A novel pharmacological target to prevent sustained-morphine-mediated pain sensit

描述(由申请人提供)：长期药物治疗后的吗啡停药暴露出对疼痛(痛觉过敏)和无害(痛觉异常)刺激的矛盾的增强敏感性。脊髓小胶质细胞的激活在痛敏调节中起重要作用。有趣的是，最近发现CB2选择性大麻素激动剂有效地拮抗神经胶质细胞的激活。此外，我们的初步研究表明，吗啡和CB2激动剂的全身联合给药完全减轻了吗啡戒断大鼠的痛觉过敏和触觉过敏。因此，我们打算探索这样一种假设，即吗啡与CB2选择性激动剂联合给药可减弱吗啡介导的持续神经胶质细胞激活。本研究的具体目的是：1.确定CB2激动剂介导的减轻吗啡处理大鼠戒断、痛觉过敏和痛觉异常的量效关系；2.确定持续全身联合应用吗啡和CB2激动剂对大鼠脊髓小胶质细胞和星形胶质细胞活性的影响以及对脊髓痛神经递质(CGRP)浓度的影响；3.建立和鉴定原代新生大鼠脊髓小胶质细胞培养，作为未来详细研究CB2受体介导的小胶质细胞信号转导的分子机制的体外细胞模型。由于CB2受体在神经元中的浓度较低，这些药物对中枢神经系统的副作用很小或根本没有。因此，阿片类药物和CB2激动剂联合使用有望通过较低剂量的吗啡产生更持久的止痛效果，减少治疗慢性疼痛的危险副作用的可能性。 与公共卫生相关：长期的吗啡治疗会导致疼痛缓解的程度逐渐下降。因此，临床医生需要使用稳步增加的吗啡剂量来控制严重的慢性疼痛(止痛耐受性)。另一方面，更高剂量的吗啡更有可能造成严重的副作用，还可能导致药物成瘾。有趣的是，最近有研究表明，持续接触吗啡实际上会增加患者对疼痛(痛觉过敏)甚至对正常无痛(痛觉异常)刺激的敏感性，从而导致在治疗慢性疼痛时需要增加吗啡的剂量。脊髓胶质细胞的激活被证明在持续吗啡治疗时对感觉刺激的这种矛盾敏感化的发展中起着关键作用。因此，研究发现，胶质细胞抑制剂可以降低神经病理性疼痛敏感化和吗啡耐受性。在目前的提案中，我们打算探索一种新的药理学方法-激活CB2大麻受体类型-在持续吗啡治疗期间抑制脊髓小胶质细胞的激活。通过防止胶质细胞激活，阿片类药物和CB2激动剂联合治疗有望产生更有效的止痛效果，而不会在持续治疗期间发生适应性疼痛敏化。因此，使用这种药物组合，我们可能能够用较低剂量的吗啡实现更持久的止痛，减少在治疗慢性疼痛时出现危险副作用和阿片成瘾的可能性。

DESCRIPTION (provided by applicant): Morphine withdrawal after prolonged drug treatment unmasks a paradoxical augmented sensitivity to painful (hyperalgesia) and innocuous (allodynia) stimuli. Activation of spinal microglia was shown to play an important role in the regulation of pain sensitivity. Interestingly, recently it was found that CB2-selective cannabinoid agonists efficiently antagonize glial activation. In addition, our preliminary shows that systemic co-administration of morphine with a CB2 agonist completely attenuates hyperalgesia and tactile allodynia upon morphine withdrawal in rats. Therefore, we intend to explore the hypothesis that co- administration of morphine with a CB2-selective agonist attenuates sustained morphine- mediated glial activation. The specific aims the proposed work are: I. to determine dose- response relationships for CB2 agonist-mediated attenuation of withdrawal hyperalgesia and allodynia in morphine treated rats; II. to determine the effect of sustained systemic co- administration of morphine and a CB2 agonist on spinal microglia and astrocyte activity and on spinal pain neurotransmitter (CGRP) concentration in rats; and III. to develop and characterize a primary neonatal rat spinal microglia culture as a future in vitro cellular model for a detailed investigation of the molecular mechanism of CB2 receptor-mediated microglial signaling. Since CB2 receptors are present in low concentrations in neurons, these agents exhibit little or no CNS side effects. Therefore, co-administration of opioid and CB2 agonists is expected to produce longer-lasting pain relief using lower morphine doses, reducing the likelihood of dangerous side effects in the treatment of chronic pain. PUBLIC HEALTH RELEVANCE: Prolonged morphine treatment leads to a gradual decline of pain relief. Consequently, clinicians need to use steadily increasing morphine doses to manage severe chronic pain (analgesic tolerance). Higher morphine doses on the other hand are more likely to cause serious side effects and may also lead to drug addiction. Interestingly, recently it was demonstrated that sustained exposure to morphine actually increases the sensitivity of patients to painful (hyperalgesia) and even to normally painless (allodynia) stimuli, contributing to the need for increased morphine doses in the treatment of chronic pain. Activation of spinal glia was shown to play a crucial role in the development of such paradoxical sensitization to sensory stimuli upon sustained morphine treatment. Accordingly, glial inhibitors were found to reduce both neuropathic pain sensitization and morphine tolerance. In the present proposal we intend to explore the utility of a novel pharmacological approach -activation of the CB2 cannabinoid receptor type - to inhibit spinal microglia activation during sustained morphine treatment. By preventing glia activation, opioid and CB2 agonist co-treatment is expected to produce more efficient pain relief without adaptive pain sensitization during sustained treatment. Thus, with such drug combinations, we may be able to achieve longer- lasting pain relief with lower morphine doses, reducing the likelihood of dangerous side effects and opiate addiction in the treatment of chronic pain.