The use-dependent sodium channel blocker mexiletine is neuroprotective against global ischemic injury

Abstract

Mechanisms responsible for anoxic/ischemic cell death in mammalian CNS grey and white matter involve an increase in intracellular Ca 2+, however the routes of Ca 2+ entry appear to differ. In white matter, pathological Ca 2+ influx largely occurs as a result of reversal of Na +–Ca 2+ exchange, due to increased intracellular Na + and membrane depolarization. Na + channel blockade has therefore been logically and successfully employed to protect white matter from ischemic injury. In grey matter ischemia, it has been traditionally presumed that activation of agonist (glutamate) operated and voltage dependent Ca 2+ channels are the primary routes of Ca 2+ entry. Less attention has been directed towards Na +–Ca 2+ exchange and Na + channel blockade as a protective strategy in grey matter. This study investigates mexiletine, a use-dependent sodium channel blocker known to provide significant ischemic neuroprotection to white matter, as a grey matter protectant. Pentobarbital (65 mg/kg) anesthetized, mechanically ventilated Sprague–Dawley rats were treated with mexiletine (80 mg/kg, i.p.). Then 25 min later the animals were subjected to 10 min of bilateral carotid occlusion plus controlled hypotension to 50 Torr by temporary partial exsanguination. Animals were sacrificed with perfusion fixation after 7 days. Ischemic and normal neurons were counted in standard H&E sections of hippocampal CA1 and the ratio of ischemic to total neurons calculated. Mexiletine pre-treatment reduced hippocampal damage by approximately half when compared to control animals receiving saline alone (45 vs. 88% damage, respectively; P<0.001). These results suggest that mexiletine (and perhaps other drugs of this class) can provide protection from ischemia to grey matter as well as white matter.