Two temporal stages of oligodendroglial response to excitotoxic lesion in the gray matter of the adult rat brain.

Excitotoxic lesions in the gray matter induce profuse demyelination of passage and afferent fibers in areas of neuronal loss, independent of Wallerian degeneration. The time course of this phenomenon, which extends over weeks after the excitotoxin injection, suggests that demyelination is not related only to a direct effect of the toxin. In order to define mechanisms at work, a parallel study of myelin and oligodendrocytes was carried out following kainate injections into the adult rat thalamus. Within the 1st day postlesion, myelin alteration appeared throughout the area exhibiting neuronal loss, while the number of oligodendrocytes fell by 45%. No apoptotic oligodendrocytes were identified at that time. Over the following 2 days, there was no further loss of myelin and oligodendrocytes, but there was an increase in the number of oligodendrocytes displaying typical signs of apoptosis as revealed with TUNEL-end-labeled nuclei, Hoechst-labeled condensed chromatin bodies, or bax immunoreactivity. This resulted in a second, progressive loss of both myelin and oligodendrocytes leading to their almost complete disappearance 2 weeks postlesion. These results demonstrate two temporal stages of oligodendroglial cell death. The excitotoxin injection resulted in the rapid destruction of a first oligodendroglial population, most probably by necrosis. A second population died in a delayed manner from apoptosis. This second wave of death coincided with an activated microglia/macrophage invasion of the lesion, suggesting that delayed oligodendroglial death results from toxic microglia/macrophage effects. In addition, the longest surviving oligodendrocytes were located next to reactive astrocytes, suggesting the existence of trophic interactions between these two glial populations.