Lee Hankyu, Graham Robert D, Melikyan Diana, Smith Brennan, Mirzakhalili Ehsan, Lempka Scott F, Duan Bo
Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI, United States.
Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI, United States.
Front Mol Neurosci. 2022 Jun 16;15:937890. doi: 10.3389/fnmol.2022.937890. eCollection 2022.
Chronic itch is associated with sensitization of the somatosensory nervous system. Recent studies have identified the neural circuits transmitting acute itch; however, the mechanisms by which itch transforms into a pathological state remain largely unknown. We have previously shown that Aβ low-threshold mechanoreceptors, together with spinal urocortin 3-positive (Ucn3) excitatory interneurons and neuropeptide Y-positive (NPY) inhibitory interneurons, form a microcircuit that transmits and gates acute mechanical itch. Here, using whole-cell patch-clamp recordings, we observed increased excitability in spinal Ucn3 neurons under chronic itch conditions. In contrast to Ucn3 neurons, the excitability of spinal NPY neurons was largely reduced under chronic itch conditions. To explore the molecular mechanisms underlying sensitization of this microcircuit, we examined the mRNA expression levels of voltage-gated ion channels in recorded spinal Ucn3 and NPY neurons by single-cell quantitative real-time PCR (qRT-PCR). We found that the expression levels of Nav1.6 and Cav2.3 channels were increased in spinal Ucn3 neurons in chronic itch mice, while the expression level of SK3 channels was decreased. By contrast, the expression levels of Nav1.6 and BK channels were decreased in spinal NPY neurons in chronic itch mice. To determine the contribution of different ion channels in chronic itch sensitization, we then used a Markov Chain Monte Carlo method to parameterize a large number of biophysically distinct multicompartment models of Ucn3 and NPY neurons. These models included explicit representations of the ion channels that we found to be up- or down-regulated under chronic itch conditions. Our models demonstrated that changes in Nav1.6 conductance are predominantly responsible for the changes in excitability of both Ucn3 and NPY neurons during chronic itch pathogenesis. Furthermore, when simulating microcircuits of our Ucn3 and NPY models, we found that reduced Nav1.6 conductance in NPY models played a major role in opening the itch gate under chronic itch conditions. However, changing SK, BK, or R-type calcium channel conductance had negligible effects on the sensitization of this circuit. Therefore, our results suggest that Nav1.6 channels may play an essential role in mechanical itch sensitization. The findings presented here may open a new avenue for developing pharmaceutical strategies to treat chronic itch.
慢性瘙痒与体感神经系统的致敏作用相关。近期研究已确定了传递急性瘙痒的神经回路；然而，瘙痒转变为病理状态的机制在很大程度上仍不清楚。我们之前已经表明，Aβ低阈值机械感受器与脊髓尿皮质素3阳性（Ucn3）兴奋性中间神经元和神经肽Y阳性（NPY）抑制性中间神经元共同构成了一个传递并控制急性机械性瘙痒的微回路。在此，我们使用全细胞膜片钳记录技术，观察到在慢性瘙痒条件下脊髓Ucn3神经元的兴奋性增加。与Ucn3神经元相反，在慢性瘙痒条件下脊髓NPY神经元的兴奋性大幅降低。为了探究该微回路致敏作用的分子机制，我们通过单细胞定量实时PCR（qRT-PCR）检测了所记录的脊髓Ucn3和NPY神经元中电压门控离子通道的mRNA表达水平。我们发现，慢性瘙痒小鼠脊髓Ucn3神经元中Nav1.6和Cav2.3通道的表达水平升高，而SK3通道的表达水平降低。相比之下，慢性瘙痒小鼠脊髓NPY神经元中Nav1.6和BK通道的表达水平降低。为了确定不同离子通道在慢性瘙痒致敏中的作用，我们随后使用马尔可夫链蒙特卡罗方法对大量具有不同生物物理特性的Ucn3和NPY神经元多室模型进行参数化。这些模型明确表示了我们发现在慢性瘙痒条件下上调或下调的离子通道。我们的模型表明，在慢性瘙痒发病过程中，Nav1.6电导的变化主要负责Ucn3和NPY神经元兴奋性的变化。此外，当模拟我们的Ucn3和NPY模型的微回路时，我们发现NPY模型中Nav1.6电导的降低在慢性瘙痒条件下打开瘙痒闸门中起主要作用。然而，改变SK、BK或R型钙通道的电导对该回路的致敏作用影响可忽略不计。因此，我们的结果表明Nav1.6通道可能在机械性瘙痒致敏中起关键作用。此处呈现的研究结果可能为开发治疗慢性瘙痒的药物策略开辟一条新途径。
