Lamotte d'Incamps B, Destombes J, Thiesson D, Hellio R, Lasserre X, Kouchtir-Devanne N, Jami L, Zytnicki D
Centre National de la Recherche Scientifique Unité de Recherche Associée 1448, Université René Descartes, 75270 Paris Cedex 06, France.
J Neurosci. 1998 Dec 1;18(23):10030-6. doi: 10.1523/JNEUROSCI.18-23-10030.1998.
Confocal microscopy was used to detect GABA-immunoreactive axo-axonic appositions, indicating possible synaptic contacts, on Ib fiber terminals in the lumbosacral spinal cord. A Ib fiber from posterior biceps-semitendinosus muscles was labeled by intra-axonal ejection of tetramethylrhodamine dextran (red), and serial sections of S1-L7 spinal cord segments were processed for GABA immunocytochemistry revealed by fluorescein isothiocynate (green). Appositions between GABA-immunoreactive structures and the labeled fiber appeared as yellow spots because of the presence of both fluorochromes in small volumes (0.3 * 0.3 * 0.5 micrometer(3)) of tissue. These spots were identified as probable axo-axonic contacts when: (1) they were observed in two to four serial confocal planes, indicating that they did not occur by chance; and (2) their sizes, shapes, and locations were similar to those of axo-axonic contacts found on Ia terminals, known to bear presynaptic boutons, and resembled the axo-axonic synapses described in electron microscope studies of Ib boutons in Clarke's column. A total of 59 presumed axo-axonic contacts was observed on two Ib collaterals, representing an estimated 20% of the total complement. In a three-dimensional reconstruction of one collateral, they were mostly located in terminal positions, and some branches bore more contacts than others. Such differential distribution could not result from chance appositions between GABAergic structures and Ib arborization and further supported the identification of axo-axonic contacts. Segmental Ib collaterals bear axo-axonic synapses that might ensure differential funneling of information toward different targets.
共聚焦显微镜用于检测腰骶脊髓中Ib纤维终末上γ-氨基丁酸免疫反应性轴-轴突触并置，这表明可能存在突触联系。通过轴突内注射四甲基罗丹明葡聚糖（红色）标记来自后二头肌-半腱肌的一条Ib纤维，并对S1-L7脊髓节段的连续切片进行异硫氰酸荧光素（绿色）显示的γ-氨基丁酸免疫细胞化学处理。由于在小体积（0.3×0.3×0.5微米³）组织中同时存在两种荧光染料，γ-氨基丁酸免疫反应性结构与标记纤维之间的并置呈现为黄色斑点。当满足以下条件时，这些斑点被确定为可能的轴-轴突触联系：（1）在两到四个连续共聚焦平面中观察到它们，表明它们不是偶然出现的；（2）它们的大小、形状和位置与在Ia终末上发现的轴-轴突触联系相似，已知Ia终末带有突触前终扣，并且类似于在克拉克柱中Ib终扣的电子显微镜研究中描述的轴-轴突触。在两条Ib侧支上总共观察到59个假定的轴-轴突触联系，约占总数的20%。在对一个侧支的三维重建中，它们大多位于终末位置，并且一些分支上的突触联系比其他分支更多。这种差异分布不可能是由γ-氨基丁酸能结构与Ib分支之间的偶然并置导致的，这进一步支持了轴-轴突触联系的认定。节段性Ib侧支带有轴-轴突触，这可能确保信息向不同靶标的差异汇聚。
