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Activated microglia may contribute to the progression of neuronal injury after a wide range of CNS insults. In this study, we used two complementary methods to evaluate acute changes in the morphology and regional distribution of microglia induced by a focal hypoxic-ischemic insult in 7-d-old (P7) rats. To elicit injury, P7 rats underwent right carotid ligation followed by 3 h of 8% O2 exposure; rats were killed 10 min to 5 d later (n >= 3/group). A histochemical assay using Griffonia simplicifolia B4-isolectin enabled detection of both resting and activated microglia in tissue sections; vascular cells were also reactive. Activated microglia were also identified immunocytochemically using a macrophage-specific MAb, ED-1. In normal P7-12 brain, lectin, and ED-1 immunoreactive-activated microglia were concentrated in white matter; lectin-positive resting, ramified microglia were also detected throughout the gray and white matter. Subtle morphologic evidence of microglial activation was noted 10 min posthypoxia-ischemia in the lesioned right cerebral hemisphere; activated microglia began to accumulate within the next 4 h. Accumulation of lectin-positive activated microglia peaked at 2-4 d posthypoxia-ischemia. ED-1 immunoreactive-microglia were first noted 4 h after hypoxic-ischemic injury in the lesioned right hemisphere, and there was a corresponding increase in accumulation over the first 48 h posthypoxia-ischemia. In the left hemisphere, contralateral to the ligation, no increase in activated microglia were detected with either method. In brain sections where no neuronal injury was evident, activated microglia did not accumulate. These data demonstrate that perinatal hypoxicischemic brain injury induced rapid accumulation of activated microglia in hypoxic-ischemic forebrain.

There is increasing evidence that phagocytic cells, including microglia and macrophages, respond acutely to diverse forms of CNS injury. In the adult CNS, ischemic and excitotoxic injury stimulate a well characterized microglial response that includes both changes in morphology and increased accumulation in regions of injury (1-9). Considerable data indicate that, in addition to phagocytosis of degenerating elements, these cells secrete a wide range of soluble factors, such as cytokines (10-13), substances with excitatory amino acid agonist properties (14), and glial promoting factors (10, 11), that may influence the extent of neuronal injury, axonal growth, synaptic plasticity, and astroglial hyperplasia. Microglia also play important roles in normal CNS development (15), and activated microglia are detectable in normal immature rodent brain; yet, little is known about the response of these cells after injury in the developing brain. In the only study that has attempted to compare the microglial response in the developing and adult brain (1), lesions in the adult visual cortex induced slower cell death and a much more protracted phagocytic response than in younger animals.

(C) International Pediatrics Research Foundation, Inc. 1996. All Rights Reserved.