Repeated anodal trans‐spinal direct current stimulation results in long‐term reduction of spasticity in mice with spinal cord injury

Key points

Spasticity is a disorder of muscle tone that is associated with lesions of the motor system. This condition involves an overactive spinal reflex loop that resists passive lengthening of muscles. Previously, we established that application of anodal trans‐spinal direct current stimulation (tsDCS) for short periods of time to anaesthetized mice sustaining a spinal cord injury leads to an instantaneous reduction of spasticity. However, the long‐term effects of repeated a‐tsDCS and its mechanism of action remained unknown. Here, a‐tsDCS was performed for 7 days and this was found to cause long‐term reduction in spasticity, increase rate‐dependent depression in spinal reflexes, and improve ground and skill locomotion. Pharmacological, molecular, and cellular evidence further suggest that a novel mechanism involving Na‐K‐Cl cotransporter isoform 1 (NKCC1) mediates the observed long‐term effects of repeated a‐tsDCS.

Abstract

Spasticity can cause pain and fatigue, sleep disturbances, it can restrict daily activities such as walking, sitting, and bathing, and it can complicate rehabilitation efforts. Thus, spasticity negatively influences an individual's quality of life and novel therapeutic interventions are needed. We previously demonstrated in anaesthetized mice that a short period of trans‐spinal subthreshold direct current stimulation (tsDCS) reduces spasticity. In the present study, the long‐term effects of repeated tsDCS to attenuate abnormal muscle tone in awake female mice with spinal cord injuries were investigated. A motorized system was used to test velocity‐dependent ankle resistance and associated electromyographical activity. Analysis of ground and skill locomotion was also performed, while electrophysiological, molecular, and cellular studies were conducted to reveal a potential underlying mechanism of action. A four‐week reduction in spasticity was associated with an increase in rate‐dependent depression of spinal reflexes and ground and skill locomotion were improved following 7 d of a‐tsDCS. Secondary molecular, cellular, and pharmacological experiments further demonstrated that the expression of K‐Cl co‐transporter isoform 2 (KCC2) was not changed in animals with spasticity. However, Na‐K‐Cl cotransporter isoform 1 (NKCC1) was significantly up‐regulated in mice that exhibited spasticity. When mice were treated with a‐tsDCS, down regulation of NKCC1 was detected, and this level did not significantly differ from that in the non‐injured control mice. Thus, long lasting reduction of spasticity by a‐tsDCS via down‐regulation NKCC1 may constitute a novel therapy for spasticity following spinal cord injury.

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