Mechanisms of ischaemic damage to central white matter axons: A quantitative histological analysis using rat optic nerve

The mechanism of ischaemic injury to white matter axons was studied by tran siently depriving rat optic nerves in vitro of oxygen and glucose. Light an d electron microscopic analysis showed that increasing periods of oxygen/gl ucose deprivation (up to 1 h) caused, after a 90-min recovery period, the a ppearance of increasing numbers of swollen axons whose ultrastructure indic ated that they were irreversibly damaged. This conclusion was supported by experiments showing that the damage persisted after a longer recovery perio d (3 h). To quantify the axonal pathology, an automated morphometric method , based on measurement of the density of swollen axons, was developed. Omis sion of Ca2+ from the incubation solution during 1 h of oxygen/glucose depr ivation land for 15 min either side) completely prevented the axonopathy (a ssessed following 90 min recovery). Omission of Na+ was also effective, tho ugh less so (70% protection). The classical Na+ channel blocker, tetrodotox in (1 mu M), provided 92% protection. In view of this evidence implicating Na+ channels in the pathogenesis of the axonal damage, the effects of three different Na+ channel inhibitors, with known neuroprotective properties to wards gray matter in in vivo models of cerebral ischaemia, were tested. The compounds used were lamotrigine and the structurally-related molecules, BW 619C89 and BW1003C87. All three compounds protected the axons to varying de grees, the maximal efficacies (observed at 30 to 100 mu M) being in the ord er: BW619C89 (>95% protection) > BW1003C87 (70%)> lamotrigine (50%). At a c oncentration affording near complete protection (100 mu M), BW619C89 had no significant effect on the optic nerve compound action potential. Experimen ts in which BW619C89 was added at different times indicated that its effect s were exerted during two distinct phases, one (accounting for about 50% pr otection) was during the early stage of oxygen/glucose deprivation itself a nd the other (also about 50%) during the first 15 min of recovery in normal incubation solution. The results are consistent with a pathophysiological mechanism in which Na entry through tetrodotoxin-sensitive Na+ channels contributes to Na+ loadi ng of the axoplasm which then results in a lethal Ca2+ overload through rev ersed Na+-Ca2+ exchange. The identification of BW619C89 as a compound able to prevent oxygen/glucose deprivation-induced injury to white matter axons without affecting normal nerve function opens the way to testing the import ance of this pathway in white matter injury in vivo. (C) 1999 IBRO. Publish ed by Elsevier Science Ltd.