CD2+-INDUCED INJURY IN CNS WHITE-MATTER

1. The affect of extracellular Cd2+ on CNS white matter was studied us ing an isolated rat optic nerve preparation. A 100-min exposure to 200 mu M Cd2+ reduced the area of the compound action potential (CAP) rec orded from the optic nerve to 32.6 +/- 3.8% (mean +/- SE) of the preex posure area, compared with a reduction to 74.9 +/- 2.9% after 100 min in control conditions (P > 0,001). This CAP reduction was not reversed after 120 min of reperfusion with Cd2+-free solution, or by perfusion with Cd2+ chelators. 2. Cd2+-induced CAP loss occurred in the absence of extracellular Ca2+. increasing extracellular Ca2+ concentration to 16 mM, however, prevented Cd2+-induced CAP loss. Once evident, Cd2+ i nduced CAP reduction could not subsequently be reversed by addition of 16 mM Ca2+. 3. Low concentrations of Cd2+ (60 mu M) did not significa ntly reduce CAP area. This concentration of Cd2+ combined with high ex tracellular K+ (30 mM) caused CAP loss that was blocked by 10 mu M nif edipine, an antagonist of L-type voltage-gated Ca2+ channels. 4. Treat ment with pharmacological inhibitors of membrane proteins known to be inhibited by Cd2+ did not affect the CAP. These included inhibitors of voltage-gated Ca2+ channels, Ca2+-activated K+ channels, Ca2+-ATPase and the Na+/Ca2+ exchanger. 5. Treatment with pharmacological agents t hat inhibit calmodulin or disrupt tubulin, two intracellular proteins affected by Cd2+ did not affect CAP area. 6. The effect of Cd2+ was no t prevented by pretreatment with (+)-cyanidanol-3, an agent that preve nts Cd2+-induced lipid peroxidation. 7. Treatment with antimycin A, a inhibitor of mitochondrial respiration, resulted in irreversible CAP r eduction with a time course and extent similar to that produced by 200 mu M Cd2+. Cd2+-induced CAP reduction was prevented by 1 mM cysteine, which prevents Cd2+-induced disruption of mitochondrial respiration.8 . The ultrastructure of optic nerves exposed to 200 mu M Cd2+ for 100 min was characterized by swollen mitochondria with disrupted cristae a nd dissolution of microtubules, which were replaced by flocculent debr is. Occasional regions of axonal swelling and empty spaces beneath the myelin also were found. Qualitatively similar changes in mitochondria and cytoskeletal elements were found in optic nerves exposed to antim ycin A for 100 min. Astrocytes also displayed disrupted mitochondria a nd had an electron lucent appearance under both conditions. 9. The neu rological injury produced by exposure to Cd2+ is characterized by lesi ons of CNS white matter. Our results indicate that Cd2+-induced white matter injury in vitro results largely from disruption of mitochondria l respiration after Cd2+ influx through routes that include voltage-ga ted Ca2+ channels.