Bolzoni F, Jankowska E
Department of Neuroscience and Physiology, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden; Human Physiology Section of the DEPT, Università degli Studi di Milano, Milano, Italy.
J Physiol. 2015 Feb 15;593(4):947-66. doi: 10.1113/jphysiol.2014.285940. Epub 2014 Dec 23.
Trans-spinal DC stimulation affects both postsynaptic neurons and the presynaptic axons providing input to these neurons. In the present study, we show that intraspinally applied cathodal current replicates the effects of trans-spinal direct current stimulation in deeply anaesthetized animals and affects spinal neurons both during the actual current application and during a post-polarization period. Presynaptic effects of local cathodal polarization were expressed in an increase in the excitability of skin afferents (in the dorsal horn) and group Ia afferents (in motor nuclei), both during and at least 30 min after DC application. However, although the postsynaptic facilitation (i.e. more effective) activation of motoneurons by stimuli applied in a motor nucleus was very potent during local DC application, it was only negligible once DC was discontinued. The results suggest that the prolonged effects of cathodal polarization are primarily associated with changes in synaptic transmission.
The present study aimed to compare presynaptic and postsynaptic actions of direct current polarization in the spinal cord, focusing on DC effects on primary afferents and motoneurons. To reduce the directly affected spinal cord region, a weak polarizing direct current (0.1-0.3 μA) was applied locally in deeply anaesthetized cats and rats; within the hindlimb motor nuclei in the caudal lumbar segments, or in the dorsal horn within the terminal projection area of low threshold skin afferents. Changes in the excitability of primary afferents activated by intraspinal stimuli (20-50 μA) were estimated using increases or decreases in compound action potentials recorded from the dorsal roots or peripheral nerves as their measure. Changes in the postsynaptic actions of the afferents were assessed from intracellularly recorded monosynaptic EPSPs in hindlimb motoneurons and monosynaptic extracellular field potentials (evoked by group Ia afferents in motor nuclei, or by low threshold cutaneous afferents in the dorsal horn). The excitability of motoneurons activated by intraspinal stimuli was assessed using intracellular records or motoneuronal discharges recorded from a ventral root or a muscle nerve. Cathodal polarization was found to affect motoneurons and afferents providing input to them to a different extent. The excitability of both was markedly increased during DC application, although post-polarization facilitation was found to involve presynaptic afferents and some of their postsynaptic actions, but only negligibly motoneurons themselves. Taken together, these results indicate that long-lasting post-polarization facilitation of spinal activity induced by locally applied cathodal current primarily reflects the facilitation of synaptic transmission.
经脊髓直流电刺激会影响突触后神经元以及为这些神经元提供输入的突触前轴突。在本研究中，我们发现，在深度麻醉的动物中，脊髓内施加阴极电流可重现经脊髓直流电刺激的效果，并且在实际施加电流期间以及极化后阶段均会影响脊髓神经元。局部阴极极化的突触前效应表现为皮肤传入神经（在背角）和Ia类传入神经（在运动核）的兴奋性增加，这一效应在施加直流电期间以及至少在施加后30分钟内均存在。然而，尽管在局部直流电施加期间，运动核中施加的刺激对运动神经元的突触后易化（即更有效的）激活非常显著，但一旦停止施加直流电，其影响就微不足道了。结果表明，阴极极化的延长效应主要与突触传递的变化有关。
本研究旨在比较脊髓中直流电极化的突触前和突触后作用，重点关注直流电对初级传入神经和运动神经元的影响。为了减少直接受影响的脊髓区域，在深度麻醉的猫和大鼠中局部施加微弱的极化直流电（0.1 - 0.3 μA）；施加部位在尾段腰段的后肢运动核内，或低阈值皮肤传入神经终末投射区域内的背角。通过记录背根或周围神经复合动作电位的增加或减少来评估脊髓内刺激（20 - 50 μA）激活的初级传入神经兴奋性的变化。从后肢运动神经元细胞内记录的单突触兴奋性突触后电位以及运动核中Ia类传入神经或背角中低阈值皮肤传入神经诱发的单突触细胞外场电位来评估传入神经突触后作用的变化。通过细胞内记录或从腹根或肌肉神经记录的运动神经元放电来评估脊髓内刺激激活的运动神经元的兴奋性。发现阴极极化对运动神经元和为其提供输入的传入神经有不同程度的影响。在施加直流电期间，两者的兴奋性均显著增加，尽管极化后易化涉及突触前传入神经及其一些突触后作用，但对运动神经元本身的影响微不足道。综上所述，这些结果表明，局部施加阴极电流引起的脊髓活动的长期极化后易化主要反映了突触传递的易化。
