Department of Physiology & Biomedical Engineering, College of Medicine and Science, Mayo Clinic, Rochester, MN, United States of America.
Department of Physiology & Biomedical Engineering, College of Medicine and Science, Mayo Clinic, Rochester, MN, United States of America; Department of Anesthesiology & Perioperative Medicine, College of Medicine and Science, Mayo Clinic, Rochester, MN, United States of America.
Exp Neurol. 2022 Jul;353:114030. doi: 10.1016/j.expneurol.2022.114030. Epub 2022 Mar 2.
Upper cervical spinal cord injuries (SCI) disrupt descending inputs to phrenic motor neurons (PhMNs), impairing respiratory function. Unilateral spinal hemisection at C2 (C2SH) results in loss of ipsilateral rhythmic diaphragm muscle (DIAm) EMG activity associated with lower force behaviors accomplished by recruitment of smaller PhMNs in rats. Activity during higher force, non-ventilatory behaviors that recruit larger PhMNs is minimally impaired following C2SH. We previously showed neuroplasticity in glutamatergic receptor expression in PhMN post-C2SH with changes in NMDA receptor expression reflecting functional recovery over time. We hypothesize that C2SH-induced changes in glutamatergic receptor (AMPA and NMDA) mRNA expression in PhMNs vary with motor neuron size, with more pronounced changes in smaller PhMNs. Retrogradely-labelled PhMNs were classified in tertiles according to somal surface area and mRNA expression was measured using single-cell, multiplex fluorescence in situ hybridization. Ipsilateral to C2SH, a pronounced reduction in NMDA mRNA expression in PhMNs was evident at 3 days post-injury with similar impact on PhMNs in the lower size tertile (68% reduction) and upper tertile (60%); by 21 days, there was near complete restoration of NMDA receptor mRNA expression across all PhMNs. There were no changes in NMDA mRNA expression contralateral to C2SH. There were no changes in AMPA mRNA expression at PhMNs on either side of the spinal cord or at any time-point post-C2SH. In summary, following C2SH there is ipsilateral reduction in PhMN NMDA mRNA expression at 3 days that is not limited to smaller PhMN recruited in the generation of lower force ventilatory behaviors. The recovery of NMDA mRNA expression by 21 days post-C2SH is consistent with evidence of spontaneous recovery of ipsilateral DIAm activity at this timepoint. These findings suggest a possible role for NMDA receptor mediated glutamatergic signaling in mechanisms supporting postsynaptic neuroplasticity at the PhMN pool and recovery of DIAm activity after cervical SCI.
上颈段脊髓损伤 (SCI) 破坏膈神经运动神经元 (PhMN) 的下行输入，损害呼吸功能。C2 单侧脊髓半切 (C2SH) 导致同侧节律性膈肌肌 (DIAm) EMG 活动丧失，与大鼠较小的 PhMN 募集相关，导致较低力行为。C2SH 后，较小的 PhMN 募集时，更高力非通气行为的活动仅受到轻微损害。我们之前的研究表明，C2SH 后 PhMN 中谷氨酸能受体表达的神经可塑性增加，NMDA 受体表达的变化反映了随时间的功能恢复。我们假设，C2SH 诱导的 PhMN 中谷氨酸能受体 (AMPA 和 NMDA) mRNA 表达的变化与运动神经元大小有关，较小的 PhMN 中变化更为明显。根据胞体表面积将逆行标记的 PhMN 分为三分位数，并使用单细胞多重荧光原位杂交测量 mRNA 表达。C2SH 同侧，损伤后 3 天 PhMN 中 NMDA mRNA 表达明显减少，对较小 PhMN（68%减少）和较大 PhMN （60%）的影响相似；到 21 天时，所有 PhMN 中 NMDA 受体 mRNA 表达几乎完全恢复。C2SH 对侧 NMDA mRNA 表达无变化。C2SH 后，脊髓两侧 PhMN 的 AMPA mRNA 表达或任何时间点均无变化。总之，C2SH 后 3 天，PhMN NMDA mRNA 表达减少，对较小的 PhMN 没有影响，较小的 PhMN 在产生较低力通气行为中募集。21 天时 NMDA mRNA 表达的恢复与此时点同侧 DIAm 活动自发恢复的证据一致。这些发现表明，NMDA 受体介导的谷氨酸能信号在支持 PhMN 池突触后神经可塑性和颈段 SCI 后 DIAm 活性恢复的机制中可能发挥作用。
