Sanderson Nydahl Katarina, Skinner Kate, Julius David, Basbaum Allan I
Department of Anatomy, University of California San Francisco, San Francisco, CA 94143, USA.
Eur J Neurosci. 2004 Apr;19(7):1789-99. doi: 10.1111/j.1460-9568.2004.03284.x.
Endomorphin-2 (EM2) is a tetrapeptide with remarkable affinity and selectivity for the mu-opioid receptor. In the present study, we used double-fluorescence and electron microscopic immunocytochemistry to identify subsets of EM2-expressing neurons in dorsal root ganglia and spinal cord dorsal horn of adult rats. Within the lumbar dorsal root ganglia, we found EM2 immunoreactivity mainly in small-to-medium size neurons, most of which co-expressed the neuropeptide substance P (SP). In adult rat L4 dorsal root ganglia, 23.9% of neuronal profiles contained EM2 immunoreactivity and ranged in size from 15 to 36 microM in diameter (mean 24.3 +/- 4.3 microM). Double-labelling experiments with cytochemical markers of dorsal root ganglia neurons showed that approximately 95% of EM2-immunoreactive cell bodies also label with SP antisera, 83% co-express vanilloid receptor subtype 1/capsaicin receptor, and 17% label with isolectin B4, a marker of non-peptide nociceptors. Importantly, EM2 immunostaining persisted in mice with a deletion of the preprotachykinin-A gene that encodes SP. In the lumbar spinal cord dorsal horn, EM2 expression was concentrated in presumptive primary afferent terminals in laminae I and outer II. At the ultrastructural level, electron microscopic double-labelling showed co-localization of EM2 and SP in dense core vesicles of lumbar superficial dorsal horn synaptic terminals. Finally, 2 weeks after sciatic nerve axotomy we observed a greater than 50% reduction in EM2 immunoreactivity in the superficial dorsal horn. We suggest that the very strong anatomical relationship between primary afferent nociceptors that express SP and EM2 underlies an EM2 regulation of SP release via mu-opioid autoreceptors.
内吗啡肽-2（EM2）是一种对μ-阿片受体具有显著亲和力和选择性的四肽。在本研究中，我们使用双荧光和电子显微镜免疫细胞化学方法，鉴定成年大鼠背根神经节和脊髓背角中表达EM2的神经元亚群。在腰段背根神经节内，我们发现EM2免疫反应性主要存在于中小尺寸神经元中，其中大多数共同表达神经肽P物质（SP）。在成年大鼠L4背根神经节中，23.9%的神经元轮廓含有EM2免疫反应性，直径范围为15至36微米（平均24.3±4.3微米）。对背根神经节神经元的细胞化学标记进行的双标记实验表明，约95%的EM2免疫反应性细胞体也用SP抗血清标记，83%共同表达香草酸受体亚型1/辣椒素受体，17%用非肽伤害感受器标记物异凝集素B4标记。重要的是，在编码SP的前速激肽原-A基因缺失的小鼠中，EM2免疫染色持续存在。在腰段脊髓背角，EM2表达集中在I层和外层II层的假定初级传入终末。在超微结构水平上，电子显微镜双标记显示EM2和SP在腰段浅表背角突触终末的致密核心小泡中共定位。最后，坐骨神经切断术后2周，我们观察到浅表背角中EM2免疫反应性降低了50%以上。我们认为，表达SP和EM2的初级传入伤害感受器之间非常紧密的解剖学关系是EM2通过μ-阿片自身受体调节SP释放的基础。
