Kv7/KCNQ/M-channels in rat glutamatergic hippocampal axons and their role in regulation of excitability and transmitter release

M-current (I-M) plays a key role in regulating neuronal excitability. Mutations in Kv7/KCNQ subunits, the molecular correlates of I-M, are associated with a familial human epilepsy syndrome. Kv7/KCNQ subunits are widely expressed, and I-M has been recorded in somata of several types of neurons, but the subcellular distribution of M-channels remains elusive. By combining field-potential, whole-cell and intracellular recordings from area CA1 in rat hippocampal slices, and computational modelling, we provide evidence for functional M-channels in unmyelinated axons in the brain. Our data indicate that presynaptic M-channels can regulate axonal excitability and synaptic transmission, provided the axons are depolarized into the I-M activation range (beyond similar to - 65 mV). Here, such depolarization was achieved by increasing the extracellular K+ concentration ([K+](0)). Extracellular recordings in the presence of moderately elevated [K+](0) (7-11 mM), showed that the specific M-channel blocker XE991 reduced the amplitude of the presynaptic fibre volley and the field EPSP in a [K+](0)-dependent manner, both in stratum radiatum and in stratum lacknosum moleculare. The M-channel opener, retigabine, had opposite effects. The higher the [K+](0), the greater the effects of XE991 and retigabine. Similar pharmacological modulation of EPSPs; recorded intracellularly from CA1 pyramidal neurons, while blocking postsynaptic K+ channels with intracellular Cs+, confirmed that active M-channels are located presynaptically. Computational analysis with an axon model showed that presynaptic I-M can control Na+ channel inactivation and thereby affect the presynaptic action potential amplitude and Ca2+ influx, provided the axonal membrane potential is sufficiently depolarized. Finally, we compared the effects of blocking I-M on the spike after-depolarization and bursting in CA3 pyramidal neuron somata versus their axons. In standard [K+](0) (2.5 mM), XE991 increased the ADP and promoted burst firing at the soma, but not in the axons. However, I-M contributed to the refractory period in the axons when spikes were broadened by a low dose 4-aminopyridine (200 mu M). Our results indicate that functional Kv7/KCNQ/M-channels are present in unmyelinated axons in the brain, and that these channels may have contrasting effects on excitability depending on their subcellular localization.