Nicotine and estrogen synergistically exacerbate cerebral ischemic injury

The greater incidence of myocardial infarction, cardiac arrest, and ischemic stroke among women who smoke and use oral contraception (OC) compared to women who do not smoke and who do or do not use OC may be due in part to how nicotine influences endocrine function in women. For example, we recently demonstrated that chronic exposure to nicotine, the addictive agent in tobacco smoke responsible for the elevated risk of cardiac arrest, abolishes the endogenous or exogenous 17β-estradiol-conferred protection of the hippocampus against global cerebral ischemia (a potential outcome of cardiac arrest) in naive or ovariectomized female rats. In the current study we examined the hypotheses that (1) a synergistic deleterious effect of nicotine plus oral contraceptives exacerbates post-ischemic hippocampal damage in female rats, and (2) nicotine directly inhibits estrogen-mediated intracellular signaling in the hippocampus. To test first hypothesis and to simulate smoking behavior-induced nicotine levels in the human body, we implanted osmotic pumps containing nicotine in the female rats for 16 days. Furthermore, we mimicked the use of oral contraceptives in females by administering oral contraceptives orally to the rat. Rats exposed to either nicotine alone or in combination with oral contraceptives were subjected to an episode of cerebral ischemia and the resultant brain damage was quantified. These results showed for the first time that nicotine with oral contraceptives did indeed exacerbate post-ischemic CA1 damage as compared to nicotine alone in naive female rats. In ex vivo hippocampal slice cultures, we found that nicotine alone or with 17β-estradiol directly hinders estrogen receptors-mediated phosphorylation of cyclic-AMP element binding protein, a process required for neuronal survival and also exacerbates ischemic damage. Thus, nicotine can affect the outcome of cerebral ischemia by influencing brain endocrine function directly rather than through indirect systemic effects.

Derangements of post-ischemic cerebral blood flow by protein kinase C delta

Cerebral ischemia causes blood flow derangements characterized by hyperemia (increased cerebral blood flow, CBF) and subsequent hypoperfusion (decreased CBF). We previously demonstrated that protein kinase C delta (δPKC) plays an important role in hippocampal neuronal death after ischemia. However, whether part of this protection is due to the role of δPKC on CBF following cerebral ischemia remains poorly understood. We hypothesized that δPKC exacerbates hyperemia and subsequent hypoperfusion resulting in CBF derangements following ischemia. Sprague-Dawley (SD) rats pretreated with a δPKC specific inhibitor (δV1-1, 0.5 mg/kg) exhibited attenuation of hyperemia and latent hypoperfusion characterized by vasoconstriction followed by vasodilation of microvessels after 2-vessel occlusion plus hypotension measured by 2-photon microscopy. In an asphyxial cardiac arrest model (ACA), SD rats treated with δV1-1 (pre- and post-ischemia) exhibited improved perfusion after 24 h and less hippocampal CA1 neuronal death 7 days after ACA. These results suggest possible therapeutic potential of δPKC in modulating CBF and neuronal damage after cerebral ischemia.

Pretreatment with a single estradiol-17beta bolus activates cyclic-AMP response element binding protein and protects CA1 neurons against global cerebral ischemia

Estradiol-17beta is released from the ovaries in a cyclic manner during the normal estrous cycle in rats. During the transition from the diestrous to proestrous stage, the 17beta-estradiol increases in blood circulation. We hypothesized that a higher serum level of endogenous 17beta-estradiol would protect hippocampal pyramidal neurons against global cerebral ischemia via activation of the cyclic-AMP response element binding protein (CREB)-mediated signaling cascade. Furthermore, we asked if a single 17beta-estradiol bolus provides protection against ischemia in the absence of endogenous estradiol. To test these hypotheses, rats were subjected to global cerebral ischemia at different stages of the estrous cycle. Ischemia was produced by bilateral carotid occlusion and systemic hypotension. Brains were examined for histopathology at 7 days of reperfusion. Higher serum levels of 17beta-estradiol (at proestrus and estrus stages) correlated with increased immunoreactivity of pCREB in hippocampus and ischemic tolerance. At diestrus, when circulating gonadal hormone concentrations were lowest, the pCREB protein content of hippocampus was reduced and showed the least number of normal neurons after ischemia compared to other stages of the estrous cycle. A similar phosphorylation pattern was also observed for mitogen-activated protein kinase (MAPK) and calcium-calmodulin-dependent protein kinase (CaMKII) in hippocampus. The cyclic variation in ovarian hormones did not reflect phosphorylation of protein kinase B (Akt). To test the efficacy of a single bolus of 17beta-estradiol before ischemia, ovariectomized rats were treated with 17beta-estradiol (5/10/50 microg/kg) or vehicle (oil) and 48/72/96 h later rats were exposed to cerebral ischemia. A single 17beta-estradiol bolus treatment in ovariectomized rats significantly increased CREB mRNA activation and protected CA1 pyramidal neurons against ischemia. These results suggest that an exogenous bolus of 17beta-estradiol to ovariectomized rats protects hippocampus against ischemia via activation of the CREB pathway in a manner similar to the endogenous estrous cycle.

Ischemic preconditioning preserves mitochondrial function after global cerebral ischemia in rat hippocampus

Ischemic tolerance in brain develops when sublethal ischemic insults occur before "lethal" cerebral ischemia. Two windows for the induction of tolerance by ischemic preconditioning (IPC) have been proposed: one that occurs within 1 hour after IPC, and another that occurs 1 or 2 days after IPC. The authors tested the hypotheses that IPC would reduce or prevent ischemia-induced mitochondrial dysfunction. IPC and ischemia were produced by bilateral carotid occlusions and systemic hypotension (50 mm Hg) for 2 and 10 minutes, respectively. Nonsynaptosomal mitochondria were harvested 24 hours after the 10-minute "test" ischemic insult. No significant changes were observed in the oxygen consumption rates and activities for hippocampal mitochondrial complexes I to IV between the IPC and sham groups. Twenty-four hours of reperfusion after 10 minutes of global ischemia (without IPC) promoted significant decreases in the oxygen consumption rates in presence of substrates for complexes I and II compared with the IPC and sham groups. These data suggest that IPC protects the integrity of mitochondrial oxidative phosphorylation after cerebral ischemia.

Transient changes of brain-derived neurotrophic factor (BDNF) mRNA expression in hippocampus during moderate ischemia induced by chronic bilateral common carotid artery occlusions in the rat

Chronic bilateral common carotid artery occlusion (BCCAO) induces moderate ischemia (oligemia) in the rat forebrain in the absence of overt neuronal damage. In situ hybridization for brain-derived neurotrophic factor (BDNF) mRNA was used to search for a molecular response to moderate ischemia. BDNF mRNA was significantly increased in the hippocampal granule cells at 6 h of occlusion (ANOVA, Tukey test P<0.05). At 1, 7 and 14 days BDNF mRNA levels returned to control levels. The frequency of BDNF gene expression at 6 h was 83%, which was significantly higher than the 7% incidence of histological injury in the hippocampus (Fisher's exact test, P<0.002). Cerebral blood flow was reduced to 75% of control levels in the hippocampus after 1 week of BCCAO when measured with the autoradiographic method. Measurements of tissue flow with a microprobe for laser Doppler flow excluded decreases into the ischemic range during the period when elevated gene expression was observed. Prolonged moderate ischemia (oligemia) is a sufficient stimulus for BDNF gene expression in the hippocampus. These molecular studies provide direct evidence for an involvement of the hippocampus in the BCCAO model.

The effect of bone morphogenetic protein-7 (BMP-7) on functional recovery, local cerebral glucose utilization and blood flow after transient focal cerebral ischemia in rats

Bone morphogenetic protein-7 (BMP-7) has been shown to enhance dendritic growth and improve functional recovery after experimental stroke. In this study, we examined the effect of BMP-7 on functional recovery, local cerebral blood flow (LCBF) and local cerebral glucose utilization (LCMRglu) following transient middle cerebral artery occlusion. Sprague--Dawley rats (n=29) were anesthetized with halothane/nitrous oxide and received 2-h middle cerebral artery occlusion (MCAo) by poly-L-lysine-coated intraluminal suture. Rectal and cranial temperatures were regulated at 37.0--37.5 degrees C. BMP-7 or vehicle (volume, 25 microl) was administered intracisternally in a blinded fashion at 24 h after MCAo. Neurological status was evaluated during occlusion (60 min) and daily for 2 days after MCAo. In matched animal groups, LCMRglu was measured autoradiographically with [(14)C]2-deoxyglucose (2-DG) and LCBF with [(14)C]iodoantipyrine 48 h after MCAo. Four animals groups were studied: LCMRglu series (BMP-7, n=7; vehicle, n=8); LCBF series (BMP-7, n=6; vehicle, n=8). Average three-dimensional image data sets were constructed for each group and were compared by pixel-based statistical methods. Rectal and cranial temperatures, mean blood pressure, plasma glucose and blood gases were similar among groups. BMP-7 significantly improved the total neurological score compared to vehicle at 48 h after MCAo (7.3+/-0.4 vs. 9.0+/-0.2, respectively; P<0.0003). Compared to vehicle-rats, BMP-7 enhanced glucose utilization in the basal ganglia ipsilateral to stroke and improved LCBF in ipsilateral subthalamus, but decreased LCBF and LCMRglu in contralateral cortical regions.

Neuroprotective effect of high-dose albumin therapy against global ischemic brain injury in rats

The purpose of this study was to determine whether treatment with high-dose human serum albumin (HSA) would offer protection in a model of high-grade transient forebrain ischemia. Twenty-six fasted Wistar rats underwent bilateral common carotid artery occlusion and severe hypotension (50 mmHg) for 10 min. The agent (25% HSA) or vehicle (0.9% NaCl) was administered i.v. 5 min after termination of ischemia. HSA-treated rats showed significantly improved neurological deficits throughout a 7-day survival period. Histologically, HSA-treated rats showed 2.4- to 5.3-fold increases in numbers of surviving CA1 hippocampal pyramidal neurons compared to saline-treated animals. These results document that high-dose albumin therapy instituted 5 min after global ischemia significantly improves neurological score and reduces histological damage.

The effect of high-dose albumin therapy on local cerebral perfusion after transient focal cerebral ischemia in rats

We have shown that high-concentration albumin therapy is markedly neuroprotective in focal cerebral ischemia. The present study was conducted to ascertain the degree to which hemodynamic alterations are responsible for this therapeutic effect. Normothermic, physiologically regulated male Sprague-Dawley rats received a 2-h period of middle cerebral artery occlusion (MCAo) by insertion of an intraluminal suture coated with poly-L-lysine. Albumin (25% human serum albumin solution) or vehicle (0.9% sodium chloride) was administered intravenously at a dose of 1% of body weight immediately after suture withdrawal following 2-h MCAo. Local cerebral blood flow (LCBF) was measured autoradiographically with 14C-iodoantipyrine after 1 h of recirculation. Novel image-processing methods were used to compare average LCBF data sets against previously obtained infarction-frequency data on a pixel-by-pixel basis. Albumin therapy reduced mean hematocrit by 42% but produced no other systemic alterations. Pixel-based histopathological analysis revealed large, consistent cortical and subcortical infarcts in saline-treated rats with MCAo; albumin therapy reduced mean cortical infarct volume by 85%. Within regions showing albumin-associated neuroprotection, numbers of pixels having LCBF in the upper ischemic-core flow range (0.12-0.24 ml g-1 min-1) were reduced by 8.6-fold by albumin therapy when compared to saline-treated rats; and numbers of pixels with LCBF in the lower penumbral flow range (0.24-0.36 ml g-1 min-1) were reduced by 3. 1-fold in albumin-treated rats (p=0.04 by repeated-measures analysis of variance). Analysis of the [albumin-saline] 3-dimensional difference-image data set revealed a circumferential zone of statistically significant albumin-associated LCBF increase within the posterior portion of the ischemic hemisphere, surrounding the core-region of prior ischemia. Thus, high-concentration albumin therapy improves local perfusion to regions of critical LCBF reduction. The spatial extent of this LCBF effect, however, appears too small to account fully for the marked neuroprotective efficacy of this therapy. We suggest that other, non-hemodynamic mechanisms may also be contributory.