M current

M current is a type of noninactivating potassium current first discovered in bullfrog sympathetic ganglion cells ^[1]

The M-channel is a voltage-gated K+ channel that is named after the receptor it is influenced by. The M-channel is important in raising the threshold for firing an action potential. It is unique because it is open at rest and even more likely to be open during depolarization. Furthermore, when the muscarinic acetylcholine receptor (MAChR) is activated, the channel closes. The M-channel is a PIP₂-regulated ion channel.^[2]

Actions of M-currents: "phasic-firing"

M-channels are the reason for slow depolarizations produced by ACh and LHRH in the autonomic ganglia and other specified areas. 1. Initial depolarization of a neuron increases likelihood that M-channels will open. 2. M-channels generate an outward potassium current (iK). 3. Potassium efflux counteracts sodium influx in action potential (AP). Overall result: full action potential is prevented. ^{[2] :343}

Inhibition of M-current: "tonic-firing"

1 molecule of Acetylcholine (Ach) binds to mAchR. Potassium (K+) channels become more likely to be closed. Neuron becomes tonic-firing. ^{[2] :343} This reduction in M-current is coupled with the actions of the G_q G-protein. Specifically, the hydrolysis of PIP₂ to IP₃. This hydrolysis causes PIP₂, which is bound to the membrane, to become IP₃ and dissociate from the membrane into the cytoplasm. When M-current is restored, it moves back to the membrane. There is some evidence for different theories of how M-channel activity is directly affected by PIP₂.^{[2] :229}

References

1. BROWN, D. A. & ADAMS, P. R. (1980). Muscarinic suppression of a novel voltage-sensitive K+-current in a vertebrate neurone. Nature, Lond. 283, 673-676.
2. Nicholls, John G.; Martin, A. Robert; Fuchs, Paul A.; Brown, David A.; Diamond, Mathew E.; Weisblat, David A. (2012). From Neuron to Brain (Fifth ed.). pp. 229, 342.