Hypoxic-Ischemic Brain Injury Induces an Acute Microglial Reaction in Perinatal Rats1

Abstract

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 > or = 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 hypoxic-ischemic brain injury induced rapid accumulation of activated microglia in hypoxic-ischemic forebrain.

Lipopolysaccharide Preconditioning Reduces Neuroinflammation Against Hypoxic Ischemia and Provides Long-Term Outcome of Neuroprotection in Neonatal Rat

“…Activated microglias are the hallmark of HI-induced neuroinflammation and exacerbate injury in ischemic areas because of their participation in inflammatory responses (21). One of the earliest events to occur after neonatal HI is the appearance of abundant numbers of activated microglias at 4 h and peaked at 2-4 d after HI (22). Distinct increases of proinflammatory cytokines, such as TNFalpha, could also be found 0 -24 h post-HI (23).…”

Hypoxic-Ischemic Oligodendroglial Injury in Neonatal Rat Brain

“…Brains were removed, postfixed in 4% paraformaldehyde (16 h, at 4°C), and cryoprotected in graded (5% to 25%) sucrose solutions in PBS, as previously described (14). Coronal sections (16 m, at 64-m intervals) were collected on gelatin-coated slides; at least six sections/ brain, three at the level of the mid-striatum and three at mid dorsal hippocampus, were assayed.…”

“…In this model, lengthening the duration of hypoxic exposure results in a progressive increase in the severity of brain damage (12); the temporal threshold to elicit tissue injury is at approximately 1.5 h, and more prolonged HI (2-2.5 h) commonly elicits extensive neuronal loss in the ipsilateral cortex, hippocampus, striatum, and thalamus (13). Additional neuropathologic features of the HI lesion include an acute and sustained microglial and monocyte infiltrate (13,14), reactive gliosis (13,15), and the evolution of cortical cavitary infarcts (13).…”

“…In a widely used neonatal rat stroke model, elicited by unilateral carotid artery ligation and timed exposure to moderate hypoxia (8% oxygen) in 7-d-old (P7) rats, ipsilateral white matter injury occurs, together with more widespread tissue injury (11)(12)(13)(14)(15). In this model, lengthening the duration of hypoxic exposure results in a progressive increase in the severity of brain damage (12); the temporal threshold to elicit tissue injury is at approximately 1.5 h, and more prolonged HI (2-2.5 h) commonly elicits extensive neuronal loss in the ipsilateral cortex, hippocampus, striatum, and thalamus (13).…”

Elevated total peripheral leukocyte count may identify risk for neurological disability in asphyxiated term neonates

“…In P7 rats, exposure to hypoxia-ischemia induced rapid accumulation of activated microglia within 4 h. 27 Activated microglia, as well as astrocytes, are known to secrete proinflammatory cytokines. 27 Cytokines induce leukocyte proliferation, increase the number of circulating neutrophils and promote endothelial transmigration and chemotaxis into areas of injury. 28 In 3-week-old Fisher rats, infiltration and accumulation of neutrophils into injured brain areas was noted 6 h after the injection of pro-inflammatory cytokines.…”