Shozo Goto

Estrogen Receptor Antagonist ICI182,780 Exacerbates Ischemic Injury in Female Mouse

Recent findings in animals emphasize that experimental ischemic brain damage can be strikingly reduced by estrogen; however, the neuroprotective mechanisms are not well understood. It was hypothesized that estrogen signaling via cognate estrogen receptors (ERs) within the vasculature is an important aspect of cerebral ischemic protection in the female brain, in part by amplifying intraischemic cerebral blood flow (CBF). In the present study, the hypothesis that chronic treatment with the pure ER antagonist ICI182,780 (ICI) would increase ischemic brain damage by a blood flow-mediated mechanism was investigated. Adult C57Bl/6J mice were pretreated with either subcutaneous ICI (100 μg/day) or oil/ ethanol vehicle for 1 week before 2 hours of middle cerebral artery occlusion (MCAO) and 22 hours of reperfusion. End-ischemic regional CBF was evaluated in additional cohorts using [14C]iodoantipyrine autoradiography. Infarction volume as measured by cresyl violet histology was greater in the striatum of ICI-treated females (70 ± 3% of contralateral striatum vs. 40 ± 12% in vehicle-treated females). Cortical injury was not enhanced relative to control animals (39 ± 6% of contralateral cortex in ICI group vs. 27 ± 8% in vehicle-treated group). Physiologic variables and ischemic reduction of the ipsilateral cortical laser-Doppler flow signal were similar between groups. Further, ICI treatment did not alter end-ischemic cortical or striatal CBF. The deleterious effect of ICI was limited to females, as there were no differences in stroke damage or CBF between male treatment groups. These data suggest that estrogen inhibits ischemic brain injury in striatum of the female by receptor-mediated mechanisms that are not linked to preservation of intraischemic CBF.

Stroke in Estrogen Receptor-α–Deficient Mice

Background and Purpose —Recent evidence suggests that endogenous estrogens or hormone replacement therapy can ameliorate brain damage from experimental stroke. Protective mechanisms involve enhanced cerebral vasodilation during ischemic stress as well as direct preservation of neuronal viability. We hypothesized that if the intracellular estrogen receptor subtype-α (ERα) is important to estrogen’s signaling in the ischemic brain, then ERα-deficient (knockout) (ERαKO) female mice would sustain exaggerated cerebral infarction damage after middle cerebral artery occlusion. Methods —The histopathology of cresyl violet–stained tissues was evaluated after reversible middle cerebral artery occlusion (2 hours, followed by 22 hours of reperfusion) in ERαKO transgenic and wild-type (WT) mice (C57BL/6J background strain). End-ischemic cerebral blood flow mapping was obtained from additional female murine cohorts by using [14C]iodoantipyrine autoradiography. Results —Total hemispheric tissue damage was not altered by ERα deficiency in female mice: 51.9±10.6 mm3 in ERαKO versus 60.5±5.0 mm3 in WT. Striatal infarction was equivalent, 12.2±1.7 mm3 in ERαKO and 13.4±1.0 mm3 in WT mice, but cortical infarction was paradoxically smaller relative to that of the WT (20.7±4.5 mm3 in ERαKO versus 30.6±4.1 mm3 in WT). Intraocclusion blood flow to the parietal cortex was higher in ERαKO than in WT mice, likely accounting for the reduced infarction in this anatomic area. There were no differences in stroke outcomes by region or genotype in male animals. Conclusions —Loss of ERα does not enhance tissue damage in the female animal, suggesting that estrogen inhibits brain injury by mechanisms that do not depend on activation of this receptor subtype.

Heme oxygenase-2 is neuroprotective in cerebral ischemia.

Heme oxygenase (HO) is believed to be a potent antioxidant enzyme in the nervous system; it degrades heme from heme-containing proteins, giving rise to carbon monoxide, iron, and biliverdin, which is rapidly reduced to bilirubin. The first identified isoform of the enzyme, HO1, is an inducible heat-shock protein expressed in high levels in peripheral organs and barely detectable under normal conditions in the brain, whereas HO2 is constitutive and most highly concentrated in the brain. Interestingly, although HO2 is constitutively expressed, its activity can be modulated by phosphorylation. We demonstrated that bilirubin, formed from HO2, is neuroprotectant, as neurotoxicity is augmented in neuronal cultures from mice with targeted deletion of HO2 (HO2−/−) and reversed by low concentrations of bilirubin. We now show that neural damage following middle cerebral artery occlusion (MCAO) and reperfusion, a model of focal ischemia of vascular stroke, is substantially worsened in HO2−/− animals. By contrast, stroke damage is not significantly altered in HO1−/− mice, despite their greater debility. Neural damage following intracranial injections of N-methyl-d-aspartate (NMDA) is also accentuated in HO2−/− animals. These findings establish HO2 as an endogenous neuroprotective system in the brain whose pharmacologic manipulation may have therapeutic relevance.

Experimental stroke in the female diabetic, db/db, mouse

Diabetic hyperglycemia increases brain damage after cerebral ischemia in animals and humans, although the underlying mechanisms remain unclear. Gender-linked differences in ischemic tolerance have been described but have not been studied in the context of diabetes. In the current study, we used a model of unilateral common carotid artery ligation, combined with systemic hypoxia, to study the effects of diabetes and gender on hypoxic–ischemic (HI) brain damage in the genetic model of Type II diabetes, the db/db, mouse. Male and female, control and db/db, mice were subjected to right common carotid artery ligation followed by varying periods of hypoxia (8% oxygen/92% nitrogen) to assess mortality, infarct volume, and tissue damage by light microscopic techniques. End-ischemic regional cerebral blood flow (CBF) was determined using [14C] iodoantipyrine autoradiography. Glycolytic and high energy phosphate compounds were measured in blood and brain by enzymatic and fluorometric techniques. Gender and diabetes had significant effects on mortality from HI and extent of brain damage in the survivors. Female mice were more resistant than their male counterparts, such that the severity (mortality and infarction size) in the male diabetics > female diabetics ~ male controls > female controls. End-ischemic CBF and depletion of cerebral high energy reserves were comparable among all groups. Surprisingly, female diabetic mice were more hyperglycemic and demonstrated a greater prolonged lactacidosis than the males; however, they were more resistant to damage. The results suggest a unique pathophysiology of hypoxia–ischemia in the female diabetic brain.

Poly(ADP-Ribose) Polymerase Impairs Early and Long-Term Experimental Stroke Recovery

Background and Purpose— Poly(ADP-ribose) polymerase (PARP-1; Enzyme Commission 2.4.30) is a nuclear DNA repair enzyme that mediates early neuronal ischemic injury. Using novel 3-dimensional, fast spin-echo-based diffusion-weighted imaging, we compared acute (21 hours) and long-term (3 days) ischemic volume after middle cerebral artery (MCA) occlusion in PARP-1-null mutants (PARP−/−) versus genetically matched wild-type mice (WT mice). PARP−/− mice were also treated with viral transfection of wild-type PARP-1 to determine whether protection from MCA occlusion is lost with restoration of the gene product. Methods— Halothane-anesthetized mice were treated with reversible MCA occlusion via intraluminal suture technique. Ischemic volumes were delineated by diffusion-weighted imaging with high spatial and temporal resolution during MCA occlusion and reperfusion. Recombinant Sindbis virus carrying &bgr;-galactosidase (lacZ) or PARP-1 was injected into ipsilateral striatum, then animals underwent MCA occlusion 3 days later. Infarction volume was measured at 22 hours of reperfusion (2,3,5-triphenyltetrazolium chloride histology). Results— Reduction in regional water apparent diffusion coefficient (ADC) during occlusion or secondary ADC decline during reperfusion was not different between groups. Ischemic volume was smaller early in occlusion in PARP−/− versus WT mice and remained less at 21 hours of reperfusion. Ischemic volume then increased from 1 to 2 days in all mice, then stabilized without further change. Ischemic damage was smaller in PARP−/− than in WT mice at 3 days. Transfection of PARP-1 into PARP−/− mice increased stroke damage relative to lacZ-injected PARP−/− and increased damage to that of the WT mice. Intraischemic laser-Doppler flowmetry and physiological variables were not different among groups. Conclusions— PARP-1 deficiency provides both early and prolonged protection from experimental focal stroke. The mechanism is not linked to preservation of ADC and mitigation of secondary energy depletion during early reperfusion.

Neuroprotective Effect of σ1-Receptor Ligand 4-Phenyl-1-(4-Phenylbutyl) Piperidine (PPBP) Is Linked to Reduced Neuronal Nitric Oxide Production

Background and Purpose —The potent σ1-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) provides neuroprotection in experimental stroke. We tested the hypothesis that PPBP attenuates striatal tissue damage after middle cerebral artery occlusion (MCAO) by a mechanism involving reduction of ischemia-evoked nitric oxide (NO) production. Furthermore, we determined whether the agent fails to protect ischemic brain when neuronal nitric oxide synthase (nNOS) is genetically deleted or pharmacologically inhibited (selective nNOS inhibitor, 7-nitroindazole [7-NI]). Methods —Halothane-anesthetized adult male Wistar rats were subjected to 2 hours of MCAO by the intraluminal filament occlusion technique. All physiological variables were controlled during the ischemic insult. In vivo striatal NO production was estimated via microdialysis by quantification of local, labeled citrulline recovery after labeled arginine infusion. In a second series of experiments, nNOS null mutants (nNOSKOs) and the genetically matched wild-type (WT) strain were treated with 90 minutes of MCAO. Brains were harvested at 22 hours of reperfusion for measurement of infarction volume by triphenyltetrazolium chloride histology. Results —PPBP attenuated infarction volume at 22 hours of reperfusion in cerebral cortex and striatum and markedly attenuated NO production in ischemic and nonischemic striatum during occlusion and early reperfusion. Treatment with 7-NI mimicked the effects of PPBP. In WT mice, infarction volume was robustly decreased by both PPBP and 7-NI, but the efficacy of PPBP was not altered by pharmacological nNOS inhibition in combined therapy. In contrast, PPBP did not decrease infarction volume in nNOSKO mice. Conclusions —These data suggest that the mechanism of neuroprotection of PPBP in vivo is through attenuation of nNOS activity and ischemia-evoked NO production. Neuroprotective effects of PPBP are lost when nNOS is not present or is inhibited; therefore, PPBP likely acts upstream from NO generation and its subsequent neurotoxicity.

Rapid three-dimensional diffusion MRI facilitates the study of acute stroke in mice

MRI studies using mouse brain models of ischemia are becoming a valuable tool for understanding the mechanism of stroke, since transgenic models are now available. However, the small size of the mouse brain and the surgical complexity of creating ischemia in mice make it technically challenging to obtain high‐quality MRI data. Therefore, there are few reports of MRI studies in murine cerebral ischemia. In this project a newly developed rapid 3D diffusion‐weighted imaging (DWI) technique was applied to study experimental stroke in a mouse model of reversible middle cerebral artery occlusion (MCAO). Ischemic volumes were successfully delineated using this 3D whole‐brain imaging technique with high spatial (0.34 × 0.5 × 1.0 mm3 before zero‐filling) and temporal (7 min) resolution. The 3D observation revealed the characteristic evolution of stroke after transient MCAO. There was a temporarily high diffusion constant in the cortex during early reperfusion, followed by a secondary energy failure in the cortex and caudate‐putamen at 6 and 21 h of reperfusion. Magn Reson Med 46:183–188, 2001.