Agmatine: an endogenous clonidine-displacing substance in the brain

Imidazoline receptors: from discovery to antihypertensive therapy (facts and doubts)

The hypothesis and indirect evidence of imidazoline receptors has been promoted since some 15 years ago and it gave a substantial impetus for research in this field, resulting in a better understanding of neuronal and cardiovascular regulatory processes. The nomenclature of the imidazoline receptors has been accepted by international forums but no direct proof for the existence of these receptors has been published. Authors summarise the most important available data, including facts and doubts as far as the discovery, characterisation, and function of imidazoline receptors and their subtypes, the differences between imidazoline receptors and alpha-2 adrenoceptors, and also on their participation in regulatory processes.

Protection by imidazol(ine) drugs and agmatine of glutamate-induced neurotoxicity in cultured cerebellar granule cells through blockade of NMDA receptor

This study was designed to assess the potential neuroprotective effect of several imidazol(ine) drugs and agmatine on glutamate-induced necrosis and on apoptosis induced by low extracellular K+ in cultured cerebellar granule cells. Exposure (30 min) of energy deprived cells to L-glutamate (1-100 microM) caused a concentration-dependent neurotoxicity, as determined 24 h later by a decrease in the ability of the cells to metabolize 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) into a reduced formazan product. L-glutamate-induced neurotoxicity (EC50=5 microM) was blocked by the specific NMDA receptor antagonist MK-801 (dizocilpine). Imidazol(ine) drugs and agmatine fully prevented neurotoxicity induced by 20 microM (EC100) L-glutamate with the rank order (EC50 in microM): antazoline (13)>cirazoline (44)>LSL 61122 [2-styryl-2-imidazoline] (54)>LSL 60101 [2-(2-benzofuranyl) imidazole] (75)>idazoxan (90)>LSL 60129 [2-(1,4-benzodioxan-6-yl)-4,5-dihydroimidazole](101)>RX82 1002 (2-methoxy idazoxan) (106)>agmatine (196). No neuroprotective effect of these drugs was observed in a model of apoptotic neuronal cell death (reduction of extracellular K+) which does not involve stimulation of NMDA receptors. Imidazol(ine) drugs and agmatine fully inhibited [3H]-(+)-MK-801 binding to the phencyclidine site of NMDA receptors in rat brain. The profile of drug potency protecting against L-glutamate neurotoxicity correlated well (r=0.90) with the potency of the same compounds competing against [3H]-(+)-MK-801 binding. In HEK-293 cells transfected to express the NR1-1a and NR2C subunits of the NMDA receptor, antazoline and agmatine produced a voltage- and concentration-dependent block of glutamate-induced currents. Analysis of the voltage dependence of the block was consistent with the presence of a binding site for antazoline located within the NMDA channel pore with an IC50 of 10-12 microM at 0 mV. It is concluded that imidazol(ine) drugs and agmatine are neuroprotective against glutamate-induced necrotic neuronal cell death in vitro and that this effect is mediated through NMDA receptor blockade by interacting with a site located within the NMDA channel pore.

The Effect of Agmatine on Ischemic and Nonischemic Isolated Rat Heart

OBJECTIVE: the natural polyamines play a protective role during ischemic injury. We studied the effects of agmatine on ischemic and nonischemic isolated rat hearts. METHODS: Thirty-one rats were randomly assigned to one of four experimental groups. Sixteen rats were injected with saline (group 1, n = 9; group 3, n = 7), and 15 rats were injected with 100 mg/kg of agmatine (group 2, n = 8; group 4, n = 7). Injections were given twice: 24 hours and 1 hour before the experiment. Using the modified Langendorf model, rat hearts were perfused with Krebs-Henseleit solution for 105 minutes during phase 1 of the experiment (groups 1 and 2). During phase 2, hearts were exposed to 45 minutes of global ischemia (groups 3 and 4). RESULTS: During phase 1, no statistically significant differences were observed between the agmatine and the control groups. During phase 2, agmatine caused a significant increase in left ventricular pressure (P <.003). At the end of reperfusion, P(max) was 111% +/- 10% from the baseline levels versus only 82% +/- 5% in the control group. After 20 minutes of reperfusion, dP/dt (first-time derivative of the ventricular pressure) in the agmatine group reached full recovery of 106% +/- 12% versus only 64% +/- 14% in the saline group (P =.059). Agmatine also caused a significant increase in coronary flow rate (P <.004) throughout the reperfusion period. Quantitative immunohistochemical staining disclosed reduced cell damage in the agmatine-treated hearts (P <.02) versus the control group. CONCLUSION: Agmatine injection given before induced ischemia improves hemodynamic recovery by mechanisms that may be attributed to its vasodilatory properties.

Identification of imidazoline-receptor binding sites in cortex and medulla of the bovine adrenal gland. Colocalization with MAO-A and MAO-B

The distribution and relative densities of imidazoline-receptor binding sites (I-RBS) in bovine adrenal gland were determined using [3H]clonidine, [3H]2-(2-benzofuranyl)-2-imidazoline ([3H]2-BFI), and [3H]rilmenidine. In light of strong evidence that I-RBS and monoamine amine oxidase enzymes are linked, the selective radioligands [3H]RO41-1049 and [3H]RO19-6327 were used to label the distribution of MAO-A and -B enzymes, respectively. [3H]Clonidine (12 nM) labeled sites in two discrete regions of the bovine adrenal gland, the zona glomerulosa (39 +/- 7 fmol/mg tissue equivalent) and inner medulla (34 +/- 1 fmol/mg tissue). Binding was nonadrenergic (i.e., not inhibited by 100 nM methoxyidazoxan) and inhibited by 60-70% by 100 nM 2-BFI, the selective I2-RBS, suggesting binding predominantly to an I2-RBS. [3H]2-BFI (5 nM), the selective I2-RBS ligand, also labeled a high density of binding sites in the zona glomerulosa (57 +/- 9 fmol/mg) and chromaffin cells in the inner medulla (53 +/- 4 fmol/mg). These sites, however, were insensitive to clonidine (100 nM). By contrast, [3H]rilmenidine (40 nM) labeled I-RBS in all regions of the adrenal gland, that is, the zonae glomerulosa (59 +/- 10 fmol/mg), fasciculata (78 +/- 10 fmol/mg) and reticularis (63 +/- 7 fmol/mg), and outer and inner medullary chromaffin cells (42 +/- 1 and 55 +/- 2 fmol/mg, respectively). Binding to sites in the zona glomerulosa was partially inhibited (16%) by 100 nM 2-BFI. These results are consistent with previous studies indicating that [3H]rilmenidine labels an I2-RBS and additional I-RBS in rat brain and kidney. The distribution of [3H]RO19-6327 (5 nM) binding resembled that of [3H]2-BFI and [3H]clonidine binding with high densities of MAO-B enzyme located in the zona glomerulosa and chromaffin cells of the inner medulla (55 +/- 7 and 76 +/- 6 fmol/mg tissue, respectively), suggesting the colocalization of MAO-B enzyme with I2-RBS. [3H]RO41-1049 (20 nM) binding to MAO-A was highest in the zona reticularis (196 +/- 7 fmol/mg tissue) compared to the zonae glomerulosa and fasciculata (90 +/- 12 and 116 +/- 14 fmol/mg tissue) and inner medulla (149 +/- 38 fmol/mg tissue). Although the existence of I-RBS in bovine adrenal chromaffin cells is well established, this is the first description of I-RBS in the adrenal cortex. Further investigations are now required to determine whether imidazolines can affect adrenal function via actions at these sites.

Modulation of MAO activity by imidazoline and guanidine derivatives

I2-binding sites (I2-BS) are attributed to be a regulative site on monoamine oxidase (MAO). The in vivo and in vitro effects of various imidazoline and guanidine derivatives on MAO activity and on mitochondrial respiration were studied. Substances with high affinity for I2-BS (antazoline, idazoxan, and cirazoline: IC50 = 20.3, 33.8, and 43.4 microM) had a stronger inhibitory effect on MAO activity than did I1-ligands (efaroxan, rilmenidine, clonidine, and moxonidine: IC50 = 277, 801, 1,224, and > 10,000 microM). Substances with the highest inhibitory effects were BDF8082 (IC50 = 1.7 microM) and 2-(2-benzofuranyl)-2-imidazoline (BFI; IC50 = 4.0 microM). The enzyme is inhibited noncompetitively and is reversible, because its activity is completely or partially restored after dialysis. Agmatine, the putative endogenous ligand for IBS, also decreased MAO activity (IC50 = 168 microM), whereas its precursor, L-arginine, and its metabolite, putrescine, had no effects. In vitro inhibition of MAO and mitochondrial respiration by the IBS-ligands tested could not be correlated, suggesting no link between the function of the inner and outer mitochondrial membrane. MAO activity in vivo was significantly reduced only by pargyline (-95%), BDF8082 (-68%), BFI (-43%), and 1-(m-chlorophenyl)-biguanide (-28%). Catecholamine content of livers obtained from animals treated with different IBS-ligands was consequently increased. In conclusion, the strong inhibitory effects of I2 selective imidazoline ligands confirm the existence of I2-BS as a regulatory site on MAO.

The I1-imidazoline receptor and its cellular signaling pathways

Two primary questions are addressed. First, do I1-imidazoline binding sites fulfill all the essential criteria for identification as a true receptor? Second, what are the cellular signaling pathways coupled to this novel receptor? I1-imidazoline binding sites show specificity in binding assays, linkage to physiologic functions, appropriate anatomic, and cellular and subcellular localization. Most important, binding affinities correlate with functional drug responses. I1-imidazoline binding sites meet several additional criteria identified with functional receptors: they show physiologic regulation and endogenous ligands and, most crucially, are coupled to cellular signaling events. A series of studies have identified cellular events triggered by I1-imidazoline receptor occupancy. This receptor is not coupled to conventional pathways downstream of heterotrimeric G-proteins, such as activation or inhibition of adenylyl or guanylyl cyclases, stimulation of inositol phospholipid hydrolysis, or induction of rapid calcium fluxes. The I1-imidazoline receptor is coupled to choline phospholipid hydrolysis, leading to the generation of diacylglyceride, arachidonic acid, and eicosanoids. Additional cellular responses include inhibition of Na+/H+ exchange and induction of genes for catecholamine synthetic enzymes. The signaling pathways linked to the I1-imidazoline receptor are similar to those of the interleukin family, implying that I1-receptors may belong to the family of neurocytokine receptors.

Pharmacologic and molecular discrimination of I2-imidazoline receptor subtypes

I2-imidazoline receptors (I2-IR) are characterized by their high affinity for imidazolines and guanidines and medium affinity for imidazolidines. The differential recognition of I2-IR by amiloride led to subtype these sites as amiloride-sensitive (I2A-IR) and amiloride-insensitive (I2B-IR). I2-IR labeled with [3H]idazoxan or [3H]2-BFI in the rabbit cerebral cortex (I2A-IR) displayed higher affinities for amiloride and amiloride analogs than in the rat cerebral cortex (I2B-IR). Other drugs tested displayed biphasic curves in competition experiments, indicating the existence of high and low affinity sites for both I2-IR subtypes. The drugs (+)- and (-)-medetomidine, bromoxidine, moxonidine, and clorgyline were more potent on the high and/or low affinity sites of I2B-IR than on I2A-IR. Preincubation (30 min at 25 degrees C) with 10(-6) M isothiocyanatobenzyl imidazoline (IBI) or with 10(-6) M clorgyline reduced by 40% and 26%, respectively, the binding of [3H]2-BFI to I2B-IR, but it did not alter the binding of the radioligand to I2A-IR. These results indicated that the I2-IR subtypes differ in their pharmacologic profiles and in the nature of the imidazoline binding site involved in clorgyline and IBI alkylation. In rat cortical membranes, western blot detection of immunoreactive imidazoline receptor proteins revealed a double band of approximately 29/30 kD and three less intense bands of approximately 45, approximately 66, and approximately 85 kD. In rabbit cortical membranes the antibody detected proteins of approximately 30, approximately 57, approximately 66, and approximately 85 kD. It is suggested that I2-IR may be related to more than one receptor protein and that I2-IR subtypes differ in the nature of the proteins implicated.

Imidazoline recognition sites and stomach function

Radioligand binding experiments carried out in cell membranes from rat and human stomach revealed the existence of non-adrenoceptor [3H]clonidine and [3H]idazoxan binding sites and of [3H]DTG (1,2-di-(2-tolyl)guanidine) binding sites. In rat stomach, specific binding was inhibited by imidazolines and guanidines and by non-imidazoline sigma-site ligands, respectively, at different rank orders of affinity, suggesting the existence of non-I1/non-I2 [3H]clonidine binding sites, I2-imidazoline binding sites as well as sigma 2-like-sites. These sites are not directly related to a postsynaptic contractile effect on rat gastric smooth muscle or to acid release from isolated gastric glands. Finally, we demonstrated that the gastric pathogen Helicobacter pylori is able to form and to release the endogenous imidazoline receptor ligand agmatine and that considerable amounts of agmatine are present in human gastric juice. The quantities of agmatine were higher in gastric juice from H. pylori-positive than H. pylori-negative patients.

Plasma agmatine and platelet imidazoline receptors in depression

Plasma agmatine concentrations are elevated significantly in depressed patients compared to healthy controls. Treatment with the antidepressant bupropion normalized plasma agmatine levels. Correlational evidence is presented that a change in plasma agmatine levels may lead to similar changes in platelet I1 imidazoline receptors.

Agmatine: an endogenous ligand at imidazoline receptors is a novel neurotransmitter

Agmatine, an amine and organic cation, is an endogenous ligand at alpha 2-adrenergic and imidazoline (I-) receptors, to which it binds with high affinity. In addition, agmatine has properties of an endogenous neurotransmitter. Thus, agmatine (a) is locally synthesized in brain by a specific enzyme, arginine decarboxylase; (b) is stored in a large number of neurons with selective distribution in the CNS; (c) is associated with small vesicles in axon terminals that, at least in hippocampus, make synaptic asymmetric (excitatory) synapses on pyramidal cells; (d) is released from synaptosomes in a Ca(2+)-dependent manner; (e) can be enzymatically degraded by agmatinase in synaptosomes; (f) can be inactivated by selective reuptake; (g) blocks the ligand-gated NMDA receptor channel at sites distinct from ligand-binding and polyamine sites; and (h) has systemic actions when administered intraventricularly. Additionally, (i) agmatine is a precursor of brain putrescine and, hence, of higher polyamines, and (j) it competitively inhibits the activity of all isozymes of nitric oxide synthase. Agmatine meets most criteria to establish it as a novel neurotransmitter/neuromodulator in the CNS. However, agmatine differs from forms of clonidine displacing system with respect to distribution, bioactivity, and capacity to interact with antibodies raised to imidazoline-like drugs. Thus, there are multiple endogenous ligands of the imidazoline receptors, one of which is agmatine.

Pharmacologic characterization of imidazoline receptor proteins identified by immunologic techniques and other methods

Biochemical and pharmacologic evidence supports the heterogeneous nature of imidazoline receptors (IRs). However, only monoamine oxidase (MAO) (55- and 61-kD) isozymes have been identified as imidazoline binding site-containing proteins. Idazoxan-binding proteins of approximately 70- and approximately 45-kD of unknown amino acid sequences have been isolated from chromaffin cells and rat brain, respectively. Other proteins of approximately 27-30 to > 80 kD have been visualized by immunologic and photoaffinity labeling techniques in different tissues and species. The specific antiserum that recognizes the approximately 70-, approximately 45-, and approximately 29-kD IR proteins, but not MAO, was used to quantitate these proteins in the rat brain cortex. Treatments (7 days) with the I2-selective imidazoline drugs idazoxan (10 mg/kg), cirazoline (1 mg/kg), and LSL 60101 ([2-(2-benzofuranyl) imidazole; 10 mg/kg]) induced differential changes in these proteins: levels of the approximately 29-kD IR were increased by idazoxan and LSL 60101 (23%), levels of the approximately 45-kD protein only by cirazoline (44%), and those of the approximately 66-kD protein only by idazoxan (50%). These treatments also increased the densities of [3H]-idazoxan (I2) binding sites (32-42%). Chronic treatment with efaroxan, RX821002, and yohimbine (10 mg/kg), which possess very low affinity for I2-IRs, did not alter either their immunoreactivities or the density of I2 sites. Chronic treatment with MAO inhibitors clorgyline and phenelzine (10 mg/kg) and acute treatment with EEDQ (1.6 mg/kg, 6 h) induced decreases in the levels of these IR proteins (17-47%) and I2 sites (31-57%). Significant correlations were found when the mean percentage changes in immunoreactivity of IR proteins were related to the mean percentage changes in the density of I2 sites after treatment with the foregoing drug (r = 0.92, r = 0.69, and r = 0.75 for the approximately 29-, approximately 45-, and approximately 66-kD proteins, respectively). These results indicate that in the rat cerebral cortex, the I2 sites labeled by [3H]idazoxan are heterogeneous and that the related immunoreactive IR proteins contribute differently to the modulation of I2 sites after drug treatment.

Mapping glutamatergic drive in the vertebrate retina with a channel-permeant organic cation

Patterns of neuronal excitation in complex populations can be mapped anatomically by activating ionotropic glutamate receptors in the presence of 1-amino-4-guanidobutane (AGB), a channel-permeant guanidinium analogue. Intracellular AGB signals were trapped with conventional glutaraldehyde fixation and were detected by probing registered serial thin sections with anti-AGB and anti-amino acid immunoglobulins, revealing both the accumulated AGB and the characteristic neurochemical signatures of individual cells. In isolated rabbit retina, both glutamate and the ionotropic glutamate receptor agonists alpha-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA), kainic acid (KA), and N-methyl-D-aspartic acid (NMDA) activated permeation of AGB into retinal neurons in dose-dependent and pharmacologically specific modes. Horizontal cells and bipolar cells were dominated by AMPA/KA receptor activation with little or no evidence of NMDA receptor involvement. Strong NMDA activation of AGB permeation was restricted to subsets of the amacrine and ganglion cell populations. Threshold agonist doses for the most responsive cell groups (AMPA, 300 nm; KA, 2 microM; NMDA, 63 microm; glutamate, 1 mM) were similar to values obtained from electrophysiological and neurotransmitter release measures. The threshold for activation of AGB permeation by exogenous glutamate was shifted to <200 microM in the presence of the glutamate transporter antagonist dihydrokainate, indicating substantial spatial buffering of extracellular glutamate levels in vitro. Agonist-activated permeation of AGB into neurons persisted under blockades of Na+ -dependent transporters, voltage-activated Ca2+ and Na+ channels, and ionotropic gamma-aminobutyric acid and glycine receptors. Cholinergic agonists evoked no permeation.

Effect of intrathecal agmatine on inflammation-induced thermal hyperalgesia in rats

Agmatine, an endogenous ligand, interacts both with the alpha2-adrenoceptors and with the imidazoline binding sites. The effect of intrathecally administered agmatine on carrageenan-induced thermal hyperalgesia was investigated by means of a paw-withdrawal test in rats. The effect of agmatine on morphine-induced anti-hyperalgesia was also studied. Intrathecal agmatine in doses larger than 250 microg caused a decrease in the pain threshold, with vocalization and agitation lasting for several hours in all animals. Agmatine alone at 1-100 microg did not give rise to any change in the thermal withdrawal threshold in the contralateral non-inflamed paw. Agmatine pretreatment was found to dose-dependently attenuate the thermal hyperalgesia induced by intraplantar carrageenan. The effect of 100 microg agmatine was completely lost by 60 min, whereas the effect of 50 microg was of similar magnitude but exhibited a longer duration. Agmatine posttreatment had a slighter effect. Agmatine pretreatment (100 microg) together with 1 microg morphine (subeffective dose) has significantly higher anti-hyperalgesic effect then the individual compounds by themselves. These are the first data demonstrating the behavioral and anti-hyperalgesic effects of intrathecal agmatine. The results reveal important interactions between intrathecal agmatine and opioids in thermal hyperalgesia.

Agmatine modulates polyamine content in hepatocytes by inducing spermidine/spermine acetyltransferase

Agmatine has been proposed as the physiological ligand for the imidazoline receptors. It is not known whether it is also involved in the homoeostasis of intracellular polyamine content. To show whether this is the case, we have studied the effect of agmatine on rat liver cells, under both periportal and perivenous conditions. It is shown that agmatine modulates intracellular polyamine content through its effect on the synthesis of the limiting enzyme of the interconversion pathway, spermidine/spermine acetyltransferase (SSAT). Increased SSAT activity is accompanied by depletion of spermidine and spermine, and accumulation of putrescine and N1-acetylspermidine. Immunoblotting with a specific polyclonal antiserum confirms the induction. At the same time S-adenosylmethionine decarboxylase activity is significantly increased, while ornithine decarboxylase (ODC) activity and the rate of spermidine uptake are reduced. This is not due to an effect on ODC antizyme, which is not significantly changed. All these modifications are observed in HTC cells also, where they are accompanied by a decrease in proliferation rate. SSAT is also induced by low oxygen tension which mimics perivenous conditions. The effect is synergic with that promoted by agmatine.

alpha-2 and imidazoline receptor agonists. Their pharmacology and therapeutic role

Clonidine has proved to be a clinically useful adjunct in clinical anaesthetic practice as well as in chronic pain therapy because it has both anaesthetic and analgesic-sparing activity. The more selective alpha-2 adrenoceptor agonists, dexmedetomidine and mivazerol, may also have a role in providing haemodynamic stability in patients who are at risk of peri-operative ischaemia. The side-effects of hypotension and bradycardia have limited the routine use of alpha-2 adrenoceptor agonists. Investigations into the molecular pharmacology of alpha-2 adrenoceptors have elucidated their role in the control of wakefulness, blood pressure and antinociception. We discuss the pharmacology of alpha-2 adrenoceptors and their therapeutic role in this review. The alpha-2 adrenoceptor agonists are agonists at imidazoline receptors which are involved in central blood pressure control. Selective imidazoline agonists are now available for clinical use as antihypertensive agents and their pharmacology is discussed.

Stimulation of locus coeruleus neurons by non-I1/I2-type imidazoline receptors: an in vivo and in vitro electrophysiological study

1. Imidazoline binding sites have been reported to be present in the locus coeruleus (LC). To investigate the role of these sites in the control of LC neuron activity, we studied the effect of imidazolines using in vivo and in vitro single-unit extracellular recording techniques. 2. In anaesthetized rats, local (27 pmoles) and systemic (1 mg kg(-1), i.v.) administrations of 2-(2-benzofuranyl)-2-imidazoline (2-BFI), a selective I-imidazoline receptor ligand, increased the firing rate of LC cells (maximal increase: 22+/-5%, P<0.001 and 16+/-7%, P<0.001 respectively). Chronic pretreatment with the irreversible monoamine oxidase inhibitor clorgyline (3 mg kg(-1), i.p., every 12 h for 14 days) abolished this effect. 3. In rat midpontine brain slices containing the LC, bath application (1 mM) of the imidazolines 2-BFI, 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224), idazoxan, efaroxan, phentolamine and (2-2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline (RX821002) reversibly stimulated LC cells. The maximal effect was approximately 90% except for RX821002 and efaroxan which induced smaller maximal effects (approximately 58% and approximately 35% respectively). Simultaneous application of idazoxan and 2BFI did not lead to additive effects. 4. Bath application of the alpha2-adrenoceptor antagonists, yohimbine (1 - 10 microM) and N-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) (10 microM), failed to modify LC activity. The irreversible blockade of alpha2-adrenoceptors with EEDQ (10 microM) did not alter the effect of idazoxan or that of efaroxan. Previous application of clorgyline (10 microM) did not modify the excitatory effect of 2-BFI or efaroxan. 5. Changes in the pH of the bathing solution (6.84-7.84) did not influence the effect caused by idazoxan. Bath application of 2-BFI (1 mM) reversed the inhibition induced by diazoxide (300 microM), an ATP-sensitive K+ channel opener, whereas application of glibenclamide (3 microM), an ATP-sensitive K+ channel blocker, partially blocked the effect of 2-BFI. 6. This study shows that imidazoline compounds stimulate the firing rate of LC neurons. This effect is not mediated by alpha2-adrenoceptors nor by I1 or I2-imidazoline receptors but involves a different subtype of imidazoline receptor. Our results indicate that this receptor is located extracellularly and modulates ATP-sensitive K+ channels.

Arginine metabolism: nitric oxide and beyond

Arginine is one of the most versatile amino acids in animal cells, serving as a precursor for the synthesis not only of proteins but also of nitric oxide, urea, polyamines, proline, glutamate, creatine and agmatine. Of the enzymes that catalyse rate-controlling steps in arginine synthesis and catabolism, argininosuccinate synthase, the two arginase isoenzymes, the three nitric oxide synthase isoenzymes and arginine decarboxylase have been recognized in recent years as key factors in regulating newly identified aspects of arginine metabolism. In particular, changes in the activities of argininosuccinate synthase, the arginases, the inducible isoenzyme of nitric oxide synthase and also cationic amino acid transporters play major roles in determining the metabolic fates of arginine in health and disease, and recent studies have identified complex patterns of interaction among these enzymes. There is growing interest in the potential roles of the arginase isoenzymes as regulators of the synthesis of nitric oxide, polyamines, proline and glutamate. Physiological roles and relationships between the pathways of arginine synthesis and catabolism in vivo are complex and difficult to analyse, owing to compartmentalized expression of various enzymes at both organ (e.g. liver, small intestine and kidney) and subcellular (cytosol and mitochondria) levels, as well as to changes in expression during development and in response to diet, hormones and cytokines. The ongoing development of new cell lines and animal models using cDNA clones and genes for key arginine metabolic enzymes will provide new approaches more clearly elucidating the physiological roles of these enzymes.

Cardiovascular effects of intravenous administration of clonidine in conscious cats

We previously reported that cardiovascular effects elicited by intracerebroventricular (i.c.v.) administration of clonidine result from stimulation of central alpha2-adrenergic and/or H2-histaminergic receptors, but not via activation of I1-imidazoline receptors in conscious cats. In this study, we investigated the effects on arterial blood pressure (MAP) and heart rate (HR) following an intravenous (i.v.) administration of clonidine using conscious cats. Injection of clonidine (2-10 microgram/kg i.v.) elicited a decrease in mean arterial pressure (MAP) and heart rate (HR) dose-dependently. The dose of 10 microgram/kg of clonidine decreased MAP and HR by 30+/-4 mmHg and 62+/-15 bpm, respectively. Intravenous or i.c.v. pretreatment with yohimbine, the alpha2-adrenoceptor and 5-HT1A receptor antagonist, blocked the cardiovascular responses to a subsequent i.v. injection of 10 microgram/kg clonidine. However, i.v. or i.c.v. preadministration of cimetidine, the H2-histamine receptor antagonist, failed to antagonize the decreases in MAP and HR to a subsequent i.v. injection of 10 microgram/kg clonidine. In addition, i.c.v. or i.v. pretreatment with the I1-imidazoline receptor blocker, efaroxan, failed to inhibit the cardiovascular effects of an i.v. administration of clonidine. These results demonstrate that i.v. clonidine evokes decreases in MAP and HR possibly via central alpha2-adrenoceptor and/or 5-HT1A receptors and not through H2-histamine or I1-imidazoline receptors.

Agmatine: an endogenous ligand at imidazoline receptors may be a novel neurotransmitter in brain

Agmatine, which in other life forms serves as a metabolic intermediate for polyamine biosynthesis, appears to have properties in mammals consistent with its actions as a neurotransmitter/neuromodulator. Thus, agmatine is synthesized unequally in brain by arginine decarboxylase (ADC); is stored in neurons and axon terminals with a heterogeneous distribution; is released from synaptosomes by depolarization; is enzymatically converted by agmatinase to putrescine; interacts not only with alpha2-adrenergic and I-receptors in the CNS, but also may selectively block NMDA receptor channels; and, when administered centrally, has several potent biological actions. Clarification of its role in normal brain function, however, has not yet been fully established, in part because of the absence of agents that selectively affect its biosynthesis or degradation.

Recent advances in imidazoline receptor research: ligands--localization and isolation--signaling--functional and clinical studies

In this article we outline the highlights of this special issue of the journal containing a series of articles covering many aspects of current interest in the field of imidazoline receptor research. This volume is the result of an international symposium held in September 1997 in Melbourne as an official satellite of the inaugural meeting of the International Society of Autonomic Neurosciences held in Cairns, Australia. A wide range of topics relating to imidazoline receptors were canvassed, including endogenous and synthetic ligands, identification and localisation of binding sites, putative transduction mechanisms and experimental and clinical functional studies.

Isolation and partial structure determination of a clonidine-displacing substance from bovine lung and brain

A large scale extraction and isolation method was developed for the purification of clonidine-displacing substance (CDS) activity from bovine lung or brain. This optimised method used direct freeze drying of tissue, hexane removal of lipids, and methanol extraction of CDS activity. Using a bioassay directed isolation strategy a new CDS compound was purified from an extract of bovine lung. The isolation strategy involved subsequent steps of flash C-18 chromatography, ion exchange, size exclusion, and C-18 HPLC. An HPLC detection method was developed and applied to show that the new CDS is present in both lung and brain tissue. Spectroscopic data for this new CDS indicates that it is related to guanosine, but is not noradrenaline, adrenaline, histamine, agmatine, guanosine, GMP, GDP or GTP.

Arachidonic acid release from PC12 pheochromocytoma cells is regulated by I1-imidazoline receptors

Rat PC 12 pheochromocytoma cells lack alpha2-adrenergic receptors but express plasma membrane I1-imidazoline receptors. In response to the I1-agonist moxonidine, diglycerides are generated via phosphatidylcholine-selective phospholipase C, and prostaglandin E2 is released. This report characterizes I-receptor-mediated release of arachidonic acid, the precursor to the prostaglandins. PC12 cells were incubated with [3H]arachidonic acid for 24 h and superfused with 0.01% bovine serum albumin in Krebs' physiological buffer at 1 ml/min. Calcium ionophore increased arachidonic acid release only marginally, implying that in PC12 cells arachidonic acid release is not driven by calcium. The I1-agonist moxonidine at concentrations between 10 nM and 1.0 microM rapidly elicited up to two-fold increases in [3H]arachidonic acid release. Guanabenz, a potent alpha2-agonist and I2-ligand, had no effect. The selective I1-antagonist efaroxan blocked the action of moxonidine. The phospholipase A2 inhibitor aristolochic acid had no effect, suggesting that arachidonic acid release may be through an indirect pathway, possibly involving diglycerides. Thus, I1-imidazoline receptors in PC12 cells are coupled to arachidonic acid release through an as yet unknown pathway.

Characterization of a partial cDNA clone detected by imidazoline receptor-selective antisera

A cDNA clone has been isolated from a human hippocampal cDNA expression library by relying on the selectivity of two antisera that are specific for imidazoline binding proteins. A 1789 bp cDNA clone was sequenced and shown to contain a single open-reading frame that predicts a 66 kDa polypeptide, but it is truncated based on its lack of a stop codon and poly-A+ tail. Two regions of homology exist for the predicted amino acid sequence in common with chromogranin-A and B proteins, a zinc finger protein, and the ryanodine receptor. Northern blot analyses of poly-A+ mRNA from 36 human tissues indicated two differentially expressed transcripts of 6.0 and 9.5 kb. The 6.0 kb mRNA form was enriched in brain and endocrine tissues as compared to other tissues, but not in strict concordance with I1-imidazoline binding sites. The highest overall amounts of the combined transcripts were found in pituitary. In situ hybridization histochemistry revealed an enrichment of the message in neuronal cell bodies of the rat hippocampus and cerebellar cortex. This clone has some of the properties expected of an imidazoline receptor.

Does a second generation of centrally acting antihypertensive drugs really exist?

The site of the hypotensive action of imidazoline compounds, such as clonidine, was first identified within the rostroventrolateral part of the brainstem: the nucleus reticularis lateralis. After that, it was shown that imidazolines and related substances reduced blood pressure when applied in this area whereas catecholamines were not capable of producing such an effect. These data led us to suggest the existence of receptors specific for imidazoline-like compounds different from the alpha2-adrenoceptors. Soon after, the existence of imidazoline binding sites was reported in the brain and in a variety of peripheral tissues including the human kidney. As expected, these specific binding sites do not bind the catecholamines. The imidazoline binding sites are already subclassified in two groups: the I1-subtype sensitive to clonidine and idazoxan, and the I2-subtype, sensitive to idazoxan and nearly insensitive to clonidine. Functional studies confirmed that the hypotensive effects of clonidine-like drugs involved imidazoline receptors while their most frequent side effects only involved alpha2-adrenoceptors. However, recent functional evidence suggests that a cross talk between imidazoline receptors and alpha2-adrenoceptors is necessary to trigger a hypotensive effect within the ventral brainstem. Rilmenidine and Moxonidine are the leader compounds of a new class of antihypertensive drugs selective for imidazoline receptors. At hypotensive doses, these drugs are devoid of significant sedative effect. Rilmenidine evoked hypotension when injected within the nucleus reticularis lateralis region; it competed for [3H]-clonidine bound to specific imidazoline binding sites in human medullary membrane preparations but proved more selective for cerebral imidazoline receptors than clonidine. It is suggested that this selectivity might explain the low incidence of their side effects. Additional potentially beneficial actions on cardiac arrhythmias or congestive heart failure enlarge the therapeutic interest of imidazoline-related drugs. Recent binding and functional data throw a new light on the optimal pharmacological profile of this second generation of centrally acting antihypertensive drugs.

Clonidine-displacing substance and its putative role in control of insulin secretion: a minireview

1. Imidazoline-binding sites, or I-sites, are a class of recently defined nonadrenoceptor recognition sites whose most potent ligands are imidazolines and related compounds. 2. The pancreatic islet beta-cell I-site, which mediates imidazoline-induced stimulation of insulin release, appears to be the first site to be pharmacologically defined with selective agonists and antagonists. 3. The natural ligand for imidazoline recognition sites is still unknown. The strongest candidate is clonidine-displacing substance (CDS), originally identified in extracts of rat and bovine brain. However, the bioactive molecule has not been identified definitively. Agmatine, a decarboxylated derivative of arginine, also binds to both I-sites and alpha2-adrenoceptors (Li et al., 1994), and is, by definition, a CDS molecule. 4. In the endocrine pancreas, agmatine is a weak insulin secretagogue, which induces a slowly developing secretory response. However, this profile does not correlate with interaction at the islet I-site, and thus agmatine is unlikely to be an endogenous secretagogue acting functionally at the islet I-site. 5. Crude preparations of CDS from rat brain can potentiate glucose-induced insulin release and reverse the effects of diazoxide in rat and human islets of Langerhans. These two effects are also subject to blockade by the imidazoline antagonists RX801080 and KU14R. Furthermore, islets that were desensitized to the effects of the imidazoline secretagogue efaroxan (after 18-hr culture with imidazoline) were refractory to the actions of CDS. 6. Overall, CDS displays many characteristics expected of an endogenous regulator of insulin secretion acting through the islet beta-cell imidazoline site. This evidence strengthens the hypothesis that the islet beta-cell imidazoline site mediating control of insulin release in the endocrine pancreas is a biologically relevant receptor. Furthermore, a physiological role of CDS in the endocrine pancreas cannot be excluded.

Attenuation of tolerance to opioid-induced antinociception and protection against morphine-induced decrease of neurofilament proteins by idazoxan and other I2-imidazoline ligands

1. Agmatine, the proposed endogenous ligand for imidazoline receptors, has been shown to attenuate tolerance to morphine-induced antinociception (Kolesnikov el al., 1996). The main aim of this study was to assess if idazoxan, an alpha2-adrenoceptor antagonist that also interacts with imidazoline receptors, could also modulate opioid tolerance in rats and to establish which type of imidazoline receptors (or other receptors) are involved. 2. Antinociceptive responses to opioid drugs were determined by the tail-flick test. The acute administration of morphine (10 mg kg(-1), i.p., 30 min) or pentazocine (10 mg kg(-1), i.p., 30 min) resulted in marked increases in tail-flick latencies (TFLs). As expected, the initial antinociceptive response to the opiates was lost after chronic (13 days) treatment (tolerance). When idazoxan (10 mg kg(-1), i.p.) was given chronically 30 min before the opiates it completely prevented morphine tolerance and markedly attenuated tolerance to pentazocine (TFLs increased by 71-143% at day 13). Idazoxan alone did not modify TFLs. 3. The concurrent chronic administration (10 mg kg(-1), i.p., 13 days) of 2-BFI, LSL 60101, and LSL 61122 (valldemossine), selective and potent I2-imidazoline receptor ligands, and morphine (10 mg kg(-1), i.p.), also prevented or attenuated morphine tolerance (TFLs increased by 64 172% at day 13). This attenuation of morphine tolerance was still apparent six days after discontinuation of the chronic treatment with LSL 60101-morphine. The acute treatment with these drugs did not potentiate morphine-induced antinociception. These drugs alone did not modify TFLs. Together, these results indicated the specific involvement of I2-imidazoline receptors in the modulation of opioid tolerance. 4. The concurrent chronic (13 days) administration of RX821002 (10 mg kg(-1), i.p.) and RS-15385-197 (1 mg kg(-1), i.p.), selective alpha2-adrenoceptor antagonists, and morphine (10 mg kg(-1), i.p.), did not attenuate morphine tolerance. Similarly, the concurrent chronic treatment of moxonidine (1 mg kg(-1), i.p.), a mixed I(1)-imidazoline receptor and alpha2-adrenoceptor agonist, and morphine (10 mg kg(-1), i.p.), did not alter the development of tolerance to the opiate. These results discounted the involvement of alpha2-adrenoceptors and I(1)-imidazoline receptors in the modulatory effect of idazoxan on opioid tolerance. 5. Idazoxan and other imidazol(ine) drugs fully inhibited [3H]-(+)-MK-801 binding to N-methyl-D-aspartate (NMDA) receptors in the rat cerebral cortex with low potencies (Ki: 37-190 microM). The potencies of the imidazolines idazoxan, RX821002 and moxonidine were similar, indicating a lack of relationship between potency on NMDA receptors and ability to attenuate opioid tolerance. These results suggested that modulation of opioid tolerance by idazoxan is not related to NMDA receptors blockade. 6. Chronic treatment (13 days) with morphine (10 mg kg(-1), i.p.) was associated with a marked decrease (49%) in immunolabelled neurofilament proteins (NF-L) in the frontal cortex of morphine-tolerant rats, suggesting the induction of neuronal damage. Chronic treatment (13 days) with idazoxan (10 mg kg(-1)) and LSL 60101 (10 mg kg(-1)) did not modify the levels of NF-L proteins in brain. Interestingly, the concurrent chronic treatment (13 days) of idazoxan or LSL 60101 and morphine, completely reversed the morphine-induced decrease in NF-L immunoreactivity, suggesting a neuroprotective role for these drugs. 7. Together, the results indicate that chronic treatment with I2-imidazoline ligands attenuates the development of tolerance to opiate drugs and may induce neuroprotective effects on chronic opiate treatment. Moreover, these findings offer the I2-imidazoline ligands as promising therapeutic coadjuvants in the management of chronic pain with opiate drugs.

'Seeing through a glass darkly': casting light on imidazoline 'I' sites

Although imidazoline sites have been the subject of research for several years, there is still controversy about their structure, diversity and physiology. The I1 site is thought to exist principally as a binding site and is widely purported to play a role in controlling systemic blood pressure, although this is still unclear. The majority of I2 sites are widely accepted as being allosteric sites on monoamine oxidase; however, even with selective ligands, their exact function remains to be determined. A putative I3 site modulates insulin secretion and could represent the first functional site to be pharmacologically defined with selective agonists and antagonists. The structure and relevance of the proposed endogenous ligand 'clonidine-displacing substance' remains elusive. A potential candidate for this substance is agmatine; however, although it is capable of displacing bound clonidine from imidazoline sites, it lacks the functionality ascribed to the clonidine-displacing substance. In this review, Richard M. Eglen and colleagues assess our knowledge of imidazoline sites in the light of recent data.

Presynaptic imidazoline receptors mediate inhibition of noradrenaline release from sympathetic nerves in rat blood vessels

In rat vena cava and aorta preincubated with [3H]noradrenaline the involvement of imidazoline receptors in modulation of [3H]noradrenaline release from sympathetic nerves was investigated. In the vena cava, the guanidine 1,3-di(2-tolyl)guanidine (DTG) inhibited the electrically evoked [3H]noradrenaline release; the inhibitory effect was more pronounced in the presence than in the absence of the alpha 2-adrenoceptor antagonist rauwolscine. The concentration-response curves of BDF 6143 [4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline], and idazoxan for their facilitatory effect on electrically evoked [3H]noradrenaline release was bell-shaped; in the presence of rauwolscine, BDF 6143 inhibited the evoked [3H]noradrenaline release, whereas idazoxan did not. After blockade of alpha 2-autoreceptors by rauwolscine, the electrically evoked [3H]noradrenaline release from vena cava was inhibited not only by DTG and BDF 6143 but also by aganodine, clonidine and cirazoline; the rank order of potency of most of the drugs was similar to that found at the presynaptic imidazoline receptors in the rabbit aorta and pulmonary artery as well as in human atrial appendages. In the presence of rauwolscine, clonidine-induced inhibition of electrically evoked [3H]noradrenaline release was counteracted by 1 microM of the selective CB1 receptor antagonist SR141716A (N-[piperidin-1-yl]-5-[4-chlorophenyl]-1-[2,4-dichlorophenyl] -4-methyl-1H-pyrazole-3-carboxamide). In the aorta, BDF 6143 and cirazoline did not modify [3H]noradrenaline release in the absence of alpha 2-adrenoceptor blockade; in the presence of rauwolscine, the electrically evoked [3H]noradrenaline release from aorta was inhibited by BDF 6143, cirazoline, aganodine and clonidine with a rank order of potency similar to that in the vena cava. SR141716A 1 microM antagonized the inhibitory effect of BDF 6143 and clonidine (in the presence of rauwolscine). In conclusion, noradrenaline release in rat vena cava and aorta is inhibited via presynaptic imidazoline receptors which appear to be related to those previously characterized in rabbit and human cardiovascular tissue.

1,5-Bis(4-amidinophenoxy)pentane (pentamidine) is a potent inhibitor of [3H]idazoxan binding to imidazoline I2 binding sites

The aromatic diamidine 1,5-bis(4-amidinophenoxy)pentane (pentamidine) is used for treatment and prophylaxis of Pneumocystis carinii pneumonia in patients with Acquired Immune Deficiency Syndrome. Clinical use of pentamidine has been restricted by significant toxicity, that includes hypotension, and hypoglycemia. Although clinical toxicity is well described, the mechanisms are still poorly understood. Competitive binding analyses using [3H]idazoxan as the radioligand, and cirazoline to define non-specific binding, demonstrate that pentamidine binds to an imidazoline I2 binding site on rat liver membranes with a Ki of 1.4+/-0.22 nM. The Ki indicates that pentamidine inhibits radioligand binding at imidazoline I2 sites with an affinity approximating the most potent known ligands and may be related to pentamidine toxicity. Moreover, pentamidine analogs inhibit radioligand binding with a range of affinities that vary according to their structure. Two candidate drugs, Compounds 5 and 6, are more active than pentamidine in the corticosteroid-suppressed rat model of P. carinii pneumonia, yet have different affinities for the imidazoline I2 site (Ki 5 = 50.1+/-1.06 nM and Ki 6 = approximately 3500 nM). Affinity for this site does not correlate with antimicrobial activity (r = 0.60; p = 0.09) or the calculated log of the octanol:water partition coefficient (ClogP) (r = -0.38; p = 0.22).

Characterisation and localisation of [3H]2-(2-benzofuranyl)-2-imidazoline binding in rat brain: a selective ligand for imidazoline I2 receptors

In rat whole brain homogenates, saturation binding analysis revealed that both [3H]2-BFI (2-(2-benzofuranyl)-2-imidazoline) and [3H]idazoxan (in the presence of 5 microM rauwolscine) bound with high affinity to an apparent single population of sites. However, the Kd for [3H]2-BFI (1.74+/-0.14 nM) was significantly (P < 0.01) less than that for [3H]idazoxan (10.4+/-2.68 nM). In competition studies idazoxan, 2-BFI, BU224 (2-(4,5-dihydroimidaz-2-yl)-quinoline), amiloride and guanabenz displayed high affinity (Ki values = 7.32, 1.71, 2.08, 21.80 and 14.90 nM, respectively) for 70-80% of sites, and low microM affinity for the remaining 20-30% of sites labelled by [3H]2-BFI. In contrast, several alpha2-adrenoceptor, imidazoline I1 receptor and histamine receptor ligands exhibited only micromolar affinity for the [3H]2-BFI labelled site. Quantitative receptor autoradiography revealed high binding by [3H]2-BFI to discrete brain nuclei, notably the area postrema, interpeduncular nucleus, arcuate nucleus, mammillary peduncle, ependyma and pineal gland. These data indicate that [3H]2-BFI recognises imidazoline I2 receptors in rat brain with higher affinity and selectivity than [3H]idazoxan and thus represents a superior radioligand to [3H]idazoxan for the study of imidazoline I2 receptors.

Agmatine containing axon terminals in rat hippocampus form synapses on pyramidal cells

We examined the cellular and subcellular localization of agmatine in the hippocampal CA1 region by immunocytochemistry. By light microscopy, agmatine-like immunoreactivity (agmatine-LI) was found primarily in the perikarya and dendritic profiles of pyramidal cells and in punctate processes preponderantly in stratum radiatum. Electron microscopy revealed that agmatine-LI was cytoplasmic and concentrated in 'clusters' associated with mitochondria and tubular vesicles. In stratum radiatum, agmatine-LI was primarily in axons and axon terminals associated with small, synaptic vesicles. The terminals almost exclusively formed asymmetric synapses on the spines of dendrites, many of which originated from pyramidal cells. Some agmatine-LI also was present in shafts and spines of pyramidal cell dendrites and in astrocytic processes. The results demonstrate that agmatine in the hippocampus is found primarily in terminals forming excitatory (asymmetric) synapses on pyramidal cells, some of which contain agmatine-LI. These findings further implicate agmatine as an endogenous neurotransmitter which may be co-stored with L-glutamate and may act in part in the rat hippocampus as a blocker of the N-methyl-D-aspartate receptor and nitric oxide synthase.

Agmatine suppresses proliferation by frameshift induction of antizyme and attenuation of cellular polyamine levels

Polyamines are required for entry and progression of the cell cycle. As such, augmentation of polyamine levels is essential for cellular transformation. Polyamines are autoregulated through induction of antizyme, which represses both the rate-limiting polyamine biosynthetic enzyme ornithine decarboxylase and cellular polyamine transport. In the present study we demonstrate that agmatine, a metabolite of arginine via arginine decarboxylase (an arginine pathway distinct from that of the classical polyamines), also serves the dual regulatory functions of suppressing polyamine biosynthesis and cellular polyamine uptake through induction of antizyme. The capacity of agmatine to induce antizyme is demonstrated by: (a) an agmatine-dependent translational frameshift of antizyme mRNA to produce a full-length protein and (b) suppression of agmatine-dependent inhibitory activity by either anti-antizyme IgG or antizyme inhibitor. Furthermore, agmatine administration depletes intracellular polyamine levels to suppress cellular proliferation in a transformed cell line. This suppression is reversible with polyamine supplementation. We propose a novel regulatory pathway in which agmatine acts as an antiproliferative molecule and potential tumor suppressor by restricting the cellular polyamine supply required to support growth.

Intracellular signaling pathways confer specificity of transactivation by mineralocorticoid and glucocorticoid receptors

The glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR) bind similar ligands and target genes in vitro yet have distinct roles in vivo. With a single exception, known mechanisms conferring specificity have been limited to prereceptor mechanisms. These alone cannot account for specificity, particularly at a transcriptional level. These studies aimed to determine whether receptor-specific transcriptional regulation via physiological modulators of cellular signaling pathways, and MR-, as well as GR-specific interactions, could be demonstrated. By comparing modulation of GR- and MR-mediated transactivation in renal LLC-PK1 cells, we have identified several activators of intracellular signaling pathways that discriminate between the GR and the MR and demonstrate that differential regulation occurs at relatively specific points in the signaling pathway. The phosphatase inhibitor, okadaic acid, and the protein kinase G activator, sodium nitroprusside, stimulate only GR-mediated transactivation, in contrast to modulators of other protein kinase pathways that act in parallel on both receptors. The GR-specific effect of okadaic acid is observed only at doses where both phosphatases 1 and 2A are inhibited. MR-specific modulators include a centrally active alpha-2 adrenergic agonist and the thyroid receptor. Comparison of the interaction between the thyroid receptor and the GR, or the MR, distinguish two types of repression, only one of which is receptor-specific. These studies identify several signal transduction pathways that can differentially activate either the MR or the GR at a transcriptional level and might play physiological roles in conferring MR- or GR-specific regulation.

Inhibited expression of insulin-like growth factor I mRNA and attenuated cardiac hypertrophy in volume overloaded hearts treated with difluoromethylornithine

The present study examined whether the previously reported hypertrophy and increased expression of insulin-like growth factor I (IGF-I) mRNA in the volume-overloaded right ventricle was dependent on an intact production of polyamines. Volume overload was created in normotensive Wistar rats by means of an aorto-caval fistula. Difluoromethylornithine (DFMO) 2%, which is a specific, irreversible blocker of ornithine decarboxylase, was administered in the drinking water to intervention groups and one sham group, respectively, 24 h prior to surgery and for up to 26 days. DFMO blocked transiently the early over-expression of right ventricular IGF-I mRNA and attenuated the rapid development of both right and left ventricular hypertrophy during volume overload. Expression of IGF-I mRNA in the right ventricle in the early phase of volume overload appears to be dependent on activation of ornithine decarboxylase, whereas other pathways are involved in the later phase of cardiac structural adaptation. Thus, these findings link together early and late growth responses potentially important for compensatory cardiac hypertrophy.

Regional localization of agmatine in the rat brain: an immunocytochemical study

The distribution of agmatine (decarboxylated arginine) was mapped in the central nervous system (CNS) in the rat. Agmatine-like immunoreactivity was identified by light microscopy, exclusively in the cytoplasm of neuronal perikarya. Immunoreactive neurons were present in the cerebral cortex, predominantly within laminae VI and V and, to a lesser extent, III and mainly in retrosplenial, cingulate, primary somatosensory and auditory cortices, and the subiculum. In the lower brainstem, immunoreactivity was selectively localized to visceral relay nuclei: the nucleus tractus solitarii and pontine parabrachial complex, and periventricular areas including the laterodorsal nucleus, locus coeruleus and dorsal raphe. In the midbrain, immunolabeled cells were concentrated in the ventral tegmental area and periaqueductal gray. In the forebrain, subcortical neurons were labeled predominantly in the preoptic area, amygdala, septum, bed nucleus of the stria terminalis, midline thalamus, and the hypothalamus. Ultrastructural analysis of layer V of the somatosensory cortex demonstrated agmatine-immunoreactivity in neurons, primarily in large dense-core vesicles located in the cytoplasm. Agmatine immunoreactivity was also affiliated with endoplasmic reticulum and the plasmalemma. Cortical neurons and the subiculum were labeled in animals not administered the axonal transport inhibitor, colchicine; thus, may normally contain higher concentrations of the amine than other brain regions. The central distribution of agmatine is consistent with the hypothesis that the amine may be a novel neurotransmitter of neurons involved in behavioral and visceral control.

Metabolism of agmatine in macrophages: modulation by lipopolysaccharide and inhibitory cytokines

Agmatine is an amine derived from the decarboxylation of arginine by arginine decarboxylase (ADC) and metabolized to putrescine by agmatinase. While prevalent in bacteria and plants, agmatine and its metabolic enzymes have been recently identified in mammalian tissues. In the present study we sought to determine: (a) whether macrophages (cell line RAW 264.7) express ADC and agmatinase, and (b) if the enzymes are regulated by lipopolysaccharide (LPS), and/or by the inhibitory cytokines transforming growth factor-beta (TGF-beta), interleukin-10 (IL-10) and interleukin-4 (IL-4). LPS induced a dose-dependent stimulation of agmatinase, while it decreased ADC, the effect in both cases being maximum at 20 h. As expected, LPS dose-dependently stimulated the inducible nitric oxide synthase activity (iNOS). A strong correlation was observed between the effects of LPS on the agmatine-related enzymes and iNOS. By contrast, exposure to IL-10 and TGF-beta caused a reduction in ADC and agmatinase, whereas IL-4 was ineffective on ADC, but reverted the LPS-induced increase of agmatinase. We conclude that the agmatine pathway may be an alternative metabolic route for arginine in macrophages, suggesting a regulatory role of agmatine during inflammation.

Alpha 2-adrenoceptors and I1-imidazoline binding sites: relationship with catecholamines in women of reproductive age

A comparison is presented between plasma catecholamine concentrations and platelet [125I]-p-iodoclonidine binding sites in 16 healthy women. Blood samples were obtained at six regularly spaced intervals over two consecutive menstrual cycles from healthy women with regular menstrual periods. Although no cycle-related changes were observed per se, there were significant correlations between the platelet binding sites and plasma norepinephrine and epinephrine concentrations. The densities of platelet alpha 2-adrenoceptors were negatively correlated in an exponential fashion (r2 = 0.694, P = 0.009) with plasma epinephrine concentrations, implying agonist-induced downregulation. On the other hand, platelet I1-imidazoline binding sites were positively correlated with plasma concentrations of norepinephrine in a linear fashion (r2 = 0.326, P = 0.021). This is the first indication that I1 binding sites might be upregulated by a physiological factor. Furthermore, the data suggest that elevations in plasma norepinephrine might explain reports of upregulated I1 binding sites in depressed patients.

Hepatic zonation of the catabolism of arginine and ornithine in the perfused rat liver

The metabolism of 14C-labelled arginine and ornithine was studied in the isolated, nonrecirculating, perfused rat liver. The catabolism of these amino acids required ornithine aminotransferase since treatment of rats with gabaculine, an inhibitor of this enzyme, decreased substantially the production of 14CO2 from the 14C-labelled amino acids. In the liver, ornithine aminotransferase is restricted to a small population of hepatocytes proximal to the terminal hepatic vein [Kuo, F.C., Hwu, W.L., Valle, D. and Darnell Jr., J.E. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 9468-9472], i.e. the perivenous subpopulation of hepatocytes. Catabolism of arginine requires arginase to convert arginine to ornithine which can then be catabolized through ornithine aminotransferase. The presence of arginase activity in the perivenous hepatocytes was demonstrated by experiments in which livers were perfused with [14C]arginine in both antegrade and retrograde directions. Identical rates of 14CO2 production were obtained in these experiments, a result which could only occur if the process of arginine catabolism through ornithine aminotransferase can be carried out in its entirety in the perivenous cells.

[Nitric oxide, L-arginine and the kidney. Experimental studies of new therapy approaches]

BACKGROUND

Nitric oxide (NO) is a small gaseous molecule with multiple biological effects. NO is produced from the semi-essential amino acid L-arginine by NO synthases (NOS). In the kidney, neuronal NOS (bNOS), which is localized in the macula densa, and endothelial NOS (ecNOS) are involved in the regulation of glomerular hemodynamics. Dysfunction of these enzymes may cause glomerular hypertension and increased intraglomerular platelet aggregation. NO production in high tissue concentrations can be achieved by activation of an inducible NOS isoform (iNOS) and may act as a potent mediator of inflammation in immune-mediated renal diseases. Selective inhibition of iNOS may, therefore, become a novel anti-inflammatory approach in the treatment of glomerulonephritis. Based on experimental data, the potential importance of NO and other metabolites of L-arginine in the pathophysiology and therapy of renal diseases is summarized in this article.

CONCLUSION

Modulation of the renal L-arginine/NO-system represents a promising therapeutic target in the treatment of acute an chronic kidney diseases.

Antagonism of alpha1-adrenoceptor agonist-induced responses by rilmenidine in vascular smooth muscle

The effect of the centrally acting antihypertensive agent, rilmenidine, was examined on the contractile properties of isolated rat portal vein strips and on the free cytosolic [Ca2+] ([Ca2+]i) in isolated myocytes. Rilmenidine (1-30 microM) relaxed strips precontracted with noradrenaline. This effect was not inhibited by the alpha2-adrenoceptor antagonist, yohimbine, and was not mimicked by the alpha2-adrenoceptor agonist, 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 14,304). Rilmenidine dose dependently shifted to the right the concentration-response curves to noradrenaline and to phenylephrine but not that to carbachol. Rilmenidine alone (0.1-30 microM) caused a contraction which maximally corresponded to 18% of the maximal noradrenaline-induced contraction. This effect was not produced by UK 14,304, was not affected by yohimbine, but was inhibited by the alpha1-adrenoceptor antagonist, prazosin. In isolated myocytes, rilmenidine reduced the noradrenaline-induced [Ca2+]i increase but alone, it produced a rise in [Ca2+]i, the peak amplitude of which averaged 15% of the noradrenaline-induced transient [Ca2+]i rise. It is concluded that rilmenidine acts as a partial agonist of alpha1-adrenoceptors of vascular smooth muscle, causing relaxation of vessels precontracted by full agonists of alpha1-adrenoceptors.

Arginine decarboxylase (polyamine synthesis) mutants of Arabidopsis thaliana exhibit altered root growth

Putrescine and polyamines are produced by two alternative pathways in plants. One pathway starts with the enzyme arginine decarboxylase; the other with ornithine decarboxylase. The authors developed an in vivo screening strategy to identify mutants with low levels of arginine decarboxylase activity. The screen requires both a primary screen of the M2 generation and a secondary screen of the M3 generation. The method used was to screen 15,000 EMS-mutagenized M2 seedlings for low levels of arginine decarboxylase (ADC) activity and identified seven mutants that fall into two complementation groups. These mutants have from 20% to 50% of wild-type enzyme activity. Morphological alterations common among the mutants include increased levels of lateral root branching. The authors obtained a double mutant combining the alleles with the lowest activities from the two complementation groups; this has lower ADC enzyme activity and putrescine levels than either of the single mutants. The double mutant has highly kinked roots that form a tight cluster; it also has narrower leaves, sepals, and petals than either single mutant or wild-type, and delayed flowering. These results suggest there may be more than one ADC gene in Arabidopsis, and that ADC and polyamine levels play roles in root meristem function and in lateral growth of leaf-homolog organs.

Plasma luteinizing hormone levels in response to gonadotropin-releasing hormone agonist and clonidine in Trypanosoma congolense-infected female goats

Trypanosomiasis, a parasitic disease of humans and animals, occurs over a wide area of Africa and imposes a large socioeconomic burden on the people. In the present study, we investigated whether trypanosomiasis-induced reproductive disorders were due to pituitary or hypothalamic dysfunction by determining plasma luteinizing hormone (LH) response to gonadotropin-releasing hormone (GnRH) agonist or clonidine in Trypanosoma congolense-infected female goats. With GnRH agonist administration, the total amount of LH secreted over a 140-min sampling period on day 23 and day 60 postinfection was consistently higher (71 and 21%, respectively) in infected goats compared to controls. In contrast, clonidine administration to infected goats on day 28 and day 69 postinfection failed to significantly alter the LH pulse frequency or the mean LH pulse amplitude over a 80-min sampling period. The results, especially the lack of response to clonidine, indicate that trypanosomiasis impairs GnRH release from the hypothalamus.

Chronic imipramine treatment downregulates IR1-imidazoline receptors in rat brainstem

One subtype of imidazoline receptors (IR1) is similar to alpha 2-adrenoceptors (alpha 2 AR) based on their high affinity for clonidine and related imidazoline compounds. On the other hand, IR1 possess low affinity for norepinephrine (NE) and other catecholamines. Imidazoline receptors have also been found to be over-expressed in plasma membranes from platelets and brain tissues of depressed patients. Over-expression of IR1 in platelet membranes of depressed patients became normalized after various antidepressant treatment to the patients. Herein, the prototypic antidepressant, imipramine (IMI), has been studied in regard to its treatment effects on [125I]p-iodoclonidine binding to both alpha 2 AR and IR1 in rat brainstem membranes. No effects of chronic IMI treatment were found on brainstem alpha 2 AR binding sites (Bmax and/or KD parameters unchanged) after 25 days of daily injections (i.p. IMI 20 mg/kg/day). However, IMI induced a decrease in the density (Bmax measured under NE mask) of brainstem IR1 sites, with no change in KD. Downregulation of IR1 sites was dose-dependent (minimal effective dose of i.p. IMI was 10 mg/kg/day) and time-dependent (> 16 days of treatment). These results implicate brainstem IR1 in the chronic effects of antidepressants.

Inhibitory effect of agmatine on naloxone-precipitated abstinence syndrome in morphine dependent rats

Effects of agmatine, which is an endogenous polyamine metabolite formed by decarboxylation of L-arginine, were investigated on the morphine abstinence syndrome in rats. Two pellets containing 75 mg morphine base (total 150 mg) were implanted subcutaneously on the back of rats. Seventy-two hours after morphine implantation, agmatine sulphate (20, 30 and 40 mg/kg) or saline was injected intraperitoneally. Forty-five min later, naloxone (2 mg/kg) was injected intraperitoneally to induce precipitated withdrawal. Immediately after naloxone injection, rats were observed for 15 min, and abstinence syndrome signs, which included jumping, wet dog shake, writhing, defecation, ptosis, teeth chattering and diarrhea were counted or rated. Agmatine attenuated all of the signs of the morphine abstinence syndrome dose dependently and significantly. Our results suggest that agmatine prevents naloxone-precipitated abstinence syndrome in morphine dependent rats; thus, this drug may be beneficial in the treatment of opioid dependence.

Agmatine: a novel neurotransmitter?

Current evidence is consistent with an hypothesis that agmatine meets many criteria for a neurotransmitter-neuromodulator. It is synthesized, stored, and released in brain; is contained in neurons and axon terminals with a heterogeneous distribution; interacts with cell-specific receptors; and elicits biological actions within the central nervous system. Its role in normal brain function, however, has not yet been established, in part because of the absence of agents that selectively affect its biosynthesis or degradation.