Axonal L-Type Ca2+ Channels and Anoxic Injury in Rat CNS White Matter

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Abstract
We studied the magnitude and route(s) of Ca2+ flux from extra- to intracellular compartments during anoxia in adult rat optic nerve (RON), a central white matter tract, using Ca2+-sensitive microelectrodes to monitor extracellular [Ca2+] ([Ca2+]o). One hour of anoxia caused a rapid loss of the stimulus-evoked compound action potential (CAP), which partially recovered following re-oxygenation, indicating that irreversible injury had occurred. After an initial increase caused by extracellular space shrinkage, anoxia produced a sustained decrease of 0.42 mM (29%) in [Ca2+]o. We quantified the [Ca2+]o decrease as the area below baseline [Ca2+]o during anoxia and used this as a qualitative index of suspected Ca2+ influx. The degree of RON injury was predicted by the amount of Ca2+ leaving the extracellular space. Bepridil, 0 Na+ artificial cerebrospinal fluid or tetrodotoxin reduced suspected Ca2+ influx during anoxia implicating reversal of the Na+-Ca2+ exchanger as a route of Ca2+ influx. Diltiazem reduced suspected Ca2+ influx during anoxia, suggesting that Ca2+ influx via L-type Ca2+channels is a route of toxic Ca2+ influx into axons during anoxia. Immunocytochemical staining was used to demonstrate and localize high-threshold Ca2+channels. Only α1C and α1D subunits were detected, indicating that only L-type Ca2+ channels were present. Double labeling with anti-neurofilament antibodies or anti-glial fibrillary acidic protein antibodies, localized L-type Ca2+channels to axons and astrocytes.