Does trans-spinal and local DC polarization affect presynaptic inhibition and post-activation depression?

KEY POINTS: Trans-spinal polarization was recently introduced as a means to improve deficient spinal functions. However, only a few attempts have been made to examine the mechanisms underlying DC actions. We have now examined the effects of DC on two spinal modulatory systems, presynaptic inhibition and post-activation depression, considering whether they might weaken exaggerated spinal reflexes and enhance excessively weakened ones. Direct current effects were evoked by using local intraspinal DC application (0.3-0.4 μA) in deeply anaesthetized rats and were compared with the effects of trans-spinal polarization (0.8-1.0 mA). Effects of local intraspinal DC were found to be polarity dependent, as locally applied cathodal polarization enhanced presynaptic inhibition and post-activation depression, whereas anodal polarization weakened them. In contrast, both cathodal and anodal trans-spinal polarization facilitated them. The results suggest some common DC-sensitive mechanisms of presynaptic inhibition and post-activation depression, because both were facilitated or depressed by DC in parallel. ABSTRACT: Direct current (DC) polarization has been demonstrated to alleviate the effects of various deficits in the operation of the central nervous system. However, the effects of trans-spinal DC stimulation (tsDCS) have been investigated less extensively than the effects of transcranial DC stimulation, and their cellular mechanisms have not been elucidated. The main objectives of this study were, therefore, to extend our previous analysis of DC effects on the excitability of primary afferents and synaptic transmission by examining the effects of DC on two spinal modulatory feedback systems, presynaptic inhibition and post-activation depression, in an anaesthetized rat preparation. Other objectives were to compare the effects of locally and trans-spinally applied DC (locDC and tsDCS). Local polarization at the sites of terminal branching of afferent fibres was found to induce polarity-dependent actions on presynaptic inhibition and post-activation depression, as cathodal locDC enhanced them and anodal locDC depressed them. In contrast, tsDCS modulated presynaptic inhibition and post-activation depression in a polarity-independent fashion because both cathodal and anodal tsDCS facilitated them. The results show that the local presynaptic actions of DC might counteract both excessively strong and excessively weak monosynaptic actions of group Ia and cutaneous afferents. However, they indicate that trans-spinally applied DC might counteract the exaggerated spinal reflexes but have an adverse effect on pathologically weakened spinal activity by additional presynaptic weakening. The results are also relevant for the analysis of the basic properties of presynaptic inhibition and post-activation depression because they indicate that some common DC-sensitive mechanisms contribute to them.