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

Abstract

Large-conductance Ca2(+)-activated K+ channels (BK) regulate action potential (AP) properties and excitability in many central neurons. However, the properties and functional roles of BK channels in parasympathetic cardiac motoneurons (PCMNs) in the nucleus ambiguus (NA) 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 7-9 days. Two days later, XRITC-labeled PCMNs in brain stem slices were identified. Using excised patch single-channel recordings, we identified voltage-gated and Ca(2+)-dependent BK channels in PCMNs. The majority of BK channels exhibited persistent channel opening during voltage holding. These BK channels had a conductance of 237 pS and a 50% opening probability at +27.9 mV, the channel open time constant was 3.37 ms at +20 mV, and dwell time increased exponentially as the membrane potential depolarized. At the +20-mV holding potential, the [Ca2+]50 was 15.2 μM with a P0.5 of 0.4. Occasionally, some BK channels showed a transient channel opening and fast inactivation. Using whole cell voltage clamp, we found that BK channel mediated outward currents and afterhyperpolarization currents (IAHP). Using whole cell current clamp, we found that application of BK channel blocker iberiotoxin (IBTX) increased spike half-width and suppressed fast afterhyperpolarization (fAHP) amplitude following single APs. In addition, IBTX application increased spike half-width and reduced the spike frequency-dependent AP broadening in trains and spike frequency adaption (SFA). Furthermore, BK channel blockade decreased spike frequency. Collectively, these results demonstrate that PCMNs have BK channels that significantly regulate AP repolarization, fAHP, SFA, and spike frequency. We conclude that activation of BK channels underlies one of the mechanisms for facilitation of PCMN excitability.

Methamphetamine Regulation of Firing Activity of Dopamine Neurons

“…BK channels are involved in AP repolarization in several neuronal types (Adams et al, 1982;Lancaster and Nicoll, 1987;Storm, 1990;Lin et al, 2014). Consistent with these reports, we found BK channels are expressed in dopamine neurons (Fig.…”

“…In addition, METH exposure broadened the spike halfwidth measured at the half-maximal voltage (Fig. 1 D METH and paxilline blockade of BK channels regulate the repolarization and AHP of APs Activation of BK channels has been shown to contribute to AP repolarization and AHP (Adams et al, 1982;Lancaster and Nicoll, 1987;Faber and Sah, 2002;Lin et al, 2014). Thus, to understand whether METH regulation of intrinsic firing behavior of dopamine neurons involves BK channel activity, we examined the influence of METH and a BK channel blocker paxilline on single spike amplitudes, repolarization, and AHP in spontaneously active dopamine neurons (Fig.…”

“…Our laboratory and others have shown that acutely applied amphetamines (1-3 min) increases the firing activity of dopamine neurons (Ingram et al, 2002;Saha et al, 2014). Dopamine transporter activity and different ionic currents involved in action potentials (APs), such as L-type calcium current (Shibasaki et al, 2010), outward potassium current (Lin et al, 2010), and inwardly rectifying potassium current (Sharpe et al, 2015), are thought to be directly or indirectly associated with METH regulation of electrical properties of neurons, albeit with a mechanism that is less understood in dopamine neurons.…”

“…Large-conductance Ca 2ϩ -activated potassium channels (KCa1.1, BK, MaxiK) play important roles in controlling neuronal excitability (Shao et al, 1999;Faber and Sah, 2002;Lin et al, 2014). BK channels have been shown to mediate a link between intracellular signaling pathways and electrical signals in the neurons through a feedback regulation countering increases in intracellular Ca 2ϩ ([Ca 2ϩ ] i ) and membrane depolarization (Vergara et al, 1998;Latorre and Brauchi, 2006;Salkoff et al, 2006).…”

“…BK channels shape neuronal activity in a number of ways, including AP termination (Lovell and McCobb, 2001), modifying firing frequency and spike frequency adaptation , and terminating bursting or bistable firing (Kiehn and Harris-Warrick, 1992;Lara et al, 1999). While BK channels are normally regarded as inhibitory (i.e., able to reduce neuronal excitability), numerous studies support the idea that suppression of BK-channel activity reduces neuronal excitability (Brenner et al, 2005;Gu et al, 2007;Lin et al, 2014). For example, activation of BK channels has been shown to enhance the excitability of adrenal chromaffin cells (Solaro et al, 1995;Lovell and McCobb, 2001), CA1 hippocampal pyramidal cells (Gu et al, 2007), and parasympathetic cardiac motoneurons in the nucleus ambiguus .…”