KCNQ channels mediate I_Ks, a slow K⁺ current regulating excitability in the rat node of Ranvier

Abstract

Mutations that reduce the function of KCNQ2 channels cause neuronal hyperexcitability, manifested as epileptic seizures and myokymia. These channels are present in nodes of Ranvier in rat brain and nerve and have been proposed to mediate the slow nodal potassium current I(Ks). We have used immunocytochemistry, electrophysiology and pharmacology to test this hypothesis and to determine the contribution of KCNQ channels to nerve excitability in the rat. When myelinated nerve fibres of the sciatic nerve were examined by immunofluorescence microscopy using antibodies against KCNQ2 and KCNQ3, all nodes showed strong immunoreactivity for KCNQ2. The nodes of about half the small and intermediate sized fibres showed labelling for both KCNQ2 and KCNQ3, but nodes of large fibres were labelled by KCNQ2 antibodies only. In voltage-clamp experiments using large myelinated fibres, the selective KCNQ channel blockers XE991 (IC50 = 2.2 microm) and linopirdine (IC50 = 5.5 microm) completely inhibited I(Ks), as did TEA (IC50 = 0.22 mm). The KCNQ channel opener retigabine (10 microm) shifted the activation curve to more negative membrane potentials by -24 mV, thereby increasing I(Ks). In isotonic KCl 50% of I(Ks) was activated at -62 mV. The activation curve shifted to more positive potentials as [K+]o was reduced, so that the pharmacological and biophysical properties of I(Ks) were consistent with those of heterologously expressed homomeric KCNQ2 channels. The ability of XE991 to selectively block I(Ks) was further exploited to study I(Ks) function in vivo. In anaesthetized rats, the excitability of tail motor axons was indicated by the stimulus current required to elicit a 40% of maximal compound muscle action potential. XE991 (2.5 mg kg(-1) i.p.) eliminated all nerve excitability functions previously attributed to I(Ks): accommodation to 100 ms subthreshold depolarizing currents, the post-depolarization undershoot in excitability, and the late subexcitability after a single impulse or short trains of impulses. Due to reduced spike-frequency adaptation after XE991 treatment, 100 ms suprathreshold current injections generated long trains of action potentials. We conclude that the nodal I(Ks) current is mediated by KCNQ channels, which in large fibres of rat sciatic nerve appear to be KCNQ2 homomers.

Influence of frequency and temperature on the mechanisms of nerve conduction block induced by high-frequency biphasic electrical current

“…In the amphibian (frog) myelinated axon (Frankenhaeuser and Huxley, 1964), there is large fast potassium current at the node of Ranvier. But in the mammalian myelinated axon the nodal potassium current is very small and mainly consists of a slow potassium current (Roper and Schwarz, 1989;Schwarz et al, 2006). The SE model (Rattay and Aberham, 1993;Schwarz and Eikhof, 1987) used in this study only incorporated a small fast potassium current.…”

“…Furthermore, our study using the CRRSS model (Zhang et al, 2006b), which was derived from mammalian (rabbit) myelinated axon and did not incorporate any potassium current (Chiu et al, 1979), completely failed to simulate the nerve conduction block, further indicating a role of the potassium current. Although the small fast potassium current at the node of Ranvier of the mammalian myelinated nerve may be of importance in nerve conduction block, investigating the role of the slow potassium current is definitively warranted due to its dominant presence at the node of Ranvier (Roper and Schwarz, 1989;Schwarz et al, 2006). …”

Molecular Targets for Antiepileptic Drug Development

“…KCNQ2 is also responsible for a slow K ϩ current that regulates excitability in neurons and axons. 122 Studies with expressed mutated channel subunits show that BFNC mutations result in a loss of K ϩ currents, making BFNC a haploinsufficiency syndrome. Although the neonatal seizures in BFNC resolve by 3 months of age, BFNC is associated with an enhanced incidence (up to 16%) of various forms of epilepsy later in life, so that KCNQ2 and KCNQ3 can be considered epilepsy susceptibility genes.…”

Propofol decreases the axonal excitability in rat primary sensory afferents

“…In our experiments, late subexcitability decreased only at higher concentrations of propofol. Since previous studies showed that late subexcitability can be specifically suppressed by blocking slow potassium currents (Schwarz et al, 2006) we may assume that propofol also affects these slow potassium currents.…”

“…Underlying mechanisms are complex and involve mainly sodium conductances at short interstimulus intervals (Hodgkin and Huxley, 1952), and capacitive charging of the internode (Barrett and Barrett, 1982), periaxonal accumulation of potassium (Kocsis et al, 1983) and slow potassium conductances at longer interstimulus intervals (e.g. late subexcitability) (Stys and Waxman, 1994;Schwarz et al, 2006).…”