Nabil J. Alkayed

Gender-Linked Brain Injury in Experimental Stroke

BACKGROUND AND PURPOSEnPremenopausal women are at lower risk than men for stroke, but the comparative vulnerability to tissue injury once a cerebrovascular incident occurs is unknown. We hypothesized that female rats sustain less brain damage than males during experimental focal ischemia and that the gender difference in ischemic outcome can be eliminated by ovariectomy.nnnMETHODSnAge-matched male (M), intact female (F), and ovariectomized female (O; plasma estradiol: 4.1+/-1.6 pg/mL compared with 7.4+/-1.5 in F and 4.0+/-1.1 in M) rats from two different strains, normotensive Wistar and stroke-prone spontaneously hypertensive rats, were subjected to 2 hours of intraluminal middle cerebral artery occlusion, followed by 22 hours of reperfusion. Cerebral blood flow (CBF) was monitored throughout the ischemic period by laser-Doppler flowmetry. Infarction volume in the cerebral cortex (Ctx) and caudoputamen (CP) was determined by 2,3,5-triphenyltetrazolium chloride staining. In a separate cohort of M, F, and O Wistar rats, absolute rates of regional CBF were measured at the end of the ischemic period by quantitative autoradiography using [14C]iodoantipyrine.nnnRESULTSnF rats of either strain had a smaller infarct size in Ctx and CP and a higher laser-Doppler flow during ischemia compared with respective M and O rats. Mean end-ischemic CBF was higher in F compared with M and O rats in CP, but not in Ctx. Cerebrocortical tissue volume with end-ischemic CBF < 10 mL/100 g/min was smaller in F than M rats, but not different from O rats.nnnCONCLUSIONSnWe conclude that endogenous estrogen improves stroke outcome during vascular occlusion by exerting both neuroprotective and flow-preserving effects.

Neuroprotective Effects of Female Gonadal Steroids in Reproductively Senescent Female Rats

BACKGROUND AND PURPOSEnYoung adult female rats sustain smaller infarcts after experimental stroke than age-matched males. This sex difference in ischemic brain injury in young animals disappears after surgical ovariectomy and can be restored by estrogen replacement. We sought to determine whether ischemic brain injury continues to be smaller in middle-aged, reproductively senescent female rats compared with age-matched males and to test the effect of ovarian steroids on brain injury after experimental stroke in females.nnnMETHODSnFour groups of 16-month old Wistar rats (males [n=9], untreated females [n=9], and females pretreated with 17beta-estradiol [25-microgram pellets administered subcutaneously for 7 days; n=9] or progesterone [10-mg pellets administered subcutaneously for 7 days; n=9] were subjected to 2 hours of middle cerebral artery occlusion with the intraluminal filament technique, followed by 22 hours of reperfusion. Physiological variables and laser-Doppler cerebral cortical perfusion were monitored throughout ischemia and early reperfusion. In a separate cohort of males (n=3), untreated females (n=3), females pretreated with 17beta-estradiol (n=3), and females pretreated with progesterone (n=3), end-ischemic regional cerebral blood flow was measured by [(14)C]iodoantipyrine autoradiography.nnnRESULTSnAs predicted, infarct size was not different between middle-aged male and female rats. Cortical infarcts were 21+/-5% and 31+/-6% of ipsilateral cerebral cortex, and striatal infarcts were 44+/-7% and 43+/-5% of ipsilateral striatum in males and females, respectively. Both estrogen and progesterone reduced cortical infarct in reproductively senescent females (5+/-2% and 16+/-4% in estrogen- and progesterone-treated groups, respectively, compared with 31+/-6% in untreated group). Striatal infarct was smaller in the estrogen- but not in the progesterone-treated group. Relative change in laser-Doppler cerebral cortical perfusion from preischemic baseline and absolute end-ischemic regional cerebral blood flow were not affected by hormonal treatments.nnnCONCLUSIONSnWe conclude that the protection against ischemic brain injury found in young adult female rats disappears after reproductive senescence in middle-aged females and that ovarian hormones alleviate stroke injury in reproductively senescent female rats by a blood flow-independent mechanism. These findings support a role for hormone replacement therapy in stroke injury prevention in postmenopausal women.

17β-Estradiol Reduces Stroke Injury in Estrogen-Deficient Female Animals

BACKGROUND AND PURPOSEnThe importance of postmenopausal estrogen replacement therapy for stroke in females remains controversial. We previously showed that female rats sustain less infarction in reversible middle cerebral artery occlusion (MCAO) than their ovariectomized counterparts and that vascular mechanisms are partly responsible for improved tissue outcomes. Furthermore, exogenous estrogen strongly protects the male brain, even when administered in a single injection before MCAO injection. The present study examined the hypothesis that replacement of 17beta-estradiol to physiological levels improves stroke outcome in ovariectomized, estrogen-deficient female rats, acting through blood flow-mediated mechanisms.nnnMETHODSnAge-matched, adult female Wistar rats were ovariectomized and treated with 0, 25, or 100 microgram of 17beta-estradiol administered through a subcutaneous implant or with a single Premarin (USP) injection (1 mg/kg) given immediately before ischemia was induced (n=10 per group). Each animal subsequently underwent 2 hours of MCAO by the intraluminal filament technique, followed by 22 hours of reperfusion. Ipsilateral parietal cortex perfusion was monitored by laser-Doppler flowmetry throughout ischemia. Cortical and caudate-putamen infarction volumes were determined by 2,3, 5-triphenyltetrazolium chloride staining and digital image analysis. End-ischemic regional cerebral blood flow was measured in ovariectomized females with 0- or 25-microgram implants (n=4 per group) by (14)C-iodoantipyrine quantitative autoradiography.nnnRESULTSnPlasma estradiol levels were 3.0+/-0.6, 20+/-8, and 46+/-10 pg/mL in the 0-, 25-, and 100-microgram groups, respectively. Caudate-putamen infarction (% of ipsilateral caudate-putamen) was reduced by long-term, 25-microgram estrogen treatment (13+/-4% versus 31+/-6% in the 0-microgram group, P<0.05, and 22+/-3% in the 100-microgram group). Similarly, cortical infarction (% of ipsilateral cortex) was reduced only in the 25-microgram group (3+/-2% versus 12+/-3% in the 0-microgram group, P<0.05, and 6+/-3% in the 100-microgram group. End-ischemic striatal or cortical blood flow was not altered by estrogen treatment at the neuroprotective dose. Infarction volume was unchanged by acute treatment before MCAO when estrogen-treated animals were compared with saline vehicle-treated animals.nnnCONCLUSIONSnLong-term estradiol replacement within a low physiological range ameliorates ischemic brain injury in previously ovariectomized female rats. The neuroprotective mechanism is flow-independent, not through preservation of residual ischemic regional cerebral blood flow. Furthermore, the therapeutic range is narrow, because the benefit of estrogen in transient vascular occlusion is diminished at larger doses, which yield high, but still physiologically relevant, plasma 17beta-estradiol levels. Lastly, unlike in the male brain, single-injection estrogen exposure does not salvage ischemic tissue in the female brain. Therefore, although exogenous steroid therapy protects both male and female estrogen-deficient brain, the mechanism may not be identical and depends on long-term hormone augmentation in the female.

Functional Hyperemia in the Brain Hypothesis for Astrocyte-Derived Vasodilator Metabolites

BACKGROUNDnCerebral blood flow is tightly coupled to neuronal metabolic activity, a phenomenon referred to as functional hyperemia. The mechanisms underlying functional hyperemia in the brain have been extensively studied, but the link between neuronal activation and nutritive blood flow has yet to be defined. Recent investigations by our laboratory and others have identified a potential role for astrocytes as an intermediary cell type in this process.nnnSUMMARY OF REVIEWnThis short review will develop the hypothesis that cytochrome P450 epoxygenase activity in astrocytes catalyzes formation of epoxyeicosatrienoic acids (EETs), which act as potent dilators of cerebral vessels and are released in response to glutamate receptor activation within astrocytes. Neuronal activity stimulates release of arachidonic acid from the phospholipid pool of astrocytic membranes. We provide evidence that the arachidonic acid released on stimulation of glutamate receptors within astrocytes is metabolized by cytochrome P450 2C11 cDNA enzymes into EETs.nnnCONCLUSIONSnThe EETs thus formed will be released and activate K+ channels, increase outward K+ current, and hyperpolarize the plasma membrane. The resulting membrane hyperpolarization inhibits voltage-gated Ca2+ channels and leads to arteriolar dilation, thereby increasing regional nutritive blood flow in response to neuronal activity.

Postischemic Estrogen Reduces Hypoperfusion and Secondary Ischemia After Experimental Stroke

Background and Purpose— Estrogen is a known neuroprotective and vasoprotective agent in experimental cerebral ischemia. Preischemic steroid treatment protects animals of both sexes from focal cerebral ischemia. This study determined whether intravenous estrogen acts as a vasodilator when administered on reperfusion and whether the resulting increase in cerebral blood flow (CBF) provides tissue protection from middle cerebral artery occlusion. Methods— Adult male Wistar rats were treated with reversible middle cerebral artery occlusion (2 hours), then infused with intravenous estrogen (Premarin; 1 mg/kg) or vehicle during the first minutes of reperfusion (n=15 per group). Cortical laser-Doppler flowmetry was used to assess adequacy of occlusion. Ischemic lesion volume was determined at 22 hours after occlusion by 2,3,5-triphenyltetrazolium chloride staining and image analysis. Cortical and striatal CBF was measured by 14[C]iodoantipyrine autoradiography at 10 (n=10) or 90 (n=11) minutes of reperfusion. Results— As expected, supraphysiological plasma estrogen levels were achieved during reperfusion (estrogen, 198±45 pg/mL; vehicle, 6±5;P =0.001). Physiological variables were controlled and not different between groups. Total hemispheric infarction was reduced in estrogen-treated rats (estrogen, 49±4% of ipsilateral structure; vehicle, 33±5%;P =0.02), which was most pronounced in striatum (estrogen, 40±6% of ipsilateral striatum; vehicle, 60±3%;P =0.01). CBF recovery was strikingly increased by estrogen infusion at 10 minutes in frontal (estrogen, 102±12 mL/100 g per minute; vehicle, 45±15;P =0.01) and parietal cortex (estrogen, 74±15 mL/100 g per minute; vehicle, 22±13;P =0.028) and throughout striatum (estrogen, 87±13 mL/100 g per minute; vehicle, 25±20;P =0.02). Hemispheric volume with low CBF recovery (eg, <20 mL/100 g per minute) was smaller in estrogen-treated animals (estrogen, 73±18 mm3; vehicle, 257±46;P =0.002). However, differences in CBF recovery could not be appreciated between groups by 90 minutes of reperfusion. Conclusions— Acute estrogen therapy during reperfusion improves tissue outcome from experimental stroke. The steroid rapidly promotes CBF recovery and reduces hemispheric no-reflow zones. This beneficial effect appears only during early reperfusion and likely complements other known mechanisms by which estrogen salvages brain from focal necrosis.

Molecular Characterization of an Arachidonic Acid Epoxygenase in Rat Brain Astrocytes

BACKGROUND AND PURPOSEnBrain parenchymal tissue metabolizes arachidonic acid (AA) via the cytochrome P450 (P450) epoxygenase to epoxyeicosatrienoic acids (EETs). EETs dilate cerebral arterioles and enhance K+ current in vascular smooth muscle cells from large cerebral arteries. Because of the close association between astrocytes and the cerebral microcirculation, we hypothesized that brain epoxygenase activity originates from astrocytes. This study was designed to identify and localize an AA epoxygenase in rat brain astrocytes. We also tested the effect of EETs on whole-cell K+ current in rat cerebral microvascular smooth muscle cells.nnnMETHODSnA functional assay was used to demonstrate endogenous epoxygenase activity of intact astrocytes in culture. Oligonucleotide primers derived from the sequence of a known hepatic epoxygenase, P450 2C11, were used in reverse transcription/polymerase chain reaction of RNA isolated from cultured rat astrocytes. The appropriate size reverse transcription/polymerase chain reaction product was cloned into a plasmid vector and sequenced. A polyclonal peptide antibody was raised against P450 2C11 and used in Western blotting and immunocytochemical staining of cultured astrocytes. A voltage-clamp technique was used to test the effect of EETs on whole-cell K+ current recorded from rat cerebral microvascular muscle cells.nnnRESULTSnBased on elution time of known standards and inhibition by miconazole, an inhibitor of P450 AA epoxygenase, cultured astrocytes produce 11,12- and 14,15-EETs when incubated with AA. The sequence of a cDNA derived from RNA isolated from cultured rat astrocytes was 100% identical to P450 2C11. Immunoreactivity to glial fibrillary acidic protein, a marker for astrocytes, colocalized with 2C11 immunoreactivity in double immunochemical staining of cultured astrocytes. EETs enhanced outward K+ current in muscle cells from rat brain microvessels.nnnCONCLUSIONSnOur results demonstrate that a P450 2C11 mRNA is expressed in astrocytes and may be responsible for astrocyte epoxygenase activity. Given the vasodilatory effect of EETs, our findings suggest a role for astrocytes in the control of cerebral microcirculation mediated by P450 2C11-catalyzed conversion of AA to EETs. The mechanism of EET-induced dilation of rat cerebral microvessels may involve activation of K+ channels.

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.nnMethods —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.nnResults —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.nnConclusions —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.

Social stress exacerbates stroke outcome by suppressing Bcl-2 expression

The relationship between stressful life events and the onset of disease is well documented. However, the role of psychological stress as a risk factor for life-threatening cerebrovascular insults such as stroke remains unspecified, but could explain individual variation in stroke outcome. To discover the mechanisms through which psychological stress may alter stroke outcome, we modeled the effects of chronic social intimidation and stress on ischemia-induced bcl-2 expression and early neuronal cell loss resulting from cerebral artery occlusion in mice (C57BL/6). The bcl-2 protooncogene promotes cell survival and protects against apoptosis and cellular necrosis in numerous neurodegenerative disorders, including stroke. In our study, male mice were chronically exposed to aggressive social stimuli before induction of a controlled, mild ischemic insult. Stressed mice expressed ≈70% less bcl-2 mRNA than unstressed mice after ischemia. In addition, social stress greatly exacerbated infarct in wild-type mice but not in transgenic mice that constitutively express increased neuronal bcl-2. Despite similar postischemic concentrations of corticosterone, the major stress hormone in mice, high corticosterone concentrations were significantly correlated with larger infarcts in wild-type mice but not bcl-2 transgenic mice. Thus, enhanced bcl-2 expression offsets the potentially deleterious consequences of high postischemic plasma corticosterone concentrations. Taken together, these data demonstrate that stressful prestroke social milieu strongly compromises an endogenous molecular mechanism of neuroprotection in injured brain and offer a new behavioral target for stroke therapy.

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.