Role of HCN ion channels in neuropathic pain: a combined animal and human study

疼痛通常被分为三种不同的类型，每种类型都有不同的潜在原因。急性疼痛，如由突然受伤或烧伤引起的，是由于疼痛刺激直接刺激伤害性(疼痛敏感)末梢引起的。炎症性疼痛伴随着损伤，由炎症介质(如应激或损伤组织释放的前列腺素E_2和缓激肽)对伤害性神经末梢的作用引起。神经性疼痛是由感觉神经本身的直接损伤引起的。引起神经性疼痛的常见情况包括糖尿病神经病变、带状疱疹爆发的后遗症，以及某些形式的癌症化疗。神经性疼痛也被认为是其他常见情况下疼痛的原因之一，如下腰痛和晚期癌症疼痛。目前可用的药物治疗神经病理性疼痛的效果很差，即使是一线治疗也只有三分之一的患者得到缓解。申请实验室最近的实验表明，神经病理性疼痛是由一种名为HCN的离子通道启动的，这种通道存在于伤害性感受器中。Hcn2的激活导致内向电流流入伤害性感受器，在这些疼痛敏感神经纤维中产生低水平的神经冲动。这种活动是神经病理性疼痛的病因的证据来自两种类型的实验：当从伤害感受器中从基因上删除hcn2时，神经病理性疼痛不会启动；当它被阻断时，药理学上的神经病理性疼痛被逆转。本研究的目的是在动物和人类实验中进一步研究这些观察结果。在动物实验中，我们将研究在神经病理性疼痛被确立后，HCN2基因缺失的影响。这项实验很重要，因为有一些证据表明，神经性疼痛有一个最初的炎症阶段，然后是一个长期阶段，许多科学家将其归因于脊髓中的事件。长时间缺失hcn2能逆转神经病理性疼痛吗？如果是这样的话，这将意味着无论是长时间还是短时间的致病事件都是由外周伤害性感受器中的HCN2启动的活动。到目前为止，只有一种阻断剂ZD7288研究了HCN2的药理阻断效应。另外两种药物目前正在临床使用或处于后期试验阶段，因此可以用于缓解人类神经病理性疼痛。它们在动物身上有效吗？众所周知，这些阻滞剂可以减缓心脏的跳动，事实上，这是它们的临床用途--但有没有一个治疗窗口，可以在不影响心脏的情况下缓解疼痛？最后，在常见人类神经病理性疼痛的动物模型中，如糖尿病神经病变，是否可以通过缺失或药物阻断hcn2来缓解疼痛？在补充的人类研究中，我们将调查临床批准的hcn2阻滞剂伊夫拉定对健康志愿者和患者的神经病理性疼痛的影响。在最初的研究中，我们将通过应用辣椒素来诱导一种短暂的疼痛状态，这被认为与神经性疼痛有共同的特征，辣椒素会产生局部的灼热感。伊夫拉定能缓解机械过敏吗？如果这些研究成功，我们将在阿登布鲁克医院疼痛诊所调查伊夫拉定对患者的影响。伊夫拉定有缓解作用吗？如果只有一部分患者看到缓解，我们能从他们的疼痛特征上解释差异吗？这个项目的成功完成将使通过药物阻断HCN减轻神经病理性疼痛的可能性更近。

Pain is commonly classified into three distinct varieties, each with different underlying causes. Acute pain, such as caused by a sudden injury or burn, is due to direct excitation of nociceptive (pain-sensitive) endings by the painful stimulus. Inflammatory pain follows an injury, and is caused by an action on nociceptive nerve endings of inflammatory mediators such as prostaglandin E2 and bradykinin released from stressed or injured tissues. Neuropathic pain is caused by direct injury of the sensory nerves themselves. Common conditions which cause neuropathic pain include diabetic neuropathy, the after-effects of a Herpes Zoster outbreak, or shingles, and some forms of cancer chemotherapy. Neuropathic pain is also thought to be a contributor to the pain felt in other common conditions, such as lower back pain and end-stage cancer pain. Neuropathic pain is poorly treated by currently available pharmaceuticals, with even the first-line treatments giving relief in only one third of patients. Recent experiments in the applicant lab have shown that neuropathic pain is initiated by an ion channel, known as HCN2, which is present in nociceptors. Activation of HCN2 causes an inward current to flow into nociceptors, producing a low-level barrage of nerve impulses in these pain-sensitive nerve fibres. The evidence that this activity is the causative agent in neuropathic pain comes from two types of experiment: when HCN2 is deleted genetically from nociceptors neuropathic pain is not initiated; and when it is blocked pharmacologically neuropathic pain is reversed. The aim of the present study is to take these observations further in both animal and human studies.In animal studies we will examine the effect of genetic deletion of HCN2 after neuropathic pain has been established. This experiment is important because there is some evidence that neuropathic pain has an initial inflammatory phase followed by a long-term phase which many scientists attribute to events in the spinal cord. Does deletion of HCN2 at long times reverse neuropathic pain? If it does this will imply that the causative event at long as well as short times is activity initiated by HCN2 in peripheral nociceptors.The effect of pharmacological block of HCN2 has been to date investigated only with one blocker, ZD7288. Two others are currently in clinical use or in late-stage trials and therefore could be used to alleviate human neuropathic pain. Are they effective in animals? These blockers are known to slow the heart, and in fact this is their clinical use - but is there a therapeutic window in which pain is relieved without cardiac effects? Finally, is pain in animal models of common human causes of neuropathic pain, such as diabetic neuropathy, alleviated by deletion or pharmacological block of HCN2?In complementary human studies we will investigate the effect of a clinically approved HCN2 blocker, ivabradine, on neuropathic pain in both healthy volunteers and in patients. In initial studies we will induce a shortlived pain state, thought to share features with neuropathic pain, by application of capsaicin, which produces a localised burning sensation. Is mechanical hypersensitivity alleviated by ivabradine? If these studies are successful we will investigate the effect of ivabradine on patients in the Addenbrooke's Hospital Pain Clinic. Is some relief obtained with ivabradine? If only some patients see relief, can we explain the differences in terms of the characteristics of their pain?Successful completion of both arms of this project will bring closer the possibility of alleviating neuropathic pain by pharmacological block of HCN2.