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

疼痛通常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，例如由突然的损伤或烧伤引起的，是由于疼痛刺激直接激发伤害性（疼痛敏感）末梢。炎性疼痛在损伤之后发生，并且是由从应激或损伤组织释放的炎性介质如前列腺素E2和缓激肽对伤害性神经末梢的作用引起的。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病症包括糖尿病性神经病变、带状疱疹爆发或带状疱疹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在其他常见情况下感受到的疼痛的贡献者，例如下背痛和晚期癌症疼痛。目前可用的药物对神经性疼痛的治疗效果很差，即使是一线治疗也只有三分之一的患者得到缓解。申请人实验室最近的实验表明，神经性疼痛是由一种离子通道（称为HCN 2）引发的，该离子通道存在于伤害感受器中。HCN 2的激活导致内向电流流入伤害感受器，在这些疼痛敏感神经纤维中产生低水平的神经冲动。这种活性是神经性疼痛的病原体的证据来自两种类型的实验：当HCN 2从伤害感受器中基因缺失时，神经性疼痛不会开始;当它被阻断时，神经性疼痛被逆转。本研究的目的是在动物和人类研究中进一步观察这些结果，在动物研究中，我们将检查HCN 2基因缺失对神经病理性疼痛的影响。这个实验很重要，因为有证据表明神经性疼痛有一个最初的炎症阶段，随后是一个长期阶段，许多科学家将其归因于脊髓中的事件。长期缺失HCN 2能逆转神经性疼痛吗？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HCN 2在外周伤害感受器中引发的活动。迄今为止，HCN 2的药理学阻断作用仅用一种阻断剂ZD 7288进行了研究。另外两种药物目前正在临床使用或处于后期试验阶段，因此可用于缓解人类神经性疼痛。对动物有效吗？众所周知，这些阻滞剂可以减缓心脏跳动，事实上这就是它们的临床用途--但是否存在一个治疗窗口，在这个窗口中，疼痛可以缓解，而不会对心脏产生影响？最后，在人类常见的神经性疼痛（如糖尿病性神经病变）的动物模型中，HCN 2的缺失或药理学阻断是否能减轻疼痛？在补充人体研究中，我们将在健康志愿者和患者中研究临床批准的HCN 2阻滞剂伊伐布雷定对神经性疼痛的影响。在最初的研究中，我们将通过应用辣椒素诱导短暂的疼痛状态，这种疼痛状态被认为与神经性疼痛具有共同的特征，辣椒素会产生局部的烧灼感。伊伐布雷定是否缓解机械性超敏反应？如果这些研究成功，我们将研究伊伐布雷定对Addenbrooke医院疼痛门诊患者的影响。伊伐布雷定是否能缓解症状？如果只有一些患者看到缓解，我们能解释他们疼痛特征的差异吗？该项目两个组的成功完成将使通过药物阻断HCN 2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.