Agmatine is widely and unequally distributed in rat organs

Three Related Enzymes in Candida albicans Achieve Arginine- and Agmatine-Dependent Metabolism That Is Essential for Growth and Fungal Virulence

Amino acid metabolism is crucial for fungal growth and development. Ureohydrolases produce amines when acting on l-arginine, agmatine, and guanidinobutyrate (GB), and these enzymes generate ornithine (by arginase), putrescine (by agmatinase), or GABA (by 4-guanidinobutyrase or GBase). Candida albicans can metabolize and grow on arginine, agmatine, or guanidinobutyrate as the sole nitrogen source. Three related C. albicans genes whose sequences suggested that they were putative arginase or arginase-like genes were examined for their role in these metabolic pathways. Of these, Car1 encoded the only bona fide arginase, whereas we provide evidence that the other two open reading frames, orf19.5862 and orf19.3418, encode agmatinase and guanidinobutyrase (Gbase), respectively. Analysis of strains with single and multiple mutations suggested the presence of arginase-dependent and arginase-independent routes for polyamine production. CAR1 played a role in hyphal morphogenesis in response to arginine, and the virulence of a triple mutant was reduced in both Galleria mellonella and Mus musculus infection models. In the bloodstream, arginine is an essential amino acid that is required by phagocytes to synthesize nitric oxide (NO). However, none of the single or multiple mutants affected host NO production, suggesting that they did not influence the oxidative burst of phagocytes.IMPORTANCE We show that the C. albicans ureohydrolases arginase (Car1), agmatinase (Agt1), and guanidinobutyrase (Gbu1) can orchestrate an arginase-independent route for polyamine production and that this is important for C. albicans growth and survival in microenvironments of the mammalian host.

Neuroprotective effect of agmatine (decarboxylated l-arginine) against oxidative stress and neuroinflammation in rotenone model of Parkinson's disease

Parkinson's disease (PD) is the second most common age-related neurodegenerative disease after Alzheimer's disease, characterized by loss of dopaminergic neurons in substantia nigra pars compacta, accompanied by motor and nonmotor symptoms. The neuropathological hallmarks of PD are well reported, but the etiology of the disease is still undefined; several studies assume that oxidative stress, mitochondrial defects, and neuroinflammation play vital roles in the progress of the disease. The current study was established to investigate the neuroprotective effect of agmatine on a rotenone (ROT)-induced experimental model of PD. Adult male Sprague Dawley rats were subcutaneously injected with ROT at a dose of 2 mg/kg body weight for 35 days. Agmatine was injected intraperitoneally at 50 and 100 mg/kg body weight, 1 h prior to ROT administration. ROT-treated rats that received agmatine showed better performance on beam walking and an elevated number of rears within the cylinder test. In addition, agmatine reduced midbrain malondialdehyde as an indication of lipid peroxidation, pro-inflammatory cytokines including tumor necrosis factor alpha and interleukin-1β, and glial fibrillary acidic protein. Moreover, agmatine was responsible for preventing loss of tyrosine hydroxylase-positive neurons. In conclusion, our study showed that agmatine possesses a dose-dependent neuroprotective effect through its antioxidant and anti-inflammatory activities. These findings need further clinical investigations of agmatine as a promising neuroprotective agent for the future treatment of PD.

Structural and Mechanistic Analysis of Drosophila melanogaster Agmatine N-Acetyltransferase, an Enzyme that Catalyzes the Formation of N-Acetylagmatine

Agmatine N-acetyltransferase (AgmNAT) catalyzes the formation of N-acetylagmatine from acetyl-CoA and agmatine. Herein, we provide evidence that Drosophila melanogaster AgmNAT (CG15766) catalyzes the formation of N-acetylagmatine using an ordered sequential mechanism; acetyl-CoA binds prior to agmatine to generate an AgmNAT•acetyl-CoA•agmatine ternary complex prior to catalysis. Additionally, we solved a crystal structure for the apo form of AgmNAT with an atomic resolution of 2.3 Å, which points towards specific amino acids that may function in catalysis or active site formation. Using the crystal structure, primary sequence alignment, pH-activity profiles, and site-directed mutagenesis, we evaluated a series of active site amino acids in order to assign their functional roles in AgmNAT. More specifically, pH-activity profiles identified at least one catalytically important, ionizable group with an apparent pK_a of ~7.5, which corresponds to the general base in catalysis, Glu-34. Moreover, these data led to a proposed chemical mechanism, which is consistent with the structure and our biochemical analysis of AgmNAT.

Agmatine suppresses peripheral sympathetic tone by inhibiting N-type Ca(2+) channel activity via imidazoline I2 receptor activation

Agmatine, a putative endogenous ligand of imidazoline receptors, suppresses cardiovascular function by inhibiting peripheral sympathetic tone. However, the molecular identity of imidazoline receptor subtypes and its cellular mechanism underlying the agmatine-induced sympathetic suppression remains unknown. Meanwhile, N-type Ca(2+) channels are important for the regulation of NA release in the peripheral sympathetic nervous system. Therefore, it is possible that agmatine suppresses NA release in peripheral sympathetic nerve terminals by inhibiting Ca(2+) influx through N-type Ca(2+) channels. We tested this hypothesis by investigating agmatine effect on electrical field stimulation (EFS)-evoked contraction and NA release in endothelium-denuded rat superior mesenteric arterial strips. We also investigated the effect of agmatine on the N-type Ca(2+) current in superior cervical ganglion (SCG) neurons in rats. Our study demonstrates that agmatine suppresses peripheral sympathetic outflow via the imidazoline I2 receptor in rat mesenteric arteries. In addition, the agmatine-induced suppression of peripheral vascular sympathetic tone is mediated by modulating voltage-dependent N-type Ca(2+) channels in sympathetic nerve terminals. These results suggest a potential cellular mechanism for the agmatine-induced suppression of peripheral sympathetic tone. Furthermore, they provide basic and theoretical information regarding the development of new agents to treat hypertension.

Involvement of nitric oxide in anticompulsive-like effect of agmatine on marble-burying behaviour in mice

In view of the reports that nitric oxide modulates the neurotransmitters implicated in obsessive-compulsive disorder (OCD), patients with OCD exhibit higher plasma nitrate levels, and drugs useful in OCD influence nitric oxide. Agmatine is a polyamine and widely distributed in mammalian brain which interacts with nitrergic systems. Hence, the present study was carried out to understand the involvement of nitrergic systems in the anticompulsive-like effect of agmatine. We used marble-burying behaviour (MBB) of mice as the animal model of OCD, and nitric oxide levels in hippocampus (HC) and cortex homogenate were measured. Results revealed that, agmatine (20 and 40mg/kg, i.p) significantly inhibited the MBB. Intraperitoneal administration of nitric oxide enhancers viz. nitric oxide precursor - l-arginine (l-ARG) (400mg/kg and 800mg/kg) increased MBB as well as brain nitrites levels, whereas treatment with N(G)-nitro-l-arginine methyl ester (l-NAME) neuronal nitric oxide synthase inhibitor (30mg/kg and 50mg/kg, i.p.) and 7-nitroindazole (7-NI) (20mg/kg and 40mg/kg) attenuated MBB and nitrites levels in brain. Further, in combination studies, the anticompulsive-like effect of agmatine (20mg/kg, ip) was exacerbated by prior administration of l-ARG (400mg/kg) and conversely l-NAME (15mg/kg) or 7-NI (10.0mg/kg) attenuated OCD-like behaviour with HC and cortex changes in the levels of NO. None of the above treatment had any significant influence on locomotor activity. In conclusion, Agmatine is effective in ameliorating the compulsive-like behaviour in mice which appears to be related to nitric oxide in brain.

Agmatine, a metabolite of L-arginine, reverses scopolamine-induced learning and memory impairment in rats

Agmatine (l-amino-4-guanidino-butane), a metabolite of L-arginine through the action of arginine decarboxylase, is a novel neurotransmitter. In the present study, effects of agmatine on cognitive functions have been evaluated by using one trial step-down passive avoidance and three panel runway task. Agmatine (20, 40, 80 mg/kg i.p.) was administered either in the presence or absence of a cholinergic antagonist, scopolamine (1 mg/kg i.p.). Scopolamine significantly impaired learning and memory in both passive avoidance and three panel runway test. Agmatine did not affect emotional learning, working and reference memory but significantly improved scopolamine-induced impairment of learning and memory in a dose dependent manner. Our results indicate that agmatine, as an endogenous substance, may have an important role in modulation of learning and memory functions.

Agmatine induces gastric protection against ischemic injury by reducing vascular permeability in rats

AIM: To investigate the effect of administration of agmatine (AGM) on gastric protection against ischemia reperfusion (I/R) injury.

METHODS: Three groups of rats (6/group); sham, gastric I/R injury, and gastric I/R + AGM (100 mg/kg, i.p. given 15 min prior to gastric ischemia) were recruited. Gastric injury was conducted by ligating celiac artery for 30 min and reperfusion for another 30 min. Gastric tissues were histologically studied and immunostained with angiopoietin 1 (Ang-1) and Ang-2. Vascular endothelial growth factor (VEGF) and monocyte chemoattractant protein-1 (MCP-1) were measured in gastric tissue homogenate. To assess whether AKt/phosphatidyl inositol-3-kinase (PI3K) mediated the effect of AGM, an additional group was pretreated with Wortmannin (WM) (inhibitor of Akt/PI3K, 15 μg/kg, i.p.), prior to ischemic injury and AGM treatment, and examined histologically and immunostained. Another set of experiments was run to study vascular permeability of the stomach using Evan’s blue dye.

RESULTS: AGM markedly reduced Evan’s blue dye extravasation (3.58 ± 0.975 μg/stomach vs 1.175 ± 0.374 μg/stomach, P < 0.05), VEGF (36.87 ± 2.71 pg/100 mg protein vs 48.4 ± 6.53 pg/100 mg protein, P < 0.05) and MCP-1 tissue level (29.5 ± 7 pg/100 mg protein vs 41.17 ± 10.4 pg/100 mg protein, P < 0.01). It preserved gastric histology and reduced congestion. Ang-1 and Ang-2 immunostaining were reduced in stomach sections of AGM-treated animals. The administration of WM abolished the protective effects of AGM and extensive hemorrhage and ulcerations were seen.

CONCLUSION: AGM protects the stomach against I/R injury by reducing vascular permeability and inflammation. This protection is possibly mediated by Akt/PI3K.

Agmatine protects against scopolamine-induced water maze performance impairment and hippocampal ERK and Akt inactivation

Cholinergic brain activity plays a significant role in memory. Scopolamine a muscarinic cholinergic antagonist is known to induce impairment in Morris water maze performance, the task which is mainly dependent on the hippocampus. It is suggested that hippocampal ERK and Akt activation play roles in synaptic plasticity and some types of learning and memory. Agmatine, a polyamine derived from l-arginine decarboxylation, is recently shown to exert some neuroprotective effects. This study was aimed to investigate if agmatine could reverse scopolamine-induced memory impairment and possible hippocampal ERK and Akt activity alteration. Adult male Sprague-Dawley rats weighing 200-250 g were randomly assigned into 5 groups. The animals were trained for 3 days in Morris water maze and in day 4 their memory retention was assessed in probe trial which was consisted of a 60 s trial with no platform. Scopolamine (1 mg/kg/ip) or saline were injected 30 min and agmatine (20 or 40 mg/kg/ip) was administered 60 min before each session. The hippocampi were isolated after behavioral studies and western blotting studies on hippocampal lysates were done to determine the levels of activated ERK and Akt. Scopolamine treatment not only impaired water maze learning and memory, but also decreased the amount of phosphorylated (activated) ERK and Akt. Agmatine pre-treatment prevented both the learning impairment and hippocampal ERK and Akt inactivation induced by scopolamine. It seems that agmatine may act as a candidate substance against amnesia.

The Effect of Intra-CA1 Agmatine Microinjection on Water Maze Learning and Memory in Rat

BACKGROUND

Reports on agmatine are controversial showing that it may improve memory, it can deteriorate memory and some did not notice any interference with learning and memory. In the present study, the effect of directly intra-CA1 agmatine microinjection on water maze learning and memory has been assessed.

METHODS

The cannuls were implanted in hippocampal CA1 regions of rats in a sterotaxic frame after general anesthesia. After one week recovery period, the animals were assessed in the reference memory version of water maze. Agmatine (1, 10, 100 or 200 μg/0.5 μl) or saline were infused 20 minutes before or immediately after training.

RESULTS

Agmatine-treated rats did not show any significant difference neither in water maze acquisition nor in consolidation task in comparison with control and sham groups.

CONCLUSION

Agmatine does not affect water maze learning and memory.

ASIC3 channels integrate agmatine and multiple inflammatory signals through the nonproton ligand sensing domain

Background

Acid-sensing ion channels (ASICs) have long been known to sense extracellular protons and contribute to sensory perception. Peripheral ASIC3 channels represent natural sensors of acidic and inflammatory pain. We recently reported the use of a synthetic compound, 2-guanidine-4-methylquinazoline (GMQ), to identify a novel nonproton sensing domain in the ASIC3 channel, and proposed that, based on its structural similarity with GMQ, the arginine metabolite agmatine (AGM) may be an endogenous nonproton ligand for ASIC3 channels.

Results

Here, we present further evidence for the physiological correlation between AGM and ASIC3. Among arginine metabolites, only AGM and its analog arcaine (ARC) activated ASIC3 channels at neutral pH in a sustained manner similar to GMQ. In addition to the homomeric ASIC3 channels, AGM also activated heteromeric ASIC3 plus ASIC1b channels, extending its potential physiological relevance. Importantly, the process of activation by AGM was highly sensitive to mild acidosis, hyperosmolarity, arachidonic acid (AA), lactic acid and reduced extracellular Ca²⁺. AGM-induced ASIC3 channel activation was not through the chelation of extracellular Ca²⁺ as occurs with increased lactate, but rather through a direct interaction with the newly identified nonproton ligand sensing domain. Finally, AGM cooperated with the multiple inflammatory signals to cause pain-related behaviors in an ASIC3-dependent manner.

Conclusions

Nonproton ligand sensing domain might represent a novel mechanism for activation or sensitization of ASIC3 channels underlying inflammatory pain-sensing under in vivo conditions.

Agmatine prevents LPS-induced spatial memory impairment and hippocampal apoptosis

Neuroinflammation is associated with a number of neurodegenerative diseases. It is known that lipopolysaccharide (LPS) treatment induces neuroinflammation and memory deterioration. Agmatine, the metabolite of arginine by arginine decarboxylase, is suggested to be a neuroprotective agent. The aim of this study was to explore if agmatine can prevent LPS-induced spatial memory impairment and hippocampal apoptosis. Adult male Wistar rats (200-250 g) were trained in water maze for 4 days (3 days in hidden platform and the last day in visible platform task). Saline, LPS (250 microg/kg/ip) or (and) agmatine (5 or 10 mg/kg) were administered 4h before every training session. LPS treatment impaired water maze place learning while agmatine co-administration prevented it. Also western blot studies revealed that LPS induces hippocampal caspase-3 activation while agmatine treatment prevented it.

Repeated immobilization stress alters rat hippocampal and prefrontal cortical morphology in parallel with endogenous agmatine and arginine decarboxylase levels

Agmatine, an endogenous amine derived from decarboxylation of L-arginine catalyzed by arginine decarboxylase, has been proposed as a neurotransmitter or neuromodulator in the brain. In the present study, we examined whether agmatine has neuroprotective effects against repeated immobilization-induced morphological changes in brain tissues and possible effects of immobilization stress on endogenous agmatine levels and arginine decarboxylase expression in rat brains. Sprague-Dawley rats were subjected to 2h immobilization stress daily for 7 days. This paradigm significantly increased plasma corticosterone levels, and the glutamate efflux in the hippocampus as measured by in vivo microdialysis. Immunohistochemical staining with beta-tubulin III showed that repeated immobilization caused marked morphological alterations in the hippocampus and medial prefrontal cortex that were prevented by simultaneous treatment with agmatine (50mg/kg/day), i.p.). Likewise, endogenous agmatine levels measured by high-performance liquid chromatography in the prefrontal cortex, hippocampus, striatum and hypothalamus were significantly increased by immobilization, as compared to controls. The increased endogenous agmatine levels, ranging from 92 to 265% of controls, were accompanied by a significant increase of arginine decarboxylase protein levels in the same regions. These results demonstrate that the administration of exogenous agmatine protects the hippocampus and medial prefrontal cortex against neuronal insults caused by repeated immobilization. The parallel increase in endogenous brain agmatine and arginine decarboxylase protein levels triggered by repeated immobilization indicates that the endogenous agmatine system may play an important role in adaptation to stress as a potential neuronal self-protection mechanism.

Exogenous agmatine has neuroprotective effects against restraint-induced structural changes in the rat brain

Agmatine is an endogenous amine derived from decarboxylation of arginine catalysed by arginine decarboxylase. Agmatine is considered a novel neuromodulator and possesses neuroprotective properties in the central nervous system. The present study examined whether agmatine has neuroprotective effects against repeated restraint stress-induced morphological changes in rat medial prefrontal cortex and hippocampus. Sprague-Dawley rats were subjected to 6 h of restraint stress daily for 21 days. Immunohistochemical staining with beta-tubulin III showed that repeated restraint stress caused marked morphological alterations in the medial prefrontal cortex and hippocampus. Stress-induced alterations were prevented by simultaneous treatment with agmatine (50 mg/kg/day, i.p.). Interestingly, endogenous agmatine levels, as measured by high-performance liquid chromatography, in the prefrontal cortex and hippocampus as well as in the striatum and hypothalamus of repeated restraint rats were significantly reduced as compared with the controls. Reduced endogenous agmatine levels in repeated restraint animals were accompanied by a significant increase of arginine decarboxylase protein levels in the same regions. Moreover, administration of exogenous agmatine to restrained rats abolished increases of arginine decarboxylase protein levels. Taken together, these results demonstrate that exogenously administered agmatine has neuroprotective effects against repeated restraint-induced structural changes in the medial prefrontal cortex and hippocampus. These findings indicate that stress-induced reductions in endogenous agmatine levels in the rat brain may play a permissive role in neuronal pathology induced by repeated restraint stress.

Chronic treatment with glucocorticoids alters rat hippocampal and prefrontal cortical morphology in parallel with endogenous agmatine and arginine decarboxylase levels

In the present study, we examined the possible effect of chronic treatment with glucocorticoids on the morphology of the rat brain and levels of endogenous agmatine and arginine decarboxylase (ADC) protein, the enzyme essential for agmatine synthesis. Seven-day treatment with dexamethasone, at a dose (10 and 50 mug/kg/day) associated to stress effects contributed by glucocorticoids, did not result in obvious morphologic changes in the medial prefrontal cortex and hippocampus, as measured by immunocytochemical staining with beta-tubulin III. However, 21-day treatment (50 mug/kg/day) produced noticeable structural changes such as the diminution and disarrangement of dendrites and neurons in these areas. Simultaneous treatment with agmatine (50 mg/kg/day) prevented these morphological changes. Further measurement with HPLC showed that endogenous agmatine levels in the prefrontal cortex and hippocampus were significantly increased after 7-day treatments with dexamethasone in a dose-dependent manner. On the contrary, 21-day treatment with glucocorticoids robustly reduced agmatine levels in these regions. The treatment-caused biphasic alterations of endogenous agmatine levels were also seen in the striatum and hypothalamus. Interestingly, treatment with glucocorticoids resulted in a similar change of ADC protein levels in most brain areas to endogenous agmatine levels: an increase after 7-day treatment versus a reduction after 21-day treatment. These results demonstrated that agmatine has neuroprotective effects against structural alterations caused by glucocorticoids in vivo. The parallel alterations in the endogenous agmatine levels and ADC expression in the brain after treatment with glucocorticoids indicate the possible regulatory effect of these stress hormones on the synthesis and metabolism of agmatine in vivo.

Interaction of endogenous ligands mediating antinociception

It is well known that a multitude of transmitters and receptors are involved in the nociceptive system, some of them increasing and others inhibiting the pain sensation both peripherally and centrally. These substances, which include neurotransmitters, hormones, etc., can modify the activity of nerves involved in the pain pathways. Furthermore, the organism itself can express very effective antinociception under different circumstances (e.g. stress), and, during such situations, the levels of various endogenous ligands change. A very exciting field of pain research relates to the roles of endogenous ligands. Most of them have been suggested to influence pain transmission, but only a few studies have been performed on the interactions of different endogenous ligands. This review focuses on the results of antinociceptive interactions after the co-administration of endogenous ligands. The data based on 55 situations reveal that the interactions between the endogenous ligands are very different, depending on the substances, the pain tests, the species of animals and the route of administrations. It is also revealed that only a few of the possible interactions between endogenous ligands have been investigated to date, in spite of the fact that the type of antinociceptive interaction between different endogenous ligands could hardly be predicted. The results indicate that the combination of endogenous ligands should not be omitted from the pain therapy arsenal. Attention will hopefully be drawn to the complex interdependence of endogenous ligands and their potential use in clinical practice.

Investigation of the dose-dependent neuroprotective effects of agmatine in experimental spinal cord injury: a prospective randomized and placebo-control trial

OBJECT

No definitive treatment for spinal cord injuries (SCIs) exists, and more research is required. The use of agmatine [4-(aminobutyl)-guanidine-NH2-CH2-CH2-CH2-CH2-NH-C(-NH2)(=NH)], a guanidinium compound formed by decarboxylation of L-arginine by arginine decarboxylase, is a neurotransmitter-neuromodulator with both N-methyl-D-aspartate receptor (NMDAR)-antagonizing and nitric oxide synthase (NOS)-inhibiting activities. The purpose of this study was to demonstrate the dose-dependent activity of agmatine, an inducible NOS (iNOS) inhibitor and selective NMDAR antagonist, on biochemical and functional recovery in an experimental rat SCI model.

METHODS

This study involved 40 Wistar albino male rats. The rats were subjected to sleep-awake cycles for 7 days before surgery. In each group, general anesthesia was induced by a 60-mg/kg ketamine injection. For the surgical SCI model, a Yaşargil aneurysm clip was placed in the spinal cord. The study was conducted in the following four main groups: Group I (control group) laminectomy only; Group II, trauma-only group and SCI; Group III, laminectomy, SCI and agmatine 50 mg/kg for 10 days; and Group IV, laminectomy, SCI, and agmatine 100 mg/kg for 10 days. On Day 1, no statistical difference was observed in any group (p < 0.005, analysis of variance [ANOVA] and the Fisher protected least significant difference [PLSD]). On Day 2, no statistical difference was noted among Groups II, III, and IV (p = 0.27, p = 0.42, and p = 0.76, respectively; ANOVA and Fisher PLSD). Beginning on Day 3, recovery in Groups III and IV differed significantly from that in Group II (p < 0.005, ANOVA and Fisher PLSD), and a statistically significant difference between Groups III and IV was observed, which also was present on Days 5, 7, and 10 (p = 0.003, p = 0.0024, and p = 0.0036, respectively; ANOVA and Fisher PLSD). Several observations were noteworthy: motor function scores were reduced significantly in the spinal cord-injured rats compared with the controls (p < 0.005); on Day 1, the agreement of motor function scores in rats in each SCI group indicated that the traumatic event had been replicated equally across all groups (p = 0.59, p = 0.59, and p = 0.28); a statistically significant difference in motor function scores developed on Day 3 between the rats subjected to trauma alone (Group II) and those treated with agmatine (Groups III and IV) (p < 0.005); and no statistically significant intergroup difference in motor function existed at any postinjury interval between the 50- and 100-mg/kg/day agmatine-treated rats (p > 0.005).

CONCLUSIONS

Agmatine administration following SCI was shown to reduce NO levels significantly. No statistically significant intergroup difference in the reduction of NO levels was found between rats treated with 50- and 100-mg/kg/day doses of agmatine. Administration of a 100-mg/kg/day dose of agmatine reduced the NO levels to those measured in controls. The authors conclude that with additional studies into the role of agmatine, this drug may be helpful in the treatment of patients with SCIs.

Arginine: beyond protein

Arginine, a semiessential or conditionally essential amino acid in humans, is one of the most metabolically versatile amino acids and serves as a precursor for the synthesis of urea, nitric oxide, polyamines, proline, glutamate, creatine, and agmatine. Arginine is metabolized through a complex and highly regulated set of pathways that remain incompletely understood at both the whole-body and the cellular levels. Adding to the metabolic complexity is the fact that limited arginine availability can selectively affect the expression of specific genes, most of which are themselves involved in some aspect of arginine metabolism. This overview highlights selected aspects of arginine metabolism, including areas in which our knowledge remains fragmentary and incomplete.

Effects of agmatine accumulation in human colon carcinoma cells on polyamine metabolism, DNA synthesis and the cell cycle

Putrescine, spermidine and spermine are low molecular polycations that play important roles in cell growth and cell cycle progression of normal and malignant cells. Agmatine (1-amino-4-guanidobutane), another polyamine formed through arginine decarboxylation, has been reported to act as an antiproliferative agent in several non-intestinal mammalian cell models. Using the human colon adenocarcinoma HT-29 Glc(-/+) cell line, we demonstrate that agmatine, which markedly accumulated inside the cells without being metabolised, exerted a strong cytostatic effect with an IC50 close to 2 mM. Agmatine decreased the rate of L-ornithine decarboxylation and induced a 70% down-regulation of ornithine decarboxylase (ODC) expression. Agmatine caused a marked decrease in putrescine and spermidine cell contents, an increase in the N1-acetylspermidine level without altering the spermine pool. We show that agmatine induced the accumulation of cells in the S and G2/M phases, reduced the rate of DNA synthesis and decreased cyclin A and B1 expression. We conclude that the anti-metabolic action of agmatine on HT-29 cells is mediated by a reduction in polyamine biosynthesis and induction in polyamine degradation. The decrease in intracellular polyamine contents, the reduced rate of DNA synthesis and the cell accumulation in the S phase are discussed from a causal perspective.

Agmatine signaling: odds and threads

Agmatine is a metabolite of L-arginine. It is formed by the decarboxylation of L-arginine via arginine decarboxylase in bacteria, plants and mammals. It is becoming clear that it has multiple physiological functions as a potential transmitter. Agmatine binds to alpha2-adrenoceptors and to imidazoline binding sites. It blocks NMDA receptors and other ligand-gated cation channels. It also inhibits nitric oxide synthase, induces release of peptide hormones and antizyme and plays a role during cell proliferation by interacting with the generation and transport of polyamines. Although the precise function of endogenously released agmatine is presently still unclear, this review will summarize several aspects concerning the biological function of agmatine.

An in vivo evaluation of the antiseizure activity and acute neurotoxicity of agmatine

Agmatine, an endogenous cationic amine, exerts a wide range of biological effects, including modulation of glutamate-activated N-methyl-D-aspartate (NMDA) receptor function in the central nervous system (CNS). Since glutamate and the NMDA receptor have been implicated in the initiation and spread of seizure activity, the capacity of agmatine to inhibit seizure spread was evaluated in vivo. Orally administered agmatine (30 mg/kg) protected against maximal electroshock seizure (MES)-induced seizure spread in rats as rapidly as 15 min and for as long as 6 h after administration. Inhibition of MES-induced seizure spread was also observed when agmatine was administered intraperitoneally. Agmatine's antiseizure activity did not appear to be dose-dependent. An in vivo neurotoxicity screen indicated that agmatine was devoid of any acute neurological toxicity at the doses tested. These preliminary data suggest that agmatine has promising anticonvulsant activity.

Imidazoline receptors in the heart: characterization, distribution, and regulation

Imidazoline receptors were identified in cardiac tissues of various species. Imidazoline receptors were immunolocalized in the rat heart. Membrane binding and autoradiography on frozen heart sections using 0.5 nM para-iodoclonidine (125I-PIC) revealed that binding was equally and concentration-dependently inhibited by epinephrine and imidazole-4-acetic acid (IAA), implying 125I-PIC binding to cardiac alpha2-adrenergic and I1-receptors, respectively. After irreversible blockade of alpha2-adrenergic receptors, binding was inhibited by the selective I1-agonist, moxonidine, and the I1-antagonist, efaroxan, in a concentration-dependent (10-12 to 10-5 M) manner. Calculation of kinetic parameters revealed that in canine left and right atria, I1-receptor Bmax was 13.4 +/- 1.7 and 20.1 +/- 3.0 fmol/mg protein, respectively. Compared to age-matched normotensive Wistar Kyoto rats, I1-receptors were increased in 12-week-old hypertensive rat (SHR) right (22.6 +/- 0.3 to 43.7 +/- 4.4 fmol/unit area, p < 0.01) and left atria (13.3 +/- 0.6 to 30.2 +/- 4.1 fmol/unit area, p < 0.01). Also, compared to corresponding normal controls, Bmax was increased in hearts of hamsters with advanced cardiomyopathy (13.9 +/- 0.4 to. 26.0 +/- 2.3 fmol/unit area, p < 0.01) and in human ventricles with heart failure (12.6 +/- 1.3 to 35.5 +/- 2.9 fmol/mg protein, p < 0.003). These studies demonstrate that the heart possesses imidazoline I1-receptors that are up-regulated in the presence of hypertension or heart failure, which would suggest their involvement in cardiovascular regulation.

Polyamine transport system mediates agmatine transport in mammalian cells

Agmatine is a biogenic amine with the capacity to regulate a number of nonreceptor-mediated functions in mammalian cells, including intracellular polyamine content and nitric oxide generation. We observed avid incorporation of agmatine into several mammalian cell lines and herein characterize agmatine transport in mammalian cells. In transformed NIH/3T3 cells, agmatine uptake is energy dependent with a saturable component indicative of carrier-mediated transport. Transport displays an apparent Michaelis-Menten constant of 2.5 microM and a maximal velocity of 280 pmol x min(-1) x mg(-1) protein and requires a membrane potential across the plasma membrane for uptake. Competition with polyamines, but not cationic molecules that utilize the y+ system transporter, suppresses agmatine uptake. Altering polyamine transporter activity results in parallel changes in polyamine and agmatine uptake. Furthermore, agmatine uptake is abrogated in a polyamine transport-deficient human carcinoma cell line. These lines of evidence demonstrate that agmatine utilizes, and is dependent on, the polyamine transporter for cellular uptake. The fact that this transport system is associated with proliferation could be of consequence to the antiproliferative effects of agmatine.

An emerging role for agmatine

Polyamines, required components of proliferation, are autoregulated by the protein antizyme. To date, agmatine is the only molecule other than the polyamines that can induce antizyme, and thus influence cell homeostasis and growth. Agmatine has effectively suppressed proliferation in immortalized and transformed cell lines. An increased sensitivity to the anti-proliferative effects of agmatine observed in Ras transformed versus native cells paralleled an increase in agmatine uptake in the transformed cells. We hypothesize that agmatine may target transformed cells via selective transporters.

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.

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.

Clonidine and ST-91 may activate imidazoline binding sites in the heart to release atrial natriuretic peptide

It is well established that the antihypertensive drug clonidine acts through specific imidazoline receptors in the brain and kidney to increase diuresis, natriuresis, and kaliuresis. We have previously shown that the effects of clonidine are associated with elevated plasma atrial natriuretic peptide (ANP). Similar to clonidine, ST-91, a clonidine analogue that does not cross the blood-brain barrier, evokes renal responses that are also associated with elevated plasma ANP. The mechanisms of ANP increase elicited by these imidazoline drugs are unclear. Since ANP is primarily released from the cardiac atria, we investigated the direct effect of the imidazoline drugs on ANP release by incubating left and right atrial sections with 10(-6) mol/L ST-91 in the presence and absence of efaroxan, a selective imidazoline I1 receptor antagonist, for 30 minutes at 37 degrees C. ST-91 significantly stimulated ANP release, and the effect was inhibited by 10(-6) mol/L efaroxan. Further studies using heart perfusion with the imidazoline drugs with and without antagonists over 30 minutes revealed that both clonidine and ST-91 gradually stimulated ANP release. Also, perfusion with these compounds resulted in a gradual decrease in heart rate, but bradycardia was significant only with clonidine. The effects of ST-91 were inhibited by 10(-6) mol/L efaroxan and to a lesser extent by 10(-6) mol/L yohimbine, implying that the actions of ST-91 were mainly mediated by I1 receptors. On the other hand, the actions of clonidine were inhibited by 10(-5) mol/L efaroxan and by 10(-6) mol/L yohimbine, an alpha2-adrenoceptor antagonist, which may suggest that the actions of clonidine were preferentially mediated by alpha2-adrenoceptors in the heart. These results indicate that the peripheral actions of clonidine are probably mediated by alpha2 and imidazoline receptors and may involve direct stimulation of ANP release by the cardiac atria--an effect that may account for the increase in plasma ANP levels and diuresis and natriuresis observed in vivo after administration of clonidine and its analogues.

Endogenous ligands of imidazoline receptors: classic and immunoreactive clonidine-displacing substance and agmatine

1. There are several endogenous ligands that bind to I-receptors of both the I1 and I2 subclass. These include: (a) classic CDS, a partially purified entity isolated by the criteria that it displaces binding ligands to alpha 2- and I-receptors; (b) immunoreactive (ir)-CDS, a moiety that binds to antibodies raised against clonidine, para-amino-clonidine, or idazoxan; and (c) agmatine. 2. Classic-CDS, not yet defined structurally, binds to I1, I2, and alpha 2-adrenergic receptors, is neither a peptide nor a catecholamine, and has purportedly a molecular weight of 588 Da. By ligand binding assays, it was found in brain, serum, CSF, and placenta and in a neural-glial cell line. Partially purified classic CDS is bioactive. Like clonidine, it contracts aorta and vas deferens and inhibits platelet aggregation, effects largely attributable to agonism at alpha 2-adrenergic receptors. Unlike clonidine, it contracts rat gastric fundus and releases catecholamines from chromaffin cells, effects attributable to actions at I-receptors. Injected into the RVL, classic CDS alters arterial pressure, but the direction of change of pressure has differed between groups of investigators. However, in the absence of structure, it is possible that ligand binding and bioactivity may be attributable to different molecules. 3. Ir-CDS, also of unknown structure, is a material(s) that binds to antibodies raised against clonidine, PAC, or idazoxan. Ir-CDS, measured by radioimmunoassay, is unevenly distributed in brain with highest concentrations in the hypothalamus, midbrain, and dorsal medulla. It is contained in the gastric fundus, adrenal gland, heart, kidney, and serum in amounts substantially higher than found in brain. Ir-CDS may be elevated in the serum of some patients with hypertension and in the CSF of patients with structural brain disease. The concentration of ir-CDS and bioactivity on gastric fundus directly correlates, suggesting that it may share similarities with classic-CDS. However, until the structure of classic and ir-CDS is determined, the possibility that ligand binding and antibody recognition are properties of different molecules must be considered. 4. Agmatine (decarboxylated arginine) is the only endogenous molecule that, like CDS, binds to alpha 2- and I-receptors of both classes. It and its biosynthetic enzyme arginine decarboxylase are present in brain, and agmatine is widely distributed throughout the body. However, the distribution of agmatine and ir-CDS differs, whereas the biological actions of agmatine do not mimic those of classic CDS. Its presence raises the possibility of an alternative pathway for polyamine biosynthesis.(ABSTRACT TRUNCATED AT 400 WORDS)