Conduction properties of the M-channel in rat sympathetic neurons.

We have investigated the conduction properties of the M-channel in rat superior cervical ganglion neurons. Reversal potentials measured under bi-ionic conditions yielded a permeation sequence of Tl > K > Rb > Cs > NH4 > Na. Slope conductances gave a conductance sequence of K > Tl > NH4 > Rb > Cs. M-current was shown to exhibit a number of features atypical of potassium channels. First, the conduction of monovalent cations relative to K was very low. Second, the nature of the permeant ion did not affect the deactivation kinetics. Third, M-current did not exhibit anomalous mole-fraction behavior, a property suggestive of a multi-ion pore. Finally, external Ba, which is a blocker of M-current, showed a preferential block of outward current and had much less effect on inward current. The permeability sequence of the M-channel is very similar to other K-selective channels, implying a high degree of conservation in the selectivity filter. However, other conduction properties suggest that the pore structure outside of the selectivity filter is very different from previously cloned potassium channels.