Voltage-gated calcium channels in CNS white matter: Role in anoxic injury

1. The effect of Ca²⁺ channel antagonists on the extent of anoxia- induced white matter injury was studied in the rat optic nerve, a white matter tract. Compound action potentials (CAPs) were recorded before and after a standard 60-min anoxic period to assess the extent of anoxic injury. 2. The L-type Ca²⁺ channel antagonists verapamil (90 μM), diltiazem (50 μM), and nifedepine (2.5 μM) significantly protected the rat optic nerve from anoxic injury. Mean recovery of CAP area was 51.3 ± 3.0% (mean ± SE, n = 8, P < 0.01), 65.6 ± 5.3% (n = 8, P < 0.01), and 54.3 ± 6.1% (n = 8, P < 0.01), respectively. Mean CAP recovery under control conditions was 35.2 ± 0.3 (n = 33). 3. Simultaneous block of L-type and N-type Ca²⁺ channels by coapplication of 50 μM diltiazem and 1 μM SNX-124 [synthetic ω-conotoxin (CgTx) GVIA], resulted in postanoxic CAP recovery of 73.6 ± 6.0% (n = 12), significantly larger than CAP recovery in diltiazem alone (P < 0.001). Block of CgTx MVIIC-sensitive channels in addition to L-type and N-type channels by coapplication of 50 μM diltiazem + 1 μM SNX-230 + 1 μM SNX-124 failed to produce any additional increase in CAP recovery (71.3 ± 5.6%, n = 8). Application of 1 μM SNX-124 alone did not significantly protect against anoxic injury (CAP recovery, 36.3 ± 2.9%, n = 10). 4. While N-type and L- type Ca²⁺ channels were involved in the development of anoxic injury, perfusion with 50 μM diltiazem or 1-2 μM SNX-124 had no effect on the normoxic CAP. Perfusion with 1 μM SNX-230, however, produced a gradual increase in CAP area over 130 min. CAP area reached an average of 114.2 ± 5.8% of the initial value (n = 8), compared with a reduction of CAP area to 88.9 ± 5.13% of the initial value (n = 8) found after 130 min under control conditions (P < 0.005). 5. Concentrations of Co²⁺ and Cd²⁺ known to block Ca²⁺ channels irreversibly reduced CAP area. This effect of Cd²⁺ and Co²⁺ was potentiated by co-perfusion with 30 mM K⁺. Low concentrations of Cd²⁺ (2 μM) and Co²⁺ (200 μM), which were below concentrations that reduced CAP area in 30 mM K⁺, had a protective action against anoxic injury but probably did not block the majority of Ca²⁺ channels. 6. We conclude that L-type and N-type Ca²⁺ channels are involved in the development of anoxic injury in CNS white matter. Ca²⁺ channels are therefore involved in anoxic injury in both CNS gray matter and white matter, suggesting that strategies directed against Ca²⁺ influx via Ca²⁺ channels may provide protection for a broad spectrum of CNS regions during stroke. In addition, we observed changes in the CAP following block of CgTx MVIIC-sensitive Ca²⁺ channels, indicating the presence of three pharmacologically distinct Ca²⁺ channels in CNS white matter.