The role of arachidonic acid and its metabolites in the release of neuropeptides

Inflammation in Bipolar Disorder (BD): Identification of new therapeutic targets

Bipolar disorder (BD) is a chronic and cyclic mental disorder, characterized by unusual mood swings between mania/hypomania and depression, raising concern in both scientific and medical communities due to its deleterious social and economic impact. Polypharmacy is the rule due to the partial effectiveness of available drugs. Disease course is often unremitting, resulting in frequent cognitive deficits over time. Despite all research efforts in identifying BD-associated molecular mechanisms, current knowledge remains limited. However, the involvement of inflammation in BD pathophysiology is increasingly consensual, with the immune system and neuroinflammation playing a key role in disease course. Evidence includes altered levels of cytokines and acute-phase proteins, pathological microglial activation, deregulation of Nrf2-Keap1 system and changes in biogenic amines neurotransmitters, whose expression is regulated by TNF-α, a pro-inflammatory cytokine highly involved in BD, pointing out inflammation as a novel and attractive therapeutic target for BD. As result, new therapeutic agents including non-steroidal anti-inflammatory drugs, N-acetylcysteine and GSK3 inhibitors have been incorporated in BD treatment. Taking into consideration the latest pre-clinical and clinical trials, in this review we discuss recent data regarding inflammation in BD, unveiling potential therapeutic approaches through direct or indirect modulation of inflammatory response.

Prostaglandin E2 modulates presynaptic regulation of GnRH neurons via EP4 receptors in accordance with estrogen milieu

Prostaglandin E2 (PGE2) promotes gonadotropin secretion by regulating the activity of neurons that release gonadotropin-releasing hormone (GnRH) in the hypothalamus. However, the mechanisms of action of PGE2 at these neurons have yet to be fully explored. We examined the effects of PGE2 on the generation of miniature excitatory postsynaptic currents (mEPSCs) at GnRH neurons as measured by whole-cell, patch-clamp recordings. GnRH neurons were identified in slices prepared from the preoptic areas of female GnRH-EGFP rats. Exposure to PGE2 significantly increased the frequency, but not the amplitude, of the mEPSCs generated on the day of proestrus, but neither frequency nor amplitude was altered on day 1 of diestrus. These data suggest that the action of PGE2 on mEPSC frequency varies depending on the stage of estrous. An estrogen-dependence of PGE2's action was further supported by the increased frequency, but not amplitude, of mEPSCs generated at GnRH neurons prepared from estrogen-primed ovariectomized rats. Conversely, PGE2 had no effect on mEPSC frequency or amplitude at GnRH neurons in cholesterol-treated rats. Subsequent experiments to identify candidate receptors for PG2E's action revealed that exposure to a PGE2 receptor 4 (EP4) agonist, but not EP1 or EP2 agonists, mimicked the effects achieved by PGE2 exposure. These effects of mEPSCs could be reversed using an EP4 antagonist, illustrating the specificity of the effect. Collectively, these data demonstrate that PGE2 can alter excitatory synaptic neurotransmission at GnRH neurons via EP4 signaling at presynaptic site(s) in an estrogen-dependent fashion during proestrus.

Cytochrome P450 2J2: distribution, function, regulation, genetic polymorphisms and clinical significance

Cytochrome P450 2J2 (CYP2J2) is an enzyme mainly found in human extrahepatic tissues, with predominant expression in the cardiovascular systems and lower levels in the intestine, kidney, lung, pancreas, brain, liver, etc. During the past 15 years, CYP2J2 has attracted much attention for its epoxygenase activity in arachidonic acid (AA) metabolism. It converts AA to four epoxyeicosatrienoic acids (EETs) that have various biological effects, especially in the cardiovascular systems. In recent publications, CYP2J2 is shown highly expressed in various human tumor cells, and its EET metabolites are demonstrated to implicate in the pathologic development of human cancers. CYP2J2 is also a human CYP that involved in phase I xenobiotics metabolism. Antihistamine drugs and many other compounds were identified as the substrates of CYP2J2, and studies have demonstrated that these substrates have a broad structural diversity. CYP2J2 is found not readily induced by known P450 inducers; however, its expression could be regulated in some pathological conditions, might through the activator protein-1(AP-1), the AP-1-like element and microRNA let-7b. Several genetic mutations in the CYP2J2 gene have been identified in humans, and some of them have been shown to have potential associations with some diseases. With the increasing awareness of its roles in cancer disease and drug metabolism, studies about CYP2J2 are still going on, and various inhibitors of CYP2J2 have been determined. Further studies are needed to delineate the roles of CYP2J2 in disease pathology, drug development and clinical practice.

Neuronal overexpression of cyclooxygenase-2 does not alter the neuroinflammatory response during brain innate immune activation

Neuroinflammation is a critical component in the progression of several neurological and neurodegenerative diseases and cyclooxygenases (COX)-1 and -2 are key regulators of innate immune responses. We recently demonstrated that COX-1 deletion attenuates, whereas COX-2 deletion enhances, the neuroinflammatory response, blood-brain barrier permeability and leukocyte recruitment during lipopolysaccharide (LPS)-induced innate immune activation. Here, we used transgenic mice, which overexpressed human COX-2 via neuron-specific Thy-1 promoter (TgCOX-2), causing elevated prostaglandins (PGs) levels. We tested whether neuronal COX-2 overexpression affects the glial response to a single intracerebroventricular injection of LPS, which produces a robust neuroinflammatory reaction. Relative to non-transgenic controls (NTg), 7 month-old TgCOX-2 did not show any basal neuroinflammation, as assessed by gene expression of markers of inflammation and oxidative stress, neuronal damage, as assessed by Fluoro-JadeB staining, or systemic inflammation, as assessed by plasma levels of IL-1beta and corticosterone. Twenty-four hours after LPS injection, all mice showed increased microglial activation, as indicated by Iba1 immunostaining, neuronal damage, mRNA expression of cytokines (TNF-alpha, IL-6), reactive oxygen expressing enzymes (iNOS and NADPH oxidase subunits), endogenous COX-2, cPLA(2) and mPGES-1, and hippocampal and cortical IL-1beta levels. However, the increases were similar in TgCOX-2 and NTg. In NTg, LPS increased brain PGE(2) to the levels observed in TgCOX-2. These results suggest that PGs derived from neuronal COX-2 do not play a role in the neuroinflammatory response to acute activation of brain innate immunity. This is likely due to the direct effect of LPS on glial rather than neuronal cells.

Possible involvement of microglia containing cyclooxygenase-1 in the accumulation of gonadotrophin-releasing hormone in the preoptic area in female rats

Prostaglandins (PGs), especially PGE(2), are involved in the hypothalamic control of gonadotrophin-releasing hormone (GnRH) release, acting at least in part on the terminal of GnRH axons in the median eminence. The present study aimed: (i) to clarify the role of PG(s) in regulating GnRH cell function at the level of the perikarya in the preoptic area; (ii) to determine the cyclooxygenase (COX) isozyme responsible for producing PG(s) that regulates GnRH perikarya; and (iii) to identify cell types that contain the responsible COX isozyme in female rats. A surge of luteinising hormone (LH) secretion was induced by oestrogen and progesterone in ovariectomised rats. Treatment of the rat before the LH surge with indomethacin, a nonselective COX inhibitor, or NS-398, a selective COX-2 inhibitor, did not interfere with the surge. However, treatment with indomethacin or flurbiprofen, a selective COX-1 inhibitor, significantly reduced the number of GnRH-immunoreactive cells in the preoptic area at the time of peak LH secretion during the surge. NS-398 did not affect the GnRH immunoreactivity. Double-labelled immunofluorescent histochemistry revealed COX-1 immunoreactivity in the vicinity of, but not within, GnRH containing neurones in the preoptic area. COX-2 immunoreactivity was not found in the same area. The COX-1 immunoreactivity was almost entirely localised in microglia in the preoptic area, but not in neurones or astrocytes. These results suggest that microglia in the preoptic area containing COX-1 are responsible for producing PG(s), which, in turn, facilitates the accumulation of GnRH during the gonadotrophin surge in female rats.

Intracellular- and extracellular-derived Ca(2+) influence phospholipase A(2)-mediated fatty acid release from brain phospholipids

Docosahexaenoic acid (DHA) and arachidonic acid (AA) are found in high concentrations in brain cell membranes and are important for brain function and structure. Studies suggest that AA and DHA are hydrolyzed selectively from the sn-2 position of synaptic membrane phospholipids by Ca(2+)-dependent cytosolic phospholipase A(2) (cPLA(2)) and Ca(2+)-independent phospholipase A(2) (iPLA(2)), respectively, resulting in increased levels of the unesterified fatty acids and lysophospholipids. Cell studies also suggest that AA and DHA release depend on increased concentrations of Ca(2+), even though iPLA(2) has been thought to be Ca(2+)-independent. The source of Ca(2+) for activation of cPLA(2) is largely extracellular, whereas Ca(2+) released from the endoplasmic reticulum can activate iPLA(2) by a number of mechanisms. This review focuses on the role of Ca(2+) in modulating cPLA(2) and iPLA(2) activities in different conditions. Furthermore, a model is suggested in which neurotransmitters regulate the activity of these enzymes and thus the balanced and localized release of AA and DHA from phospholipid in the brain, depending on the primary source of the Ca(2+) signal.

Neuroinflammatory response to lipopolysaccharide is exacerbated in mice genetically deficient in cyclooxygenase-2

Background

Cyclooxygenases (COX) -1 and -2 are key mediators of the inflammatory response in the central nervous system. Since COX-2 is inducible by inflammatory stimuli, it has been traditionally considered as the most appropriate target for anti-inflammatory drugs. However, the specific roles of COX-1 and COX-2 in modulating a neuroinflammatory response are unclear. Recently, we demonstrated that COX-1 deficient mice show decreased neuroinflammatory response and neuronal damage in response to lipopolysaccharide (LPS).

Methods

In this study, we investigated the role of COX-2 in the neuroinflammatory response to intracerebroventricular-injected LPS (5 μg), a model of direct activation of innate immunity, using COX-2 deficient (COX-2^-/-) and wild type (COX-2^+/+) mice, as well as COX-2^+/+ mice pretreated for 6 weeks with celecoxib, a COX-2 selective inhibitor.

Results

Twenty-four hours after LPS injection, COX-2^-/- mice showed increased neuronal damage, glial cell activation, mRNA and protein expression of markers of inflammation and oxidative stress, such as cytokines, chemokines, iNOS and NADPH oxidase. Brain protein levels of IL-1β, NADPH oxidase subunit p67^phox, and phosphorylated-signal transducer and activator of transcription 3 (STAT3) were higher in COX-2^-/- and in celecoxib-treated mice, compared to COX-2^+/+ mice. The increased neuroinflammatory response in COX-2^-/- mice was likely mediated by the upregulation of STAT3 and suppressor of cytokine signaling 3 (SOCS3).

Conclusion

These results show that inhibiting COX-2 activity can exacerbate the inflammatory response to LPS, possibly by increasing glial cells activation and upregulating the STAT3 and SOCS3 pathways in the brain.

Arachidonic acid metabolism in brain physiology and pathology: lessons from genetically altered mouse models

The arachidonic acid (AA) cascade involves the release of AA from the membrane phospholipids by a phospholipase A(2), followed by its subsequent metabolism to bioactive prostanoids by cyclooxygenases coupled with terminal synthases. Altered brain AA metabolism has been implicated in neurological, neurodegenerative, and psychiatric disorders. The development of genetically altered mice lacking specific enzymes of the AA cascade has helped to elucidate the individual roles of these enzymes in brain physiology and pathology. The roles of AA and its metabolites in brain physiology, with a particular emphasis on the phospholipase A(2)/cyclooxygenases pathway, are summarized, and the specific phenotypes of genetically altered mice relevant to brain physiology and neurotoxic models are discussed.

Polymorphisms in human CYP2C8 decrease metabolism of the anticancer drug paclitaxel and arachidonic acid

Cytochrome P450 (CYP) 2C8 is the principal enzyme responsible for the metabolism of the anti-cancer drug paclitaxel (Taxol). It is also the predominant P450 responsible for the metabolism of arachidonic acid to biologically active epoxyeicosatrienoic acids (EETs) in human liver and kidney. In this study, we describe two new CYP2C8 alleles containing coding changes: CYP2C8*2 has an Ile269Phe substitution in exon 5 and CYP2C8*3 includes both Arg139Lys and Lys399Arg amino acid substitutions in exons 3 and 8. CYP2C8*2 was found only in African-Americans, while CYP2C8*3 occurred primarily in Caucasians. Neither occurred in Asians. The frequency of the CYP2C8*2 allele was 0.18 in African-Americans, and that of CYP2C8*3 was 0.13 in Caucasians. CYP2C8*1 (wild-type), CYP2C8*2 and CYP2C8*3 cDNAs were expressed in Escherichia coli, and the ability of these enzymes to metabolize both paclitaxel and arachidonic acid was assessed. Recombinant CYP2C8*3 was defective in the metabolism of both substrates. The turnover number of CYP2C8*3 for paclitaxel was 15% of CYP2C8*1. CYP2C8*2 had a two-fold higher Km and two-fold lower intrinsic clearance for paclitaxel than CYP2C8*1. CYP2C8*3 was also markedly defective in the metabolism of arachidonic acid to 11,12- and 14,15-EET (turnover numbers 35-40% that of CYP2C8*1). Thus, CYP2C8*3 is defective in the metabolism of two important CYP2C8 substrates: the anticancer drug paclitaxel and the physiologically important compound arachidonic acid. This polymorphism has important clinical and physiological implications in individuals homozygous for this allele.

CYP2C40, a unique arachidonic acid 16-hydroxylase, is the major CYP2C in murine intestinal tract

We recently identified five different murine CYP2C cDNAs from a murine cDNA library. When expressed in a bacterial cDNA expression system, all five recombinant proteins metabolized arachidonic acid but produced distinctly different profiles. In addition, some CYP2C mRNAs were found in extrahepatic tissues, as well as in liver. Immunoblots with an antibody raised against recombinant CYP2C38, which recognizes all five murine CYP2Cs, demonstrated that among extrahepatic tissues, colon and cecum contained the highest amount of CYP2Cs. The highest concentration of CYP2Cs occurred in cecum and colon (cecum >/= proximal colon >> distal colon), with lower levels in duodenum, jejunum, and ileum. Immunohistochemical studies revealed that CYP2Cs were localized principally in epithelial cells and autonomic ganglia in gut and colon. Polymerase chain reaction amplification of reverse-transcribed mRNA using murine CYP2C-specific primers followed by cloning and sequencing identified CYP2C40 as the major CYP2C isoform expressed in murine intestinal tract. Recombinant CYP2C40 metabolized arachidonic acid in a regio- and stereospecific manner to 16(R)-HETE (hydroxyeicosatetraenoic acid) as the major product. To our knowledge, CYP2C40 is the first enzyme known to produce primarily 16-HETE. We conclude that CYP2C40 is one of the major cytochrome P450 proteins in the mouse intestinal tract. In the light of vasoactive and anti-neutrophilic effects of 16-HETE, we hypothesize that CYP2C40 may play an important role in endogenous biological functions in intestine.

P450 subfamily CYP2J and their role in the bioactivation of arachidonic acid in extrahepatic tissues

Historically, there has been intense interest in P450 metabolic oxidation, peroxidation, and reduction of xenobiotics. More recently, there has been a growing appreciation for the role of P450s in the oxidation of lipophilic endobiotics, such as bile acids, fat-soluble vitamins, and eicosanoids. This review details the emerging CYP2J subfamily of P450s and their role as catalysts of arachidonic acid metabolism.

CYP2J subfamily cytochrome P450s in the gastrointestinal tract: expression, localization, and potential functional significance

Our laboratory recently described a new human cytochrome P450 arachidonic acid epoxygenase (CYP2J2) and the corresponding rat homologue (CYP2J3), both of which were expressed in extrahepatic tissues. Northern analysis of RNA prepared from the human and rat intestine demonstrated that CYP2J2 and CYP2J3 mRNAs were expressed primarily in the small intestine and colon. In contrast, immunoblotting studies using a polyclonal antibody raised against recombinant CYP2J2 showed that CYP2J proteins were expressed throughout the gastrointestinal tract. Immunohistochemical staining of formalin-fixed, paraffin-embedded intestinal sections using anti-CYP2J2 IgG and avidin-biotin-peroxidase detection revealed that CYP2J proteins were present at high levels in nerve cells of autonomic ganglia, epithelial cells, intestinal smooth muscle cells, and vascular endothelium. The distribution of this immunoreactivity was confirmed by in situ hybridization using a CYP2J2-specific antisense RNA probe. Microsomal fractions prepared from human jejunum catalyzed the NADPH-dependent metabolism of arachidonic acid to epoxyeicosatrienoic acids as the principal reaction products. Direct evidence for the in vivo epoxidation of arachidonic acid by intestinal cytochrome P450 was provided by documenting, for the first time, the presence of epoxyeicosatrienoic acids in human jejunum by gas chromatography/mass spectrometry. We conclude that human and rat intestine contain an arachidonic acid epoxygenase belonging to the CYP2J subfamily that is localized to autonomic ganglion cells, epithelial cells, smooth muscle cells, and vascular endothelium. In addition to the known effects on intestinal vascular tone, we speculate that CYP2J products may be involved in the release of intestinal neuropeptides, control of intestinal motility, and/or modulation of intestinal fluid/electrolyte transport.

Predominant expression of an arachidonate epoxygenase in islets of Langerhans cells in human and rat pancreas

Our laboratory recently described a new human cytochrome P450 arachidonic acid epoxygenase (CYP2J2) and the corresponding rat homolog (CYP2J3). Immunoblotting studies using a polyclonal antibody raised against recombinant human CYP2J2 confirmed CYP2J protein expression in human and rat pancreatic tissues. Immunohistochemical staining of formalin-fixed paraffin-embedded rat and human pancreas using the anti-CYP2J2 IgG and avidin-biotin-peroxidase detection revealed that CYP2J2 protein expression was highly localized to cells in the islets of Langerhans, with minimal staining in pancreatic exocrine cells. Colocalization studies using antibodies to the glucagon, insulin, somatostatin, and pancreatic polypeptide as markers for alpha-, beta-, delta-, and PP cells, respectively, showed that CYP2J protein expression was abundantly present in all four cell types, but was highest in the glucagon-producing alpha-cells. Direct evidence for the epoxidation of arachidonic acid by pancreatic cytochrome P450 was provided by documenting, for the first time, the presence of epoxyeicosatrienoic acids in vivo in human and rat pancreas by gas chromatography/mass spectrometry. Importantly, the levels of immunoreactive CYP2J2 in different human pancreatic tissues were highly correlated with endogenous epoxyeicosatrienoic acid concentrations. We conclude that human and rat pancreas contain an arachidonic acid epoxygenase belonging to the CYP2J subfamily that is highly localized to islet cells. These data together with previous work showing effects of epoxyeicosatrienoic acids in stimulating insulin and glucagon secretion from isolated rat pancreatic islets support the hypothesis that epoxygenase products may be involved in stimulus-secretion coupling in the pancreas.

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.

Changes in hypothalamic prostaglandin E2 may predict the occurrence of sleep or wakefulness as assessed by parallel EEG and microdialysis in the rat

Prostaglandin (PG) E2 is produced by mammalian hypothalamus and when administered exogenously prolongs wakefulness. In order to study the relation of endogenous hypothalamic PGE2 to sleep and wakefulness, we have used microdialysis in freely moving rats associated with EEG recording. Male Wistar rats were implanted with three cortical electrodes and with a guide cannula for microdialysis in the space between the paraventricular nucleus (PVN) and the ventromedial hypothalamus (VMH). PGE2 was measured by RIA in 3- or 6-min dialysates 15 days after surgery, when sleep patterns were normal again and PGE2 production stabilised. PGE2 levels were significantly higher during wakefulness (601 +/- 35 pg/ml, 5 experiments, 35 samples) than during slow-wave sleep (487 +/- 24 pg/ml, 5 experiments, 49 samples). Samples corresponding to paradoxical sleep showed a tendency towards higher PGE2 values compared to slow-wave sleep but lower compared to wakefulness. In epochs of wakefulness or sleep lasting at least 12 min, high PGE2 levels in the middle of wakefulness regularly dropped, thus announcing the occurrence of sleep. During sleep, PGE2 first went on dropping and then reincreased towards the values that characterize early periods of wakefulness. In its turn, this reincrease in PGE2 announced the end of sleep and the imminent occurrence of wakefulness. It is the first study to our knowledge showing that the evolvement in endogenous PG profile may predict the occurrence of sleep or wakefulness.

Specific delta-opioid antagonists exert an agonist-independent inhibitory effect, similar to the agonist, on the release of GnRH in vitro

1. In in vitro studies with adult male rats we have recently shown that the delta-opioid agonist DTLET inhibits the release of the Gonadotropin-Releasing Hormone (GnRH) from hypothalamic fragments containing the arcuate nucleus and the median eminence. This effect is receptor mediated and eicosanoid dependent (Gerozissis et al., 1993). 2. In the present study we report that the delta-opioid antagonists with negative intrinsic activity, Diallyl-G and ICI 174864, applied under the same experimental conditions (30 min static incubations at 37 degrees C, in a potassium rich milieu), in the absence of the agonist DTLET, also exert a similar to the agonist inhibitory effect on the release of GnRH. 3. The dose-dependent inhibitory effect of Diallyl-G on GnRH release is reversed by increasing concentrations of DTLET. The mu and delta opioid antagonist, naloxone is without effect in the absence of DTLET. However, naloxone acts as an antagonist on the Diallyl-G-induced inhibition of GnRH release. 4. Diallyl-G also inhibits the release of prostaglandin E2 (PGE2). In the presence of indomethacin or nordihydroguaiaretic acid, Diallyl-G is ineffective to further inhibit the release of GnRH. These latter observations taken together with the results of eicosanoid estimation suggest that PGE2 but not leukotrienes participate in the agonist-independent effects of Diallyl-G on GnRH release. 5. Therefore these results support the hypothesis that delta-opioid antagonists with negative intrinsic activity exert agonist-independent biological responses similar to those of the agonists.

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)

The delta-opioid signal transduction on the gonadotropin-releasing hormone release is eicosanoid dependent

In static incubations, the K+ induced release of gonadotropin-releasing hormone (GnRH) and of prostaglandin (PG) E2, was 2-3 times higher in the isolated median eminence (ME) compared to the hypothalamic area containing the arcuate nucleus (ARN) plus the ME. The delta-opioid agonist DTLET, induced a parallel, dose-dependent reduction of GnRH and PGE2 release in the ARN plus ME. Both effects of DTLET were blocked by the delta-opioid antagonist Diallyl-G. In the isolated ME, DTLET reduced the secretion of PGE2 but enhanced the release of GnRH. In this area Diallyl-G had no effect on the PGE2 release but blocked the GnRH secretion. When the PGE2 production was blocked by indomethacin in the ARN plus ME preparation, DTLET increased the release of GnRH and induced the production of leukotrienes (LTs). On the other hand, DTLET decreased the release of both GnRH and PGE2 in the presence of nordihydroguaiaretic acid (NDGA), an inhibitor of the production of LTs. The above results suggest that: (a) the delta-opioid agonist DTLET modulates GnRH release differentially in the hypothalamic areas examined; and (b) the arachidonic acid metabolites are involved in the mode of action of DTLET on the release of GnRH in the ARN plus ME hypothalamic fragment.

Indomethacin, an inhibitor of prostaglandin synthesis attenuates alteration in spinal cord evoked potentials and edema formation after trauma to the spinal cord: an experimental study in the rat

The potential efficacy of indomethacin (a potent inhibitor of endogenous prostaglandin synthesis) on spinal cord-evoked potentials and edema formation occurring after a focal trauma to the spinal cord was examined in a rat model. The spinal cord evoked potentials were recorded in urethane-anesthetized male rats using monopolar electrodes placed epidurally over the T9 (rostral) and T12 (caudal) segments after stimulation of the ipsilateral right tibial and sural nerves. Reference electrodes were placed in the corresponding paravertebral muscles. The spinal cord evoked potential consisted of a small positive peak followed by a broad and high negative peak. Amplitudes and latencies of the maximal positive peak and the maximal negative peak were measured. The latencies and amplitudes 30 min before injury were used as references (100%). A complete loss was denoted as 0%. All the potentials were quite stable during 30 min of recording before injury. Infliction of trauma to the T10-T11 segments of the spinal cord with a sterile scalpel blade (about 5 mm longitudinal and 2 mm deep incision into the right dorsal horn extending to Rexed's laminae VII) in untreated animals resulted in an immediate depression of the rostral maximal negative peak amplitude (60-100%) which persisted during 5 h of recording. The latencies of the rostral as well as caudal maximal negative and positive peaks increased successively from 2 h post-trauma. In this group of animals, 5 h after injury the spinal cord water content in the traumatized segments was increased by more than 6% as compared with a group of uninjured animals. Pretreatment with indomethacin (10 mg/kg body weight i.p. 30 min before injury) markedly attenuated the immediate decrease in the maximal negative peak amplitude after injury, but did not influence the successive latency increase. However, the increase in the water content of the traumatized cord after 5 h was less pronounced compared with untreated injured rats. Our results show a beneficial effect of indomethacin on trauma-induced spinal cord evoked potential changes and edema formation. Prostaglandins may thus influence early bioelectrical changes occurring in traumatized spinal cord not reported earlier. The findings support the view that early recording of spinal cord evoked potential may be useful to predict the outcome in some forms of spinal cord injuries.

Early perifocal cell changes and edema in traumatic injury of the spinal cord are reduced by indomethacin, an inhibitor of prostaglandin synthesis. Experimental study in the rat

The possibility that prostaglandins participate in the formation of perifocal edema and cell changes following a localized trauma to the spinal cord was investigated in a rat model. A laminectomy was performed in urethane-anesthetized animals at the thoracic T10-11 segment. Using a scalpel blade a unilateral lesion, about 2 mm deep and 5 mm long was made 1 mm to the right of the midline. The deepest part of the injury occupied Rexed's lamina VII of the dorsal horn. Animals were pretreated with the prostaglandin synthesis inhibitor, indomethacin (10 mg/kg, i.p. 30 min prior to trauma). Five hours after the injury the water content was determined and cell changes in and around the primary lesion were examined by light and electron microscopy. Normal and injured rats without indomethacin pretreatment served as controls. Untreated injured rats showed a profound increase of water content in the traumatized T10-11, the rostral (T9) and caudal (T12) segments compared with normal rats. These segments also exhibited marked cell changes in ipsilateral and contralateral dorsal and ventral horns. The gray matter had a spongy appearance and some nerve cells were condensed and distorted. The white matter contained many distorted fibers. Immunostaining for myelin basic protein showed a marked reduction of reaction product in the injured animals compared with normal rats. Ultrastructurally widened extracellular spaces, cytoplasmic vacuolation, swollen and condensed neurons, swollen astrocytes and vesiculation of myelin were frequent findings. Pretreatment of rats with indomethacin significantly reduced the accumulation of water in the traumatized and in the rostral and caudal segments. The structural changes were less pronounced particularly in the cranial and caudal segments.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide mediates norepinephrine-induced prostaglandin E2 release from the hypothalamus

Nitric oxide (NO), formed by conversion of arginine to citrulline and NO by NO synthase, mediates relaxation of vascular smooth muscle. NO synthase has been demonstrated by immunocytochemical methods in neurons in various parts of the central nervous system including the hypothalamus. The latter finding suggested to us that NO might play a role in controlling the release of hypothalamic peptides. We have previously shown that norepinephrine mediates the release of luteinizing hormone-releasing hormone (LHRH) from LHRH terminals in the median eminence into the hypophyseal portal veins, which transport LHRH to the anterior pituitary gland to trigger release of luteinizing hormone from gonadotrophs. LHRH release from these terminals requires increased release of prostaglandin E2 (PGE2). PGE2 activates adenylate cyclase to produce cAMP, and then cAMP induces the exocytosis of LHRH secretory granules. In view of the evidence above and because of the developing evidence for the importance of NO in the central nervous system, it occurred to us that NO might be involved in this process. Consequently, we evaluated the role of NO in the release of PGE2 from medial basal hypothalamic fragments. As previously reported, norepinephrine (10 microM) increased PGE2 release from the hypothalamic fragments. The inhibitor of NO synthase NG-monomethyl-L-arginine (NMMA, 300 microM) blocked the stimulation of PGE2 release induced by norepinephrine but had no effect on the basal release of PGE2. Sodium nitroprusside (100 microM), which liberates NO, also elevated PGE2 release from the hypothalamic fragments. This elevation was not affected by NMMA, presumably because NMMA blocks enzymatic generation of NO but does not alter NO liberated by nitroprusside. When the NO liberated by nitroprusside was inactivated by hemoglobin (2 micrograms/ml), the effect of nitroprusside on PGE2 release was completely inhibited. Neither NMMA nor hemoglobin altered the basal release of PGE2, which indicates that NO is not responsible for basal PGE2 release. Addition of L-arginine (10 microM to 1 mM), the substrate for NO synthase, had no effect on basal PGE2 production. These results indicate that NO synthase is not activated in unstimulated hypothalamic fragments in vitro. The results suggest that norepinephrine activates NO synthase leading to the production of NO, which subsequently activates cyclooxygenase and results in the production of PGE2. PGE2 then activates adenylate cyclase leading to generation of increased cAMP, which induces exocytosis of secretory granules of LHRH and other neuropeptides released by PGE2. The indication that NO is essential to norepinephrine-induced release of PGE2 from hypothalamic fragments provides insight into the mechanism of LHRH release and the results open the possibility that the importance of NO to neuronal functions may be widespread in the nervous system.

Prostaglandin E2 and leukotriene C4-induced luteinizing hormone-releasing hormone release from immature and adult male rat median eminences in vitro: eicosanoid formation and binding parameters

The amounts of prostaglandin E2 formed in vitro by the median eminences of adult male rats were greater than those produced by the median eminences of immature, 22 day-old rats. However, the amount of leukotriene C4 produced by the adult rat median eminences was lower than that produced by the immature rat median eminences. Analysis of the prostaglandin E2 binding parameters of hypothalamic P2 membrane fractions indicates that there are two binding components, one high affinity (RH) and one low affinity (RL) in both adult and immature rats. The maximal binding capacity of RH from adult rat membranes was significantly lower than that of immature rat membranes, correlating with greater prostaglandin E2 production by the adult rat median eminence. Only one leukotriene C4 binding site was detected in both adult and immature rat membranes. Exogenous prostaglandin E2 and leukotriene C4 both stimulated, the release of luteinizing hormone-releasing hormone to the same extent from both the adult and immature median eminences.

Interleukin 1 alpha inhibits prostaglandin E2 release to suppress pulsatile release of luteinizing hormone but not follicle-stimulating hormone

Interleukin 1 alpha (IL-1 alpha), a powerful endogenous pyrogen released from monocytes and macrophages by bacterial endotoxin, stimulates corticotropin, prolactin, and somatotropin release and inhibits thyrotropin release by hypothalamic action. We injected recombinant human IL-1 alpha into the third cerebral ventricle, to study its effect on the pulsatile release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in conscious, freely moving, ovariectomized rats. Intraventricular injection of 0.25 pmol of IL-1 alpha caused an almost immediate reduction of plasma LH concentration; this decrease was statistically significant 20 min after injection and occurred through a highly significant reduction in the number of LH pulses, with no effect on pulse amplitude. In contrast, there was no change in pulse frequency but a small significant elevation in amplitude of FSH pulses. Intraventricular injection of the diluent had no effect on gonadotropin release. The results provide further evidence for separate hypothalamic control mechanisms for FSH and LH release. To determine the mechanism of the suppression of LH release, mediobasal hypothalamic fragments were incubated in vitro with IL-1 alpha (10 pM) and the release of LH-releasing hormone (LHRH) and prostaglandin E2 into the medium was measured by RIA in the presence or absence of norepinephrine (50 microM). IL-1 alpha reduced basal LHRH release and blocked LHRH release induced by norepinephrine. It had no effect on the basal release of prostaglandin E2; however, it completely inhibited the release of PGE2 evoked by norepinephrine. To evaluate the possibility that IL-1 alpha might also interfere with the epoxygenase pathway of arachidonic acid metabolism, epoxyeicosatrienoic acids were also measured. IL-1 alpha had no effect on the content of epoxyeicosatrienoic acids in the hypothalamic fragments as measured by gas chromatography and mass spectrometry. In conclusion, IL-1 alpha suppresses LH but not FSH release by an almost complete cessation of pulsatile release of LH in the castrated rat. The mechanism of this effect appears to be by inhibition of prostaglandin E2-mediated release of LHRH.

Lipoxygenase metabolites of arachidonic acid in neuronal transmembrane signalling

Studies of invertebrate and vertebrate nervous tissue have demonstrated that free arachidonic acid and its lipoxygenase metabolites are produced in a receptor-dependent fashion. The intracellular actions of these compounds include the regulation of activity of membrane ion channels and protein kinases. In this article Daniele Piomelli and Paul Greengard review the evidence that these lipophilic molecules constitute a novel class of intracellular second messenger, possibly involved in the modulation of neurotransmitter release.