Long-term activation of spinal opioid analgesia after inflammation

描述(由申请人提供)：严重的组织损伤会产生中枢敏感化(中枢伤害性神经元对正常或阈值以下传入输入的反应性增加)，从而导致痛觉过敏。潜伏性敏化(LS)是一种静默形式的中枢敏化，在组织愈合和明显的痛觉过敏症状消失后持续存在。可以用阿片受体拮抗剂或反向激动剂“重新点燃”或恢复痛觉过敏来揭示LS。因此，LS期间的疼痛缓解可能是通过紧张性阿片受体活性来维持的，这种活性掩盖了LS的前伤害性成分。LS很重要，因为它启动了伤害性感受系统，当抑制系统失效时，疼痛发作就会随之而来。理解LS的关键第一步是证明其翻译意义，我们现在证明了阿片受体反向激动剂纳洛酮在一级烫伤后继发性痛敏消失后1wk给药时可以恢复实验性疼痛。特定目标1测试烧伤或手术在人类中触发LS和长期阿片类止痛的假设。为了进一步研究LS的神经生物学机制，我们还将使用持续时间长、功能强大、范围广、可重复的小鼠模型，并将其转化为人类研究。我们发现，MU阿片受体(MOR)反向激动剂即使在组织损伤后几个月给药，也能恢复痛觉过敏的行为和分子迹象，这需要腺酰环化酶1型(AC1)的NMDA受体激活。我们的结果很重要，因为它们表明，在LS期间，任何干扰吗啡镇痛的事件，如应激，都会导致人类慢性疼痛综合征痛觉过敏的复发。特异靶2测试假设，吗啡结构性活动(MORCA)和/或激活吗啡受体，增量(DOR)，或kappa(KOR)受体的阿片肽维持内源性镇痛，从而限制LS的疼痛缓解状态。具体目的3确定在LS过程中，MORS抑制双侧丘脑的空间协调神经活动的程度(使用一种创新的通道场记录系统)以及初级传入伤害性感受器突触前终末或双侧丘神经元上的突触强度(使用膜片钳电生理)。然后测试MORS是否特异性地抑制脊髓NMDA受体亚单位(GluN2A或GluN2B)和/或Epac1(由cAMP直接激活的交换蛋白，最近被发现有助于外周疼痛感觉)来阻断LS期间的疼痛。该项目的完成将使我们更接近缓解慢性疼痛的长期目标：a)促进内源性阿片类止痛，从而将LS限制在缓解状态；或b)完全消除LS，例如使用选择性AC1或Epac1抑制剂。我们的一般模型和假设与异物作用的概念有相似之处：通过相互反馈相互促进的对立过程(MOR和LS)之间的病理性平衡。我们的长期愿景是慢性疼痛治疗的一个新的概念性战略，恢复动态平衡，既没有中枢敏化，也没有MOR代偿反应。

DESCRIPTION (provided by applicant): Severe tissue injury generates central sensitization (increased responsiveness of CNS nociceptive neurons to normal or sub-threshold afferent input) that contributes to hyperalgesia. Latent sensitization (LS) is a silent form of central sensitization that persists after tissue has healed and overt signs of hyperalgesia have resolved. LS can be revealed with opioid receptor antagonists or inverse agonists that "rekindle" or reinstate hyperalgesia. Thus, pain remission during LS is likely maintained by tonic opioid receptor activity that masks the pronociceptive components of LS. LS is important because it primes nociceptive systems such that, when inhibitory systems fail, a pain episode ensues. A key first step in understanding LS is to demonstrate the translational significance, and we now show that the opioid receptor inverse agonist, naloxone, can reinstate experimental pain when delivered 1 wk after the resolution of secondary hyperalgesia following first degree thermal injury. Specific Aim 1 tests the hypothesis that burn or surgery triggers LS and long-term opioid analgesia in humans. To further study the neurobiological mechanisms of LS, we will also use a mouse model that is long-lasting, powerful, broad range, repeatable, and translates to human studies. We found that mu opioid receptor (MOR) inverse agonists reinstated behavioral and molecular signs of hyperalgesia, even when administered months after tissue injury, and this required NMDA receptor activation of adenylyl cyclase type 1 (AC1). Our results are important because they suggest that any event, such as stress, that interferes with MOR analgesia during LS will lead to relapse of hyperalgesia in chronic pain syndromes in humans. Specific Aim 2 tests the hypothesis that MOR constitutive activity (MORCA) and/or activation of MOR, delta (DOR), or kappa (KOR) receptors by opioid peptides in the DH or rostroventromedial medulla maintains endogenous analgesia and thereby restricts LS to a state of pain remission. Specific Aim 3 determines the extent to which MORs inhibit spatially coordinated neural activity in the DH (using an innovative 64-channel field recording system) and synaptic strength in presynaptic terminals of primary afferent nociceptors or on DH neurons (using patch clamp electrophysiology) during LS. Specific Aim 4 then tests whether MORs specifically inhibit spinal NMDA receptor subunits (GluN2A or GluN2B) and/or Epac1 (exchange protein directly activated by cAMP, recently found to contribute to peripheral pain senstization) to block pain during LS. Completion of this project will bring us closer to our long-term goal of alleviating chronic pain b either: a) facilitating endogenous opioid analgesia, thus restricting LS within a state of remission; or b) extinguishing LS altogether, for example with a selective AC1 or Epac1 inhibitor. Our general model and hypothesis shares similarities with the concept of allostasis: a pathologically-elevated balance between opposing processes (MOR and LS) that facilitate each other by mutual feedback. Our long-term vision is a new conceptual strategy for chronic pain therapy, to restore homeostasis, where there is neither central sensitization nor MOR compensatory responses.