conductance Ca²⁺-activated K⁺ channels regulate firing properties and excitability in parasympathetic cardiac motoneurons in the nucleus ambiguus

Abstract

Small conductance Ca(2+)-activated K(+) channels (SK) regulate action potential (AP) firing properties and excitability in many central neurons. However, the functional roles of SK channels of parasympathetic cardiac motoneurons (PCMNs) in the nucleus ambiguus have not yet been well characterized. In this study, the tracer X-rhodamine-5 (and 6)-isothiocyanate (XRITC) was injected into the pericardial sac to retrogradely label PCMNs in FVB mice at postnatal days 7-9. Two days later, XRITC-labeled PCMNs in brain stem slices were identified. With the use of whole cell current clamp, single APs and spike trains of different frequencies were evoked by current injections. We found that 1) PCMNs have two different firing patterns: the majority of PCMNs (90%) exhibited spike frequency adaptation (SFA) and the rest (10%) showed less or no adaptation; 2) application of the specific SK channel blocker apamin significantly increased spike half-width in single APs and trains and reduced the spike frequency-dependent AP broadening in trains; 3) SK channel blockade suppressed afterhyperpolarization (AHP) amplitude following single APs and trains and abolished spike-frequency dependence of AHP in trains; and 4) SK channel blockade increased the spike frequency but did not alter the pattern of SFA. Using whole cell voltage clamp, we measured outward currents and afterhyperpolarization current (I(AHP)). SK channel blockade revealed that SK-mediated outward currents had both transient and persistent components. After bath application of apamin and Ca(2+)-free solution, we found that apamin-sensitive and Ca(2+)-sensitive I(AHP) were comparable, confirming that SK channels may contribute to a major portion of Ca(2+)-activated K(+) channel-mediated I(AHP). These results suggest that PCMNs have SK channels that significantly regulate AP repolarization, AHP, and spike frequency but do not affect SFA. We conclude that activation of SK channels underlies one of the mechanisms for negative control of PCMN excitability.

Characteristics of single large-conductance Ca2+-activated K+ channels and their regulation of action potentials and excitability in parasympathetic cardiac motoneurons in the nucleus ambiguus

“…This confirmed our previous observation (38). In contrast, SK channels regulate both fHAP and mAHP but not sAHP (36). Of note: blockade of BK channels with IBTX noticeably increased AHP in trains.…”

“…This resulted in an increase of intracellular free Ca 2ϩ and, in turn, activation of SK channels. As we showed previously (36), activation of SK channels may increase fAHP and mAHP which may contribute to the increase of mAHP in trains after blockade of BK channels as seen in Figs. 9A and 10B.…”

“…We used a 10-ms duration because this stimulus duration has been commonly used to evoke single APs in other central neurons (11,17,36). Within minutes after good recording conditions were established (i.e., resting membrane potential was less than Ϫ60 mV and AP peak amplitude was Ͼ70 mV), AP half-width was measured as the spike width at the half-maximal voltage using Clampfit 9.2.…”

“…In the literature, both small conductance and large conductance Ca 2ϩ -activated K ϩ channels (SK and BK channels, respectively) were reported to mediate the firing properties of action potentials (APs) and excitability of central neurons (14 -16, 59, 67). Previously, we have characterized the functional roles of SK channels in regulating firing properties of APs and excitability of PCMNs (36). However, the membrane properties and the functional roles of BK channels of PCMNs have not yet been fully determined.…”

CaV1.3-Driven SK Channel Activation Regulates Pacemaking and Spike Frequency Adaptation in Mouse Chromaffin Cells

“…Upon sustained current injections, SK channels in various neurons lead to SFA (Madison and Nicoll, 1984;Benda and Herz, 2003;Chen and Toney, 2009;Peron and Gabbiani, 2009;Lin et al, 2010;Vandecasteele et al, 2011). This holds as well for MCCs where SK channels slow down the steady-state firing frequency.…”