Role of L- and N-type calcium channels in the pathophysiology of traumatic spinal cord white matter injury

Recent work has suggested a potential role for voltage-gated Ca2+ channels in the pathophysiology of anoxic central nervous system whits matter injury. To examine the relevance of these findings to neurotrauma, we conducted electrophysiological studies with inorganic Ca2+ channels blockers and L- and N-subtype-specific calcium channel antagonists in an in vitro model of spinal cord injury. Confocal immunohistochemistry was used to examine for localization of L- and N-type calcium channels in spinal cord white matter tracts. A 30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber and injured with a modified clip (2 g closing force) for 15 s. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential at two points with glass microelectrodes. The compound action potential decreased to 71.4 +/- 2.0% of control (P < 0.05) after spinal cord injury. Removal of extracellular Ca2+ promoted significantly greater recovery of compound action potential amplitude (86.3 +/- 7.6% of control; P < 0.05) after injury. Partial blockade of voltage-gated Ca2+ channels with cobalt (20 mu M) or cadmium (200 mu M) conferred improvement in compound action potential amplitude. Application of the L-type Ca2+ channel blockers diltiazem (50 mu M) or verapamil (90 mu M), and the N-type antagonist omega-conotoxin GVIA (1 mu M), significantly enhanced the recovery of compound action potential amplitude postinjury. Go-application of the L-type antagonist diltiazem with the N-type blocker omega-conotoxin GVIA showed significantly greater (P < 0.05) improvement in compound action potential amplitude than application of either drug alone. Confocal immunohistochemistry with double labelling for glial fibrillary acidic protein, GalC and NF200 demonstrated L- and N-type Ca2+ channels on astrocytes and oligodendrocytes, but not axons, in spinal cord white matter. In conclusion, the injurious effects of Ca2+ in traumatic central nervous system white matter injury appear to be partially mediated by voltage-gated Ca2+ channels. The presence of L- and N-type Ca2+ channels on periaxonal astrocytes and oligodendrocytes suggests a role for these cells in post-traumatic axonal conduction failure. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.