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

Abstract

Experiments were conducted on decerebrate adult cats to examine the effect of brainstem-evoked fictive locomotion on the threshold voltage (Vth) at which action potentials were initiated in hindlimb motoneurones. Measurements of the voltage threshold of the first spike evoked by intracellular injection of depolarizing ramp currents or square pulses were compared during control and fictive locomotor conditions. The sample of motoneurones included flexor and extensor motoneurones, and motoneurones with low and high rheobase currents. In all 38 motoneurones examined, action potentials were initiated at more hyperpolarized membrane potentials during fictive locomotion than in control conditions (mean hyperpolarization -8.0 +/- 5.5 mV; range -1.8 to -26.6 mV). Hyperpolarization of Vth occurred immediately at the onset of fictive locomotion and recovered in seconds (typically < 60 s) following the termination of locomotor activity. The Vth of spikes occurring spontaneously without intracellular current injection was also reduced during locomotion. Superimposition of rhythmic depolarizing current pulses on current ramps in the absence of locomotion did not lower Vth to the extent seen during fictive locomotion. We suggest that Vth hyperpolarization results from an as yet undetermined neuromodulatory process operating during locomotion and is not simply the result of the oscillations in membrane potential occurring during locomotion.The hyperpolarization of Vth for action potential initiation during locomotion is a state-dependent increase in motoneurone excitability. This Vth hyperpolarization may be a fundamental process in the generation of motoneurone activity during locomotion and perhaps other motor tasks.

Figure 1. Firing was elicited from antidromically identified lumbar motoneurones by intracellular current injection, prior to and during MLR-evoked fictive locomotion

A shows a trial for a SmAB motoneurone, where a 50 nA ramp of current was injected, after which the brainstem stimulation was started (indicated by the bar under the ENG trace). Fictive locomotion was evident as rhythmic activity that alternated between extensor and flexor ENGs (not illustrated). Discontinuous current clamp recording allowed accurate measurement of the membrane potential during simultaneous current injection. Bars labelled B and C denote the time periods expanded in panels B and C. B shows that the voltage threshold for production of action potentials (V_th) before fictive locomotion was -46.5 mV. C shows that during fictive locomotion, less current was required to fire the neurone (compare current at +) and the V_th was hyperpolarized compared to B. Note that the neurone fired at 29 Hz before locomotion, and at 59 Hz at the same membrane potential during locomotion (see bracketed areas). The Y-axes in B also apply to C, and the time bar shown below C also applies to B. V_m, membrane potential; I_m, membrane current.

Figure 5. Membrane potential oscillations produced by current pulses can hyperpolarize V_th, but not to the same degree as seen during fictive locomotion

A1 shows a recording from an extensor motoneurone during a current ramp in the absence of fictive locomotion. The V_th in this control condition was -48.2 mV. A2 shows that 10 nA, 300 ms current pulses produced approximately 10 mV depolarizations of the membrane potential that were subthreshold for spiking, until they were superimposed on a current ramp. Fictive locomotion was then elicited with MLR stimulation. This neurone required the injection of hyperpolarizing current to be kept from firing spontaneously. As the current ramp was increased, the neurone began firing on the depolarizing portion of the LDPs. The first action potential of the repetitive firing for A1-3 is shown in B1-3 on expanded time and voltage scales to better illustrate the measured V_th. This motoneurone had a control rheobase of 9 nA and a resting V_m of -69.2 mV.

Figure 2. The V_th recovers after the cessation of fictive locomotion

This SmAB motoneurone had a V_th of -32.7 mV prior to locomotion (A1) and -42.2 mV during MLR-evoked fictive locomotion (A2; see ENG activity). As in Fig. 1, the current required to elicit firing was reduced during fictive locomotion (compare + in A1 and 2) and the neurone fired at a higher rate (32 Hz) during fictive locomotion than at the same membrane potential in the control condition (20 Hz). Within 60 s following the cessation of locomotion the V_th had depolarized back to -31.8 mV (A3). The time scale shown in A2 applies to all traces of A1-3. B1-3 shows the first action potential of the corresponding firing shown in A1-3 on expanded scales to better illustrate the V_th value (the point where the V_m dV/dt≤ 10 V s⁻¹). The scale bar in B1 also applies to B2 and 3.

Figure 3. There is no relationship between the amount of V_th hyperpolarization during fictive locomotion and the rheobase of the motoneurone

Figure 4. During fictive locomotion the V_th is hyperpolarized during both phases of the fictive step cycle

In this extensor motoneurone, current pulses were delivered at approximately 1 Hz during fictive locomotion (lower traces show MLR-evoked ENG activity). One current pulse occurring during the inactive phase of the fictive step cycle elicited an action potential (marked by the arrow) with a V_th of -53.0 mV. This was the same as the V_th seen during the active phase, and was -3.0 mV hyperpolarized compared to control.

Figure 6. Hyperpolarization of V_th during fictive locomotion does not depend on the amplitude of the LDPs in the motoneurone

A shows firing evoked by the injection of current pulses (15 nA, 200 ms) into this MG motoneurone in the absence of fictive locomotion. The V_th for this control condition was -31.0 mV and is denoted by the dotted line. B shows a recording from this same motoneurone after MLR stimulation had been initiated (Locomotion - 1). The cell exhibited spontaneous firing linked to the fictive step cycle, but had only small LDPs. The V_th for this fictive locomotion-induced firing (no current injection) was -37.1 mV. C shows the same neurone approximately 3 min later when the fictive locomotion had become more robust (Locomotion - 2). During this period, the cell exhibited approximately 8 mV LDPs and locomotor-related firing that had a V_th of -37.4 mV.