Contribution of epoxyeicosatrienoic acids to the hypoxia-induced activation of Ca2+-activated K+ channel current in cultured rat hippocampal astrocytes

Regulatory lipid vicinal diols counteract the biological activity of epoxy fatty acids and can act as biomarkers and mechanisms for disease progression

Polyunsaturated fatty acids (PUFAs) are essential fatty acids required for human health and are obtained primarily from food or synthesized in the body by highly regulated processes. The metabolites of these lipids, formed largely through the action of cyclooxygenase, lipoxygenase, or cytochrome P450 (CYP450) enzymes, are responsible for multiple biological functions including inflammation, tissue repair, cell proliferation, blood vessel permeability, and immune cell behavior. The role of these regulatory lipids in disease has been well studied since their discovery as druggable targets; however, the metabolites generated downstream of these pathways have only recently gained attention for regulating biology. Specifically, the biological activity of lipid vicinal diols formed from the metabolism of CYP450-generated epoxy fatty acids (EpFA) by epoxide hydrolases were previously thought to have little biological activity but increasingly are recognized as promoting inflammation and brown fat adipogenesis, and exciting neurons through the regulation of ion channel activity at low concentrations. These metabolites also appear to balance the action of the EpFA precursor. For example, EpFA demonstrate the ability to resolve inflammation and reduce pain, while some lipid diols, through opposing mechanisms, promote inflammation and pain. This review describes recent studies that highlight the role of regulatory lipids, focusing on the balance between EpFA and their diol metabolites in promoting or resolving disease.

Adaptation to oxidative stress at cellular and tissue level

Several in vitro and in vivo investigations have already proved that cells and tissues, when pre-exposed to low oxidative stress by different stimuli such as chemical, physical agents and environmental factors, display more resistance against subsequent stronger ischaemic injuries, resulting in an adaptive response known as ischaemic preconditioning (IPC). The aim of this review is to report the most recent knowledge about the complex adaptive mechanisms, including signalling transduction pathways, antioxidant systems, apoptotic and inflammation pathways, underlying cell protection against oxidative damage. In addition, an update about in vivo adaptation strategies in response to ischaemic/reperfusion episodes and brain trauma is also given.

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

Neurologic effects of short-term treatment with a soluble epoxide hydrolase inhibitor after cardiac arrest in pediatric swine

BACKGROUND

Cardiac arrest (CA) is the most common cause of acute neurologic insult in children. Many survivors have significant neurocognitive deficits at 1 year of recovery. Epoxyeicosatrienoic acids (EETs) are multifunctional endogenous lipid signaling molecules that are involved in brain pathobiology and may be therapeutically relevant. However, EETs are rapidly metabolized to less active dihydroxyeicosatrienoic acids by soluble epoxide hydrolase (sEH), limiting their bioavailability. We hypothesized that sEH inhibition would improve outcomes after CA in an infant swine model. Male piglets (3-4 kg, 2 weeks old) underwent hypoxic-asphyxic CA. After resuscitation, they were randomized to intravenous treatment with an sEH inhibitor (TPPU, 1 mg/kg; n = 8) or vehicle (10% poly(ethylene glycol); n = 9) administered at 30 min and 24 h after return of spontaneous circulation. Two sham-operated groups received either TPPU (n = 9) or vehicle (n = 8). Neurons were counted in hematoxylin- and eosin-stained sections from putamen and motor cortex in 4-day survivors.

RESULTS

Piglets in the CA + vehicle groups had fewer neurons than sham animals in both putamen and motor cortex. However, the number of neurons after CA did not differ between vehicle- and TPPU-treated groups in either anatomic area. Further, 20% of putamen neurons in the Sham + TPPU group had abnormal morphology, with cell body attrition and nuclear condensation. TPPU treatment also did not reduce neurologic deficits.

CONCLUSION

Treatment with an sEH inhibitor at 30 min and 24 h after resuscitation from asphyxic CA does not protect neurons or improve acute neurologic outcomes in piglets.

Propentdyopents as Heme Degradation Intermediates Constrict Mouse Cerebral Arterioles and Are Present in the Cerebrospinal Fluid of Patients With Subarachnoid Hemorrhage

RATIONALE

Delayed ischemic neurological deficit is the most common cause of neurological impairment and unfavorable prognosis in patients with subarachnoid hemorrhage (SAH). Despite the existence of neuroimaging modalities that depict the onset of the accompanying cerebral vasospasm, preventive and therapeutic options are limited and fail to improve outcome owing to an insufficient pathomechanistic understanding of the delayed perfusion deficit. Previous studies have suggested that BOXes (bilirubin oxidation end products), originating from released heme surrounding ruptured blood vessels, are involved in arterial vasoconstriction. Recently, isolated intermediates of oxidative bilirubin degradation, known as PDPs (propentdyopents), have been considered as potential additional effectors in the development of arterial vasoconstriction.

OBJECTIVE

To investigate whether PDPs and BOXes are present in hemorrhagic cerebrospinal fluid and involved in the vasoconstriction of cerebral arterioles.

METHODS AND RESULTS

Via liquid chromatography/mass spectrometry, we measured increased PDP and BOX concentrations in cerebrospinal fluid of SAH patients compared with control subjects. Using differential interference contrast microscopy, we analyzed the vasoactivity of PDP isomers in vitro by monitoring the arteriolar diameter in mouse acute brain slices. We found an arteriolar constriction on application of PDPs in the concentration range that occurs in the cerebrospinal fluid of patients with SAH. By imaging arteriolar diameter changes using 2-photon microscopy in vivo, we demonstrated a short-onset vasoconstriction after intrathecal injection of either PDPs or BOXes. Using magnetic resonance imaging, we observed a long-term PDP-induced delay in cerebral perfusion. For all conditions, the arteriolar narrowing was dependent on functional big conductance potassium channels and was absent in big conductance potassium channels knockout mice.

CONCLUSIONS

For the first time, we have quantified significantly higher concentrations of PDP and BOX isomers in the cerebrospinal fluid of patients with SAH compared to controls. The vasoconstrictive effect caused by PDPs in vitro and in vivo suggests a hitherto unrecognized pathway contributing to the pathogenesis of delayed ischemic deficit in patients with SAH.

P450 Eicosanoids and Reactive Oxygen Species Interplay in Brain Injury and Neuroprotection

Significance: Eicosanoids are endogenous lipid mediators that play important roles in brain function and disease. Acute brain injury such as that which occurs in stroke and traumatic brain injury increases the formation of eicosanoids, which, in turn, exacerbate or diminish injury. In chronic neurodegenerative diseases such as Alzheimer's disease and vascular dementia (VD), eicosanoid synthetic and metabolizing enzymes are altered, disrupting the balance between neuroprotective and neurotoxic eicosanoids. Recent Advances: Human and experimental studies have established the opposing roles of hydroxy- and epoxyeicosanoids and their potential utility as diagnostic biomarkers and therapeutic targets in neural injury. Critical Issues: A gap in knowledge remains in understanding the cellular and molecular mechanisms underlying the neurovascular actions of specific eicosanoids, such as specific isomers of epoxyeicosatrienoic (EETs) and hydroxyeicosatetraenoic acids (HETEs). Future Directions: EETs and HETEs exert their actions on brain cells by targeting multiple mechanisms, which include surface G-protein coupled receptors. The identification of high-affinity receptors for EETs and HETEs and their cellular localization in the brain will be a breakthrough in our understanding of these eicosanoids as mediators of cell-cell communications and contributors to brain development, function, and disease. Antioxid. Redox Signal. 28, 987-1007.

Regulation of Cerebral Blood Flow: Response to Cytochrome P450 Lipid Metabolites

There have been numerous reviews related to the cerebral circulation. Most of these reviews are similar in many ways. In the present review, we thought it important to provide an overview of function with specific attention to details of cerebral arterial control related to brain homeostasis, maintenance of neuronal energy demands, and a unique perspective related to the role of astrocytes. A coming review in this series will discuss cerebral vascular development and unique properties of the neonatal circulation and developing brain, thus, many aspects of development are missing here. Similarly, a review of the response of the brain and cerebral circulation to heat stress has recently appeared in this series (8). By trying to make this review unique, some obvious topics were not discussed in lieu of others, which are from recent and provocative research such as endothelium-derived hyperpolarizing factor, circadian regulation of proteins effecting cerebral blood flow, and unique properties of the neurovascular unit. © 2018 American Physiological Society. Compr Physiol 8:801-821, 2018.

Cytochrome P450 eicosanoids in cerebrovascular function and disease

Cytochrome P450 eicosanoids play important roles in brain function and disease through their complementary actions on cell-cell communications within the neurovascular unit (NVU) and mechanisms of brain injury. Epoxy- and hydroxyeicosanoids, respectively formed by cytochrome P450 epoxygenases and ω-hydroxylases, play opposing roles in cerebrovascular function and in pathological processes underlying neural injury, including ischemia, neuroinflammation and oxidative injury. P450 eicosanoids also contribute to cerebrovascular disease risk factors, including hypertension and diabetes. We summarize studies investigating the roles P450 eicosanoids in cerebrovascular physiology and disease to highlight the existing balance between these important lipid signaling molecules, as well as their roles in maintaining neurovascular homeostasis and in acute and chronic neurovascular and neurodegenerative disorders.

Linoleic acid participates in the response to ischemic brain injury through oxidized metabolites that regulate neurotransmission

Linoleic acid (LA; 18:2 n-6), the most abundant polyunsaturated fatty acid in the US diet, is a precursor to oxidized metabolites that have unknown roles in the brain. Here, we show that oxidized LA-derived metabolites accumulate in several rat brain regions during CO2-induced ischemia and that LA-derived 13-hydroxyoctadecadienoic acid, but not LA, increase somatic paired-pulse facilitation in rat hippocampus by 80%, suggesting bioactivity. This study provides new evidence that LA participates in the response to ischemia-induced brain injury through oxidized metabolites that regulate neurotransmission. Targeting this pathway may be therapeutically relevant for ischemia-related conditions such as stroke.

Augmentation of poly(ADP-ribose) polymerase-dependent neuronal cell death by acidosis

Tissue acidosis is a key component of cerebral ischemic injury, but its influence on cell death signaling pathways is not well defined. One such pathway is parthanatos, in which oxidative damage to DNA results in activation of poly(ADP-ribose) polymerase and generation of poly(ADP-ribose) polymers that trigger release of mitochondrial apoptosis-inducing factor. In primary neuronal cultures, we first investigated whether acidosis per sé is capable of augmenting parthanatos signaling initiated pharmacologically with the DNA alkylating agent, N-methyl- N'-nitro- N-nitrosoguanidine. Exposure of neurons to medium at pH 6.2 for 4 h after N-methyl- N'-nitro- N-nitrosoguanidine washout increased intracellular calcium and augmented the N-methyl- N'-nitro- N-nitrosoguanidine-evoked increase in poly(ADP-ribose) polymers, nuclear apoptosis-inducing factor , and cell death. The augmented nuclear apoptosis-inducing factor and cell death were blocked by the acid-sensitive ion channel-1a inhibitor, psalmotoxin. In vivo, acute hyperglycemia during transient focal cerebral ischemia augmented tissue acidosis, poly(ADP-ribose) polymers formation, and nuclear apoptosis-inducing factor , which was attenuated by a poly(ADP-ribose) polymerase inhibitor. Infarct volume from hyperglycemic ischemia was decreased in poly(ADP-ribose) polymerase 1-null mice. Collectively, these results demonstrate that acidosis can directly amplify neuronal parthanatos in the absence of ischemia through acid-sensitive ion channel-1a . The results further support parthanatos as one of the mechanisms by which ischemia-associated tissue acidosis augments cell death.

Inhibition of soluble epoxide hydrolase augments astrocyte release of vascular endothelial growth factor and neuronal recovery after oxygen-glucose deprivation

Epoxyeicosatrienoic acids (EETs) are synthesized in astrocytes, and inhibitors of soluble epoxide hydrolase (sEH), which hydrolyzes EETs, reduce infarct volume in ischemic stroke. Astrocytes can release protective neurotrophic factors, such as vascular endothelial growth factor (VEGF). We found that addition of sEH inhibitors to rat cultured astrocytes immediately after oxygen-glucose deprivation (OGD) markedly increased VEGF concentration in the medium 48 h later and the effect was blocked by an EET antagonist. The sEH inhibitors increased EET concentrations to levels capable of increasing VEGF. When the sEH inhibitors were removed from the medium at 48 h, the increase in VEGF persisted for an additional 48 h. Neurons exposed to OGD and subsequently to astrocyte medium previously conditioned with OGD plus sEH inhibitors showed increased phosphorylation of their VEGF receptor-2, less TUNEL staining, and increased phosphorylation of Akt, which was blocked by a VEGF receptor-2 antagonist. Our findings indicate that sEH inhibitors, applied to cultured astrocytes after an ischemia-like insult, can increase VEGF secretion. The released VEGF then enhances Akt-enabled cell survival signaling in neurons through activation of VEGF receptor-2 leading to less neuronal cell death. These results suggest a new strategy by which astrocytes can be leveraged to support neuroprotection.

Expression of CYP 4A ω-hydroxylase and formation of 20-hydroxyeicosatetreanoic acid (20-HETE) in cultured rat brain astrocytes

Astrocytes secrete vasodilator and vasoconstrictor factors via end feet processes, altering blood flow to meet neuronal metabolic demand. Compared to what is known about the ability of astrocytes to release factors that dilate local cerebral vasculature, very little is known regarding the source and identity of astrocyte derived constricting factors. The present study investigated if astrocytes express CYP 4A ω-hydroxylase and metabolize arachidonic acid (AA) to 20-hydroxyeicotetraenoic acid (20-HETE) that regulates KCa channel activity in astrocytes and cerebral arterial myocyte contractility. Here we report that cultured astrocytes express CYP 4A2/3 ω-hydroxylase mRNA and CYP 4A protein and produce 20-HETE and the CYP epoxygenase metabolites epoxyeicosatrienoic acids (EETs) when incubated with AA. The production of 20-HETE and EETs was enhanced following stimulation of metabotropic glutamate receptors (mGluR) on the astrocytes. Exogenous application of 20-HETE attenuated, whereas inhibition of 20-HETE production with HET-0016 increased the open state probabilities (NPo) of 71pS and 161pS KCa single-channel currents recorded from astrocytes. Exposure of isolated cerebral arterial myocytes to conditioned media from cultured astrocytes caused shortening of the length of freshly isolated cerebral arterial myocytes that was not evident following inhibition of astrocyte 20-HETE synthesis and action. These findings suggest that astrocytes not only release vasodilator EETs in response to mGluR stimulation but also synthetize and release the cerebral arterial myocyte constrictor 20-HETE that also functions as an endogenous inhibitor of the activity of two types of KCa channel currents found in astrocytes.

Cyclooxygenase- and cytochrome P450-derived eicosanoids in stroke

Arachidonic acid (AA) is metabolized by cyclooxygenase (COX) and cytochrome P450 (CYP) enzymes into eicosanoids, which are involved in cardiovascular diseases and stroke. Evidence has demonstrated the important functions of these eicosanoids in regulating cerebral vascular tone, cerebral blood flow, and autoregulation of cerebral circulation. Although COX-2 inhibitors have been suggested as potential treatments for stroke, adverse events, including an increased risk of stroke, occur following long-term use of coxibs. It is important to note that prolonged treatment with rofecoxib increased circulating levels of 20-hydroxyeicosatetraenoic acid (20-HETE), and 20-HETE blockade is a possible strategy to prevent coxib-induced stroke events. It appears that 20-HETE has detrimental effects in the brain, and that its blockade exerts cerebroprotection against ischemic stroke and subarachnoid hemorrhage (SAH). There is clear evidence that activation of EP2 and EP4 receptors exerts cerebroprotection against ischemic stroke. Several elegant studies have contributed to defining the importance of stabilizing the levels of epoxyeicosatrienoic acids (EETs), by inhibiting or deleting soluble epoxide hydrolase (sEH), in stroke research. These reports support the notion that sEH blockade is cerebroprotective against ischemic stroke and SAH. Here, we summarize recent findings implicating these eicosanoid pathways in cerebral vascular function and stroke. We also discuss the development of animal models with targeted gene deletion and specific enzymatic inhibitors in each pathway to identify potential targets for the treatment of ischemic stroke and SAH.

Contribution of epoxyeicosatrienoic acids to the cerebral blood flow response to hypoxemia

Adenosine A2A receptors and ATP-activated K(+) (KATP) channels contribute to part of the cerebral vasodilatory response to systemic hypoxia, but other mediators are likely involved. Epoxyeicosatrienoic acids (EETs) are cerebral vasodilators and are released from astrocytes exposed to hypoxia. Moreover, stimulation of metabotropic glutamate receptors (mGluR) produces vasodilation by an EET-dependent mechanism. Here, we tested the hypothesis that EET signaling and mGluR activation contribute to hypoxic vasodilation. Laser-Doppler flow was measured over cerebral cortex of anesthetized rats subjected to stepwise reductions in arterial oxygen saturation to 50-70%. Hypoxic reactivity was calculated as the slope of the change in laser-Doppler flow vs. the reciprocal of arterial oxygen content. Hypoxic reactivity significantly decreased from 9.2 ± 1.9 (±95% confidence interval) in controls with vehicle treatment to 2.6 ± 1.4 with the EET antagonist 14,15-epoxyeicosa-5(Z)-enoic acid, to 3.0 ± 1.5 with the EET synthesis inhibitor MS-PPOH, to 1.9 ± 2.3 with the combined mGluR subtype 1 and 5 antagonists 2-methyl-6-(phenylethynyl)pyridine and LY367385, to 5.6 ± 1.2 with the KATP channel inhibitor glibenclamide, and to 5.8 ± 2.3 with the A2A receptor antagonist SCH58261. However, reactivity was not significantly altered by the A2B receptor antagonist MRS1754 (6.7 ± 1.8; P = 0.28 Dunnett's test) or by the 20-hydroxyeicosatetraenoic acid synthesis inhibitor HET0016 (7.5 ± 2.3; P = 0.6). These data indicate that, in addition to the known contributions of A2A receptors and KATP channels to the increase in cerebral blood flow during hypoxia, EETs and mGluRs make a major contribution, possibly by mGluR stimulation and hypoxia-induced release of EETs. In contrast, A2B receptors do not make a major contribution, and 20-hydroxyeicosatetraenoic acid does not significantly limit hypoxic vasodilation.

Integrative regulation of human brain blood flow

Herein, we review mechanisms regulating cerebral blood flow (CBF), with specific focus on humans. We revisit important concepts from the older literature and describe the interaction of various mechanisms of cerebrovascular control. We amalgamate this broad scope of information into a brief review, rather than detailing any one mechanism or area of research. The relationship between regulatory mechanisms is emphasized, but the following three broad categories of control are explicated: (1) the effect of blood gases and neuronal metabolism on CBF; (2) buffering of CBF with changes in blood pressure, termed cerebral autoregulation; and (3) the role of the autonomic nervous system in CBF regulation. With respect to these control mechanisms, we provide evidence against several canonized paradigms of CBF control. Specifically, we corroborate the following four key theses: (1) that cerebral autoregulation does not maintain constant perfusion through a mean arterial pressure range of 60-150 mmHg; (2) that there is important stimulatory synergism and regulatory interdependence of arterial blood gases and blood pressure on CBF regulation; (3) that cerebral autoregulation and cerebrovascular sensitivity to changes in arterial blood gases are not modulated solely at the pial arterioles; and (4) that neurogenic control of the cerebral vasculature is an important player in autoregulatory function and, crucially, acts to buffer surges in perfusion pressure. Finally, we summarize the state of our knowledge with respect to these areas, outline important gaps in the literature and suggest avenues for future research.

Modeling secondary messenger pathways in neurovascular coupling

Neurovascular coupling is the well-documented link between neural stimulation and constriction or dilation of the surrounding vasculature. Glial cells mediate this response via their unique anatomy, which connects neurons to arterioles. It is believed that calcium transients and the release of secondary messengers by these cells influence the vascular response. We present a model of intracellular calcium dynamics in an astrocyte (glial cell) and show that stable oscillatory behaviour is possible under certain conditions. We then couple this to a novel model for the relationship between calcium concentration and the production of vasoactive secondary messengers through a fatty-acid intermediate. The two secondary messengers modelled are epoxyeicosatrienoic and 20-hydroxyeicosatetraenoic acids (EET and 20-HETE, respectively). These secondary messengers are produced on different time scales, and we show how this supports the observation that the vasculature dilates rapidly in response to neural stimulation, before returning to baseline levels on a slower time scale.

The complex contribution of NOS interneurons in the physiology of cerebrovascular regulation

Following the discovery of the vasorelaxant properties of nitric oxide (NO) by Furchgott and Ignarro, the finding by Bredt and coll. of a constitutively expressed NO synthase in neurons (nNOS) led to the presumption that neuronal NO may control cerebrovascular functions. Consequently, numerous studies have sought to determine whether neuraly-derived NO is involved in the regulation of cerebral blood flow (CBF). Anatomically, axons, dendrites, or somata of NO neurons have been found to contact the basement membrane of blood vessels or perivascular astrocytes in all segments of the cortical microcirculation. Functionally, various experimental approaches support a role of neuronal NO in the maintenance of resting CBF as well as in the vascular response to neuronal activity. Since decades, it has been assumed that neuronal NO simply diffuses to the local blood vessels and produce vasodilation through a cGMP-PKG dependent mechanism. However, NO is not the sole mediator of vasodilation in the cerebral microcirculation and is known to interact with a myriad of signaling pathways also involved in vascular control. In addition, cerebrovascular regulation is the result of a complex orchestration between all components of the neurovascular unit (i.e., neuronal, glial, and vascular cells) also known to produce NO. In this review article, the role of NO interneuron in the regulation of cortical microcirculation will be discussed in the context of the neurovascular unit.

Cytochrome P450-derived epoxyeicosatrienoic acids and pulmonary hypertension: central role of transient receptor potential C6 channels

Hypoxia induces the constriction of pulmonary resistance arteries, which results in the redistribution of blood from poor to better ventilated areas, thus optimizing its oxygenation. Many different oxygen-sensing mechanisms have been proposed to regulate this process, including cytochrome P450 enzymes. These enzymes, which convert substrates such as arachidonic acid into bioactive epoxides (the epoxyeicosatrienoic acids [EETs]), are highly expressed in the lung as is the soluble epoxide hydrolase which metabolizes the epoxides to their less active diols. The EETs play a well-documented role as endothelium-derived vasodilators in the systemic vasculature, but in the pulmonary circulation, they are generated in vascular smooth muscle cells and potentiate vasoconstriction. Preventing the breakdown of 11,12-EET by the inhibition or genetic deletion of the soluble epoxide hydrolase strongly augments the response to hypoxia. Mechanistically, 11,12-EET potentiates the contractile response by recruiting transient receptor potential C6 channels to caveolae. Indeed, neither 11,12-EET nor hypoxia is able to elicit pulmonary vasoconstriction in TRPC6 knockout mice. The cytochrome and soluble epoxide hydrolase enzymes are also implicated in the vascular remodeling associated with chronic hypoxia and pulmonary hypertension. Thus, targeting this pathway may be in an attractive new therapeutic approach to treat this incapacitating disease.

Brain P450 epoxygenase activity is required for the antinociceptive effects of improgan, a nonopioid analgesic

The search for the mechanism of action of improgan (a nonopioid analgesic) led to the recent discovery of CC12, a compound that blocks improgan antinociception. Because CC12 is a cytochrome P450 inhibitor, and brain P450 mechanisms were recently shown to be required in opioid analgesic signaling, pharmacological and transgenic studies were performed in rodents to test the hypothesis that improgan antinociception requires brain P450 epoxygenase activity. Intracerebroventricular (i.c.v.) administration of the P450 inhibitors miconazole and fluconazole, and the arachidonic acid (AA) epoxygenase inhibitor N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH) potently inhibited improgan antinociception in rats at doses that were inactive alone. MW06-25, a new P450 inhibitor that combines chemical features of CC12 and miconazole, also potently blocked improgan antinociception. Although miconazole and CC12 were weakly active at opioid and histamine H(3) receptors, MW06-25 showed no activity at these sites, yet retained potent P450-inhibiting properties. The P450 hypothesis was also tested in Cpr(low) mice, a viable knock-in model with dramatically reduced brain P450 activity. Improgan (145 nmol, i.c.v.) antinociception was reduced by 37% to 59% in Cpr(low) mice, as compared with control mice. Moreover, CC12 pretreatment (200 nmol, i.c.v.) abolished improgan action (70% to 91%) in control mice, but had no significant effect in Cpr(low) mice. Thus, improgan's activation of bulbospinal nonopioid analgesic circuits requires brain P450 epoxygenase activity. A model is proposed in which (1) improgan activates an unknown receptor to trigger downstream P450 activity, and (2) brainstem epoxygenase activity is a point of convergence for opioid and nonopioid analgesic signaling.

Epoxyeicosatrienoic acid-dependent cerebral vasodilation evoked by metabotropic glutamate receptor activation in vivo

Group I metabotropic glutamate receptors (mGluR) on astrocytes have been shown to participate in cerebral vasodilation to neuronal activation in brain slices. Pharmacological stimulation of mGluR in brain slices can produce arteriolar constriction or dilation depending on the initial degree of vascular tone. Here, we examined whether pharmacological stimulation of mGluR in vivo increases cerebral blood flow. A 1-mM solution of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) superfused at 5 μl/min over the cortical surface of anesthetized rats produced a 30 ± 2% (±SE) increase in blood flow measured by laser-Doppler flowmetry after 15-20 min. The response was completely blocked by superfusion of group I mGluR antagonists and attenuated by superfusion of an epoxyeicosatrienoic acid (EET) antagonist (5 ± 4%), an EET synthesis inhibitor (11 ± 3%), and a cyclooxygenase-2 inhibitor (15 ± 3%). The peak blood flow response was not significantly affected by administration of inhibitors of cyclooxygenase-1, neuronal nitric oxide synthase, heme oxygenase, adenosine A(2B) receptors, or an inhibitor of the synthesis of 20-hydroxyeicosatetraenoic acid (20-HETE). The blood flow response gradually waned following 30-60 min of DHPG superfusion. This loss of the flow response was attenuated by a 20-HETE synthesis inhibitor and was prevented by superfusion of an inhibitor of epoxide hydrolase, which hydrolyzes EETs. These results indicate that pharmacological stimulation of mGluR in vivo increases cerebral blood flow and that the response depends on the release of EETs and a metabolite of cyclooxygenase-2. Epoxide hydrolase activity and 20-HETE synthesis limit the duration of the response to prolonged mGluR activation.

Hypoxia. 4. Hypoxia and ion channel function

The ability to sense and respond to oxygen deprivation is required for survival; thus, understanding the mechanisms by which changes in oxygen are linked to cell viability and function is of great importance. Ion channels play a critical role in regulating cell function in a wide variety of biological processes, including neuronal transmission, control of ventilation, cardiac contractility, and control of vasomotor tone. Since the 1988 discovery of oxygen-sensitive potassium channels in chemoreceptors, the effect of hypoxia on an assortment of ion channels has been studied in an array of cell types. In this review, we describe the effects of both acute and sustained hypoxia (continuous and intermittent) on mammalian ion channels in several tissues, the mode of action, and their contribution to diverse cellular processes.

Role of epoxyeicosatrienoic acids as autocrine metabolites in glutamate-mediated K+ signaling in perivascular astrocytes

Epoxyeicosatrienoic acids (EETs), synthesized and released by astrocytes in response to glutamate, are known to play a pivotal role in neurovascular coupling. In vascular smooth muscle cells (VSMC), EETs activate large-conductance, Ca(2+)-activated K(+) (BK) channels resulting in hyperpolarization and vasodilation. However, the functional role and mechanism of action for glial-derived EETs are still to be determined. In this study, we evaluated the effect of the synthetic EET analog 11-nonyloxy-undec-8(Z)-enoic acid (NUD-GA) on outward K(+) currents mediated by calcium-activated K(+) channels. Addition of NUD-GA significantly increased intracellular Ca(2+) and outward K(+) currents in perivascular astrocytes. NUD-GA-induced currents were significantly inhibited by BK channel blockers paxilline and tetraethylammonium (TEA) (23.4 ± 2.4%; P < 0.0005). Similarly, NUD-GA-induced currents were also significantly inhibited in the presence of the small-conductance Ca(2+)-activated K(+) channel inhibitor apamin along with a combination of blockers against glutamate receptors (12.8 ± 2.70%; P < 0.05). No changes in outward currents were observed in the presence of the channel blocker for intermediate-conductance K(+) channels TRAM-34. Blockade of the endogenous production of EETs with N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH) significantly blunted (dl)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD)-induced outward K(+) currents (P < 0.05; n = 6). Both NUD-GA and t-ACPD significantly increased BK channel single open probability; the later was blocked following MS-PPOH incubation. Our data supports the idea that EETs are potent K(+) channel modulators in cortical perivascular astrocytes and further suggest that these metabolites may participate in NVC by modulating the levels of K(+) released at the gliovascular space.

Ca2+- and thromboxane-dependent distribution of MaxiK channels in cultured astrocytes: from microtubules to the plasma membrane

Large-conductance, voltage- and Ca2+-activated K+ channels (MaxiK) are broadly expressed ion channels minimally assembled by four pore-forming alpha-subunits (MaxiKalpha) and typically observed as plasma membrane proteins in various cell types. In murine astrocyte primary cultures, we show that MaxiKalpha is predominantly confined to the microtubule network. Distinct microtubule distribution of MaxiKalpha was visualized by three independent labeling approaches: (1) MaxiKalpha-specific antibodies, (2) expressed EGFP-labeled MaxiKalpha, and (3) fluorophore-conjugated iberiotoxin, a specific MaxiK pore-blocker. This MaxiKalpha association with microtubules was further confirmed by in vitro His-tag pulldown, co-immunoprecipitation from brain lysates, and microtubule depolymerization experiments. Changes in intracellular Ca2+ elicited by general pharmacological agents, caffeine or thapsigargin, resulted in increased MaxiKalpha labeling at the plasma membrane. More notably, U46619, an analog of thromboxane A2 (TXA2), which triggers Ca2+-release pathways and whose levels increase during cerebral hemorrhage/trauma, also elicits a similar increase in MaxiKalpha surface labeling. Whole-cell patch clamp recordings of U46619-stimulated cells develop a approximately 3-fold increase in current amplitude indicating that TXA2 stimulation results in the recruitment of additional, functional MaxiK channels to the surface membrane. While microtubules are largely absent in mature astrocytes, immunohistochemistry results in brain slices show that cortical astrocytes in the newborn mouse (P1) exhibit a robust expression of microtubules that significantly colocalize with MaxiK. The results of this study provide the novel insight that suggests that Ca2+ released from intracellular stores may play a key role in regulating the traffic of intracellular, microtubule-associated MaxiK stores to the plasma membrane of developing murine astrocytes.

Soluble Epoxide Hydrolase Inhibition: Targeting Multiple Mechanisms of Ischemic Brain Injury with a Single Agent

Soluble epoxide hydrolase (sEH) is a key enzyme in the metabolic conversion and degradation of P450 eicosanoids called epoxyeicosatrienoic acids (EETs). Genetic variations in the sEH gene, designated EPHX2, are associated with ischemic stroke risk. In experimental studies, sEH inhibition and gene deletion reduce infarct size after focal cerebral ischemia in mice. Although the precise mechanism of protection afforded by sEH inhibition remains under investigation, EETs exhibit a wide array of potentially beneficial actions in stroke, including vasodilation, neuroprotection, promotion of angiogenesis and suppression of platelet aggregation, oxidative stress and post-ischemic inflammation. Herein we argue that by capitalizing on this broad protective profile, sEH inhibition represents a prototype "combination therapy" targeting multiple mechanisms of stroke injury with a single agent.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Astrocytes and the regulation of cerebral blood flow

Moment-to-moment changes in local neuronal activity lead to dynamic changes in cerebral blood flow. Emerging evidence implicates astrocytes as one of the key players in coordinating this neurovascular coupling. Astrocytes are poised to sense glutamatergic synaptic activity over a large spatial domain via activation of metabotropic glutamate receptors and subsequent calcium signaling and via energy-dependent glutamate transport. Astrocyte foot processes can signal vascular smooth muscle by arachidonic acid pathways involving astrocytic cytochrome P450 epoxygenase, astrocytic cyclooxygenase-1 and smooth muscle cytochrome P450 omega-hydroxylase activities, and by astrocytic and smooth muscle potassium channels. Non-glutamatergic transmitters released from neurons, such as nitric oxide, cyclooxygenase-2 metabolites and vasoactive intestinal peptide, might modulate neurovascular signaling at the level of the astrocyte or smooth muscle. Thus, astrocytes have a pivotal role in dynamic signaling within the neurovascular unit. Important questions remain on how this signaling is integrated with other pathways in health and disease.

Arachidonic acid and ion channels: an update

Arachidonic acid (AA), a polyunsaturated fatty acid with four double bonds, has multiple actions on living cells. Many of these effects are mediated by an action of AA or its metabolites on ion channels. During the last 10 years, new types of ion channels, transient receptor potential (TRP) channels, store-operated calcium entry (SOCE) channels and non-SOCE channels have been studied. This review summarizes our current knowledge about the effects of AA on TRP and non-SOCE channels as well as classical ion channels. It aims to distinguish between effects of AA itself and effects of AA metabolites. Lipid mediators are of clinical interest because some of them (for example, leukotrienes) play a role in various diseases, others (such as prostaglandins) are targets for pharmacological therapeutic intervention.

Interaction of nitric oxide, 20-HETE, and EETs during functional hyperemia in whisker barrel cortex

Nitric oxide (NO) modulates vasodilation in cerebral cortex during sensory activation. NO is known to inhibit the synthesis of 20-HETE, which has been implicated in arteriolar constriction during astrocyte activation in brain slices. We tested the hypothesis that the attenuated cerebral blood flow (CBF) response to whisker stimulation seen after NO synthase (NOS) inhibition requires 20-HETE synthesis and that the ability of an epoxyeicosatrienoic acids (EETs) antagonist to reduce the CBF response is blunted after NOS inhibition but restored with simultaneous blockade of 20-HETE synthesis. In anesthetized rats, the increase in CBF during whisker stimulation was attenuated after the blockade of neuronal NOS with 7-nitroindazole. Subsequent administration of the 20-HETE synthesis inhibitor N-hydroxy-N'-(4-n-butyl-2-methylphenyl)formamidine (HET0016) restored the CBF response to control levels. After the administration of 7-nitroindazole, the inhibitory effect of an EETs antagonist 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE) on the CBF response was lost, whereas the simultaneous administration of 7-nitroindazole and HET0016 restored the inhibitory effect of 14,15-EEZE. The administration of HET0016 alone had only a small effect on the evoked CBF response in rats. Furthermore, in neuronal NOS(+/+) and NOS(-/-) mice, HET0016 administration did not increase the CBF response to whisker stimulation. In neuronal NOS(+/+) mice, HET0016 also blocked the reduction in the response seen with acute NOS inhibition. These results indicate that 20-HETE synthesis normally does not substantially restrict functional hyperemia. Increased NO production during functional activation may act dynamically to suppress 20-HETE synthesis or downstream signaling and permit EETs-dependent vasodilation. With the chronic loss of neuronal NOS in mice, other mechanisms apparently suppress 20-HETE synthesis or signaling.

Origins of blood volume change due to glutamatergic synaptic activity at astrocytes abutting on arteriolar smooth muscle cells

The cellular mechanisms that couple activity of glutamatergic synapses with changes in blood flow, measured by a variety of techniques including the BOLD signal, have not previously been modelled. Here we provide such a model, that successfully accounts for the main observed changes in blood flow in both visual cortex and somatosensory cortex following their stimulation by high-contrast drifting grating or by single whisker stimulation, respectively. Coupling from glutamatergic synapses to smooth muscle cells of arterioles is effected by astrocytes releasing epoxyeicosatrienoic acids (EETs) onto them, following glutamate stimulation of the astrocyte. Coupling of EETs to the smooth muscle of arterioles is by means of potassium channels in their membranes, leading to hyperpolarization, relaxation and hence an increase in blood flow. This model predicts a linear increase in blood flow with increasing numbers of activated astrocytes, but a non-linear increase with increasing glutamate release.

Neurovascular coupling in the mammalian brain

Normal brain function requires proper supply of oxygen and glucose in a timely and local manner. This is achieved through an orchestrated intercellular communication between neurones, astrocytes and microvessels that results in a rapid and restricted increase in cerebral blood flow, a process known as neurovascular coupling. Astrocytic end-feet make close contacts with neuronal synapses and blood vessels and, given their ability to release vasoactive signals following neuronal activation, have been recognized as key intermediaries in the neurovascular response. Both dilating and constricting signals appear to be released from astrocytes upon increases in intracellular Ca(2+) concentration, and both dilatation and constriction of brain vessels have been observed in previous studies. In this article, we discuss the various astrocyte-derived vasodilating and vasoconstricting signals, their interactions and effects on astrocytes and vascular smooth muscle cells, and suggest the importance of the intrinsic properties of the latter cell type on the overall neurovascular response. We present a working model in which the rise in astrocytic Ca(2+) following neuronal activation leads not only to the rapid activation of calcium-activated K(+) channels in astrocytic end-feet, but also to their modulation by metabolites of the arachidonic acid pathway, which in general have been proposed to act on vascular smooth muscle cells rather than on astrocytes. We propose that this latter mechanism may in turn modulate K(+) signalling from astrocytes to smooth muscle cells, influencing the overall effects of the vasodilating and vasoconstricting signals released during neuronal activation.

Toxic effects of cobalt in primary cultures of mouse astrocytes

Cobalt is suspected to cause memory deficit in humans and was reported to induce neurotoxicity in animal models. We have studied the effects of cobalt in primary cultures of mouse astrocytes. CoCl(2) (0.2-0.8mM) caused dose-dependent ATP depletion, apoptosis (cell shrinkage, phosphatidylserine externalization and chromatin rearrangements) and secondary necrosis. The mitochondria appeared to be a main target of cobalt toxicity, as shown by the loss of mitochondrial membrane potential (DeltaPsi(m)) and release from the mitochondria of apoptogenic factors, e.g. apoptosis inducing factor (AIF). Pre-treatment with bongkrekic acid reduced ATP depletion, implicating the involvement of the mitochondrial permeability transition (MPT) pore. Cobalt increased the generation of oxygen radicals, but antioxidants did not prevent toxicity. There was also an impaired response to ATP stimulation, evaluated as a lower raise in intracellular calcium. Similarly to hypoxia and dymethyloxallyl glycine (DMOG), cobalt triggered stabilization of the alpha-subunit of hypoxia-inducible factor HIF-1 (HIF-1alpha). This early event was followed by an increased expression of HIF-1 regulated genes, e.g. stress protein HO-1, pro-apoptotic factor Nip3 and iNOS. Although all of the three stimuli activated the HIF-1alpha pathway and decreased ATP levels, the downstream effects were different. DMOG only inhibited cell proliferation, whereas the other two conditions caused cell death by apoptosis and necrosis. This points to cobalt and hypoxia not only inducing HIF-1alpha regulated genes but also affecting similarly other cellular functions, including metabolism.