Inosine, Calcium Channels, and Neuroprotection Against Ischemic Brain Injury
To the Editor:
We read with great interest the article by Shen et al,1 dealing with neuroprotective effects of inosine against ischemic brain injury. The results of their study demonstrated that intracerebroventricular administration of inosine before middle cerebral artery occlusion in rats resulted in a higher level of locomotor activity and less cerebral infarction. In addition, they indicated that coadministration of selective A3 receptor antagonist MRS1191 significantly attenuated inosine-mediated protection. In the electrophysiological study, it was shown that inosine antagonized glutamate-induced excitation in cerebral cortical neurons. The authors proposed that inosine may inhibit glutamate postsynaptic responses and reduce cerebral infarction via the activation of A3 receptors.
Several studies have shown the mechanisms for glutamate-induced excitation of neural cells. In a study we presented earlier, changes in acetylcholine (ACh) release evoked by glutamate was investigated in rat central nervous system.2,3 In an in vitro study, we showed that glutamate increased the release of ACh from rat striatum, which was inhibited by the N-methyl-d-aspartate type of glutamate receptor antagonist MK-801.3 Furthermore, it was demonstrated that the calcium-channel blocker verapamil significantly reduced glutamate-evoked ACh release and that the inhibitory effect was more pronounced in the presence of magnesium.2 It might be possible that the calcium-channel blocker could inhibit the N-methyl-d-aspartate receptor–dependent calcium current in neural cells. It was also shown that the calcium-channel blocker nilvadipine blocked glutamate-evoked intracellular calcium increase with a concomitant reduction in glutamate-induced apoptosis in purified retinal ganglion cells,4 suggesting that the calcium-channel blocker might inhibit glutamate-induced apoptotic cell death by blocking calcium influx. Therefore, we would like to know whether inosine and calcium-channel blockers might have a synergistic effect in neural protection in the present study by Shen et al. Although the authors demonstrated that inosine did not alter basal glutamate release, nor did it reduce ischemia-evoked glutamate overflow in the central nervous system, it can be speculated that inosine might interfere with glutamate receptor–mediated calcium conductance in neural cells and might have therapeutic potential as a neuroprotective agent against stroke-induced damage.
We thank Dr Tsuda for expressing interest in our article.1 As indicated by Dr Tsuda, increasing evidence has suggested that calcium channels are involved in glutamate-mediated neurodegeneration. In our study, we reported that inosine suppresses glutamate-evoked neuronal excitation in cerebral cortex. It is possible that inosine may have indirect actions on calcium channels through the inhibition of glutamate response during cerebral ischemia. Currently, there are no reports suggesting that calcium-channel blockers have synergistic effects on inosine-mediated response in the central nervous system. Our preliminary studies showed that an N-type calcium channel blocker had a trend toward increasing inosine-mediated protection against ischemic brain injury in adult Sprague-Dawley rats. Animals were injected intracerebroventricularly, before middle cerebral artery occlusion (MCAo), with ω-conotoxin GVIA (0.1 μg/10 μL, n=6) or ω-conotoxin GVIA+inosine (25 nmol, n=7). Compared with our previous data,1 ω-conotoxin GVIA alone did not significantly reduce the size of infarction. Coadministration of ω-conotoxin GVIA and inosine significantly reduced volume of infarction compared with the stroke animals pretreated with vehicle (52.1±28.9 mm3 versus 187.1±9.0 mm3; P<0.05; 1-way ANOVA). Although the mean of infarction volume is further reduced by ω-conotoxin GVIA in the animals pretreated with inosine, the difference in protection between ω-conotoxin GVIA+inosine (52.1±28.9 mm3) and inosine alone (118.5±18.4 mm3) is not statistically significant. We observed that the majority of animals that received ω-conotoxin GVIA with or without inosine developed seizures after MCAo. Other studies have shown nifedipine or felodipine suppressed hypoxia-evoked inosine release in rat cerebral cortex.2 The combined effects of inosine and calcium channel blockers in cerebral ischemia are complicated by such observations. Further studies are needed to definitively address the interaction of inosine and calcium channels.