A quantitative in situ hybridization and polymerase chain reaction study of microglial-macrophage expression of interleukin-1β mRNA following permanent middle cerebral artery occlusion in mice

Interleukin-1β (IL-1β) is known to play a central role in ischemia-induced brain damage in rodents. In comparison to the rat, however, the available data on the cellular synthesis of IL-1β mRNA and protein in the mouse are very limited. Here, we report on the time profile, the topography and the quantitative, cellular expression of IL-1β mRNA in mice subjected to permanent occlusion of the distal middle cerebral artery (MCA). The in situ hybridization analysis showed that IL-1β mRNA was expressed during the first post-surgical hour in a small number of high-expressing macrophage-like cells, located in cortical layers I and II of the future infarct. At 2 h, a significant number of faintly labeled IL-1β mRNA-expressing cells had appeared in the developing peri-infarct, and the number remained constant at 4 h and 6 h, when the hybridization signal began to distribute to the cellular processes. Quantitative PCR performed on whole hemispheres showed a significant 20-fold increase in the relative level of IL-1β mRNA at 12 h and a highly significant 42-fold increase at 24 h, at which time single IL-1β mRNA-expressing cells were supplemented by aggregates and perivascular infiltrates of intensely labeled IL-1β mRNA-expressing cells. Immunohistochemistry and double immunohistochemical stainings in addition to combined in situ hybridization, confirmed that the intensely labeled IL-1β mRNA-expressing and IL-1β protein synthesizing cells predominantly were glial fibrillary acidic protein-immunonegative, macrophage associated antigen-1-immunopositive microglia-macrophages. By day 5 there was a dramatic decline in the relative level of IL-1β mRNA in the ischemic hemisphere. In summary, the data provide evidence that permanent occlusion of the distal MCA in mice results in expression of IL-1β mRNA and IL-1β synthesis in spatially and temporally segregated subpopulations of microglia and macrophages.