Dudel Josef, Schramm Marion
Physiologisches Institut der Ludwig-Maximilians-Universität München, Schillerstrasse 46, D-80336 Münich, Germany.
Eur J Neurosci. 2003 Aug;18(4):902-10. doi: 10.1046/j.1460-9568.2003.02799.x.
Monoquantal excitatory postsynaptic currents were recorded by means of a perfused macropatch electrode from 9 to 15 micro m stretches of crayfish neuromuscular junctions. The excitatory transmitter l-glutamate superfused to a terminal inhibits quantal release by activating autoreceptors [Parnas et al. (1996) Eur. J. Neurosci., 8, 116-126]. Substances related to glutamate that do not activate glutamatergic postsynaptic channels, but are substrates of glutamate transporters, elicited analogous inhibitions, e.g. l- and d-aspartate and some other glutamate transport blockers. As expected, all transport blockers prolonged synaptic currents. Blockers that bind to the transporter receptors but are not transported did not inhibit release, but prevented inhibition by the transport substrates. It appears that autoinhibition is elicited by transport of glutamate or its analogues. Transport into cells is powered by symport of three Na+. To block the transport step electrochemically, extracellular Na+ concentration was lowered to one-quarter, but this surprisingly left the inhibition of release by glutamate unaffected, showing inhibition to be associated to a step between binding and transport. After binding a substrate, glutamate transporters open a parallel Cl- channel. Replacement of extracellular Cl- prevented Cl- current, and release inhibition by glutamate or aspartate was blocked. It is suggested that the flow of Cl- across the cell membrane, after binding a transport substrate, mediates autoinhibition. We measured a related reduction of presynaptic action potentials.
通过灌注式大膜片电极，在小龙虾神经肌肉接头9至15微米的节段上记录单量子兴奋性突触后电流。兴奋性递质L-谷氨酸超灌流到终末时，通过激活自身受体抑制量子释放[帕尔纳斯等人（1996年）《欧洲神经科学杂志》，8，116 - 126]。与谷氨酸相关但不激活谷氨酸能突触后通道、却是谷氨酸转运体底物的物质，也引发类似的抑制作用，例如L-和D-天冬氨酸以及一些其他谷氨酸转运体阻滞剂。正如预期的那样，所有转运体阻滞剂都延长了突触电流。与转运体受体结合但不被转运的阻滞剂并不抑制释放，但能阻止转运体底物的抑制作用。似乎自身抑制是由谷氨酸或其类似物的转运引发的。转运进入细胞是由三个Na⁺的同向转运驱动的。为了从电化学角度阻断转运步骤，将细胞外Na⁺浓度降至四分之一，但令人惊讶的是，这并未影响谷氨酸对释放的抑制作用，表明抑制作用与结合和转运之间的一个步骤相关。在结合底物后，谷氨酸转运体打开一个平行的Cl⁻通道。细胞外Cl⁻的置换阻止了Cl⁻电流，并且谷氨酸或天冬氨酸对释放的抑制作用也被阻断。有人提出，在结合转运体底物后，Cl⁻跨细胞膜的流动介导了自身抑制。我们测量了突触前动作电位的相应降低。
