The intra-arterial injection of microglia protects hippocampal CA1 neurons against global ischemia-induced functional deficits in rats.

In the present study, we have attempted to elucidate the effects of the intra-arterial injection of microglia on the global ischemia-induced functional and morphological deficits of hippocampal CA1 neurons. When PKH26-labeled immortalized microglial cells, GMIR1, were injected into the subclavian artery, these exogenous microglia were found to accumulate in the hippocampus at 24 h after ischemia. In hippocampal slices prepared from medium-injected rats subjected to ischemia 48 h earlier, synaptic dysfunctions including a significant reduction of synaptic responses and a marked reduction of long-term potentiation (LTP) of the CA3-CA1 Schaffer collateral synapses were observed. At this stage, however, neither significant neuronal degeneration nor gliosis was observed in the hippocampus. At 96 h after ischemia, there was a total loss of the synaptic activity and a marked neuronal death in the CA1 subfield. In contrast, the basal synaptic transmission and LTP of the CA3-CA1 synapses were well preserved after ischemia in the slices prepared from the microglia-injected animals. We also found the microglial-conditioned medium (MCM) to significantly increase the frequency of the spontaneous postsynaptic currents of CA1 neurons without affecting the amplitude, thus indicating that MCM increased the provability of the neurotransmitter release. The protective effect of the intra-arterial injected microglia against the ischemia-induced neuronal degeneration in the hippocampus was substantiated by immunohistochemical and immunoblot analyses. Furthermore, the arterial-injected microglia prevented the ischemia-induced decline of the brain-derived neurotrophic factor (BDNF) levels in CA1 neurons. These observations strongly suggest that the arterial-injection of microglia protected CA1 neurons against the ischemia-induced neuronal degeneration. The restoration of the ischemia-induced synaptic deficits and the resultant reduction of the BDNF levels in CA1 neurons, possibly by the release of diffusible factor(s), might thus contribute to the protective effect of the arterial-injection of microglia against ischemia-induced neuronal degeneration.

Arundic acid (ONO-2506) ameliorates delayed ischemic brain damage by preventing astrocytic overproduction of S100B.

After focal cerebral ischemia, the infarct volume increases rapidly within acute infarct expansion (initial 12 to 24 h) and continues slowly during delayed infarct expansion (25 to 168 h). While acute infarct expansion represents progressive necrosis within the ischemic core, delayed infarct expansion starts as disseminated apoptotic cell death in a narrow rim surrounding the infarct border, which gradually coalesces to form a larger infarct. Discovery of a distinct correlation between reactive astrogliosis along the infarct border and delayed infarct expansion in the rodent ischemia model led us to investigate the possible causal relationship between the two events. Specifically, the calcium binding protein S100B exerts detrimental effects on cell survival through activation of various intracellular signaling pathways, resulting in altered protein expression. Arundic acid [(R)-(-)-2-propyloctanoic acid, ONO-2506] is a novel agent that inhibits S100B synthesis in cultured astrocytes. In the rodent ischemia model, this agent was shown to inhibit both the astrocytic overexpression of S100B and the subsequent activation of signaling pathways in the peri-infarct area. Concurrently, delayed infarct expansion was prevented, and neurologic deficits were promptly ameliorated. The results of subsequent studies suggest that the efficacy of arundic acid is mediated by restoring the activity of astroglial glutamate transporters via enhanced genetic expression.

[Activation of astrocytes and ischemic damage following the transient focal ischemia].

Astrocytes play vital roles not only in the mechanical support of the central nervous system but also in the metabolism of neurotransmitters and in the transfer of nutritive substances to neuron. After ischemic brain injuries, it has been known that gliosis appears around degenerative regions and repairs these regions. Recently, accumulating evidence indicates that overexpression of S-100 protein, astrocyte-derived protein, is detrimental to neuronal cells in various pathological conditions. To confirm the astrocytic activation in cerebral ischemia, we examined immunohistochemical changes in S-100 protein and glial fibrillary acidic protein (GFAP) in the transient focal ischemia. Cerebral infarction determined by hematoxylin-eosin staining was slight on day 1 and further expanded on day 2 and 3. Thereafter, GFAP immunoreactivity was observed in boundary zone of the infarct area at 72 hours after the transient focal ischemia. On the other hand, S-100 protein immunoreactivities were markedly increased at 9 hours after the transient focal ischemia. After the infarct formation, the increase of S-100 immunoreactivity was observed in outside boundary of infarct area. These results suggest that astrocytic activation, which we would like to be called "pre-mitotic S-100 peak (PSP)", precedes the neurodegeneration following the transient focal ischemia, and should be distinguished from so-called gliosis observed in the post-neurodegeneration and GFAP-dependent astrocytic proliferation.