An M-like outward current regulates the excitability of spinal motoneurones in the adult turtle.

The excitatory action of muscarine on spinal motoneurones was investigated with intracellular recordings in a slice preparation from adult turtles. In these cells muscarine is known to facilitate a persistent inward current mediated by L-type Ca(2+) channels. When this effect was blocked by nifedipine, muscarine still increased the excitability. In voltage clamp, a slowly activating outward current, generated during depolarizing voltage commands and deactivating as a tail current on return to the holding voltage, was reduced by muscarine. This outward current was activated when the voltage was stepped to potentials positive to -60 mV, was voltage sensitive and had a deactivation time constant of approximately 80 ms. These findings are compatible with an M-current. This possibility was also supported by the finding that the current was reduced by XE-991 - a selective blocker of the KCNQ potassium channels underlying M-currents in other cell types. Our findings suggest that an M-like current, mediated by a KCNQ channel, contributes to the intrinsic response properties of motoneurones in the adult spinal cord by increasing adaptation of repetitive firing and decreasing the slope of the frequency-current relation.