Fukuoka Tetsuo, Tokunaga Atsushi, Kondo Eiji, Miki Kenji, Tachibana Toshiya, Noguchi Koichi
Department of Anatomy and Neuroscience, Hyogo College of Medicine, 1-1 Mukogawa-cho, Nishinomiya, Hyogo 663-8501, Japan Department of Anesthesiology, Osaka University Medical School, 2-2 Yamadaoka, Suita, Osaka 565-0871, Japan.
Pain. 1998 Oct;78(1):13-26. doi: 10.1016/S0304-3959(98)00111-0.
We examined two possible mechanisms of neuropathic pain: contribution of adjacent intact nerves and decrease in presynaptic inhibition at the central terminal of the injured primary afferent. To this end, we examined the effects of unilateral L5 spinal nerve ligation, which causes mechanical allodynia and heat hyperalgesia in the ipsilateral hind paw, on gene expression in L4 and L5 dorsal root ganglion (DRG) neurons using in situ hybridization (ISH). Specifically, we examined changes in the expression of messenger RNAs (mRNAs) for neuropeptides which have been reported to be up- or down-regulated in the axotomized DRG neurons and for gamma-aminobutyric acid (GABA)A receptor (GABA(A)-R) subunits which contribute to presynaptic inhibition at the primary afferent terminals. Seven days following ligation, ISH demonstrated an increase in signal intensity for calcitonin gene-related peptide (CGRP) mRNA in the subpopulation of small-to medium-sized L4 DRG neurons ipsilateral to the ligation which were not directly injured as compared to the contralateral side, although the overall percentages and the size distribution of positively labelled neurons for CGRP mRNA were not different between the bilateral L4 DRGs. This suggests that the L4 DRG neurons which express CGRP mRNA constitutively up-regulated the gene expression and the functional importance of these neurons has increased following L5 spinal nerve ligation. However, the mRNAs for other neuropeptides such as preprotachykinin (PPT), vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), and galanin (GAL), were not different between the bilateral L4 DRGs. The mRNA for the GABA(A)-Rgamma2 subunit was significantly down-regulated in the medium- to large-sized L5 DRG neurons ipsilateral to the ligation as compared to the contralateral side. GABA(A)-Ralpha2 subunit mRNA also decreased in the ipsilateral L5 DRG neurons but did not reach statistical significance. There was no difference in mRNAs between the bilateral L4 DRGs. These data suggest that the presynaptic disinhibition of the ipsilateral L5 primary afferent terminals may be explained at least partly by the down-regulation of GABA(A)-R following L5 spinal nerve ligation. Thus, both the up-regulation of CGRP in adjacent intact nerves and the decrease in presynaptic inhibition at the central terminal of the injured primary afferent could cause the hyper-excitability of dorsal horn neurons and contribute to the molecular mechanisms of this neuropathic pain model.
相邻完整神经的作用以及受损初级传入神经中枢终末的突触前抑制减弱。为此，我们使用原位杂交（ISH）技术，研究了单侧L5脊神经结扎对L4和L5背根神经节（DRG）神经元基因表达的影响，该结扎会导致同侧后爪出现机械性异常性疼痛和热痛觉过敏。具体而言，我们检测了神经肽信使核糖核酸（mRNA）表达的变化，这些神经肽据报道在轴突切断的DRG神经元中表达上调或下调，以及γ-氨基丁酸（GABA）A受体（GABA(A)-R）亚基的表达变化，这些亚基有助于初级传入神经终末的突触前抑制。结扎后7天，ISH显示，与对侧相比，结扎同侧未直接受损的中小尺寸L4 DRG神经元亚群中，降钙素基因相关肽（CGRP）mRNA的信号强度增加，尽管双侧L4 DRG中CGRP mRNA阳性标记神经元的总体百分比和大小分布没有差异。这表明，组成性表达CGRP mRNA的L4 DRG神经元上调了基因表达，并且在L5脊神经结扎后这些神经元的功能重要性增加。然而，双侧L4 DRG中其他神经肽的mRNA，如前速激肽原（PPT）、血管活性肠肽（VIP）、神经肽Y（NPY）和甘丙肽（GAL），没有差异。与对侧相比，结扎同侧中大型L5 DRG神经元中GABA(A)-Rγ2亚基的mRNA显著下调。同侧L5 DRG神经元中GABA(A)-Rα2亚基的mRNA也有所下降，但未达到统计学意义。双侧L4 DRG的mRNA没有差异。这些数据表明，同侧L5初级传入神经终末的突触前去抑制至少部分可以用L5脊神经结扎后GABA(A)-R的下调来解释。因此，相邻完整神经中CGRP的上调和受损初级传入神经中枢终末的突触前抑制减弱，都可能导致背角神经元的过度兴奋，并促成这种神经性疼痛模型的分子机制。
