Motoneuron firing properties are modified by trans-spinal direct current stimulation in rats

Spinal polarization evoked by direct current stimulation [trans-spinal direct current stimulation (tsDCS)] is a novel method for altering spinal network excitability; however, it remains not well understood. The aim of this study was to determine whether tsDCS influences spinal motoneuron activity. Twenty Wistar rats under general pentobarbital anesthesia were subjected to 15 min anodal (n = 10) or cathodal (n = 10) tsDCS of 0.1 mA intensity, and the electrophysiological properties of their motoneurons were intracellularly measured before, during, and after direct current application. The major effects of anodal intervention included increased minimum firing frequency and the slope of the frequency-current (f-I) relationship, as well as decreased rheobase and currents evoking steady-state firing (SSF). The effects of cathodal polarization included decreased maximum SSF frequency, decreased f-I slope, and decreased current evoking the maximum SSF. Notably, the majority of observed effects appeared immediately after the current onset, developed during polarization, and outlasted it for at least 15 min. Moreover, the effects of anodal polarization were generally more pronounced and uniform than those evoked by cathodal polarization. Our study is the first to present polarity-dependent, long-lasting changes in spinal motoneuron firing following tsDCS, which may aid in the development of more safe and accurate application protocols in medicine and sport. NEW & NOTEWORTHY Trans-spinal direct current stimulation induces significant polarity-dependent, long-lasting changes in the threshold and firing properties of spinal motoneurons. Anodal polarization potentiates motoneuron firing whereas cathodal polarization acts mainly toward firing inhibition. The alterations in rheobase and rhythmic firing properties are not restricted to the period of current application and can be observed long after the current offset.