Brain ischemia and reperfusion: molecular mechanisms of neuronal injury

Brain ischemia and reperfusion engage multiple independently-fatal terminal pathways involving loss of membrane integrity in partitioning ions, progressive proteolysis, and inability to check these processes because of loss of general translation competence and reduced survival signal-transduction. Ischemia results in rapid loss of high-energy phosphate compounds and generalized depolarization, which induces release of glutamate and, in selectively vulnerable neurons (SVNs), opening of both voltage-dependent and glutamate-regulated calcium channels. This allows a large increase in cytosolic Ca(2+) associated with activation of mu-calpain, calcineurin, and phospholipases with consequent proteolysis of calpain substrates (including spectrin and eIF4G), activation of NOS and potentially of Bad, and accumulation of free arachidonic acid, which can induce depletion of Ca(2+) from the ER lumen. A kinase that shuts off translation initiation by phosphorylating the alpha-subunit of eukaryotic initiation factor-2 (eIF2alpha) is activated either by adenosine degradation products or depletion of ER lumenal Ca(2+). Early during reperfusion, oxidative metabolism of arachidonate causes a burst of excess oxygen radicals, iron is released from storage proteins by superoxide-mediated reduction, and NO is generated. These events result in peroxynitrite generation, inappropriate protein nitrosylation, and lipid peroxidation, which ultrastructurally appears to principally damage the plasmalemma of SVNs. The initial recovery of ATP supports very rapid eIF2alpha phosphorylation that in SVNs is prolonged and associated with a major reduction in protein synthesis. High catecholamine levels induced by the ischemic episode itself and/or drug administration down-regulate insulin secretion and induce inhibition of growth-factor receptor tyrosine kinase activity, effects associated with down-regulation of survival signal-transduction through the Ras pathway. Caspase activation occurs during the early hours of reperfusion following mitochondrial release of caspase 9 and cytochrome c. The SVNs find themselves with substantial membrane damage, calpain-mediated proteolytic degradation of eIF4G and cytoskeletal proteins, altered translation initiation mechanisms that substantially reduce total protein synthesis and impose major alterations in message selection, down-regulated survival signal-transduction, and caspase activation. This picture argues powerfully that, for therapy of brain ischemia and reperfusion, the concept of single drug intervention (which has characterized the approaches of basic research, the pharmaceutical industry, and clinical trials) cannot be effective. Although rigorous study of multi-drug protocols is very demanding, effective therapy is likely to require (1) peptide growth factors for early activation of survival-signaling pathways and recovery of translation competence, (2) inhibition of lipid peroxidation, (3) inhibition of calpain, and (4) caspase inhibition. Examination of such protocols will require not only characterization of functional and histopathologic outcome, but also study of biochemical markers of the injury processes to establish the role of each drug.

Gender differences in labetalol kinetics: importance of determining stereoisomer kinetics for racemic drugs

STUDY OBJECTIVE

To evaluate the impact of gender on labetalol kinetics.

DESIGN

Part of a randomized, crossover study.

SETTING

Academic medical center.

PATIENTS

Nineteen hypertensive patients (14 men, 5 women; 6 blacks, 13 whites).

INTERVENTIONS

Participants had labetalol dosages titrated to a specific antihypertensive response, then underwent ambulatory blood pressure monitoring (ABPM) and a pharmacokinetic study. Labetalol plasma concentrations were measured by high-performance liquid chromatography (HPLC) and labetalol stereoisomer ratios were determined in a single plasma sample by chiral HPLC, both with fluorescence detection.

MEASUREMENTS AND MAIN RESULTS

Labetalol concentrations were 80% higher in women (area under the concentration-time curve [AUC]/dose x 1000: 6.79 +/- 2.11 in women vs 3.82 +/- 1.37 hr/L in men, p<0.05), yet both genders had a similar antihypertensive response by 24-hour ABPM. Dose-corrected AUC (AUC/dose x 1000) for labetalol's stereoisomers in women and men, respectively, were S,R-labetalol 7.55 +/- 1.47 and 4.83 +/- 1.54 hr/L (p<0.05), S,S-labetalol 8.23 +/- 2.93 and 4.65 +/- 1.78 hr/L (p<0.05), R,S-labetalol 6.99 +/- 3.30 and 4.25 +/- 2.35 hr/L (p=0.11), and R,R-labetalol 3.91 +/- 2.57 and 3.55 +/- 3.08 hr/L (NS).

CONCLUSION

The higher labetalol concentration in women than in men was explained largely by differences in inactive and alpha1-blocking stereoisomers. However, concentrations were similar between genders for the beta-blocking stereoisomer (R,R-labetalol), possibly explaining the similarity in antihypertensive response to the drug. This study highlights the importance of determining stereoisomer kinetics for agents administered as racemates, particularly when relating concentrations to pharmacologic response.

Electrical approach to study rhodopsin activation in single cells with early receptor current assay

The ERC is a conformation-dependent charge motion similar to the gating currents of ionic channels. Both the waveforms and bandwidth of ERCs and ionic channel gating currents are similar, providing support to the initial suggestion that the ERP was a kind of gating current. In ionic channels the electrostatic field promotes motion of alpha-helical elements that stimulate large-scale molecular events that promote opening of the ionic pore. In ionic channels gating currents of expressed channel mutants has contributed significantly to understanding the mechanism of activation. Given the known role of electrical processes to rhodopsin activation, the ERC approach applied to mutant and wild-type visual pigments is likely to lead to a fuller understanding of the mechanism of conformational activation. This method is currently well suited to investigate the later phases of rhodopsin activation that are thought to be electrostatic in nature. We anticipate that ERC studies will make significant contributions to understanding how the breakdown of the electrostatic interaction between the PSB and its counterion is initiated and propagated to induce the proton uptake on the cytoplasmic surface of the pigment and the shaping of the transducin docking domain. We encourage collaboration to apply the ERC methodology to interesting mutant pigments and retinal analogs. We expect that the ERC methodology can soon be applied to understand rapid charge displacements associated with photochemistry (i.e., R1), the effects of transduction proteins on R2, and the measurement of electrical processes during cone visual pigment activation.

Development of stable cell lines expressing high levels of point mutants of human opsin for biochemical and biophysical studies

Stable HEK293S cell lines expressing high levels of normal and mutant human rod opsins were generated. Cellular expression is uniform across a population. Secondary overexpression of the same opsin transgene linked to a different drug selection marker (hygro(R)) yielded expression clones with increased opsin levels compared to the neo(R) parent strain. Wild-type and mutant human opsins regenerate with native chromophore and demonstrate spectroscopic properties consistent with previous reports of bovine opsin mutants. HEK293S cells can be grown in larger scale suspension culture (10(9) cells/liter) or in roller bottles (10(8) cells/bottle) to facilitate milligram-order preparations of purified pigments. These cell lines should be useful in any time-resolved spectroscopic or biophysical experiments that require either uniform cellular levels of opsin protein or regenerable pigment, or large amounts of purified visual pigment. They should also be useful in experiments where uniform constitutive levels of a given mutant human visual pigment are needed in each cell. These and similar types of constitutive or inducible cell lines may also be useful for studying mechanisms of human cell death that occur by mutations in the human rod opsin gene.

Cellular and molecular mechanisms underlying learning and memory impairments produced by cannabinoids

Why does smoking marijuana impair learning and memory? Behavioral studies suggest that a disruption of normal hippocampal function contributes to these deficits. In vitro experiments find that cannabinoid receptor activation reduces neurotransmitter release below the levels required to trigger long-term changes in synaptic strength in the hippocampus. Cannabinoids reduce glutamate release through a G-protein-mediated inhibition of the calcium channels responsible for neurotransmitter release from hippocampal neurons. These mechanisms likely play a role in the learning and memory impairments produced by cannabinoids and by endogenous cannabinoid receptor ligands.

Insulin induces dephosphorylation of eukaryotic initiation factor 2alpha and restores protein synthesis in vulnerable hippocampal neurons after transient brain ischemia

Brain reperfusion causes prompt, severe, and prolonged protein synthesis suppression and increased phosphorylation of eukaryotic initiation factor 2alpha [eIF2alpha(P)] in hippocampal CA1 and hilar neurons. The authors hypothesized that eIF2alpha(P) dephosphorylation would lead to recovery of protein synthesis. Here the effects of insulin, which activates phosphatases, were examined by immunostaining for eIF2alpha(P) and autoradiography of in vivo 35S amino acid incorporation. Rats resuscitated from a 10-minute cardiac arrest were given 0, 2, 10 or 20 U/kg of intravenous insulin, underwent reperfusion for 90 minutes, and were perfusion fixed. Thirty minutes before perfusion fixation, control and resuscitated animals received 500 microCi/kg of 35S methionine/cysteine. Alternate 30-microm brain sections were autoradiographed or immunostained for eIF2alpha(P). Controls had abundant protein synthesis and no eIF2alpha(P) in hippocampal neurons. Untreated reperfused neurons in the CA1, hilus, and dentate gyrus had intense staining for eIF2alpha(P) and reduced protein synthesis; there was little improvement with treatment with 2 or 10 U/kg of insulin. However, with 20 U/kg of insulin, these neurons recovered protein synthesis and were free of eIF2alpha(P). These results show that the suppression of protein synthesis in the reperfused brain is reversible; they support a causal association between eIF2alpha(P) and inhibition of protein synthesis, and suggest a mechanism for the neuroprotective effects of insulin.

Mechanisms of cannabinoid-receptor-mediated inhibition of synaptic transmission in cultured hippocampal pyramidal neurons

Cannabinoids, such as marijuana, are known to impair learning and memory perhaps through their actions in the hippocampus where cannabinoid receptors are expressed at high density. Although cannabinoid receptor activation decreases glutamatergic synaptic transmission in cultured hippocampal neurons, the mechanisms of this action are not known. Cannabinoid receptor activation also inhibits calcium channels that support neurotransmitter release in these cells, making modulation of these channels a candidate for cannabinoid-receptor-mediated effects on synaptic transmission. Whole cell patch-clamp recordings of glutamatergic neurons cultured from the CA1 and CA3 regions of the hippocampus were used to identify the mechanisms of the effects of cannabinoids on synaptic transmission. Cannabinoid receptor activation reduced excitatory postsynaptic current (EPSC) size by approximately 50% but had no effect on the amplitude of spontaneous miniature EPSCs (mEPSCs). This reduction in EPSC size was accompanied by an increase in paired-pulse facilitation measured in low (1 mM) extracellular calcium and by a decrease in paired-pulse depression measured in normal (2.5 mM) extracellular calcium. Together, these results strongly support the hypothesis that cannabinoid receptor activation decreases EPSC size by reducing release of neurotransmitter presynaptically while having no effect on postsynaptic sensitivity to glutamate. Further experiments were done to identify the molecular mechanisms underlying this cannabinoid-receptor-mediated decrease in neurotransmitter release. Cannabinoid receptor activation had no effect on the size of the presynaptic pool of readily releasable neurotransmitter-filled vesicles, eliminating reduction in pool size as a mechanism for cannabinoid-receptor-mediated effects. After blockade of Q- and N-type calcium channels with omega-agatoxin TK and omega-conotoxin GVIA; however, activation of cannabinoid receptors reduced EPSC size by only 14%. These results indicate that cannabinoid receptor activation reduces the probability that neurotransmitter will be released in response to an action potential via an inhibition of presynaptic Q- and N-type calcium channels. This molecular mechanism most likely contributes to the impairment of learning and memory produced by cannabinoids and may participate in the analgesic, antiemetic, and anticonvulsive effects of these drugs as well.

Time-resolved rhodopsin activation currents in a unicellular expression system

The early receptor current (ERC) is the charge redistribution occurring in plasma membrane rhodopsin during light activation of photoreceptors. Both the molecular mechanism of the ERC and its relationship to rhodopsin conformational activation are unknown. To investigate whether the ERC could be a time-resolved assay of rhodopsin structure-function relationships, the distinct sensitivity of modern electrophysiological tools was employed to test for flash-activated ERC signals in cells stably expressing normal human rod opsin after regeneration with 11-cis-retinal. ERCs are similar in waveform and kinetics to those found in photoreceptors. The action spectrum of the major R(2) charge motion is consistent with a rhodopsin photopigment. The R(1) phase is not kinetically resolvable and the R(2) phase, which overlaps metarhodopsin-II formation, has a rapid risetime and complex multiexponential decay. These experiments demonstrate, for the first time, kinetically resolved electrical state transitions during activation of expressed visual pigment in a unicellular environment (single or fused giant cells) containing only 6 x 10(6)-8 x 10(7) molecules of rhodopsin. This method improves measurement sensitivity 7 to 8 orders of magnitude compared to other time-resolved techniques applied to rhodopsin to study the role particular amino acids play in conformational activation and the forces that govern those transitions.

Mechanism of cannabinoid effects on long-term potentiation and depression in hippocampal CA1 neurons

Cannabinoids, the active constituents of marijuana, are known to impair learning and memory. Receptors for cannabinoids are highly expressed in the hippocampus, a brain region that is believed to play an important role in certain forms of learning and memory. To investigate the possible contribution of cannabinoid receptor-mediated deficits in hippocampal function to the learning and memory impairments produced by marijuana, we studied the effects of cannabinoid receptor activation on two models of learning and memory, long-term potentiation (LTP) and long-term depression (LTD), in hippocampal slices. Although LTP and LTD of CA1 field potentials were blocked by cannabinoid receptor activation in the presence of Mg(2+), they could be induced after Mg(2+) was removed. Similarly, LTP and LTD of whole-cell EPSCs were unimpaired in the presence of cannabinoid receptor agonist when the postsynaptic membrane was depolarized during the LTP or LTD induction protocol. Cannabinoid receptor activation also reduced EPSCs and enhanced paired-pulse facilitation, while having no effect on the amplitude of spontaneous miniature EPSCs. Finally, as with cannabinoid receptor activation, inhibition of LTP by adenosine receptor activation could be overcome by removal of Mg(2+) or depolarization of the postsynaptic membrane during tetanus. Our results indicate that cannabinoid receptor activation does not directly inhibit the molecular mechanisms responsible for long-term synaptic plasticity but instead impairs LTP and LTD by reducing presynaptic neurotransmitter release to a level below that required to depolarize the postsynaptic membrane to relieve Mg(2+) blockade of NMDA receptors.

Process modeling for health care organizations

Process models can be useful tools for managers seeking to correct problems within their organization. This paper explores how two such process models can be utilized in a medical practice.

Comparison of hepatic lesions in veal calves with concentrations of copper, iron and zinc in liver and kidney

Veal calf producers in Indiana have reported condemnation of carcasses due to icterus as well as condemnation of livers because of yellow discoloration, hepatomegaly and fibrosis. This study assessed the degree of hepatic injury in affected veal calves and correlated it with copper, iron and zinc concentrations in the liver and kidney. Tissues examined histopathologically were from slaughtered and necropsied veal calves. Hepatic lesions were divided into histopathologic categories of severity (minimal, moderate, marked or severe) based upon the degree of fibrosis, biliary epithelial hyperplasia, and inflammation. Hepatic copper levels decreased as the severity of lesions increased. The clinical observations and morphologic changes suggested initial hepatic damage before 9 w-of-age. The affected calves either died of acute copper toxicosis or survived to develop hepatomegaly, hepatic discoloration and/or fibrosis at the time of slaughter.

Limited literacy revisited implications for patient education

PURPOSE

It is important to determine whether teaching materials are understood and deemed accurate by the cancer patients being served. The authors used a series of patient participatory interviews to evaluate two brochures: Chemotherapy: What It Is and How It Helps by the American Cancer Society; and Helping Yourself During Chemotherapy: 4 Steps for Patients by the National Cancer Institute. The authors sought to determine whether 1) the brochures were clear; 2) differences in presentation were perceived by patients; and 3) differences influenced respondents' understanding and opinions of the brochures.

DESCRIPTION OF STUDY

A qualitative, focus-group methodology was used. Twelve cancer patients, eight of whom were women and six of whom were African American, were recruited from three outpatient clinics. Participants ranged in age from 31 to 62 years of age, and in education from completion of ninth grade to graduate school. Patients participated in a series of five groups, with a series of semistructured questions about content and format being asked of each group. All sessions were audiotaped. The investigators reviewed data both independently and together to identify content and format evaluations.

RESULTS

Findings showed that most participants were drawn first by the National Cancer Institute format; the American Cancer Society brochure was seen as having more information; the two brochures were seen as complementary; some confusion arose from the content of both brochures; and the discussions of emotional and sexual aspects were important.

CLINICAL IMPLICATIONS

These findings support the need for simple, clearly written brochures rather than brochures of varying literacy levels. The confusion caused by differences in advice given by the two organizations indicates the need to address specific areas in which dissimilarities exist. Finally, the project demonstrates the utility of patient focus groups for evaluation of patient education material.

Eukaryotic initiation factor 2alpha kinase and phosphatase activity during postischemic brain reperfusion

When ischemic brain is reperfused, there is in vulnerable neurons immediate inhibition of protein synthesis associated with a large increase in phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 [eIF2alpha, phosphorylated form eIF2alpha(P)]. We examined eIF2alpha kinase and eIF2alpha(P) phosphatase activity in brain homogenate postmitochondrial supernatants obtained from rats after 3 to 30 min of global brain ischemia (cardiac arrest), after 5 min of ischemia and 5 min of reperfusion (5R), and after 10 min of ischemia and 90 min reperfusion (90R). Because it has been suggested that PKR might be specifically responsible for producing eIF2alpha(P) during reperfusion, we also examined in brain homogenates from wild-type and PKR0/0 C57BL/6J x 129/SV mice the effect of 5 min of ischemia and 5 min of reperfusion on eIF2alpha(P). Cytosolic brain eIF2alpha(P) in the 5R and 90R rats was 18- and 23-fold that of nonischemic controls without any change in the rate of eIF2alpha(P) dephosphorylation. There was no change in eIF2alpha kinase activity between 3 and 30 min of ischemia but an 85% decrease in the 5R group; the 90R group was similar to controls. In wild-type and PKR0/0 mice total eIF2alpha was identical, and there was an identical 16-fold increase in eIF2alpha(P) at 5 min of reperfusion. Our observations contradict hypotheses that PKR activation, loss of eIF2alpha(P) phosphatase activity, or any general increase in eIF2alpha kinase activity are responsible for reperfusion-induced phosphorylation of eIF2alpha, and we suggest that the mechanism may involve regulation of the availability of eIF2alpha to a kinase.

Apoptosis

Apoptosis is a process of cell suicide, the mechanisms of which are encoded in the genomes of all higher eukaryotes. The mechanisms involved in apoptosis suggest that the process is based on a viral defense originally developed in primitive multicelled eukaryotes and that the fundamental execution platform of the process involves 1) inhibition of protein synthesis at the level of translation initiation, 2) proteolysis specifically involving degradation of DNA repair mechanisms, and 3) polynucleotide degradation. In mammals this execution platform is regulated by a complex molecular signaling system that includes feedback mechanisms tending toward activation of all elements of the execution platform if only one element is initially engaged. Tissue ischemia and reperfusion activate elements of the apoptosis system, which thus represents a therapeutic target for emerging treatment approaches to preserve cellular integrity in critical organs such as the heart and brain.

Regulation of the readily releasable vesicle pool by protein kinase C

Modulation of the size of the readily releasable vesicle pool has recently come under scrutiny as a candidate for the regulation of synaptic strength. Using electrophysiological and optical measurement techniques, we show that phorbol esters increase the size of the readily releasable pool at glutamatergic hippocampal synapses in culture through a protein kinase C (PKC)-dependent mechanism. Phorbol ester activation of PKC also increases the rate at which the pool refills. These results identify two powerful ways that activation of the PKC pathway may regulate synaptic strength by modulating the readily releasable pool of vesicles.

Doctorally-prepared nurses: different practice settings, different views

Nurse researchers in academic and clinical settings have the ultimate goal of improving nursing care while balancing the demands of education, their institution, research, and nursing service. The setting shapes the focus of research, the choice of research models, and the parameters of the position. The setting also influences available resources and the services rendered to the institution and the community. The functions of doctorally-prepared nurses in academic and clinical settings are compared. A better understanding of the functions of nurses in each setting could lead to better informed employment choices and improved collaborative efforts.

Effect of brain ischemia and reperfusion on the localization of phosphorylated eukaryotic initiation factor 2 alpha

Postischemic brain reperfusion is associated with a substantial and long-lasting reduction of protein synthesis in selectively vulnerable neurons. Because the overall translation initiation rate is typically regulated by altering the phosphorylation of serine 51 on the alpha-subunit of eukaryotic initiation factor 2 (eIF-2 alpha), we used an antibody specific to phosphorylated eIF-2 alpha [eIF-2(alpha P)] to study the regional and cellular distribution of eIF-2(alpha P) in normal, ischemic, and reperfused rat brains. Western blots of brain postmitochondrial supernatants revealed that approximately 1% of all eIF-2 alpha is phosphorylated in controls, eIF-2(alpha P) is not reduced by up to 30 minutes of ischemia, and eIF-2(alpha P) is increased approximately 20-fold after 10 and 90 minutes of reperfusion. Immunohistochemistry shows localization of eIF-2(alpha P) to astrocytes in normal brains, a massive increase in eIF-2(alpha P) in the cytoplasm of neurons within the first 10 minutes of reperfusion, accumulation of eIF-2(alpha P) in the nuclei of selectively vulnerable neurons after 1 hour of reperfusion, and morphology suggesting pyknosis or apoptosis in neuronal nuclei that continue to display eIF-2(alpha P) after 4 hours of reperfusion. These observations, together with the fact that eIF-2(alpha P) inhibits translation initiation, make a compelling case that eIF-2(alpha P) is responsible for reperfusion-induced inhibition of protein synthesis in vulnerable neurons.

Effect on survival of estrogen replacement therapy after coronary artery bypass grafting

We examined the relation between postmenopausal estrogen placement therapy (ERT) and survival in 1,098 women who underwent coronary artery bypass grafting (CABG). Patients were selected for the study if their age was > or = 55 years at the time of preoperative coronary angiography or if they had previously undergone bilateral oophorectomy. Life-table analysis was used to compare survival after surgery in 92 women who received ERT and 1,006 women who did not. Five-year survival was 98.8% in the estrogen users and 82.3% in the non-users. Ten-year survival was 81.4% in the users and 65.1% in the nonusers (p = 0.0001 by Lee Desu test). The women who did not take estrogen were significantly older (p < 0.001), had more vessels with significant stenosis (p = 0.033), lower ejection fractions (p = 0.051), and more prior myocardial infarctions (p = 0.054). However, a Cox proportional-hazards model selected the number of coronary arteries narrowed (RR 1.43, p < 0.0001), estrogen use (RR 0.38, p = 0.001), left main coronary stenosis (RR 1.83, p = 0.001), and diabetes mellitus (RR 1.57, p = 0.003) as the significant independent predictors of survival. These data suggest that ERT improves survival significantly after CABG in postmenopausal women with coronary artery disease.

Learning the baby: a maternal thinking and problem-solving process

PURPOSE

To describe a maternal problem-solving process from new mothers' day-to-day experience of caring for and developing a relationship with their babies.

DESIGN

Qualitative study using grounded theory approach.

SETTING

Mothers' homes (n = 23) and workplaces (n = 2).

PARTICIPANTS

25 mothers of babies from 2 weeks to 7 months.

MAIN OUTCOME MEASURES

Semi-structured interviews.

RESULTS

"Learning the Baby" was a major thinking process that emerged. Systematic thinking dominated mothers' problem-solving. Even after the babies were comforted, mothers were not certain their efforts alleviated the babies' problems, because they usually were not certain of the problem.

CONCLUSIONS

Mothers' thinking directs their caregiving actions. The findings were related to previous problem-solving and women's thinking perspectives. Nurses must use planned and unplanned encounters effectively to collaborate with and teach mothers about baby behavior and health care.