Melatonin (Mel) and its receptors (MT1 and MT2) have a well-documented efficacy in treating different pain conditions. However, the anti-nociceptive effects of Mel and Mel receptors in neuropathic pain (NP) are poorly understood. To elucidate this process, pain behaviors were measured in a dorsal root ganglia (DRG)-friendly sciatic nerve cuffing model. We detected up-regulation of MT2 expression in the DRGs of cuff-implanted mice and its activation by the agonist 8-M-PDOT (8MP). Also, Mel attenuated the mechanical and thermal allodynia induced by cuff implantation. Immunohistochemical analysis demonstrated the expression of MT2 in the DRG neurons, while MT1 was expressed in the satellite cells. In cultured primary neurons, microarray analysis and gene knockdown experiments demonstrated that MT2 activation by 8MP or Mel suppressed calcium signaling pathways via MAPK1, which were blocked by RAR-related orphan receptor alpha (RORα) activation with a high dose of Mel. Furthermore, expression of nitric oxide synthase 1 (NOS1) was down-regulated upon Mel treatment regardless of MT2 or RORα. Application of Mel or 8MP in cuff-implanted models inhibited the activation of peptidergic neurons and neuro-inflammation in the DRGs by down-regulating c-fos, calcitonin gene-related peptide [CGRP], and tumor necrosis factor-1α [TNF-1α] and interleukin-1β [IL-1β]. Addition of the MT2 antagonist luzindole blocked the effects of 8MP but not those of Mel. In conclusion, only MT2 was expressed in the DRG neurons and up-regulated upon cuff implantation. The analgesic effects of Mel in cuff-implanted mice were closely associated with both MT2-dependent (MAPK-calcium channels) and MT2-independent (NOS1) pathways in the DRG.

中文翻译：

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褪黑素通过小鼠背根神经节神经元的MT2依赖性和非依赖性途径抑制神经性疼痛。

褪黑激素（Mel）及其受体（MT1和MT2）在治疗不同的疼痛状况方面具有公认的功效。但是，人们对Mel和Mel受体在神经性疼痛（NP）中的抗伤害感受作用了解甚少。为了阐明此过程，在友好的背根神经节（DRG）坐骨神经袖带模型中测量了疼痛行为。我们检测到袖带植入小鼠的DRG中MT2表达的上调及其被激动剂8-M-PDOT（8MP）激活。而且，梅尔减弱了袖套植入引起的机械和热异常性疼痛。免疫组织化学分析表明，MT2在DRG神经元中表达，而MT1在卫星细胞中表达。在培养的原代神经元中 微阵列分析和基因敲低实验表明，通过8MP或Mel激活MT2可以抑制MAPK1的钙信号通路，而高剂量的Mel可以通过RAR相关的孤儿受体α（RORα）激活来阻断钙信号通路。此外，无论MT2或RORα，Mel处理后一氧化氮合酶1（NOS1）的表达均下调。在袖套植入模型中应用Mel或8MP可通过下调抑制DRG中的肽能神经元激活和神经炎症c-fos，降钙素基因相关肽[CGRP]和肿瘤坏死因子1α[TNF-1α]和白介素1β[IL-1β]。MT2拮抗剂luzindole的加入阻止了8MP的作用，但没有阻止Mel的作用。总之，仅MT2在DRG神经元中表达，并在套囊植入后上调。Mel对袖带植入小鼠的镇痛作用与DRG中依赖MT2的（MAPK-钙通道）和依赖MT2的（NOS1）通路密切相关。