Axonal L-type Ca2+ channels and anoxic injury in rat CNS white matter

We studied the magnitude and route(s) of Ca2+ flux from extra- to intracell ular compartments during anoxia in adult rat optic nerve (RON), a central w hite matter tract, using Ca2+-sensitive microelectrodes to monitor extracel lular [Ca2+] ([ Ca2+](o)). One hour of anoxia caused a rapid loss of the st imulus-evoked compound action potential (CAP), which partially recovered fo llowing re-oxygenation, indicating that irreversible injury had occurred. A fter an initial increase caused by extracellular space shrinkage, anoxia pr oduced a sustained decrease of 0.42 mM (29%) in [Ca2+](o). We quantified th e [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 R ON injury was predicted by the amount of Ca2+ leaving the extracellular spa ce. Bepridil, 0 Na+ artificial cerebrospinal fluid or tetrodotoxin reduced suspected Ca2+ influx during anoxia implicating reversal of the Na+-Ca2+ ex changer as a route of Ca2+ influx. Diltiazem reduced suspected Ca2+ influx during anoxia, suggesting that Ca2+ influx via L-type Ca2+ channels is a ro ute of toxic Ca2+ influx into axons during anoxia. Immunocytochemical stain ing was used to demonstrate and localize high-threshold Ca2+ channels. Only alpha1(C) and alpha1(D) subunits were detected, indicating that only L-typ e Ca2+ channels were present. Double labeling with anti-neurofilament antib odies or anti-glial fibrillary acidic protein antibodies, localized L-type Ca2+ channels to axons and astrocytes.