Application of functional imaging techniques to animal models is vital to understand pain mechanisms, but is often confounded by the need to limit movement artefacts with anaesthesia, and a focus on evoked responses rather than clinically relevant spontaneous pain and related hyperalgesia. The aim of the present study was to investigate the potential of manganese-enhanced magnetic resonance imaging (MEMRI) to measure neural responses during on-going pain that underpins hyperalgesia in pre-clinical models of nociception. As a proof of concept that MEMRI is sensitive to the neural activity of spontaneous, intermittent behaviour, we studied a separate positive control group undergoing a voluntary running wheel experiment. In the pain models, pain behaviour (weight bearing asymmetry and hindpaw withdrawal thresholds (PWTs)) was measured at baseline and following either intra-articular injection of nerve growth factor (NGF, 10µg/50µl; acute pain model, n=4 rats per group), or the chondrocyte toxin monosodium iodoacetate (MIA, 1mg/50µl; chronic model, n=8 rats per group), or control injection. Separate groups of rats underwent a voluntary wheel running protocol (n=8 rats per group). Rats were administered with paramagnetic ion Mn2+ as soluble MnCl2 over seven days (subcutaneous osmotic pump) to allow cumulative activity-dependent neural accumulation in the models of pain, or over a period of running. T1-weighted MR imaging at 7T was performed under isoflurane anaesthesia using a receive-only rat head coil in combination with a 72mm volume coil for excitation. The pain models resulted in weight bearing asymmetry (NGF: 20.0 ± 5.2%, MIA: 15 ± 3%), and a reduction in PWT in the MIA model (8.3 ± 1.5g) on the final day of assessment before undergoing MR imaging. Voxel-wise and region-based analysis of MEMRI data did not identify group differences in T1 signal. However, MnCl2 accumulation in the VTA, right Ce amygdala, and left cingulate was negatively correlated with pain responses (greater differences in weight bearing), similarly MnCl2 accumulation was reduced in the VTA in line with hyperalgesia (lower PWTs), which suggests reduced regional activation as a result of the intensity and duration of pain experienced during the 7 days of MnCl2 exposure. Motor cortex T1-weighted signal increase was associated with the distance ran in the wheel running study, while no between group difference was seen. Our data suggest that on-going pain related signal changes identified using MEMRI offers a new window to study the neural underpinnings of spontaneous pain in rats.

译文

:功能成像技术在动物模型中的应用对于理解疼痛机制至关重要,但通常因需要限制运动伪影并进行麻醉,并且侧重于诱发反应而不是临床相关的自发性疼痛和相关的痛觉过敏而感到困惑。本研究的目的是研究锰增强磁共振成像(MEMRI)在持续疼痛过程中测量神经痛的潜在能力,该疼痛在临床前伤害感受模型中是痛觉过敏的基础。作为MEMRI对自发性,间歇性行为的神经活动敏感的概念的证明,我们研究了一个单独的阳性对照组,他们接受了一项自愿的自转轮实验。在疼痛模型中,在基线时以及关节内注射神经生长因子(NGF,10µg / 50µl;急性疼痛模型,每只n = 4只大鼠)时,测量基线时的疼痛行为(负重不对称和后爪缩回阈值(PWTs))。组)或软骨细胞毒素碘乙酸单钠(MIA,1mg / 50µl;慢性模型,每组n = 8只大鼠)或对照注射液。单独的大鼠组接受自愿轮转方案(每组n = 8只大鼠)。在7天(皮下渗透泵)中,给大鼠施用了顺磁性离子Mn2作为可溶的MnCl2,以允许在疼痛模型中或在一段时间内累积依赖于活动的神经蓄积。在异氟烷麻醉下,使用仅接收的大鼠头部线圈和72mm体积线圈进行激励,在异氟烷麻醉下进行7T的T1加权MR成像。疼痛模型导致负重不对称(NGF:20.0±5.2%,MIA:15±3%),并且在进行MR成像前的评估的最后一天,MIA模型中的PWT减少(8.3±1.5g)。 MEMRI数据的基于体素分析和基于区域的分析未发现T1信号中的组差异。然而,VTA,右Cy杏仁核和左扣带中的MnCl2积累与疼痛反应呈负相关(负重差异更大),与痛觉过敏一致,VTA中的MnCl2积累减少(PWT较低),这表明区域性减少MnCl2暴露7天期间疼痛的强度和持续时间的结果是激活。运动皮层T1加权信号的增加与车轮行驶研究中的跑步距离相关,而两组之间未见差异。我们的数据表明,使用MEMRI识别的与疼痛相关的持续信号变化为研究大鼠自发性疼痛的神经基础提供了新的窗口。

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