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

Brown, Angus M., Ruth E. Westenbroek, William A. Catterall, and Bruce R. Ransom.  Axonal L-type Ca²⁺channels and anoxicinjury in rat CNS white matter.  J Neurophysiol  85: 900 –911, 2001.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 fo  llowing 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 α1_C and α1_D 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.