Elevated Expression and Activity of Sodium Leak Channel Contributes to Neuronal Sensitization of Inflammatory Pain in Rats.

Inflammatory pain encompasses many clinical symptoms, and there is no satisfactory therapeutic target. Neuronal hyperexcitability and/or sensitization of the primary nociceptive neurons in the dorsal root ganglion (DRG) and spinal dorsal horn are critical to the development and maintenance of inflammatory pain. The sodium leak channel (NALCN), a non-selective cation channel, mediates the background Na+ leak conductance and controls neuronal excitability. It is unknown whether abnormal activity of NALCN mediates the pathological process of inflammatory pain. Complete Freund’s adjuvant (CFA) was injected into the left footpad of rats to induce inflammatory pain. The thresholds of mechanical and thermal sensation and spontaneous pain behaviors were assessed. The expression of NALCN in DRG and spinal dorsal cord was measured. NALCN currents and the contribution of NALCN to neuronal excitability in the DRG and spinal dorsal cord were recorded using whole-cell patch-clamping recording. NALCN was abundantly expressed in neurons of the DRG and spinal dorsal cord. In acutely isolated DRG neurons and spinal cord slices from rats with CFA-induced inflammatory pain, NALCN currents and neuronal excitability were increased. Subsequently, intrathecal and sciatic nerve injection of NALCN-small interfering RNA (siRNA) decreased NALCN mRNA and reverted NALCN currents to normal levels, and then reduced CFA-induced neuronal excitability and alleviated pain symptoms. Furthermore, pain-related symptoms were significantly prevented by the NALCN-shRNA-mediated NALCN knockdown in DRG and spinal cord. Therefore, increased expression and activity of NALCN contributed to neuronal sensitization in CFA-induced inflammatory pain. NALCN may be a novel molecular target for the control of inflammatory pain.

炎性疼痛包含多种临床症状，且目前尚无令人满意的治疗靶点。背根神经节（DRG）和脊髓背角中初级伤害性感受神经元的过度兴奋和/或敏化，对于炎性疼痛的发生和维持至关重要。钠泄漏通道（NALCN）是一种非选择性阳离子通道，介导背景钠泄漏电导并控制神经元兴奋性。目前尚不清楚NALCN的异常活动是否介导炎性疼痛的病理过程。将完全弗氏佐剂（CFA）注射到大鼠左后足垫以诱导炎性疼痛。评估机械和热感觉阈值以及自发疼痛行为。检测DRG和脊髓背侧NALCN的表达。采用全细胞膜片钳记录法记录DRG和脊髓背侧的NALCN电流以及NALCN对神经元兴奋性的影响。NALCN在DRG和脊髓背侧的神经元中大量表达。在急性分离的DRG神经元以及CFA诱导炎性疼痛大鼠的脊髓切片中，NALCN电流和神经元兴奋性均升高。随后，鞘内和坐骨神经注射NALCN - 小干扰RNA（siRNA）可降低NALCN信使核糖核酸（mRNA）水平，使NALCN电流恢复至正常水平，进而降低CFA诱导的神经元兴奋性并缓解疼痛症状。此外，通过在DRG和脊髓中利用NALCN - 短发夹RNA（shRNA）介导的NALCN基因敲低，可显著预防疼痛相关症状。因此，NALCN表达和活性的增加导致了CFA诱导的炎性疼痛中神经元的敏化。NALCN可能是控制炎性疼痛的一个新分子靶点。