Lee Seul Yi, Kim Jun Hee
Department of Physiology.
Centre for Biomedical Neuroscience, University of Texas Health Science Centre, San Antonio, Texas, 78229, USA.
J Physiol. 2015 Jul 1;593(13):2793-806. doi: 10.1113/JP270060. Epub 2015 May 22.
Here we demonstrate presynaptic responses and mechanisms of increased vesicular glutamate release during in vitro ischaemia in the calyx of Held terminal, an experimentally accessible presynaptic terminal in the CNS. The ischaemia-induced increase in presynaptic Ca(2+) was mediated by both Ca(2+) influx and Ca(2+) -induced Ca(2+) release from intracellular stores. The reverse operation of the plasma membrane Na(+) /Ca(2+) exchanger (NCX) plays a key role in Ca(2+) influx for triggering Ca(2+) release from intracellular stores at presynaptic terminals during in vitro ischaemia. Ca(2+) uptake via NCX underlies the ischaemia-induced Ca(2+) rise and the consequent increase in vesicular glutamate release from presynaptic terminals in the early phase of brain ischaemia.
An early consequence of brain ischaemia is an increase in vesicular glutamate release from presynaptic terminals. However, the mechanisms of this increased glutamate release are not fully understood. Here we studied presynaptic responses and mechanisms of increased glutamate release during in vitro ischaemia, using pre- and postsynaptic whole-cell recordings and presynaptic Ca(2+) imaging at the calyx of Held synapse in rat brainstem slices. Consistent with results from other brain regions, in vitro ischaemia significantly increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) without affecting their amplitude, suggesting that ischaemia enhances vesicular glutamate release from presynaptic terminals. We found that ischaemia-induced vesicular glutamate release was dependent on a rise in basal Ca(2+) at presynaptic terminals, which resulted from extracellular Ca(2+) influx and Ca(2+) release from intracellular stores. During early ischaemia, increased Ca(2+) influx into presynaptic terminals was due to reverse operation of the plasma membrane Na(+) /Ca(2+) exchanger (NCX) rather than presynaptic depolarization or voltage-activated Ca(2+) currents. KB-R7943, an inhibitor of NCX, prevented the ischaemia-induced increases in presynaptic Ca(2+) and vesicular glutamate release. In addition, the removal of extracellular Na(+) completely inhibited the ischaemia-induced Ca(2+) rise. It therefore appears that a link between Na(+) accumulation and Ca(2+) uptake via NCX underlies the ischaemia-induced Ca(2+) rise and the consequent increase in vesicular glutamate release from presynaptic terminals in the early phase of brain ischaemia.
在此我们展示了在体外缺血期间，在Held终扣（中枢神经系统中一个可通过实验研究的突触前终扣）中突触前的反应以及囊泡谷氨酸释放增加的机制。缺血诱导的突触前Ca(2+)增加是由Ca(2+)内流和细胞内钙库的Ca(2+)诱导的Ca(2+)释放介导的。质膜Na(+) /Ca(2+)交换体（NCX）的反向运转在体外缺血期间，在突触前终扣触发细胞内钙库释放Ca(2+)的Ca(2+)内流中起关键作用。通过NCX的Ca(2+)摄取是脑缺血早期缺血诱导的Ca(2+)升高以及随后突触前终扣囊泡谷氨酸释放增加的基础。
脑缺血的一个早期后果是突触前终扣囊泡谷氨酸释放增加。然而，这种谷氨酸释放增加的机制尚未完全阐明。在此我们利用大鼠脑干切片中Held突触的突触前和突触后全细胞记录以及突触前Ca(2+)成像，研究了体外缺血期间突触前的反应以及谷氨酸释放增加的机制。与其他脑区的结果一致，体外缺血显著增加了自发性兴奋性突触后电流（sEPSCs）的频率，而不影响其幅度，这表明缺血增强了突触前终扣的囊泡谷氨酸释放。我们发现缺血诱导的囊泡谷氨酸释放依赖于突触前终扣基础Ca(2+)的升高，这是由细胞外Ca(2+)内流和细胞内钙库的Ca(2+)释放导致的。在缺血早期，突触前终扣Ca(2+)内流增加是由于质膜Na(+) /Ca(2+)交换体（NCX）的反向运转，而不是突触前去极化或电压激活的Ca(2+)电流。NCX抑制剂KB-R7943可阻止缺血诱导的突触前Ca(2+)增加和囊泡谷氨酸释放。此外，去除细胞外Na(+)完全抑制了缺血诱导的Ca(2+)升高。因此，似乎Na(+)积累与通过NCX的Ca(2+)摄取之间的联系是脑缺血早期缺血诱导的Ca(2+)升高以及随后突触前终扣囊泡谷氨酸释放增加的基础。
