Since the first documentation of the induction of heat shock protein following transient cerebral ischemia, much experimental evidence suggested that all of the cellular elements in the central nervous system show dynamic stress responses depending on the degree of environmental changes induced by ischemia and reperfusion. In this review, first 1 focused on the importance of the usage of an appropriate experimental model for brain ischemia and reperfusion, and I presented our work on mouse models of transient global and focal ischemia. Next, I reviewed the pathogenic role of microvascular stasis (i.e., secondary ischemia) caused by the primary ischemic event and demonstrated the important role of cell adhesion molecules through the experiments using ICAM-1 knock-out mouse as a model of brain ischemia/reperfusion. Thirdly, I discussed the ischemia-induced neuronal cell responses in relation to the apoptosis-like selective neuronal death and the induction of adopted stress responses including stress protein synthesis and ‘ischemic tolerance’ phenomenon. A variety of stress proteins induced by ischemic stress have been reviewed and a pivotal role of tyrosine kinase system in selective neuronal death has been suggested in the gerbil model of transient forebrain ischemia. Finally, I showed the important pathophysiological roles of glial cells such as astrocytes and oligodendrocytes in the celluar cross-talk triggered by an ischemic event. For the development of a novel therapeutic agent against ischemic stroke, it is quite important to clarify both the negative and positive cellular responses induced by brain ischemia/reperfusion.
Brain ischemia produces morphologic and biochemical alterations in astrocytes. This mini-review summarizes astrocytic responses to brain ischemia including our studies on the neuronal and astrocytic Na+-Ca2+ exchanger (NCX). NCX is considered to cause Ca2+ efflux (forward mode) or Ca2+ influx (reverse mode), depending on the electrochemical gradient of Na+ across the plasma membranes and membrane potential. We demonstrated that NCX is present in cultured neurons and astrocytes and that there are differences in their properties and distribution ratio of the isoforms between neurons and astrocytes. We also found that Ca2+ depletion followed by reperfusion with Ca2+-containing medium caused cell death in cultured astrocytes (Call paradox-like injury), but not in neurons. The study, carried out by the use of a specific antisense oligomer, provides direct evidence that Ca2+ paradoxlike injury is mediated by NCX in the reverse mode. The injury was attenuated by inhibitors of the Na+-Ca2+ exchanger, heat shock protein and the calcineurin inhibitor FK506. In a preliminary experiment, we found that brain ischemia decreases the mRNA level of NCX in the hippocampus. Further studies on activation and cell injury of astrocytes will contribute to development of new drugs that modulate the function of astrocytes.
Treatment with FK506, an inhibitor of Ca2+/calmodulin dependent phosphatase (calcineurin, CaN), within 1 hr after transient ischemia afforded protection from apoptotic death in CA1 pyramidal neurons. To investigate isoformspecific roles of CaN in the neuronal cell death, we localized CaN Aα and CaN Aβ in the gerbil hippocampus using isoform-specific, antibodies. In control gerbils, immunoreactions of both isoforms were highly enriched in hippocampal CA1 pyramidal neurons. Four to seven days after the induced ischemia, immunoreactivities of both isoforms were markedly reduced in the CA1 pyramidal cell and lacunosum-molecular layers. The CaN Aα immunoreactivity was increased in the CA1 radiatum and oriens layers, whereas that of CaN Aβ was enhanced in reactive astrocytes in the CA1 region. These findings suggest that CaN Aα is involved in sprouting of afferent fibers in CA1 and that CaN Aβ is involved in the reaction of astrocytes such as assembly of glial fibril acidic protein.
Both endothelin and nitric oxide (NO) have been proposed to act as pathophysiological factors in ischemia-related neural damage. This review is concerned with the participation of the glial endothelin-NO system in ischemia-related neuronal cell death. In the rat brain with cerebral apoplexy, endothelin, endothelin receptors and NO synthase (NOS) were rich in the glial cells of damaged brain areas. The brain subjected to transient forebrain ischemia contained astrocytic endothelins and microglial expressions of the ETB-receptor and NOS aggregating in the damaged CA1 subfield of the hippocampus at 7 days after the ischemia. Astrocytic endothelin, ETB-receptor and NOS became more apparent at 28 days after the ischemia, corresponding to a time when neural tissue-repair/-remodeling after damage occurs, whereas no activities of the endothelin-NO system are observed in microglia. In the in vitro experiment, endothelin was found to modulate the release of NO from the hippocampal slices subjected to transient forebrain ischemia. There may be a cross-talk between the endothelin system and NO in the astrocytes and microglia during the process of ischemia-related neuronal cell death and neural tissue-remodeling.
Inflammatory/immunological processes underlie the survival/damage of neurons after brain ischemia. In glial cells, cytokines such as IL-1β and TNF-α are produced following ischemic stresses. On the other hand, it is suggested that NO/iNOS is involved in neuronal apoptosis. We here review the ischemia-induced production of cytokine/iNOS and the neurotrophic/neurotoxic effects. It is not clear whether or not the neuronal death after brain ischemia is apoptosis or necrosis. Under the condition of transient forebrain ischemia, however, we obtained results suggesting apoptosis in the delayed neuronal death of the CA1 pyramidal neurons. The time course and cellular localization of postischemic iNOS expression depend on the properties of the ischemic insult. The iNOS induction is detected primarily in astrocytes after the transient forebrain ischemia when the neuronal apoptosis is observed. We discuss a variety of cytokines with neurotrophic/neurotoxic actions that are produced by ischemia or environmental stresses in glial cells. From the neurotoxicological aspect of the neuro-glial interaction, we also review recent findings on signaling pathways of the iNOS induction in glial cells and the mechanisms of the cytotoxic actions of NO.
We have already reported that the concentration of nitric oxide (NO) increases during and after cerebral ischemia and a selective inhibitor of neuronal NO synthase (nNOS) suppresses this increase and subsequently mitigates brain damage in rats. Although the selective inhibition of nNOS is a promising pharmacological strategy for the treatment of stroke, the role of inducible NOS (iNOS) remains to be clarified. Toward this end, we investigated temporal alterations in iNOS mRNA by the RT-PCR method in a rat model of middle cerebral artery (MCA) occlusion. We found that iNOS mRNA in the ischemic hemisphere began to increase at 3 hr and reached the maximum level at 24 hr of reperfusion following 3 hr of MCA occlusion. However, quantitative analysis revealed that no significant difference existed between 6 hr or 24 hr reperfusion group and their respective time-matched sham operation group. In addition, neither Western blotting nor immunocytochemical study disclosed an apparent induction of iNOS at any time points examined. Similar results were obtained at 24 hr of permanent MCA occlusion. Taken together, these data indicate that iNOS induction during and after MCA occlusion may be not a critical event for the development of infarction caused by ischemia itself.
Long-term potentiation (LTP) has been widely studied as a form of synaptic plasticity that represents a cellular mechanism of learning and memory. Among numerous processes and molecules that may be involved in LTP formation, a great many of them including neurotrophic and transcription factors have been described as those involved in neural death after ischemic insult. Nitric oxide (NO) is a molecule that is known to also exert double-edged effects on LTP formation. Here we will be describing recent advances with respect to the LTP mechanisms in the hippocampal synapses, a critical brain region for learning and memory function. In another context, we described our study elucidating the changes in hippocampal LTP as a functional response to transient cerebral ischemic insult, from the viewpoint of its relevance to NO production. As indices of NO production, nitrite and nitrate levels were determined by in vivo microdialysis. It was demonstrated that hippocampal LTP deficiency after transient cerebral ischemia was preceded by an increase in hippocampal NO production. Direct or indirect inhibition of an inducible NO synthase restored ischemia-induced LTP deficiency. These findings suggest that NO production, in part via inducible NO synthase, is responsible for LTP deficiency after transient cerebral ischemia in the rat hippocampus.
The axial movements of rat incisors were recorded continuously for over 20 hr. The rats were anesthetized with halothane delivered by intratracheal intubation using an artificial respirator. A hemostatic clamp was used to immobilize the jaw. The displacement detector that detects changes in the electric eddy current exhibited high resolution, good linearity and low drift. The average eruption rates of the rat incisor were estimated to be 406 and 516 μm/24 hr at 34 and 37 °C of the rectal temperature, respectively. The values were within the normal range. We also measured the force needed to restrain eruptive movement of the rat incisor using the same apparatus combined with a load cell. The maximum pushing force was estimated to be 9 mN or 29 mmHg (converted value) on average. Then the effects of adrenaline on axial movement of the incisor and arterial blood pressure were examined. Adrenaline caused a rapid extrusive tooth movement with a nearly simultaneous increase in the blood pressure, followed by a marked intrusive tooth movement and a decrease in blood pressure. These results support the view that the systemic arterial blood pressure and vasculature in the tooth socket play important roles to determine the position of teeth. We assume that our method would be useful to clarify the precise relationship between tooth displacement and vascular change in the tooth socket.