Date Published: April 12, 2019
Publisher: Public Library of Science
Author(s): Shohei Yokoyama, Ichiro Nakagawa, Yoichi Ogawa, Yudai Morisaki, Yasushi Motoyama, Young Su Park, Yasuhiko Saito, Hiroyuki Nakase, Alexander A. Mongin.
A mild ischemic load applied after a lethal ischemic insult reduces the subsequent ischemia–reperfusion injury, and is called ischemic postconditioning (PostC). We studied the effect of ischemic PostC on synaptic glutamate release using a whole-cell patch-clamp technique. We recorded spontaneous excitatory post-synaptic currents (sEPSCs) from CA1 pyramidal cells in mouse hippocampal slices. The ischemic load was perfusion of artificial cerebrospinal fluid (ACSF) equilibrated with mixed gas (95% N2 and 5% CO2). The ischemic load was applied for 7.5 min, followed by ischemic PostC 30 s later, consisting of three cycles of 15 s of reperfusion and 15 s of re-ischemia. We found that a surging increase in sEPSCs frequency occurred during the immediate-early reperfusion period after the ischemic insult. We found a significant positive correlation between cumulative sEPSCs and the number of dead CA1 neurons (r = 0.70; p = 0.02). Ischemic PostC significantly suppressed this surge of sEPSCs. The mitochondrial KATP (mito-KATP) channel opener, diazoxide, also suppressed the surge of sEPSCs when applied for 15 min immediately after the ischemic load. The mito-KATP channel blocker, 5-hydroxydecanoate (5-HD), significantly attenuated the suppressive effect of both ischemic PostC and diazoxide application on the surge of sEPSCs. These results suggest that the opening of mito-KATP channels is involved in the suppressive effect of ischemic PostC on synaptic glutamate release and protection against neuronal death. We hypothesize that activation of mito-KATP channels prevents mitochondrial malfunction and breaks mutual facilitatory coupling between glutamate release and Ca2+ entry at presynaptic sites.
In the heart and brain, a brief sublethal ischemic load prior to lethal ischemia induces tolerance to the subsequent ischemic insult, a phenomenon known as ischemic preconditioning . Ischemic preconditioning provides powerful protection against ischemia-reperfusion injury in both the heart and nervous system [2–6]. A previous study demonstrated that ATP-dependent potassium (KATP) channel opener-induced chemical preconditioning is involved in neuroprotection . Although preconditioning is an effective treatment for protection against cerebral and myocardial damage, its clinical use is limited, because the onset of an ischemic stroke or cardiac infarction is extremely difficult to predict. In contrast, the onset of reperfusion following symptomatic ischemia is relatively easy to predict. Therefore, the concept of postconditioning (PostC) in the brain and heart has been proposed [8–10]. PostC consists of several repeated cycles of brief re-occlusion and reperfusion, which is started early in the reperfusion period after a lethal ischemic load . Zhao and colleagues first reported that PostC reduces cerebral ischemia–reperfusion injury .
In the present electrophysiological study using mouse hippocampal slices, we found that reperfusion after an ischemic insult caused a surge of synaptic glutamate release. Ischemic PostC suppressed this surge. Excessive glutamate release during the post-ischemia-reperfusion period tended to promote neuronal death, even early during the post-ischemic period, demonstrating that PostC in this study may have had a neuroprotective effect. Chemical PostC with the mito-KATP channel opener, DZX, also suppressed the surge of synaptic glutamate release. In addition, the mito-KATP channel blocker, 5-HD, attenuated the suppressive actions of ischemic and chemical PostC on the surge of synaptic glutamate release. These results suggest that the opening of mito-KATP channels is involved in suppression of the surge of glutamate release and in protection against neuronal death.
The present electrophysiological study demonstrated that synaptic glutamate release was drastically increased during the immediate-early reperfusion period after an ischemic insult, and that ischemic PostC suppressed this surge of glutamate release. Chemical PostC with DZX equivalently reduced the increase in glutamate release. These actions were weakened by pre-application of 5-HD, suggesting that the opening of mito-KATP channels is essential for suppression of the surge in glutamate release.