Breukel A I, Besselsen E, Lopes da Silva F H, Ghijsen W E
Graduate School Neurosciences, Institute for Neurobiology, University of Amsterdam, The Netherlands.
Eur J Neurosci. 1998 Jan;10(1):106-14. doi: 10.1046/j.1460-9568.1998.00008.x.
A possible role of the N-methyl-D-aspartate receptor (NMDA-R) as a presynaptic autoreceptor was investigated using Percoll-purified hippocampus nerve terminals (synaptosomes). This preparation contained only a neglectable amount of postsynaptic structures. Two main effects of NMDA were observed. First, NMDA dose-dependently (10-100 microM) and in the absence of Mg2+, stimulated basal release of aspartate and glutamate, but not of GABA. MK801 (10 microM), an open NMDA-R-channel blocker, reduced this effect even below control levels, indicating endogenous NMDA-R activation. By superfusing synaptosomes, which prevents a tonic receptor occupation, also basal GABA release was stimulated by NMDA. The NMDA-induced potentiation of amino acid superfusate levels was blocked both by MK801 and Mg2+ (1 mM), was slow in onset and returned to baseline after NMDA-removal. The NMDA-effect was also found in the absence of extracellular Ca2+, suggesting that amino acids were released from a non-vesicular (cytoplasmic) pool. Secondly, in KCl-depolarized synaptosomes exposed to 1 mM Mg2+, NMDA did not affect the release of the amino acids. MK801, however, reduced the KCl-evoked Ca2+-independent release of aspartate and glutamate, but not of GABA. L-trans-PDC, the selective inhibitor of the glutamate/aspartate transporter, prevented this MK801-effect, suggesting a coupling between NMDA-Rs and these transporters. These data provide evidence for a presynaptic NMDA autoreceptor in rat hippocampus. We speculate on the role of this NMDA-R to depolarize the presynaptic membrane by Na+-entry, which may induce reversal of amino acid transporters and thereby releasing amino acids from a cytoplasmic pool.
利用Percoll纯化的海马神经末梢（突触体）研究了N-甲基-D-天冬氨酸受体（NMDA-R）作为突触前自身受体的可能作用。该制剂仅含有可忽略量的突触后结构。观察到NMDA的两种主要作用。首先，在不存在Mg2+的情况下，NMDA剂量依赖性地（10-100 microM）刺激天冬氨酸和谷氨酸的基础释放，但不刺激GABA的基础释放。开放的NMDA-R通道阻滞剂MK801（10 microM）将这种作用降低至甚至低于对照水平，表明内源性NMDA-R激活。通过灌注突触体（这可防止受体的持续性占据），NMDA也刺激基础GABA释放。NMDA诱导的氨基酸灌流液水平增强被MK801和Mg2+（1 mM）阻断，起效缓慢且在去除NMDA后恢复至基线。在不存在细胞外Ca2+的情况下也发现了NMDA效应，表明氨基酸是从非囊泡（细胞质）池中释放的。其次，在暴露于1 mM Mg2+的KCl去极化突触体中，NMDA不影响氨基酸的释放。然而，MK801减少了KCl诱发的天冬氨酸和谷氨酸的Ca2+非依赖性释放，但不影响GABA的释放。谷氨酸/天冬氨酸转运体的选择性抑制剂L-反式-吡咯烷二羧酸（L-trans-PDC）阻止了这种MK801效应，表明NMDA-R与这些转运体之间存在偶联。这些数据为大鼠海马中的突触前NMDA自身受体提供了证据。我们推测这种NMDA-R通过Na+内流使突触前膜去极化的作用，这可能诱导氨基酸转运体的逆转，从而从细胞质池中释放氨基酸。
