The spinal dorsal horn contains vesicular glutamate transporter-2 (VGluT2)-expressing excitatory neurons and vesicular GABA transporter (VGAT)-expressing inhibitory neurons, which normally have different roles in nociceptive transmission. Spinal glutamate NMDAR hyperactivity is a crucial mechanism of chronic neuropathic pain. However, it is unclear how NMDARs regulate primary afferent input to spinal excitatory and inhibitory neurons in neuropathic pain. Also, the functional significance of presynaptic NMDARs in neuropathic pain has not been defined explicitly. Here we showed that paclitaxel treatment or spared nerve injury (SNI) similarly increased the NMDAR-mediated mEPSC frequency and dorsal root-evoked EPSCs in VGluT2 dorsal horn neurons in male and female mice. By contrast, neither paclitaxel nor SNI had any effect on mEPSCs or evoked EPSCs in VGAT neurons. In mice with conditional Grin1 (gene encoding GluN1) KO in primary sensory neurons (Grin1-cKO), paclitaxel treatment failed to induce pain hypersensitivity. Unexpectedly, SNI still caused long-lasting pain hypersensitivity in Grin1-cKO mice. SNI increased the amplitude of puff NMDA currents in VGluT2 neurons and caused similar depolarizing shifts in GABA reversal potentials in WT and Grin1-cKO mice. Concordantly, spinal Grin1 knockdown diminished SNI-induced pain hypersensitivity. Thus, presynaptic NMDARs preferentially amplify primary afferent input to spinal excitatory neurons in neuropathic pain. Although presynaptic NMDARs are required for chemotherapy-induced pain hypersensitivity, postsynaptic NMDARs in spinal excitatory neurons play a dominant role in traumatic nerve injury-induced chronic pain. Our findings reveal the divergent synaptic connectivity and functional significance of spinal presynaptic and postsynaptic NMDARs in regulating cell type-specific nociceptive input in neuropathic pain with different etiologies.

SIGNIFICANCE STATEMENT Spinal excitatory neurons relay input from nociceptors, whereas inhibitory neurons repress spinal nociceptive transmission. Chronic nerve pain is associated with aberrant NMDAR activity in the spinal dorsal horn. This study demonstrates, for the first time, that chemotherapy and traumatic nerve injury preferentially enhance the NMDAR activity at primary afferent–excitatory neuron synapses but have no effect on primary afferent input to spinal inhibitory neurons. NMDARs in primary sensory neurons are essential for chemotherapy-induced chronic pain, whereas nerve trauma causes pain hypersensitivity predominantly via postsynaptic NMDARs in spinal excitatory neurons. Thus, presynaptic and postsynaptic NMDARs at primary afferent–excitatory neuron synapses are differentially engaged in chemotherapy- and nerve injury-induced chronic pain and could be targeted respectively for treating these painful conditions.

脊髓背角含有表达囊泡谷氨酸转运蛋白 2 (VGluT2) 的兴奋性神经元和表达囊泡 GABA 转运蛋白 (VGAT) 的抑制性神经元，它们通常在伤害性传递中发挥不同的作用。脊髓谷氨酸 NMDAR 过度活跃是慢性神经性疼痛的重要机制。然而，目前尚不清楚 NMDAR 如何调节神经性疼痛中脊髓兴奋性和抑制性神经元的初级传入输入。此外，突触前 NMDAR 在神经性疼痛中的功能意义尚未明确定义。在这里，我们发现，紫杉醇治疗或免神经损伤（SNI）同样增加了雄性和雌性小鼠 VGluT2 背角神经元中 NMDAR 介导的 mEPSC 频率和背根诱发的 EPSC。相比之下，紫杉醇和 SNI 对 VGAT 神经元中的 mEPSC 或诱发 EPSC 均没有任何影响。在初级感觉神经元条件性Grin1（编码 GluN1 的基因）KO ( Grin1 -cKO) 的小鼠中，紫杉醇治疗未能诱导疼痛超敏反应。出乎意料的是，SNI 仍然引起Grin1 -cKO 小鼠持久的疼痛超敏反应。SNI 增加了 VGluT2 神经元中喷发 NMDA 电流的幅度，并引起 WT 和Grin1 -cKO 小鼠中 GABA 反转电位的类似去极化变化。相应地，脊髓Grin1敲低可减轻 SNI 诱导的疼痛超敏反应。因此，在神经性疼痛中，突触前 NMDAR 优先放大脊髓兴奋性神经元的初级传入输入。虽然化疗引起的疼痛超敏反应需要突触前 NMDAR，但脊髓兴奋性神经元中的突触后 NMDAR 在创伤性神经损伤引起的慢性疼痛中起主导作用。我们的研究结果揭示了脊髓突触前和突触后 NMDAR 在调节不同病因的神经病理性疼痛中细胞类型特异性伤害性输入方面的不同突触连接性和功能意义。

意义陈述脊髓兴奋性神经元传递来自伤害性感受器的输入，而抑制性神经元则抑制脊髓伤害性传递。慢性神经痛与脊髓背角 NMDAR 活性异常有关。这项研究首次证明，化疗和创伤性神经损伤优先增强初级传入兴奋性神经元突触的 NMDAR 活性，但对脊髓抑制性神经元的初级传入输入没有影响。初级感觉神经元中的 NMDAR 对于化疗引起的慢性疼痛至关重要，而神经创伤主要通过脊髓兴奋性神经元中的突触后 NMDAR 引起疼痛超敏反应。因此，初级传入兴奋神经元突触的突触前和突触后 NMDAR 与化疗和神经损伤引起的慢性疼痛有不同的作用，可以分别作为治疗这些疼痛的目标。