Shupliakov O, Atwood H L, Ottersen O P, Storm-Mathisen J, Brodin L
Nobel Institute for Neurophysiology, Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden.
J Neurosci. 1995 Nov;15(11):7168-80. doi: 10.1523/JNEUROSCI.15-11-07168.1995.
Synaptic glutamate release involves the accumulation of cytoplasmic glutamate in synaptic vesicles, whereafter it is released by triggered exocytosis. As glutamatergic terminals are known to be functionally diverse it was of interest to examine whether the presynaptic glutamate supply differs between individual axon terminals with distinct release properties. The glutamatergic terminals in the crustacean neuromuscular system system comprise a "phasic" type which shows fatigue of release during repetitive stimulation, and a "tonic" type which can maintain transmission for long periods. Quantitative immunogold analysis showed that the axons in a tonic nerve innervating slow muscles in the abdomen contained two times higher levels of glutamate labeling over axoplasmic matrix and over mitochondria, as compared to the corresponding elements in a phasic nerve. Similar results were obtained when adjacent phasic and tonic axons in a mixed nerve innervating leg muscles were compared. In the terminal regions of tonic and phasic axons the glutamate labeling differed correspondingly over axoplasmic matrix and mitochondria, while the synaptic vesicles showed a similar strong accumulation of labeling in both types of terminal. The level of labeling for glutamine, a glutamate precursor, was closely similar in phasic and tonic axons. The axoplasmic glutamate concentration was estimated to be in the low millimolar range, through comparison with coprocessed conjugates with known glutamate concentration. These results show that fatigue-resistant tonic axons and terminals contain higher levels of glutamate than fatiguable phasic axons, presumably representing an adaptation to the markedly different impulse activities in the two types of neuron. The axonal glutamate concentrations are in the range of the Km value for vesicular glutamate transport. Thus in tonic axons the high glutamate level appears to promote an efficient refilling of synaptic vesicles during sustained release, while in phasic axons the refilling should be slower which is compatible with an infrequent release.
突触谷氨酸释放涉及细胞质谷氨酸在突触小泡中的积累，随后通过触发胞吐作用释放。由于已知谷氨酸能终末在功能上具有多样性，因此研究具有不同释放特性的单个轴突终末之间突触前谷氨酸供应是否存在差异很有意义。甲壳类神经肌肉系统中的谷氨酸能终末包括一种“相位性”类型，在重复刺激期间表现出释放疲劳，以及一种“紧张性”类型，能够长时间维持传递。定量免疫金分析表明，支配腹部慢肌的紧张性神经中的轴突，其轴浆基质和线粒体上的谷氨酸标记水平比相位性神经中的相应成分高两倍。当比较支配腿部肌肉的混合神经中相邻的相位性和紧张性轴突时，也得到了类似的结果。在紧张性和相位性轴突的终末区域，轴浆基质和线粒体上的谷氨酸标记相应不同，而两种类型终末中的突触小泡都显示出类似的强烈标记积累。谷氨酸前体谷氨酰胺的标记水平在相位性和紧张性轴突中非常相似。通过与已知谷氨酸浓度的共处理缀合物比较，估计轴浆谷氨酸浓度处于低毫摩尔范围内。这些结果表明，抗疲劳的紧张性轴突和终末比易疲劳的相位性轴突含有更高水平的谷氨酸，这可能代表了对两种神经元中明显不同的冲动活动的一种适应。轴突谷氨酸浓度处于囊泡谷氨酸转运的Km值范围内。因此，在紧张性轴突中，高谷氨酸水平似乎促进了持续释放期间突触小泡的有效再填充，而在相位性轴突中，再填充应该较慢，这与不频繁释放是一致的。
