Glial changes following an excitotoxic lesion in the CNS—I. Microglia/macrophages

Abstract

When an area of the adult rat CNS is depleted of neurons by an in situ excitotoxic injection, afferent axons to the area exhibit morphological alterations reminiscent of growth cones. These morphological changes are likely to be related to the deprivation of target cells. In addition, however, the area of neuronal loss is itself the site of profound changes in glial cell content, and altered axon-glial interactions may play a role in the axonal changes. In an attempt to define these interactions, we have undertaken a systemic study of glial populations in excitotoxically lesioned CNS over time. The microglial/macrophagic response is analysed in this paper; the astrocytic response is described in the companion paper [Dusart et al. (1991) Neuroscience 45, 541–549]. The microglial/macrophagic response was studied following kainic acid-induced neuronal loss in the thalamus of the adult rat. These microglial/macrophagic cells were labeled with the B4 isolectin from Griffonia simplicifolia, and the time-course of their response was studied between one day and one year post-lesion. This time-course study revealed different stages in the evolution of the response. At one day post-lesion, cell counts indicated that there was no increase in the number of non-neuronal cells in the neuron-depleted area. However, activated labeled cells were present in the entire thalamus on the side of the lesion, neuron-depleted or not. They were characterized by both increased lectin-binding and altered morphology when compared to quiescent microglia. In the absence of recruitment and/or proliferation, this result indicates that the early response consisted solely of the activation of resident microglia. By contrast, we observed a progressive increase in the number of non-neuronal cells in the lesion from four to 15 days post-lesion. A recruitment of blood-borne monocytes was apparent, and the observation of mitotic labeled cells indicated a proliferation of microglial/macrophagic cells in situ. There was a progressive decrease in the microglial/macrophagic reaction that began one month after lesion. In a thin band of parenchyma surrounding the neuron-depleted area, activated microglial/macrophagic cells were seen contacting neurons, and clusters of glial cells were observed around neurons up to one year post-lesion. These results suggest that neurons around the lesion site itself may be injured, secondarily, from a long term deleterious effect of the inflammatory process. This study allows us to conclude that activated microglia/macrophages are the predominant glial cell type in the excitotoxically lesioned CNS over the first weeks. It is known that, in their activated stage, these cells are able to release numerous factors acting on other cell types. It is proposed, therefore, that microglia/macrophages may play a key role in the evolution of cellular populations observed in the area of neuronal loss.