State-dependent hyperpolarization of voltage threshold enhances motoneurone excitability during fictive locomotion in the cat

1. Experiments were conducted on decerebrate adult cats to examine the effe ct of brainstem-evoked fictive locomotion on the threshold voltage (V-th) a t which action potentials were initiated in hindlimb motoneurones. Measurem ents of the voltage threshold of the first spike evoked by intracellular in jection of depolarizing ramp currents or square pulses were compared during control and fictive locomotor conditions. The sample of motoneurones inclu ded flexor and extensor motoneurones, and motoneurons with low and high rhe obase currents. 2. In all 38 motoneurones examined, action potentials were initiated at mor e hyperpolarized membrane potentials during fictive locomotion than in cont rol conditions (mean hyperpolarization -8.0 +/- 5.5 mV; range -1.8 to -26.6 mV). Hyperpolarization of V-th occurred immediately at the onset of fictiv e locomotion and recovered in seconds (typically < 60 s) following the term ination of locomotor activity. 3. The V-th of spikes occurring spontaneously without intracellular current injection was also reduced during locomotion. 4. Superimposition of rhythmic depolarizing current pulses on current ramps in the absence of locomotion did not lower V-th to the extent seen during fictive locomotion. We suggest that V-th hyperpolarization results from an as yet undetermined neuromodulatory process operating during locomotion and is not simply the result of the oscillations in membrane potential occurri ng during locomotion. The hyperpolarization of V-th, for action potential i nitiation during locomotion is a state-dependent increase in motoneurone ex citability. This V-th hyperpolarization may be a Fundamental process in til e generation of motoneurone activity during locomotion and perhaps other mo tor tasks.