Epoxygenase products of arachidonic acid are endogenous constituents of the hypothalamus involved in D2 receptor-mediated, dopamine-induced release of somatostatin

Eicosanoid Signaling in Insect Immunology: New Genes and Unresolved Issues

This paper is focused on eicosanoid signaling in insect immunology. We begin with eicosanoid biosynthesis through the actions of phospholipase A₂, responsible for hydrolyzing the C18 polyunsaturated fatty acid, linoleic acid (18:2n-6), from cellular phospholipids, which is subsequently converted into arachidonic acid (AA; 20:4n-6) via elongases and desaturases. The synthesized AA is then oxygenated into one of three groups of eicosanoids, prostaglandins (PGs), epoxyeicosatrienoic acids (EETs) and lipoxygenase products. We mark the distinction between mammalian cyclooxygenases and insect peroxynectins, both of which convert AA into PGs. One PG, PGI₂ (also called prostacyclin), is newly discovered in insects, as a negative regulator of immune reactions and a positive signal in juvenile development. Two new elements of insect PG biology are a PG dehydrogenase and a PG reductase, both of which enact necessary PG catabolism. EETs, which are produced from AA via cytochrome P450s, also act in immune signaling, acting as pro-inflammatory signals. Eicosanoids signal a wide range of cellular immune reactions to infections, invasions and wounding, including nodulation, cell spreading, hemocyte migration and releasing prophenoloxidase from oenocytoids, a class of lepidopteran hemocytes. We briefly review the relatively scant knowledge on insect PG receptors and note PGs also act in gut immunity and in humoral immunity. Detailed new information on PG actions in mosquito immunity against the malarial agent, Plasmodium berghei, has recently emerged and we treat this exciting new work. The new findings on eicosanoid actions in insect immunity have emerged from a very broad range of research at the genetic, cellular and organismal levels, all taking place at the international level.

Biosynthesis and immunity of epoxyeicosatrienoic acids in a lepidopteran insect, Spodoptera exigua

Eicosanoids mediate both cellular and humoral immune responses in insects. Epoxyeicosatrienoic acids (EETs) are a group of eicosanoids containing epoxide formed by epoxygenase (EPX) activity of cytochrome P450 (CYP). Although EETs have been considered to mediate immune responses in some insects, their synthetic machinery was little understood in insects. This study monitored EETs in a lepidopteran insect, Spodoptera exigua, immunized with bacteria and found all four EETs (5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET) from larval fat body at 247-1,736 pg/g levels. Then to predict EPXs, 140 CYPs were collected from S. exigua transcriptomes and compared with human EPXs. Four CYPs (SeEPX1-SeEPX4) sharing homologies with human EPXs were chosen and assessed in subsequent expression and functional analyses. All four EPXs were expressed in all development stages. In larval stage, all four EPXs were expressed in immune-associated tissues such as fat body and hemocytes. Furthermore, their expression levels were highly enhanced by bacterial challenge in different tissues. RNA interference (RNAi) using gene-specific double stranded RNA injection suppressed their expression levels by more than 55%. RNAi treatments interfered with hemocyte-spreading behavior and nodule formation upon bacterial challenge except RNAi treatment against SeEPX2. All four EETs stimulated cellular immune response measured by nodule formation in S. exigua. The suppressed immune responses by the RNAi treatments against three SeEPXs were rescued by the addition of 8,9-EET. However, other three EETs gave their specific rescue effect depending on SeEPX types under RNAi. In humoral immune response, all four RNAi treatments suppressed expression of antimicrobial peptide genes. This study reports the presence of all four EETs in larval fat body of S. exigua and suggests that four SeEPXs are associated with immune responses mediated by EETs.

Soluble Epoxide Hydrolase and Brain Cholesterol Metabolism

The bifunctional enzyme soluble epoxide hydrolase (sEH) is found in all regions of the brain. It has two different catalytic activities, each assigned to one of its terminal domains: the C-terminal domain presents hydrolase activity, whereas the N-terminal domain exhibits phosphatase activity. The enzyme’s C-terminal domain has been linked to cardiovascular protective and anti-inflammatory effects. Cholesterol-related disorders have been associated with sEH, which plays an important role in the metabolism of cholesterol precursors. The role of sEH’s phosphatase activity has been so far poorly investigated in the context of the central nervous system physiology. Given that brain cholesterol disturbances play a role in the onset of Alzheimer’s disease (AD) as well as of other neurodegenerative diseases, understanding the functions of this enzyme could provide pivotal information on the pathophysiology of these conditions. Moreover, the sEH phosphatase domain could represent an underexplored target for drug design and therapeutic strategies to improve symptoms related to neurodegenerative diseases. This review discusses the function of sEH in mammals and its protein structure and catalytic activities. Particular attention was given to the distribution and expression of sEH in the human brain, deepening into the enzyme’s phosphatase activity and its participation in brain cholesterol synthesis. Finally, this review focused on the metabolism of cholesterol and its association with AD.

14,15-epoxyeicosatrienoic acid produced by cytochrome P450s enhances neurite outgrowth of PC12 and rat hippocampal neuronal cells

Polyunsaturated fatty acids, such as arachidonic acid, are accumulated in brain and induce neuronal differentiation. Arachidonic acid is metabolized to epoxyeicosatrienoic acids (EETs) and hydroxyeicosatetraenoic acids (HETEs) by cytochrome P450s. In this study, we found that 14,15-EET and 20-HETE-enhanced NGF-induced rat pheochromocytoma PC12 cell neurite outgrowth even at the concentration of 100 nmol L-1. LC-MS analysis revealed that 14,15-EET was effectively produced from arachidonic acid by rat CYP2C11, 2C13, and 2C23, and these P450s were expressed in PC12 cells. An inhibitor of these P450s, ketoconazole, inhibited neurite outgrowth, whereas inhibition of soluble epoxide hydrolase, which hydrolyzes EETs to their corresponding diols enhanced neurite outgrowth. To determine the mechanism of neurite formation enhancement by arachidonic acid metabolites, we focused on transient receptor potential (TRP) channels expressed in PC12 cells. The TRPV4 inhibitor HC067047, but not the TRPV1 inhibitor capsazepine, inhibited the effects of 14,15-EET on neurite outgrowth of PC12. Furthermore, 14,15-EET increased the cytosolic calcium ion concentration and this increase was inhibited by HC067047. 14,15-EET also enhanced neurite outgrowth of primary cultured neuron from rat hippocampus. This study suggests that arachidonic acid metabolites produced by P450 contribute to neurite outgrowth through calcium influx.

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.

8,9-Epoxyeicosatrienoic acid inhibits antibody production of B lymphocytes in mice

Epoxyeicosatrienoic acids (EETs), synthesized from arachidonic acid by cytochrome P450 epoxygenases, are converted to dihydroxyeicosatrienoic acids by soluble epoxide hydrolase. EETs exert anti-inflammatory effects. However, the effect of EETs on humoral immunity is poorly understood. The present study is to investigate the potential role of EETs on B cell function and mechanisms. We examined the role of EETs on antibody production of splenic B cells from C57BL/6 and apolipoprotein E-deficient (ApoE−/−) mice by means of ELISA. Of the 4 EET regioisomers, 8,9-EET decreased basal and activation-induced B cell antibody secretion. As well, 8,9-EET significantly inhibited B-cell proliferation and survival, plasma cell differentiation and class-switch recombination. Western blot analysis revealed that lipopolysaccharide-induced nuclear translocation of NF-κB could be attenuated by 8,9-EET. Furthermore, germinal center formation was impaired by 8,9-EET in mice in vivo. 8,9-EET may inhibit B-cell function in vitro and in vivo, which suggests a new therapeutic strategy for diseases with excess B cell activation.

Epoxygenated fatty acids and soluble epoxide hydrolase inhibition: novel mediators of pain reduction

The soluble epoxide hydrolase (sEH) enzyme was discovered while investigating the metabolism of xenobiotic compounds in the Casida laboratory. However, an endogenous role of sEH is to regulate the levels of a group of potent bioactive lipids, epoxygenated fatty acids (EFAs), that have pleiotropic biological activities. The EFAs, in particular the arachidonic acid derived epoxy eicosatrienoic acids (EETs), are established autocrine and paracrine messengers. The most recently discovered outcome of inhibition of sEH and increased EFAs is their effects on the sensory system and in particular their ability to reduce pain. The inhibitors of sEH block both inflammatory and neuropathic pain. Elevation of EFAs, in both the central and peripheral nervous systems, blocks pain. Several laboratories have now published a number of potential mechanisms of action for the pain-reducing effects of EFAs. This paper provides a brief history of the discovery of the sEH enzyme and argues that inhibitors of sEH through several independent mechanisms display pain-reducing effects.

Characterization of transgenic mice with neuron-specific expression of soluble epoxide hydrolase

Soluble epoxide hydrolase (sEH) is the major enzyme responsible for the metabolism and inactivation of epoxyeicosatrienoic acids (EETs). EETs are produced by the cytochrome P450 (CYP) epoxygenase pathway of arachidonic acid (AA) metabolism and tend to be anti-hypertensive, anti-inflammatory and protective against ischemic injury. Since the metabolism of EETs by sEH reduces or eliminates their bioactivity, inhibition of sEH has become a therapeutic strategy for hypertension and inflammation. sEH is found in nearly all tissues so the systemic application of inhibitors is likely to affect more than blood pressure and inflammation. In the central nervous system, EETs are thought to play a role in the regulation of local blood flow, protection from ischemic injury, inhibition of inflammation, the release of peptide hormones and modulation of fever. However, little is known about region- and cell-specific expression of sEH in the brain. In the mouse brain, expression of sEH was found widely in cortical and hippocampal astrocytes and also in a few specific neuron types in the cortex, cerebellum, and medulla. To assess the functional significance of neuronal sEH, we generated a transgenic mouse model, which over-expresses sEH specifically in neurons. Transgenic mice showed increased neuron labeling in cortex and hippocampus with little change in labeling of other brain regions. Despite a 3-fold increase in sEH activity in the brain, there was no change in arterial pressure. This data provides new information required for studying the central roles of the cytochrome P450 epoxygenase pathway.

Epoxyeicosanoid signaling in CNS function and disease

Epoxyeicosatrienoic acids (EETs) are arachidonic acid metabolites of cytochrome P450 epoxygenase enzymes recognized as key players in vascular function and disease, primarily attributed to their potent vasodilator, anti-inflammatory and pro-angiogenic effects. Although EETs' actions in the central nervous system (CNS) appear to parallel those in peripheral tissue, accumulating evidence suggests that epoxyeicosanoid signaling plays different roles in neural tissue compared to peripheral tissue; roles that reflect distinct CNS functions, cellular makeup and intercellular relationships. This is exhibited at many levels including the expression of EETs-synthetic and -metabolic enzymes in central neurons and glial cells, EETs' role in neuro-glio-vascular coupling during cortical functional activation, the capacity for interaction between epoxyeicosanoid and neuroactive endocannabinoid signaling pathways, and the regulation of neurohormone and neuropeptide release by endogenous EETs. The ability of several CNS cell types to produce and respond to EETs suggests that epoxyeicosanoid signaling is a key integrator of cell-cell communication in the CNS, coordinating cellular responses across different cell types. Under pathophysiological conditions, such as cerebral ischemia, EETs protect neurons, astroglia and vascular endothelium, thus preserving the integrity of cellular networks unique to and essential for proper CNS function. Recognition of EETs' intimate involvement in CNS function in addition to their multi-cellular protective profile has inspired the development of therapeutic strategies against CNS diseases such as cerebral ischemia, tumors, and neural pain and inflammation that are based on targeting the cellular actions of EETs or their biosynthetic and metabolizing enzymes. Based upon the emerging importance of epoxyeicosanoids in cellular function and disease unique to neural systems, we propose that the actions of "neuroactive EETs" are best considered separately, and not in aggregate with all other peripheral EETs functions.

Distribution of soluble and microsomal epoxide hydrolase in the mouse brain and its contribution to cerebral epoxyeicosatrienoic acid metabolism

Epoxide hydrolases comprise a family of enzymes important in detoxification and conversion of lipid signaling molecules, namely epoxyeicosatrienoic acids (EETs), to their supposedly less active form, dihydroxyeicosatrienoic acids (DHETs). EETs control cerebral blood flow, exert analgesic, anti-inflammatory and angiogenic effects and protect against ischemia. Although the role of soluble epoxide hydrolase (sEH) in EET metabolism is well established, knowledge on its detailed distribution in rodent brain is rather limited. Here, we analyzed the expression pattern of sEH and of another important member of the EH family, microsomal epoxide hydrolase (mEH), in mouse brain by immunohistochemistry. To investigate the functional relevance of these enzymes in brain, we explored their individual contribution to EET metabolism in acutely isolated brain cells from respective EH -/- mice and wild type littermates by mass spectrometry. We find sEH immunoreactivity almost exclusively in astrocytes throughout the brain, except in the central amygdala, where neurons are also positive for sEH. mEH immunoreactivity is abundant in brain vascular cells (endothelial and smooth muscle cells) and in choroid plexus epithelial cells. In addition, mEH immunoreactivity is present in specific neuronal populations of the hippocampus, striatum, amygdala, and cerebellum, as well as in a fraction of astrocytes. In freshly isolated cells from hippocampus, where both enzymes are expressed, sEH mediates the bulk of EET metabolism. Yet we observe a significant contribution of mEH, pointing to a novel role of this enzyme in the regulation of physiological processes. Furthermore, our findings indicate the presence of additional, hitherto unknown cerebral epoxide hydrolases. Taken together, cerebral EET metabolism is driven by several epoxide hydrolases, a fact important in view of the present targeting of sEH as a potential therapeutic target. Our findings suggest that these different enzymes have individual, possibly quite distinct roles in brain function and cerebral EET metabolism.

Antinociception produced by 14,15-epoxyeicosatrienoic acid is mediated by the activation of beta-endorphin and met-enkephalin in the rat ventrolateral periaqueductal gray

Cytochrome P450 genes catalyze formation of epoxyeicosatrienoic acids (EETs) from arachidonic acid. The effects of 5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET microinjected into the ventrolateral periaqueductal gray (vlPAG) on the thermally produced tail-flick response were studied in male Sprague-Dawley rats. 14,15-EET microinjected into vlPAG (3-156 pmol) dose-dependently inhibited the tail-flick response (ED50 = 32.5 pmol). In contrast, 5,6-EET, 8,9-EET, and 11,12-EET at a dose of 156 pmol were not active when injected into the vlPAG. 14,15-EET failed to displace the radiobinding of [3H][D-Ala2,NHPe4, Gly-ol5]enkephalin (mu-opioid receptor ligand) or [3H]naltrindole (delta-opioid receptor ligand) in crude membrane fractions of rat brain. Tail-flick inhibition produced by 14,15-EET from vlPAG was blocked by intra-vlPAG pretreatment with antiserum against beta-endorphin or Met-enkephalin or the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) or the delta-opioid receptor antagonist naltrindole but not with dynorphin A[1-17] antiserum or the kappa-opioid receptor antagonist nor-binaltorphimine. In addition, tail-flick inhibition produced by 14,15-EET treatment was blocked by intrathecal pretreatment with Met-enkephalin antiserum, naltrindole, or CTOP but not with beta-endorphin antiserum. It is concluded that 1) 14,15-EET itself does not have any affinity for mu- or delta-opioid receptors and 2) 14,15-EET activates beta-endorphin and Met-enkephalin, which subsequently act on mu- and delta-opioid receptors to produce antinociception.

Soluble epoxide hydrolase: a novel therapeutic target in stroke

The P450 eicosanoids epoxyeicosatrienoic acids (EETs) are produced in brain and perform important biological functions, including protection from ischemic injury. The beneficial effect of EETs, however, is limited by their metabolism via soluble epoxide hydrolase (sEH). We tested the hypothesis that sEH inhibition is protective against ischemic brain damage in vivo by a mechanism linked to enhanced cerebral blood flow (CBF). We determined expression and distribution of sEH immunoreactivity (IR) in brain, and examined the effect of sEH inhibitor 12-(3-adamantan-1-yl-ureido)-dodecanoic acid butyl ester (AUDA-BE) on CBF and infarct size after experimental stroke in mice. Mice were administered a single intraperitoneal injection of AUDA-BE (10 mg/kg) or vehicle at 30 mins before 2-h middle cerebral artery occlusion (MCAO) or at reperfusion, in the presence and absence of P450 epoxygenase inhibitor N-methylsulfonyl-6-(2-propargyloxyphenyl) hexanamide (MS-PPOH). Immunoreactivity for sEH was detected in vascular and non-vascular brain compartments, with predominant expression in neuronal cell bodies and processes. 12-(3-Adamantan-1-yl-ureido)-dodecanoic acid butyl ester was detected in plasma and brain for up to 24 h after intraperitoneal injection, which was associated with inhibition of sEH activity in brain tissue. Finally, AUDA-BE significantly reduced infarct size at 24 h after MCAO, which was prevented by MS-PPOH. However, regional CBF rates measured by iodoantipyrine (IAP) autoradiography at end ischemia revealed no differences between AUDA-BE- and vehicle-treated mice. The findings suggest that sEH inhibition is protective against ischemic injury by non-vascular mechanisms, and that sEH may serve as a therapeutic target in stroke.

Steroid-producing cells regulate arterial tone of adrenal cortical arteries

Adrenal blood flow is coupled to adrenal hormone secretion. ACTH increases adrenal blood flow and stimulates the secretion of aldosterone and cortisol in vivo. However, ACTH does not alter vascular tone of isolated adrenal cortical arteries. Mechanisms underlying this discrepancy remain unsolved. The present study examined the effect of zona glomerulosa (ZG) cells on cortical arterial tone. ZG cells (10(5) to 10(7) cells) and ZG cell-conditioned medium relaxed preconstricted adrenal arteries (maximal relaxations = 79 +/- 4 and 66 +/- 4%, respectively). In adrenal arteries coincubated with a small number of ZG cells (0.5-1 x 10(6)), ACTH (10(-12) to 10(-8) m) induced concentration-dependent relaxations (maximal relaxation = 67 +/- 4%). Similarly, ACTH (10(-8) m) dilated (55 +/- 10%) perfused arteries embedded in adrenal cortical slices. ZG cell-dependent relaxations to ACTH were endothelium-independent and inhibited by high extracellular K(+) (60 mm); the K(+) channel blocker, iberiotoxin (100 nm); the cytochrome P450 inhibitors SKF 525A (10 microm) and miconazole (10 microm); and the epoxyeicosatrienoic acid (EET) antagonist 14,15-EEZE (2 microm). Four EET regioisomers were identified in ZG cell-conditioned media. EET production was stimulated by ACTH. We conclude that ZG cells release EETs and this release is stimulated by ACTH. Interaction of endocrine and vascular cells represents a mechanism for regulating adrenal blood flow and couples steroidogenesis to increased blood flow.

Effect of Quinine on Autoreceptor-Regulated Dopamine Release in the Rat Striatum

In vivo brain microdialysis was used to examine the role of potassium channel activation in dopamine (DA) autoreceptor function in the striatum of freely moving rats. Local application of the D2 receptor agonists quinpirole or N-0437 through the dialysis probe significantly reduced extracellular concentrations of DA. Local application of the D2 antagonist (-)-sulpiride produced significant increases in DA. Local perfusion with quinine, a K+ channel blocker, completely blocked the (-)-sulpiride-induced increases in DA but did not affect the DA agonist-induced decreases. (-)-Sulpiride completely blocked the effect of quinpirole on DA both in control and in quinine-treated animals. At the highest dose used, quinine caused a large transient increase in extracellular DA. Local application of tetrodotoxin or infusion of Mg2+ in the absence of Ca2+ did not prevent this quinine-induced transient increase in extracellular DA. These results demonstrate that DA autoreceptors in the striatum regulate DA release in awake, behaving animals. Local application of (-)-sulpiride increases DA levels by blocking the tonic activation of autoreceptors by endogenous DA. Quinine blocks the neuroleptic-induced increase in DA, perhaps by preventing the K+ channel opening that would normally accompany endogenous autoreceptor activation. The fact that exogenously applied DA receptor agonists can decrease extracellular DA levels in the presence of quinine suggests that they may be acting at extrasynaptic autoreceptors that are not tonically active in vivo. The effect of DA agonists on this site is via a DA receptor because it is blocked by (-)-sulpiride. However, this receptor does not appear to be coupled to a quinine-sensitive potassium channel.

Action of epoxyeicosatrienoic acids on cellular function

Epoxyeicosatrienoic acids (EETs), which function primarily as autocrine and paracrine mediators in the cardiovascular and renal systems, are synthesized from arachidonic acid by cytochrome P-450 epoxygenases. They activate smooth muscle large-conductance Ca(2+)-activated K(+) channels, producing hyperpolarization and vasorelaxation. EETs also have anti-inflammatory effects in the vasculature and kidney, stimulate angiogenesis, and have mitogenic effects in the kidney. Many of the functional effects of EETs occur through activation of signal transduction pathways and modulation of gene expression, events probably initiated by binding to a putative cell surface EET receptor. However, EETs are rapidly taken up by cells and are incorporated into and released from phospholipids, suggesting that some functional effects may occur through a direct interaction between the EET and an intracellular effector system. In this regard, EETs and several of their metabolites activate peroxisome proliferator-activated receptor alpha (PPARalpha) and PPARgamma, suggesting that some functional effects may result from PPAR activation. EETs are metabolized primarily by conversion to dihydroxyeicosatrienoic acids (DHETs), a reaction catalyzed by soluble epoxide hydrolase (sEH). Many potentially beneficial actions of EETs are attenuated upon conversion to DHETs, which do not appear to be essential under routine conditions. Therefore, sEH is considered a potential therapeutic target for enhancing the beneficial functions of EETs.

Stable 5,6-epoxyeicosatrienoic acid analog relaxes coronary arteries through potassium channel activation

5,6-epoxyeicosatrienoic acid (5,6-EET) is a cytochrome P450 epoxygenase metabolite of arachidonic acid that causes vasorelaxation. However, investigations of its role in biological systems have been limited by its chemical instability. We developed a stable agonist of 5,6-EET, 5-(pentadeca-3(Z),6(Z),9(Z)-trienyloxy)pentanoic acid (PTPA), in which the 5,6-epoxide was replaced with a 5-ether. PTPA obviates chemical and enzymatic hydrolysis. In bovine coronary artery rings precontracted with U46619, PTPA (1 nmol/L to 10 micromol/L) induced concentration-dependent relaxations, with maximal relaxation of 86+/-5% and EC50 of 1 micromol/L. The relaxations were inhibited by the cyclooxygenase inhibitor indomethacin (10 micromol/L; max relaxation 43+/-9%); the ATP-sensitive K+ channel inhibitor glybenclamide (10 micromol/L; max relaxation 49+/-6%); and the large conductance calcium-activated K+ channel inhibitor iberiotoxin (100 nmol/L; max relaxation 38+/-6%) and abolished by the combination of iberiotoxin with indomethacin or glybenclamide or increasing extracellular K+ to 20 mmol/L. Whole-cell outward K+ current was increased nearly 6-fold by PTPA (10 micromol/L), which was also blocked by iberiotoxin. Additionally, we synthesized 5-(pentadeca-6(Z),9(Z)-dienyloxy)pentanoic acid and 5-(pentadeca-3(Z),9(Z)-dienyloxy)pentanoic acid (PDPA), PTPA analogs that lack the 8,9 or 11,12 double bonds of arachidonic acid and therefore are not substrates for cyclooxygenase. The PDPAs caused concentration-dependent relaxations (max relaxations 46+/-13% and 52+/-7%, respectively; EC50 1micromol/L), which were not altered by glybenclamide but blocked by iberiotoxin. These studies suggested that PTPA induces relaxation through 2 mechanisms: (1) cyclooxygenase-dependent metabolism to 5-ether-containing prostaglandins that activate ATP-sensitive K+ channels and (2) activation of smooth muscle large conductance calcium-activated K+ channels. PDPAs only activate large conductance calcium-activated K+ channels.

Identification of 5,6-trans-Epoxyeicosatrienoic Acid in the Phospholipids of Red Blood Cells

A novel eicosanoid, 5,6-trans-epoxy-8Z,11Z,14Z-eicosatrienoic acid (5,6-trans-EET), was identified in rat red blood cells. Characterization of 5,6-trans-EET in the sn-2 position of the phospholipids was accomplished by hydrolysis with phospholipase A(2) followed by gas chromatography/mass spectrometry as well as electrospray ionization-tandem mass spectrometry analyses. The electron ionization spectrum of 5,6-erythro-dihydroxyeicosatrienoic acid (5,6-erythro-DHET), converted from 5,6-trans-EET in the samples, matches that of the authentic standard. Hydrogenation of the extracted 5,6-erythro-DHET with platinum(IV) oxide/hydrogen resulted in an increase of the molecular mass by 6 daltons and the same retention time shift as an authentic standard in gas chromatography, suggesting the existence of three olefins as well as the 5,6-erythro-dihydroxyl structure in the metabolite. Match of retention times by chromatography indicated identity of the stereochemistry of the red blood cell 5,6-erythro-DHET vis à vis the synthetic standard. High pressure liquid chromatography-electrospray ionization-tandem mass spectrometry analysis of the phospholipase A(2)-hydrolyzed lipid extracts from red blood cells revealed match of the mass spectrum and retention time of the compound with the authentic 5,6-trans-EET standard, providing direct evidence of the existence of 5,6-trans-EET in red blood cells. The presence of other trans-EETs was also demonstrated. The ability of both 5,6-trans-EET and its product 5,6-erythro-DHET to relax preconstricted renal interlobar arteries was significantly greater than that of 5,6-cis-EET. In contrast, 5,6-cis-EET and 5,6-trans-EET were equipotent in their capacity to inhibit collagen-induced rat platelet aggregation, whereas 5,6-erythro-DHET was without effect. We propose that the red blood cells serve as a reservoir for epoxides which on release may act in a vasoregulatory capacity.

Suppression of rat and human growth hormone and prolactin secretion by a novel somatostatin/dopaminergic chimeric ligand

As cotreatment of somatostatin (SRIF) and dopamine (DA) agonists reduces GH in acromegaly more effectively than either agonist alone, SRIF and DA receptors (SSTR and DAR) may interact with enhanced functional activity. The selective SSTR2 agonist, BIM-23023 (50% effective dose, 0.42), and the DAR2 agonist, BIM-53097 (50% effective dose, 22.1), dose- dependently inhibited GH secretion in cultured primary rat and human fetal as well as in human pituitary tumor cells derived from GH-secreting adenomas. The combination of individual SSTR2 and DAR2 agonists was additive for suppressing GH secretion in both rat and human pituitary cells. BIM-23A387 is a chimeric compound that contains structural elements of both SRIF and DA in a single molecule and retains potent, selective binding to DAR2 and SSTR2. BIM-23A387 (50% effective dose, 0.16 for SSTR2 and 24.5 for DAR2), displayed similar efficacy in suppressing GH secretion from rat pituitary cells as the combination of the two individual agonists. In contrast, the chimeric molecule was more potent than individual selective analogs in suppressing GH secretion by human fetal pituitary and GH-secreting adenoma cells (P < 0.05). Although the DAR2 antagonist, sulpiride, reversed BIM-23A387-induced GH suppression, blockade of SSTR2 by the selective SSTR antagonist, BIM-23454, did not block BIM-23A387-suppressed GH secretion. These results indicate that mechanisms by which the chimeric molecule suppresses pituitary GH secretion may not be mediated by individual SSTR2 or DAR2 signaling, respectively. Functional interaction of the two receptors may explain the clinical observation that more effective GH suppression is achieved when DAR2 and SSTR2 agonists are administered in combination. The SRIF/DA chimeric molecule, BIM-23A387, represents a novel tool for effective drug treatment of acromegaly and for prolactinomas otherwise resistant to dopaminergic therapy.

Cytochrome P450 CYP2J9, a new mouse arachidonic acid omega-1 hydroxylase predominantly expressed in brain

A cDNA encoding a new cytochrome P450 was isolated from a mouse brain library. Sequence analysis reveals that this 1,958-base pair cDNA encodes a 57-58-kDa 502-amino acid polypeptide that is 70-91% identical to CYP2J subfamily P450s and is designated CYP2J9. Recombinant CYP2J9 was co-expressed with NADPH-cytochrome P450 oxidoreductase (CYPOR) in Sf9 cells using a baculovirus system. Microsomes of CYP2J9/CYPOR-transfected cells metabolize arachidonic acid to 19-hydroxyeicosatetraenoic acid (HETE) thus CYP2J9 is enzymologically distinct from other P450s. Northern analysis reveals that CYP2J9 transcripts are present at high levels in mouse brain. Mouse brain microsomes biosynthesize 19-HETE. RNA polymerase chain reaction analysis demonstrates that CYP2J9 mRNAs are widely distributed in brain and most abundant in the cerebellum. Immunoblotting using an antibody raised against human CYP2J2 that cross-reacts with CYP2J9 detects a 56-kDa protein band that is expressed in cerebellum and other brain segments and is regulated during postnatal development. In situ hybridization of mouse brain sections with a CYP2J9-specific riboprobe and immunohistochemical staining with the anti-human CYP2J2 IgG reveals abundant CYP2J9 mRNA and protein in cerebellar Purkinje cells. Importantly, 19-HETE inhibits the activity of recombinant P/Q-type Ca(2+) channels that are known to be expressed preferentially in cerebellar Purkinje cells and are involved in triggering neurotransmitter release. Based on these data, we conclude that CYP2J9 is a developmentally regulated P450 that is abundant in brain, localized to cerebellar Purkinje cells, and active in the biosynthesis of 19-HETE, an eicosanoid that inhibits activity of P/Q-type Ca(2+) channels. We postulate that CYP2J9 arachidonic acid products play important functional roles in the brain.

Actions of epoxyeicosatrienoic acid on large-conductance Ca(2+)-activated K(+) channels in pituitary GH(3) cells

Epoxyeicosatrienoic acids (EETs), a family of cytochrome P450 epoxygenase metabolites of arachidonic acid, are believed to have an autocrine or paracrine role in the regulation of neurons or neuroendocrine cells. The effects of 14,15-EET on ionic currents were investigated in rat pituitary GH(3) cells. In the whole-cell configuration, 14,15-EET (3 microM) reversibly increased the amplitude of the Ca(2+)-activated K(+) current (I(K(Ca))). The 14, 15-EET-induced increase in I(K(Ca)) was unaffected in the presence of 10 microM thyrotropin-releasing hormone externally or 10 microM inositol trisphosphate in the recording pipette. In cells preincubated with pertussis toxin or herbimycin A, the 14, 15-EET-induced increase in I(K(Ca)) was also not changed. In the inside-out configuration, 14,15-EET applied intracellularly did not change single-channel conductance, but did increase the opening probability of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels. 14,15-EET (3 microM) shifted the activation curve of BK(Ca) channels to less positive membrane potential by approximately 15 mV. The change in the kinetic behavior of BK(Ca) channels caused by 14,15-EET is explained by a lengthening of open and a shortening of closed times. 14,15-EET increased the activity of BK(Ca) channels in a concentration-dependent manner with an EC(50) value of 1 microM. However, 14,15-EET did not affect the Ca(2+) sensitivity of BK(Ca) channels. The present study indicates that 14,15-EET is an opener of BK(Ca) channels in GH(3) cells and that the stimulatory effect of 14, 15-EET on these channels may, at least in part, contribute to the underlying cellular mechanisms by which EETs affect neuronal or neuroendocrine function.

Identification, functional characterization, and regulation of a new cytochrome P450 subfamily, the CYP2Ns

The screening of liver and heart cDNA libraries from the teleost Fundulus heteroclitus with degenerate oligonucleotide probes to conserved alpha-helical regions in mammalian P450s resulted in the identification of two cDNAs that together represent a novel P450 subfamily, the CYP2Ns. Northern analysis demonstrated that CYP2N1 transcripts are most abundant in liver and intestine, whereas CYP2N2 mRNAs are most abundant in heart and brain. CYP2N1 and CYP2N2 proteins were co-expressed with NADPH-cytochrome P450 oxidoreductase in Sf9 insect cells, and their ability to metabolize arachidonic acid and xenobiotic substrates was examined. Both CYP2N1 and CYP2N2 metabolize arachidonic acid to epoxyeicosatrienoic acids. Epoxidation is highly regio- and enantioselective with preferential formation of (8R,9S)-epoxyeicosatrienoic acid (optical purities are 91 and 90% for CYP2N1 and CYP2N2, respectively) and (11R, 12S)-epoxyeicosatrienoic acid (optical purities are 92 and 70% for CYP2N1 and CYP2N2, respectively). CYP2N1 and CYP2N2 also catalyze the formation of a variety of hydroxyeicosatetraenoic acids. Both P450s have benzphetamine N-demethylase activities but show minimal alkoxyresorufin O-dealkylase activities. To investigate factors affecting CYP2N expression in vivo, CYP2N transcripts were examined following starvation and/or treatment with 12-O-tetradecanoyl phorbol-13-acetate. Intestinal CYP2N1 mRNAs decrease in starved and/or phorbol ester-treated fish, whereas intestinal CYP2N2 transcripts decrease only following phorbol ester treatment. Interestingly, cardiac CYP2N2 expression decreases following phorbol ester treatment but increases following starvation. These results demonstrate that members of this novel P450 subfamily encode early vertebrate forms of arachidonic acid catalysts that are widely expressed and are regulated by environmental factors. Given the wealth of information on the functional role of P450-derived arachidonate metabolites in mammals, we postulate that CYP2N1 and CYP2N2 products have similar biological functions in early vertebrates. The identity of the mammalian orthologue(s) of the CYP2Ns remains unknown.

Dopaminergic regulation of three somatostatin mRNAs in goldfish brain

Three distinct somatostatin cDNAs characterized previously from goldfish brain encode three preprosomatostatins (PSS), designated as PSS-I, PSS-II and PSS-III. In this study, dopaminergic regulation of PSS gene expression was examined by Northern blot analysis in the forebrain of goldfish. Intraperitoneal injection of the non-selective dopamine (DA) agonist, apomorphine, significantly decreased the levels of all three PSS mRNAs, indicating an inhibitory regulation of PSS gene expression by DA. The involvement of DA receptor subtypes in the regulation of PSS gene expression was examined using the D1 receptor agonist and antagonist drugs SKF 38393 and SCH 23390, and the D2 agonist and antagonist drugs LY 171555 and pimozide, respectively. The results provide evidence for inhibitory and/or stimulatory regulation of PSS gene expression by DA through both D1 and D2 receptors, which are dependent on the temporal pattern of dopamine input and reproductive stage of the fish. Demonstration of involvement of both DA D1 and D2 receptors in the dopaminergic regulation of goldfish brain PSS gene expression is a novel finding, distinct from the observations in mammalian models.

Dopamine receptor-mediated regulation of expression of Fos and its related antigens (FRA) in somatostatin neurons in the hypothalamic periventricular nucleus

Somatostatin (SS)-containing perikarya located within the hypothalamic periventricular nucleus (PeVN) comprise a heterogenous population of neurons with both local intrahypothalamic and distant extrahypothalamic axonal projection sites. The close proximity of SS perikarya and their dendrites to dopaminergic (DA) neuronal processes in the PeVN suggests that these peptidergic neurons may be regulated by DA receptor-mediated mechanisms. To test this, the effects of the D1 agonist SKF 38393 and D2/3 agonist quinelorane were examined on expression of the immediate early gene products Fos and its related antigens (FRA) in SS-immunoreactive (IR) neurons in the PeVN. SS-IR neurons were located in the most medial portion of the PeVN bordered medially by the third ventricle and laterally by tyrosine hydroxylase (TH)-IR neurons. In control rats, 10-15% of all SS-IR neurons contained FRA-IR. Activation of D1 receptors with SKF 38393 had no effect on either the total number of SS-IR neurons or the number of SS-IR neurons containing FRA-IR. In contrast, activation of D2/3 receptors with quinelorane decreased the number of SS-IR neurons containing FRA-IR, without affecting the total number of SS-IR neurons. The D2/3 antagonist raclopride had no effect per se, but prevented the quinelorane-induced decrease in the number of SS neurons expressing FRA-IR. These results reveal that activation of D2/3 (but not D1) receptors inhibits expression of the immediate early gene products FRA in SS-containing neurons in the PeVN, but expression of FRA in SS neurons is not tonically inhibited by dopamine acting on D2/3 receptors.

Cytochrome P450 metabolites of arachidonic acid in human placenta

Little is known about the epoxygenase pathway of the arachidonic acid cascade in uterine tissues. In this paper, we describe the formation of epoxyeicosatrienoic acids (EETs) and dihydroxyeicosatrienoic acids (DHETs) in human term placenta after uncomplicated pregnancies. Metabolism of [3H]-arachidonic acid was analyzed in short term tissue cultures of placenta by reverse phase HPLC. Major metabolites coeluted with authentic EETs and DHETs. The formation of EETs was inhibited by carbon monoxide. In non-radioactive incubations with biopsies from seven different placentas, sufficient material for GC/MS analysis was obtained. The combined media were purified by solid phase extraction and reverse phase HPLC. The fraction coeluting with DHETs was derivatized with pentafluorobenzylbromide (PFB) and bis-(trimethylsilyl)-trifluoroacetylacetamide (BSTFA) and analyzed by GC/NICI/MS/MS. 11, 12-DHET and 14, 15-DHET were identified by their mass spectra displaying specific fragments at m/z 149 and m/z 189, respectively. Our results suggest that the epoxygenase pathway is active in human term placenta.

Dopamine enhances somatostatin receptor-mediated inhibition of adenylate cyclase in rat striatum and hippocampus

Although there is evidence that suggests that dopamine (DA) has stimulatory effects on somatostatinergic transmission, it is unknown to date if DA increases the activity of the somatostatin (SS) receptor-effector system in the rat brain. In this study, we evaluated the effects of the administration of DA and the DA D1-like (D1, D5) receptor antagonist SCH 23390 and the D2-like (D2, D3, D4) receptor antagonist spiperone on the SS receptor-adenylate cyclase (AC) system in the Sprague-Dawley rat striatum and hippocampus. An intracerebroventricular injection of DA (0.5 microgram/rat) increased the number of SS receptors and decreased their apparent affinity in the striatum and hippocampus 15 hr after its administration. The simultaneous administration of the DA receptor antagonists SCH 23390 (0.25 mg/kg, ip) and spiperone (0.1 mg/kg, ip) before DA injection partially prevented the DA-induced increase in SS binding. The administration of SCH 23390 plus spiperone alone produced a significant decrease in the number of SS receptors in both brain areas studied at 15 hr after injection, an effect that disappeared at 24 hr. The increased number of SS receptors in the DA-treated rats was associated with an increased capacity of SS to inhibit basal and forskolin (FK)-stimulated (AC) activity in the striatum and hippocampus at 15 hr after injection. This effect had disappeared at 24 hr. By contrast, basal and FK-stimulated enzyme activities were unaltered after DA injection. No significant changes in the levels of the alpha i (alpha i1 + alpha i2) subunits were found in DA-treated rats as compared with control rats. In addition, the immunodetection of the alpha i1 or alpha i2 subunits showed no significant changes in their levels in DA-treated rats when compared with controls. DA injection also induced an increase in SS-like immunoreactive content in the rat striatum but not hippocampus at 15 hr after administration and returned to control values at 24 hr. These results provide direct evidence of a functional linkage between the dopaminergic and somatostatinergic systems at the molecular level.

Bisallylic hydroxylation and epoxidation of polyunsaturated fatty acids by cytochrome P450

Polyunsaturated fatty acids can be oxygenated by cytochrome P450 to hydroxy and epoxy fatty acids. Two major classes of hydroxy fatty acids are formed by hydroxylation of the omega-side chain and by hydroxylation of bisallylic methylene carbons. Bisallylic cytochrome P450-hydroxylases transform linoleic acid to 11-hydroxylinoleic acid, arachidonic acid to 13-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid, 10-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid and 7-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid and eicosapentaenoic acid to 16-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid, 13-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid and 10-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid as major metabolites. The bisallylic hydroxy fatty acids are chemically unstable and decompose rapidly to cis-trans conjugated hydroxy fatty acids during acidic extractive isolation. Bisallylic hydroxylase activity appears to be augmented in microsomes induced by the synthetic glucocorticoid dexamethasone and by some other agents, but the P450 gene families of these hydroxylases have yet to be determined. The fatty acid epoxides, which are formed by cytochrome P450, are chemically stable, but are hydrolyzed to diols by soluble epoxide hydrolases. Epoxidation of polyunsaturated fatty acids is a prominent pathway of metabolism in the liver and the renal cortex and epoxy-genase activity appears to be under homeostatic control in the kidney. Many arachidonate epoxygenases have been identified belonging to the CYP2C gene subfamily. Epoxygenases have also been found in the central nervous system, endocrine organs, the heart and endothelial cells. Epoxides of arachidonic acid have been found to exert pharmacological effects on many cells.

Molecular characterization of an arachidonic acid epoxygenase in rat brain astrocytes

BACKGROUND AND PURPOSE

Brain 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.

METHODS

A 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.

RESULTS

Based 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.

CONCLUSIONS

Our 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.

Effect of calcitonin gene-related peptide on sodium absorption through isolated skin of Rana esculenta

Calcitonin gene-related peptide (CGRP) added to the internal fluid bathing the isolated skin of Rana esculenta strongly stimulates the active sodium absorption. This action is dose-dependent, the dose eliciting the maximal effect being 2 . 10(-7) M; alpha and beta CGRP exhibit the same potency. The CGRP action on sodium transport is mainly due to its interaction with CGRP1 receptors, since it is inhibited by CGRP8-37, its specific antagonist. The second messengers probably involved in the action of CGRP are cAMP and Ca+2, since this action is reduced by SQ22536 and W7, which are inhibitors of adenyl cyclase and calmodulin respectively. On the contrary, inhibitors of protein kinase C (1-O-hexadecyl-2-O-methyl-sn-glycerol) and nitric oxide synthase (L-NAME) do not modify the action on sodium transport. ETYA, an inhibitor of all the metabolic pathways of arachidonic acid, decreases the CGRP action by 38%. In order to search for the arachidonic acid metabolites involved in the CGRP action, the effect of the following inhibitors was tested: aspirin and naproxen (for cyclooxygenases), NDGA (for cyclooxygenases), NDGA (for lipoxygenases) clotrimazole (for epoxygenases). None of these substances is able to inhibit the CGRP action on sodium transport. Moreover, adding arachidonic acid inhibits the CGRP action, but this effect was also obtained by another unsaturated fatty acid, oleic acid. Since unsaturated fatty acids are able to inhibit the protein kinase A, these results indirectly support the role of cAMP as a second messenger of the CGRP action on sodium transport.

Quantitative chromatographic analysis of inositol phospholipids and related compounds

The metabolism of phospholipids and the mobilization of second messengers such as inositol-1,4,5-trisphosphate, 1,2-diacylglycerol (DAG) and arachidonic acid (AA) from phospholipids is commonly studied by radiolabelling phospholipids with [3H]myo-inositol or [32P]ATP and measuring the incorporation of radioactivity in different phospholipids or their hydrolysis products. However, for the radiolabelling method to accurately reflect changes in the compound's mass, it is essential that the tissue is labelled to isotopic equilibrium which is difficult to achieve. To circumvent the disadvantages of the radiolabelling method, several analytical procedures have been developed for the mass analysis of phospholipids and inositolphosphates (IPs). Quantitation of the mass or the radiolabelling of phospholipids is a complex multi-step procedure that involves quantitative isolation of phospholipids, fractionation of individual phospholipids and either determination of radioactivity in each component or the measurement of their mass. Phospholipids, DAG and AA are extracted from tissue sample with organic solvents such as chloroform-methanol (2:1) containing HCl or formic acid. The extract is separated by TLC, cartridge-column chromatography or HPLC on a reversed-phase column. Phospholipids are quantitated by measuring inorganic phosphate, absorption at 200 nm or mass spectrometry. Inositol phosphates are extracted with perchloric acid or trichloroacetic acid and separated by ion-exchange cartridge-column or HPLC with an ion-exchange column. IPs are quantitated by measuring inorganic phosphate or by using enzymatic reaction, metal-dye coupling, NMR or mass spectrometry.

Arachidonic acid as a neurotoxic and neurotrophic substance

In this article we summarize a wide variety of properties of arachidonic acid (AA) in the mammalian nervous system especially in the brain. AA serves as a biologically-active signaling molecule as well as an important component of membrane lipids. Esterified AA is liberated from the membrane by phospholipase activity which is stimulated by various signals such as neurotransmitter-mediated rise in intracellular Ca2+. AA exerts many biological actions which include modulation of the activities of protein kinases and ion channels, inhibition of neurotransmitter uptake, and enhancement of synaptic transmission. AA serves also as a precursor of a variety of eicosanoids, which are formed by oxidative metabolism of AA. AA cascade is activated under several pathological conditions in the brain such as ischemia and seizures, and may be involved in irreversible tissue damage. On the other hand, AA can show beneficial influences on brain tissues and cells in several situations. In a recent study using cultured brain neurons, we have found that AA shows quite distinct actions at a narrow concentration range, such as induction of cell death, promotion of cell survival and enhancement of neurite extension. The neurotoxic action is mediated by free radicals generated by AA metabolism, whereas the neurotrophic actions are exerted by AA itself. The observed in vitro actions of AA might be related to important roles of AA in brain pathogenesis and neural development.

Optimization of epoxyeicosatrienoic acid syntheses to test their effects on cerebral blood flow in vivo

Epoxyeicosatrienoic acids (EETs), normally present in brain and blood, appear to be released from atherosclerotic vessels in large amounts. Once intravascular, EETs can constrict renal arteries in vivo and dilate cerebral and coronary arteries in vitro. Whether EETs in blood will alter cerebral blood flow (CBF) in vivo is unknown. In the present study, the chemical synthesis of four EET regioisomers was optimized, and their identity and structural integrity established by chromatographic and mass spectral methods. The chemically labile EETs were converted to a sodium salt, complexed with albumin, and infused into anesthetized rats via the common carotid. The objective was to test whether sustained, high levels of intravascular EETs alter CBF. The CBF (cortical H2 clearance) was measured before and 30 min after the continuous infusion of 14,15- (n = 5), 11,12- (n = 5), 8,9- (n = 7) and 5,6-EET (unesterified or as the methyl ester, n = 5 for each). Neither the CBF nor the systemic blood pressure was affected by EETs. Because the infusions elevated the plasma concentrations of EETs about 700-fold above normal levels (1.0 nM), it is unlikely that EETs released from atherosclerotic vessels will alter CBF.

Experimental and/or genetically controlled alterations of the renal microsomal cytochrome P450 epoxygenase induce hypertension in rats fed a high salt diet

Excess dietary salt induces a cytochrome P450 arachidonic acid epoxygenase isoform in rat kidneys (Capdevila, J. H., S. Wei, J. Yang, A. Karara, H. R. Jacobson, J. R. Falck, F. P. Guengerich, and R. N. Dubois. 1992. J. Biol. Chem. 267:21720-21726). Treatment of rats on a high salt diet with the epoxygenase inhibitor, clotrimazole, produces significant increases in mean arterial blood pressure (122 +/- 2 and 145 +/- 4 mmHg for salt and salt- and clotrimazole-treated rats, respectively). The salt- and clotrimazole-dependent hypertension is accompanied by reductions in the urinary excretion of epoxygenase metabolites and by a selective inhibition of the renal microsomal epoxygenase reaction. The prohypertensive effects of clotrimazole are readily reversed when either the salt or clotrimazole treatment is discontinued. The indication that a salt-inducible renal epoxygenase protects against hypertension, are supported by studies with the Dahl rat model of genetic salt-sensitive hypertension. Dahl resistant animals responded to excess dietary salt by inducing the activity of their kidney microsomal epoxygenase(s) (0.102 +/- 0.01 and 0.240 +/- 0.04 nmol of products formed/min per mg of microsomal protein for control and salt-treated rats, respectively). Despite severe hypertension during excess dietary salt intake (200 +/- 20 mmHg), Dahl salt-sensitive rats demonstrated no increase in renal epoxygenase activity. These studies indicate that acquired or inherited abnormalities in renal epoxygenase activities and/or regulation can be related to salt-sensitive hypertension in rodents. Studies on the human renal epoxygenase and its relationship to salt hypertension may prove useful.

Localization of drug-metabolizing enzyme activities to blood-brain interfaces and circumventricular organs

The brain, with the exception of the choroid plexuses and circumventricular organs, is partially protected from the invasion of blood-borne chemicals by the specific morphological properties of the cerebral micro-vessels, namely, the tight junctions of the blood-brain barrier. Recently, several enzymes that are primarily involved in hepatic drug metabolism have been shown to exist in the brain, albeit at relatively low specific activities. In the present study, the hypothesis that these enzymes are located primarily at blood-brain interfaces, where they form an "enzymatic barrier," is tested. By using microdissection techniques or a gradient-centrifugation isolation procedure, the activities of seven drug-metabolizing enzymes in isolated microvessels, choroid plexuses, meningeal membranes, and tissue from three circumventricular organs (the neural lobe of the hypophysis, pineal gland, and median eminence) were assayed. With two exceptions, the activities of these enzymes were higher in the three circumventricular organs and cerebral microvessel than in the cortex. Very high membrane-bound epoxide hydrolase and UDP-glucuronosyltransferase activities (approaching those in liver) and somewhat high 7-benzoxyresorufin-O-dealkylase and NADPH-cytochrome P-450 reductase activities were determined in the choroid plexuses. The pia-arachnoid membranes, but not the dura matter, displayed drug-metabolizing enzyme activities, notably that of epoxide hydrolase. The drug-metabolizing enzymes located at these nonparenchymal sites may function to protect brain tissue from harmful compounds.

Oxygenation of polyunsaturated fatty acids by cytochrome P450 monooxygenases

Polyunsaturated fatty acids can be oxygenated by P450 in different ways--by epoxidation, by hydroxylation of the omega-side chain, by allylic and bis-allylic hydroxylation and by hydroxylation with double bond migration. Major organs for these oxygenations are the liver and the kidney. P450 is an ubiquitous enzyme. It is therefore not surprising that some of these reactions have been found in other organs and tissues. Many observations indicate that P450 oxygenates arachidonic acid in vivo in man and in experimental animals. This is hardly surprising. omega-Oxidation was discovered in vivo 60 years ago. It was more unexpected that biological activities have been associated with many of the P450 metabolites of arachidonic acid, at least in pharmacological doses. Epoxygenase metabolites of arachidonic acid have attracted the largest interest. In their critical review on epoxygenase metabolism of arachidonic acid in 1989, Fitzpatrick and Murphy pointed out some major differences between the PGH synthase, the lipoxygenase and the P450 pathways of arachidonic acid metabolism. Their main points are still valid and have only to be modified slightly in the light of recent results. First, lipoxygenases show a marked regiospecificity and stereospecificity, while many P450 seem to lack this specificity. There are, however, P450 isozymes which catalyse stereospecific epoxidations or hydroxylations. Many hydroxylases and at least some epoxygenases also show regiospecificity, i.e. oxygenate only one double bond or one specific carbon of the fatty acid substrate. In addition, preference for arachidonic acid and eicosapentaenoic acid may occur in the sense that other fatty acids are oxygenated with less regiospecificity. A more important difference is that prostaglandins and leukotrienes affect specific and well characterised receptors in cell membranes, while receptors for epoxides of arachidonic acid or other P450 metabolites have not been characterised. Nevertheless, epoxides of arachidonic acid have been found to induce a large number of different pharmacological effects. In some systems, effects have been noted at pm concentrations which might conceivably be in the physiological concentration range of these epoxides, e.g. after release from phospholipids by phospholipase A2. An intriguing possibility is that the effects of [Ca]i on different ion channels might possibly explain their biological actions. In situations when pharmacological doses are used, metabolism to epoxyprostanoids or other interactions with PGH synthase could also be of importance. Finally, one report on a specific receptor for 14R,15S-EpETrE in mononuclear cell membranes has just been published.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Neurotransmitter regulation of somatostatin secretion by fetal rat cerebral cortical cells in culture

Extensive studies exploring the regulation of hypothalamic somatostatin GHRIH release have been reported, but the factors regulating GHRIH release in the cerebral cortex have not been well defined. We have studied the effects of central neurotransmitters on GHRIH secretion by cultured fetal rat cerebral cortical cells and on intracellular GHRIH levels. Cells maintained in vitro for 15-20 days were incubated with dopamine (DA), acetylcholine (ACh), gamma-aminobutyric acid (GABA), norepinephrine (NE), serotonin (SE) or histamine (His) (10(-11) M to 10(3) M) for 30 minutes. Following incubation, immunoreactive GHRIH was measured by RIA in cell extracts and incubation media. DA increased intracellular GHRIH content but have no effect on GHRIH in the media. Both media and intracellular GHRIH content were significantly reduced by GABA and SE. The effect of NE was stimulatory at low (10(-9) M) and inhibitory at high (10(-5) M to 10(-3) M) concentrations. ACh was found to increase media GHRIH and to decrease intracellular GHRIH content; 30 min exposure to His did not significantly modify either media or intracellular GHRH. Our findings with fetal rat cerebral cortical cells in culture demonstrate that endogenous neurotransmitters do have the capacity to directly influence GHRIH regulation.

Metabolism of arachidonic acid to epoxyeicosatrienoic acids, hydroxyeicosatetraenoic acids, and prostaglandins in cultured rat hippocampal astrocytes

We have recently shown that brain slices are capable of metabolizing arachidonic acid by the epoxygenase pathway. The purpose of this study was to begin to determine the ability of individual brain cell types to form epoxygenase metabolites. We have examined the astrocyte epoxygenase pathway and have also confirmed metabolism by the cyclooxygenase and lipoxygenase enzyme systems. Cultured rat hippocampal astrocyte homogenate, when incubated with radiolabeled [3H]arachidonic acid, formed products that eluted in four major groups designated as R17-30, R42-50, R51-82, and R83-90 based on their retention times in reverse-phase HPLC. These fractions were further segregated into as many as 13 peaks by normal-phase HPLC and a second reverse-phase HPLC system. The principal components in each peak were structurally characterized by gas chromatography/electron impact-mass spectrometry. Based on HPLC retention times and gas chromatography/electron impact-mass spectrometry analysis, the more polar fractions (R17-30) contained prostaglandin D2 as the major cyclooxygenase product. Minor products included 6-keto prostaglandin F1 alpha, prostaglandin E2, prostaglandin F2 alpha, and thromboxane B2. Fractions R42-50, R51-82, and R83-90 contained epoxygenase and lipoxygenase-like products. The major metabolite in fractions R83-90 was 5,6-epoxyeicosatrienoic acid (EET). Fractions R51-82 contained 14,15- and 8,9-EETs, 12- and 5-hydroxyeicosatetraenoic acids, and 8,9- and 5,6-dihydroxyeicosatrienoic acids (DHETs). In fractions R42-50, 14,15-DHET was the major product. When radiolabeled [3H]14,15-EET was incubated with astrocyte homogenate, it was rapidly metabolized to [3H]14,15-DHET. The metabolism was inhibited by submicromolar concentration of 4-phenylchalcone oxide, a potent inhibitor of epoxide hydrolase activity. Formation of other polar metabolites such as triols or epoxy alcohols from 14,15-DHET was not observed. In conclusion, astrocytes readily metabolize arachidonic acid to 14,15-EET, 5,6-EET, and their vicinal-diols. Previous studies suggest these products may affect neuronal function and cerebral blood flow.

Participation of prostaglandin E2 in dopamine D2 receptor-dependent potentiation of arachidonic acid release

Stimulation of dopamine D2 receptors potentiates Ca2+ ionophore- or ATP-induced arachidonic acid (AA) release in D2 receptor cDNA-transfected Chinese hamster ovary (CHO) cells [CHO(D2)]. By using a combination of chromatographic, biochemical, and radioimmunochemical techniques, we show here that prostaglandin (PG) E2 is a major product of AA metabolism in CHO(D2) cells stimulated with the Ca2+ ionophore A23187. Formation of this PG was markedly increased by the concomitant application of quinpirole, a D2 receptor agonist. In addition, PGE2 enhanced D2-dependent amplification of AA release, either when it was added (EC50 = 100 nM) or when it was produced endogenously, as shown by experiments carried out with the cyclooxygenase inhibitor indomethacin. The results suggest that PGE2 may participate in D2 receptor-mediated potentiation of AA release in CHO(D2) cells. They also support a functional role for this PG in the modulation of dopaminergic transmission in areas of the CNS, such as amygdala and hypothalamus, where high levels of both PGE2 and dopamine D2 receptors are found.

Brain synthesis and cerebrovascular action of epoxygenase metabolites of arachidonic acid

The purpose of this study was to determine if whole brain makes epoxygenase metabolites of arachidonic acid and, if so, whether they are vasoactive on the cerebral microcirculation. Blood-free mouse brain slices were incubated with exogenous radiolabeled arachidonic acid, and the extracted metabolites were resolved by HPLC. Metabolite structures were confirmed by gas chromatography/mass spectrometry. In addition to prostaglandins, leukotriene B4, and hydroxyeicosatetraenoic acids, mouse brain metabolized arachidonic acid into several other compounds. Among them, we identified 5,6- and 14,15-epoxyeicosatrienoic acid. Next, we tested the effect of topical application of brain-synthesized 5,6-epoxyeicosatrienoic acid and synthetic epoxyeicosatrienoic acids on in vivo rabbit cerebral arteriolar diameter using the cranial window technique and in vivo microscopy. Brain-synthesized 5,6-epoxyeicosatrienoic acid caused a transient 28% arteriolar dilation, similar to that produced by 5 micrograms/ml of synthetic 5,6-epoxyeicosatrienoic acid. A concentration of synthetic 14,15- and 11,12-epoxyeicosatrienoic acid of 5 micrograms/ml CSF had little or no effect on diameter, whereas 8,9-epoxyeicosatrienoic acid caused a maximum dilation of 8%. These studies suggest that brain-synthesized 5,6-epoxyeicosatrienoic acid may play a role in the normal or pathophysiological regulation of the cerebral microcirculation.

Urinary excretion of diols derived from eicosapentaenoic acid during n-3 fatty acid ingestion by man

Epoxides and fatty acid diols derived from arachidonate by the action of cytochrome P-450 appear in human urine and have biological activities. Dietary eicosapentaenoic acid gives rise to prostaglandins in vivo, but vascular effects of n-3 supplements do not all correlate with altered types or amounts of in vivo cyclooxygenase products. We investigated whether dietary eicosapentaenoic acid could also be metabolized by cytochrome P-450, by assessing the excretion of its vicinal diols. Utilizing gas chromatography/negative chemical ionization mass spectrometry, we have found that humans ingesting n-3 fatty acids excrete vicinal diols of eicosapentaenoic acid in substantial quantities.