Brandstätter J H, Seyan H S, Meinertzhagen I A
Neuroscience Institute, Dalhousie University, Halifax, Nova Scotia, Canada.
J Neurocytol. 1992 Oct;21(10):693-705. doi: 10.1007/BF01181585.
The sensitivity of sensory neurons to target cell denervation varies in the CNS. We have examined the effects of surgically interrupting the output axons of the first optic neuropil, or lamina, in the optic lobe of the fly (Musca domestica), upon the receptor terminal inputs to the lamina. Two of the output interneurons are the monopolar cells L1 and L2, which are found as a pair in each of the unit modules or cartridges of the lamina neuropil. The lamina axons of L1 and L2 degenerate rapidly (within 0.5 h) in a retrograde direction from their lesion site, but there is no sign of retrograde transneuronal degeneration to the receptor terminals, across the input synapse. At each of these synaptic sites, L1 and L2 are invariable contributors to two of the four elements of a postsynaptic tetrad. Not only do the receptor terminals persist, but the presynaptic ribbons at the tetrad sites do also, opposite the degenerated spines of L1 and L2, indicating their lack of target dependence at least over the longest period of post-lesion recovery (48 h) examined. The areal density of presynaptic sites was conserved in the face of the degenerative loss of L1 and L2, as were the numbers of capitate projections (glial invaginations into receptor terminals). The stability of both synaptic density and capitate projection number indicates that they are predominantly influenced by the receptor terminals, which are still intact. A reduction in the number of mitochondrial profiles was one of the few observed changes in the receptor terminals. The results reflect the autonomy which the terminals have, during development, from their interneurons; they especially reflect the role of the terminals in the adult, in maintaining the presynaptic site of their afferent synapses, the tetrads.
在中枢神经系统中，感觉神经元对靶细胞去神经支配的敏感性各不相同。我们研究了手术切断果蝇（家蝇）视叶中第一个视神经节层（即神经层）的输出轴突，对神经层受体终末输入的影响。其中两个输出中间神经元是单极细胞L1和L2，在神经层神经节的每个单元模块或神经元簇中，它们都是成对存在的。L1和L2的神经层轴突从损伤部位开始迅速（在0.5小时内）逆向退化，但没有迹象表明跨输入突触向受体终末发生逆向跨神经元变性。在这些突触位点中的每一个，L1和L2都是突触后四分体四个元件中两个元件的恒定贡献者。不仅受体终末持续存在，而且四分体位点处的突触前带也持续存在，与L1和L2退化的棘相对，这表明至少在所检查的损伤后最长恢复时期（48小时）内，它们不依赖于靶细胞。面对L1和L2的退化性丧失，突触前位点的面密度保持不变，头状突起（胶质细胞向受体终末的内陷）的数量也保持不变。突触密度和头状突起数量的稳定性表明，它们主要受仍然完整的受体终末的影响。线粒体轮廓数量的减少是在受体终末观察到的少数变化之一。这些结果反映了终末在发育过程中相对于中间神经元的自主性；它们尤其反映了终末在成体中在维持其传入突触（四分体）的突触前位点方面的作用。
