Diffusion, not uptake, limits glycine concentration in the synaptic cleft.
Diffusion, not uptake, limits glycine concentration in the synaptic cleft.
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扩散而非摄取限制了突触间隙中的甘氨酸浓度。
DOI：
10.1152/jn.1996.75.4.1738
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
Faber,DS
中科院分区：
文献类型：
--
作者：
Titmus,MJ;Korn,H;Faber,DS
1. 主动摄取是否限制抑制性递质甘氨酸的作用持续时间的问题已在金鱼 Mauthner (M) 细胞的抑制性突触体内得到解决。在对照和预期阻止甘氨酸摄取或减慢其扩散的条件下记录逆向和第八神经刺激引起的抑制性突触后电位和抑制性突触后电流（IPSC）的动力学。基于模拟量子电流的理论考虑预测，如果扩散缓慢，则需要快速摄取甘氨酸，并且甘氨酸的阻断将延长突触反应。 2. 对于 15 至 23 摄氏度的温度，IPSC 衰减时间常数 (tau S) 的温度系数值在 2.0 范围内，表明扩散不是限速步骤。 3.用Li+、Ch+或N-甲基-D-葡萄糖胺替代细胞外液中80%的Na+，分析阻断Na(+)依赖性甘氨酸摄取的效果。这些程序增强了甘氨酸离子电渗疗法产生的最大抑制分流，从而表明摄取可能缓冲裂隙中递质的浓度。相比之下，Na+替代物对循环性侧支IPSC的tau没有影响，其涉及中间神经元池的同步激活并且具有单指数衰减（tau大约10-11毫秒）。 4.刺激对侧第八神经产生的非突触IPSC的衰减阶段有快和慢的成分，拖尾持续长达100毫秒，特别是在重复神经刺激的情况下。尾部是抑制性的，正如其对逆向动作电位的分流所揭示的那样，并且它至少部分是 Cl 依赖性的。然而，通过灌输谷氨酸受体拮抗剂 6-氰基-7-硝基喹喔啉-2,3-二酮 (CNQX) 和 DL-2-氨基-5-膦酰基戊酸 (APV) 可以加速这一过程。在存在这些阻滞剂的情况下，IPSC 衰减保持双指数（对于单次和突发刺激，tau 快 = 5.2 和 5.9 ms，tau 慢 = 94 和 130 ms）。在这种情况下阻断摄取不会改变 tau 快或 tau 慢。 5. 我们的结论是，主动摄取机制不会塑造甘氨酸能IPSC，包括可能包括由于递质的持久性而做出贡献的更持久的成分。相反，仅扩散就足以以比通道解除结合更快的速率去除甘氨酸。 6. 为了测试甘氨酸是否可以扩散到邻近的兴奋性突触并增强 N-甲基-D-天冬氨酸受体的激活，通过压力将 CNQX 和 APV 局部施加到 M 细胞胞体，但它们对第八神经诱发反应的长期衰减没有影响。因此，当添加到超融合物中时，拮抗剂的作用是在网络水平上发挥的。
1. The question of whether active uptake limits the duration of action of the inhibitory transmitter glycine has been addressed in vivo at inhibitory synapses on the goldfish Mauthner (M) cell. The kinetics of inhibitory postsynaptic potentials and inhibitory postsynaptic currents (IPSCs) evoked antidromically and by eighth-nerve stimulation were recorded in control and in conditions expected to block glycine uptake or slow its diffusion. Theoretical considerations, based on simulated quantal currents, predicted that if diffusion was slow, rapid uptake of glycine would be required and its block would prolong the synaptic responses. 2. Temperature coefficient values for IPSC decay time constants (tau S) are in the range of 2.0 for temperatures between 15 and 23 degrees C, suggesting that diffusion is not the rate-limiting step. 3. Li+, Ch+, or N-methyl-D-glucamine were substituted for 80% of the Na+ in the extracellular fluid to analyze the effects of blocking the Na(+)-dependent glycine uptake. These procedures enhanced the maximum inhibitory shunt produced by glycine iontophoresis, leading to the suggestion that uptake may buffer the concentration of the transmitter in the cleft. In contrast, the Na+ substitutes had no effect on the tau of the recurrent collateral IPSC, which involves synchronous activation of a pool of interneurons and has a monoexponential decay (tau approximately 10-11 ms). 4. The decay phase of the disynaptic IPSCs produced by stimulating the contralateral eighth nerve has fast and slow components, with a prolonged tail lasting up to 100 ms, particularly in the case of repetitive nerve stimulation. The tail is inhibitory, as revealed by its shunt of the antidromic action potential, and it is at least partially Cl- dependent. However, it can be accelerated by superfusion with the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and DL-2-amino-5-phosphonopentanoic acid (APV). In the presence of these blockers, the IPSC decay remains biexponential (tau fast = 5.2 and 5.9 ms, tau slow = 94 and 130 ms for single and burst stimuli, respectively). Blocking uptake in this condition did not modify tau fast or tau slow. 5. We conclude that an active uptake mechanism does not shape glycinergic IPSCs, including the longer-lasting components that might include a contribution due to persistence of the transmitter. Rather, diffusion alone is sufficient to remove glycine at a rate faster than channel unbinding. 6. To test whether glycine might diffuse to adjacent excitatory synapses and enhance activation of N-methyl-D-aspartate receptors, CNQX and APV were applied locally, by pressure, to the M cell soma, but they had no effect on the prolonged decay of eighth-nerve-evoked responses. Thus the effects of the antagonists when added to the superfusate are exerted at the network level.