Secreted microRNAs (miRNAs) have been detected in various body fluids including the cerebrospinal fluid, yet their direct role in regulating synaptic transmission remains uncertain. We found that intrathecal injection of low dose of let-7b (1 μg) induced short-term (<24 hours) mechanical allodynia and heat hyperalgesia, a response that is compromised in Tlr7-/- or Trpa1-/- mice. Ex vivo and in vivo calcium imaging in GCaMP6-report mice revealed increased calcium signal in spinal cord afferent terminals and doral root ganglion/dorsal root ganglia neurons following spinal perfusion and intraplantar injection of let-7b. Patch-clamp recordings also demonstrated enhanced excitatory synaptic transmission (miniature excitatory postsynaptic currents [EPSCs]) in spinal nociceptive neurons following let-7b perfusion or optogenetic activation of axonal terminals. The elevation in spinal calcium signaling and EPSCs was dependent on the presence of toll-like receptor-7 (TLR7) and transient receptor potential ion channel subtype A1 (TRPA1). In addition, endogenous let-7b is enriched in spinal cord synaptosome, and peripheral inflammation increased let-7b in doral root ganglion/dorsal root ganglia neurons, spinal cord tissue, and the cerebrospinal fluid. Notably, let-7b antagomir inhibited inflammatory pain and inflammation-induced synaptic plasticity (EPSC increase), suggesting an endogenous role of let-7b in regulating pain and synaptic transmission. Furthermore, intrathecal injection of let-7b, at a higher dose (10 μg), induced persistent mechanical allodynia for >2 weeks, which was abolished in Tlr7-/- mice. The high dose of let-7b also induced microgliosis in the spinal cord. Of interest, intrathecal minocycline only inhibited let-7b-induced mechanical allodynia in male but not female mice. Our findings indicate that the secreted microRNA let-7b has the capacity to provoke pain through both neuronal and glial signaling, thereby establishing miRNA as an emerging neuromodulator.

中文翻译：

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MicroRNA let-7b 增强脊髓伤害性突触传递，并通过神经元和小胶质细胞信号传导诱导急性和持续性疼痛。

分泌性 microRNA (miRNA) 已在包括脑脊液在内的各种体液中检测到，但它们在调节突触传递中的直接作用仍不确定。我们发现鞘内注射低剂量的let-7b（1μg）会引起短期（<24小时）机械异常性疼痛和热痛觉过敏，这种反应在Tlr7-/-或Trpa1-/-小鼠中受到损害。GCaMP6 报告小鼠的离体和体内钙成像显示，在脊髓灌注和足底内注射 let-7b 后，脊髓传入末梢和背根神经节/背根神经节神经元中的钙信号增加。膜片钳记录还表明，在let-7b灌注或轴突末端光遗传学激活后，脊髓伤害性神经元的兴奋性突触传递（微型兴奋性突触后电流[EPSC]）增强。脊髓钙信号传导和 EPSC 的升高取决于 Toll 样受体 7 (TLR7) 和瞬时受体电位离子通道亚型 A1 (TRPA1) 的存在。此外，内源性let-7b在脊髓突触体中富集，周围炎症使背根神经节/背根神经节神经元、脊髓组织和脑脊液中的let-7b增加。值得注意的是，let-7b antagomir 抑制炎症疼痛和炎症诱导的突触可塑性（EPSC 增加），表明 let-7b 在调节疼痛和突触传递中具有内源性作用。此外，鞘内注射较高剂量（10 μg）的let-7b可诱导持续超过2周的机械异常性疼痛，但在Tlr7-/-小鼠中这种现象被消除。高剂量的let-7b还会诱发脊髓中的小胶质细胞增生。有趣的是，鞘内注射米诺环素仅抑制雄性小鼠而非雌性小鼠中let-7b诱导的机械异常性疼痛。我们的研究结果表明，分泌的 microRNA let-7b 能够通过神经元和神经胶质信号传导引发疼痛，从而确立 miRNA 作为新兴神经调节剂的地位。