Eisenach J C, Zhang Y, Duflo F
Department of Anesthesiology, Center for the Study of Pharmacologic Plasticity in the Presence of Pain, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157, USA.
Neuroscience. 2005;131(1):189-97. doi: 10.1016/j.neuroscience.2004.10.017.
Nerve injury resulting in chronic pain is associated with novel excitatory effects of norepinephrine on injured peripheral nerve terminals and their cell bodies, due to actions on alpha2-adrenoceptors. Paradoxically, alpha2-adrenoceptor agonists administered near peripheral terminals or their cell bodies results in analgesia, not pain. This study tested, using intracellular Ca2+ response to stimulation, the effects of alpha2-adrenoceptor agonists on injured sensory neurons and classified their neuronal phenotype. Dorsal root ganglion cells from normal and spinal nerve-ligated rats were dissociated and activated twice with electrical field stimulation, while measuring Fura-2 fluorescence. Cells were perfused between stimulations with vehicle or alpha2-adrenoceptor agonists alone or with antagonists. Cells were considered inhibited if the ratio of their peak Ca2+ response to the second stimulus divided by the first was less than the 2.5th percentile for vehicle controls. alpha2-, But not alpha1-adrenoceptor agonists inhibited the Ca2+ response in a concentration related fashion, and this inhibition was blocked by alpha2-adrenoceptor antagonists. Clonidine inhibited a similar percentage of cells in the normal and spinal nerve-ligated group. In both groups, the large majority of clonidine-inhibited cells stained for isolectin B4. Spinal nerve ligation resulted in a 4-10-fold increase in the percentage of clonidine inhibited cells which immunostained for calcitonin gene-related peptide. These data are consistent with the known inhibition of Ca2+ currents by alpha2-adrenoceptors and suggest that, at the level of intracellular Ca2+, the key determinant of neurotransmitter release, alpha2-adrenoceptors are inhibitory after nerve injury, not excitatory. There is a shift in phenotype of sensory neurons which are inhibited by clonidine after nerve injury, which may explain clonidine's increased potency in the treatment of neuropathic compared with acute pain.
由于去甲肾上腺素作用于α2-肾上腺素能受体，导致神经损伤引起的慢性疼痛与去甲肾上腺素对损伤的外周神经末梢及其细胞体的新型兴奋作用有关。矛盾的是，在外周神经末梢或其细胞体附近给予α2-肾上腺素能受体激动剂会产生镇痛作用，而非疼痛。本研究利用细胞内Ca2+对刺激的反应，测试了α2-肾上腺素能受体激动剂对损伤感觉神经元的作用，并对其神经元表型进行了分类。将正常大鼠和脊髓神经结扎大鼠的背根神经节细胞分离，用电场刺激激活两次，同时测量Fura-2荧光。在两次刺激之间，用载体或单独的α2-肾上腺素能受体激动剂或与拮抗剂一起灌注细胞。如果细胞对第二次刺激的峰值Ca2+反应与第一次刺激的比值除以第一次刺激的比值小于载体对照的第2.5百分位数，则认为细胞受到抑制。α2-肾上腺素能受体激动剂而非α1-肾上腺素能受体激动剂以浓度相关的方式抑制Ca2+反应，并且这种抑制被α2-肾上腺素能受体拮抗剂阻断。可乐定在正常组和脊髓神经结扎组中抑制的细胞百分比相似。在两组中，绝大多数被可乐定抑制的细胞对isolectin B4染色。脊髓神经结扎导致对降钙素基因相关肽免疫染色的可乐定抑制细胞百分比增加4至10倍。这些数据与已知的α2-肾上腺素能受体对Ca2+电流的抑制作用一致，并表明，在细胞内Ca2+水平（神经递质释放的关键决定因素），α2-肾上腺素能受体在神经损伤后是抑制性的，而非兴奋性的。神经损伤后被可乐定抑制的感觉神经元的表型发生了转变，这可能解释了与急性疼痛相比，可乐定在治疗神经性疼痛方面效力增加的原因。
