Baird D H, Trenkner E, Mason C A
Department of Neurobiology and Anatomy, Medical College of Pennsylvania and Hahnemann University, Philadelphia, 19129, USA.
J Neurosci. 1996 Apr 15;16(8):2642-8. doi: 10.1523/JNEUROSCI.16-08-02642.1996.
Cerebellar granule neurons in vitro specifically arrest the extension of their appropriate presynaptic axons, mossy fibers. This "stop-growing signal" may be an essential step in the formation and specificity of synapses. Here, we have tested whether ionotropic glutamate receptors are involved in the stop-growing signal. When explants of basilar pontine nuclei, a mossy fiber source, were cultured on granule neurons, most pontine neurites terminated <200 microm from their explant of origin, a criterion for the stop-growing signal. In contrast, treatment with the NMDA antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) greatly increased the number of pontine neurites extending beyond 300 microm, whereas treatment with NMDA reduced the number of pontine neurites extending beyond 200 microm. A non-NMDA agonist (AMPA) and antagonist (6-cyano-7-nitroquinoxaline-2,3-dione) did not alter pontine neurite lengths. None of these agents affected neurite outgrowth from pontine explants in the absence of granule neurons, nor did any agent affect the survival of granule neurons. These results indicate that NMDA and D-AP5 specifically perturb an interaction between axons and target cells necessary for the stop-growing signal, and that NMDA receptors are critical for the development of a major cerebellar afferent system. These findings also suggest that NMDA-sensitive refinement of axon arbors during later development may involve the direct regulation of axon extension by target neurons.
体外培养的小脑颗粒神经元会特异性地阻止其相应的突触前轴突（苔藓纤维）的延伸。这种“生长停止信号”可能是突触形成和特异性的关键步骤。在此，我们测试了离子型谷氨酸受体是否参与生长停止信号。当将苔藓纤维来源的基底桥核的外植体培养在颗粒神经元上时，大多数桥脑神经突在距离其起源外植体<200微米处终止，这是生长停止信号的一个标准。相比之下，用NMDA拮抗剂D(-)-2-氨基-5-磷酸戊酸（D-AP5）处理可显著增加延伸超过300微米的桥脑神经突数量，而用NMDA处理则减少延伸超过200微米的桥脑神经突数量。非NMDA激动剂（AMPA）和拮抗剂（6-氰基-7-硝基喹喔啉-2,3-二酮）并未改变桥脑神经突的长度。在没有颗粒神经元的情况下，这些试剂均未影响桥脑外植体的神经突生长，任何试剂也未影响颗粒神经元的存活。这些结果表明，NMDA和D-AP5特异性地干扰了生长停止信号所需的轴突与靶细胞之间的相互作用，并且NMDA受体对于主要小脑传入系统的发育至关重要。这些发现还表明，在后期发育过程中，轴突分支的NMDA敏感型精细化可能涉及靶神经元对轴突延伸的直接调节。
