Agmatine, a novel hypothalamic amine, stimulates pituitary luteinizing hormone release in vivo and hypothalamic luteinizing hormone-releasing hormone release in vitro

Agmatine prevents the manifestation of impulsive burying and depression-like behaviour in progesterone withdrawn female rats

Premenstrual dysphoric disorder (PMDD) is characterized by various physical and affective symptoms, including anxiety, irritability, anhedonia, social withdrawal, and depression. The present study investigated the role of the agmatinergic system in animal model of progesterone withdrawal in female rats. Chronic progesterone exposure of female rats for 21 days and its abrupt withdrawal showed enhanced marble burying, increased immobility time, and reduced no. of entries in open arm as compared to control animals. The progesterone withdrawal-induced enhanced marble burying anxiety and immobility time was significantly attenuated by agmatine (5-20 mg/kg, i.p.), and its endogenous modulators like L-arginine (100 mg/kg, i.p.), amino-guanidine (25 mg/kg, i.p.) and arcaine (50 mg/kg, i.p.) by their once-daily administration from day 14-day 21 of the protocol. We have also analysed the levels of agmatine, progesterone, and inflammatory cytokines in the hippocampal region of progesterone withdrawn rats. There was a significant decline in agmatine and progesterone levels and an elevation in cytokine levels in the hippocampal region of progesterone withdrawn rats compared to the control animals. In conclusion, the present studies suggest the importance of the endogenous agmatinergic system in progesterone withdrawal-induced anxiety-like and depression-like behaviour. The data also projects agmatine as a potential therapeutic target for the premenstrual dysphoric disorder.

Distribution of nitric oxide in the rock cavy (Kerodon rupestris) brain II: The brainstem

In a recent paper, we described the distribution of Nitric oxide (NO) in the diencephalon of the rock cavy (Kerodon rupestris). This present paper follows this work, showing the distribution of NO synthesizing neurons in the rock cavy's brainstem. For this, we used immunohistochemistry against the neuronal form of nitric oxide synthase (NOS) and NADPH diaphorase histochemistry. In contrast to the diencephalon in the rock cavy, where the NOS neurons were seen to be limited to some nuclei in the thalamus and hypothalamus, the distribution of NOS in the brainstem is widespread. Neurons immunoreactive to NOS (NOS-ir) were seen as rostral as the precommissural nuclei and as caudal as the caudal and gelatinous parts of the spinal trigeminal nucleus. Places such as the raphe nuclei, trigeminal complex, superior and inferior colliculus, oculomotor complex, periaqueductal grey matter, solitary tract nucleus, laterodorsal tegmental nucleus, pedunculopontine tegmental, and other nuclei of the reticular formation are among the locations with the most NOS-ir neurons. This distribution is similar, but with some differences, to those described for other rodents, indicating that NO also has an important role in rock cavy's physiology.

Effect of agmatine on food intake in mandarin fish (Siniperca chuatsi)

Agmatine, an endogenous biogenic amine, is considered to be a central neurotransmitter. And it plays an important role in mammal feeding behavior. However, there were few studies on the effect of agmatine on feeding behavior in fishes. Here, we investigated the impact of intracerebroventricular (ICV) injections of agmatine (1.25-20 nmol/fish) on food intake in mandarin fish (Siniperca chuatsi). At 1-h post-injection, food intake showed a significant decrease in agmatine-treated fishes compared with the saline treated. Furthermore, the food intake in agmatine treatment mostly did not differ from that in saline treatment at 4--24-h post-injection as well as the results of genes expression of neuropeptide Y (NPY), agouti-regulated peptide (AgRP), and anorexigenic melanocortin 4 receptor (MC4R). In accordance with the insulin level increasing in liver, the gene expression of insulin receptor substrate (IRS2) was significantly higher in agmatine treatment compared to saline treatment at 1-h post-injection. Thus, the anorexigenic effect of agmatine is likely to decrease NPY and AgRP expression levels and increase MC4R and IRS2 levels which was coupled with stimulation of insulin secretion. Although these initial findings are limited in dose, the data firstly provides evidence for the anorectic effects of agmatine in fish.

Agmatine: multifunctional arginine metabolite and magic bullet in clinical neuroscience?

Agmatine, the decarboxylation product of arginine, was largely neglected as an important player in mammalian metabolism until the mid-1990s, when it was re-discovered as an endogenous ligand of imidazoline and α2-adrenergic receptors. Since then, a wide variety of agmatine-mediated effects have been observed, and consequently agmatine has moved from a wallflower existence into the limelight of clinical neuroscience research. Despite this quantum jump in scientific interest, the understanding of the anabolism and catabolism of this amine is still vague. The purification and biochemical characterization of natural mammalian arginine decarboxylase and agmatinase still are open issues. Nevertheless, the agmatinergic system is currently one of the most promising candidates in order to pharmacologically interfere with some major diseases of the central nervous system, which are summarized in the present review. Particularly with respect to major depression, agmatine, its derivatives, and metabolizing enzymes show great promise for the development of an improved treatment of this common disease.

Agmatine in the hypothalamic paraventricular nucleus stimulates feeding in rats: involvement of neuropeptide Y

BACKGROUND AND PURPOSE

Agmatine, a multifaceted neurotransmitter, is abundantly expressed in the hypothalamic paraventricular nucleus (PVN). Our aim was to assess (i) the effect of agmatine on feeding behaviour and (ii) its association, if any, with neuropeptide Y (NPY).

EXPERIMENTAL APPROACH

Satiated rats fitted with intra-PVN cannulae were administered agmatine, alone or jointly with (i) α₂-adrenoceptor agonist, clonidine, or antagonist, yohimbine; (ii) NPY, NPY Y₁ receptor agonist, [Leu³¹, Pro³⁴]-NPY, or antagonist, BIBP3226; or (iii) yohimbine and NPY. Cumulative food intake was monitored at different post-injection time points. Furthermore, the expression of hypothalamic NPY following i.p. treatment with agmatine, alone or in combination with yohimbine (i.p.), was evaluated by immunocytochemistry.

KEY RESULTS

Agmatine robustly increased feeding in a dose-dependent manner. While pretreatment with clonidine augmented, yohimbine attenuated the orexigenic response to agmatine. Similarly, NPY and [Leu³¹, Pro³⁴]-NPY potentiated the agmatine-induced hyperphagia, whereas BIBP3226 inhibited it. Moreover, yohimbine attenuated the synergistic orexigenic effect induced by the combination of NPY and agmatine. Agmatine increased NPY immunoreactivity in the PVN fibres and in the cells of the hypothalamic arcuate nucleus (ARC) and this effect was prevented by pretreatment with yohimbine. NPY immunoreactivity in the fibres of the ARC, dorsomedial, ventromedial and lateral nuclei of the hypothalamus was not affected by any of the above treatments.

CONCLUSIONS AND IMPLICATIONS

The orexigenic effect of agmatine is coupled to increased NPY activity mediated by stimulation of α₂-adrenoceptors within the PVN. This signifies the importance of agmatine or α₂-adrenoceptor modulators in the development of novel therapeutic agents to treat feeding-related disorders.

Expression and localization of an agmatinase-like protein in the rat brain

Agmatinase catalyzes the hydrolysis of agmatine into putrescine and urea, and agmatine (decarboxylated L: -arginine) plays several roles in mammalian tissues, including neurotransmitter/neuromodulatory actions in the brain. Injection of agmatine in animals produces anticonvulsant, antineurotoxic and antidepressant-like actions. Information regarding the enzymatic aspects of agmatine metabolism in mammals, especially related to its degradation, is relatively scarce. The explanation for this is the lack of enzymatically active preparations of mammalian agmatinase. Recently, we have cloned a protein from a cDNA rat brain library having agmatinase activity although its amino acid sequence greatly differs from all known agmatinases, we called agmatinase-like protein. In this work, we analyzed the expression of this enzyme in the rat brain by means of RT-PCR and immunohistochemical analysis using a polyclonal antibody generated against the recombinant agmatinase-like protein. The agmatinase-like protein was detected in the hypothalamus in glial cells and arcuate nucleus neurons, and in hippocampus astrocytes and neurons, but not in brain cortex. In general, detected localization of agmatinase-like protein coincides with that described for its substrate agmatine and our results help to explain several reported effects of agmatine in the brain. Concretely, a role in the regulation of intracellular concentrations of the neurotransmitter/neuromodulator agmatine is suggested for the brain agmatinase-like protein.

Repeated agmatine treatment attenuates nicotine sensitization in mice: modulation by alpha2-adrenoceptors

Agmatine [2-(4-aminobutyl)guanidine] is an endogenous amine proposed as a neurotransmitter/neuromodulator that binds to multiple target receptors in brain. Besides, many central and peripheral functions, agmatine have been implicated in the process of drug addiction. The purpose of the present study was to examine the effects of centrally injected agmatine on nicotine induced locomotor sensitization in Swiss male mice. Our data shows that repeated injections of nicotine (0.4 mg/kg, sc, twice daily for 7 days) gradually increased locomotion during 7 days development period or after 3 days (nicotine) withdrawal phase challenged with nicotine (0.4 mg/kg, sc) on day 11. Mice were pretreated with agmatine (40-80 microg, icv) or agents known to increase endogenous brain agmatine levels [e.g. an agmatine biosynthetic precursor, L-arginine (80 microg, icv), ornithine decarboxylase inhibitor, difluoromethyl-ornithine (50 microg, icv), diamine oxidase inhibitor, aminoguanidine (25 microg, icv) and agmatinase inhibitor, arcaine (50 microg, icv)] 30 min before daily first nicotine injection or during nicotine withdrawal phase. All these treatments attenuated the development as well as incubation of locomotor sensitization to nicotine. Coadministration of agmatine (20 microg, icv) and alpha(2)-adrenoreceptors agonist, clonidine (0.1 microg, icv) evoked synergistic inhibition of nicotine sensitization. Conversely, prior administration of alpha(2)-adrenoceptor antagonist, yohimbine (5mg/kg, ip) or idazoxan (0.4 mg/kg, ip) reversed the inhibitory effect of agmatine on nicotine sensitization. There was no significant difference in activity between mice injected with any of these agents/saline alone and saline/saline groups. These data indicate that agmatine attenuates nicotine induced locomotor sensitization via a mechanism which may involve alpha(2)-adrenergic receptors. Thus, agmatine might have therapeutic implications in the treatment of nicotine addiction and deserve further investigations.

Ocular hypotensive effects of topically administered agmatine in a chronic ocular hypertensive rat model

Agmatine, a primary polyamine and potential neuromodulator, exhibits a high affinity to the alpha2-adrenergic receptor as well as imidazoline receptors. As alpha2-adrenergic receptor agonists display positive ocular hypotensive effects, we assessed whether agmatine effectively lowers intraocular pressure (IOP) using a chronic ocular hypertensive rat model. We raised IOP in unilateral eyes of Sprague-Dawley rats by cauterizing three episcleral veins per eye. Four weeks later, we topically administered 10(-3) M agmatine solution 4 times a day for 6 consecutive weeks. After confirming the recovery of IOP to pretreatment level at 13 weeks after cauterization, the retinal ganglion cells (RGCs) were retrogradely labeled and counted. Eyes subjected to episcleral vein cauterization (EVC) demonstrated significant increases in IOP (48.39% increase over baseline IOP), and the elevated IOP was well maintained until 12 weeks. Topically administered agmatine powerfully lowered IOP to 30.29% of its pretreatment level, and the associated washout period was about two weeks. EVC was associated with a 55.44% loss of RGCs in the control group, but agmatine appeared to attenuate this RGC loss to 18.65%. Overall, topically administered agmatine appeared to effectively lower IOP and rescue RGCs in a chronic ocular hypertensive rat model. Although the mechanism underlying these effects is not yet established, it is possible that agmatine offers a powerful new ocular hypotensive agent for eyes with chronic ocular hypertension and/or glaucoma.

Agmatine enhances cannabinoid action in the hot-plate assay of thermal nociception

Agmatine-cannabinoid interactions are supported by the close association between cannabinoid CB(1) receptors and agmatine immunoreactive neurons and evidence that shared brain mechanisms underlie the pharmacological effects of agmatine and cannabinoid agonists. In the present study, we used the hot-plate assay of thermal nociception to determine if agmatine alters cannabinoid action through activation of imidazoline sites and/or alpha(2)-adrenoceptors. WIN 55212-2 (1, 2 or 3 mg/kg, i.p.) or CP55,940 (1, 2 or 3 mg/kg, i.p.) administration increased hot-plate response latency. Agmatine (50 or 100 mg/kg, i.p.) was ineffective. Administration of agmatine (50 mg/kg, i.p.) with WIN 55212-2 (1, 2 or 3 mg/kg, i.p.) or CP55,940 (1, 2 or 3 mg/kg, i.p.) produced response-latency enhancement. Regression analysis indicated that agmatine increased the potency of WIN 55212-2 and CP55,940 by 3- and 4.4-fold, respectively, indicating synergy for both drug interactions. Idazoxan, a mixed imidazoline site/alpha(2)-adrenoceptor antagonist, but not yohimbine (5 mg/kg, i.p.), a selective alphia(2)-adrenoceptor antagonist, blocked response-latency enhancement produced by a combination of WIN 55212-2 (2 mg/kg) and agmatine. Response-latency enhancement produced by WIN 55212-2 (2 mg/kg) was blocked by SR 141716A (5 mg/kg, i.p.), a cannabinoid CB(1) receptor antagonist; attenuated by idazoxan (2 and 5 mg/kg); and not affected by yohimbine (5 mg/kg). These results demonstrate a synergistic interaction between agmatine and cannabinoid agonists and suggest that agmatine administration enhances cannabinoid action in vivo.

Agmatine: identification and inhibition of methamphetamine, kappa opioid, and cannabinoid withdrawal in planarians

Agmatine blocks morphine physical dependence in mammals, but its effects on withdrawal signs caused by other abused drugs have been less studied. One of the reasons is that withdrawal to some of these drugs is difficult to quantify in mammals. An alternative to mammals is planarians, a type of flatworm. Planarians possess mammalian-like neurotransmitters and display withdrawal from amphetamines, benzodiazepines, cannabinoids, cocaine, and opioids. The withdrawal is manifested as a reduction in locomotor behavior following discontinuation of drug exposure. In the present study, our goal was to identify agmatine in planarians and to determine if planarians exposed to agmatine display withdrawal to methamphetamine, a cannabinoid receptor agonist (WIN 55,212-2), or a kappa-opioid receptor agonist (U-50,488H). Neurochemical experiments revealed that the concentration of agmatine in planarians was 185 +/- 33.7 pmol per mg of planarian weight (dry weight). In behavioral experiments, withdrawal (i.e., reduced locomotor activity) was observed when planarians exposed to each drug (10 microM) for 60 min were placed into water. The withdrawal was attenuated when methamphetamine- or U-50,488H-exposed planarians were tested in agmatine (100 microM). Withdrawal was inhibited similarly when planarians coexposed to agmatine (100 microM) plus methamphetamine (10 microM), WIN 55,212-2 (10 microM), or U-50,488H (10 microM) were tested in water. Arginine, the metabolic precursor to agmatine, was ineffective. Our results identify endogenous agmatine in planarians and demonstrate that agmatine exposure blocks withdrawal to three different drugs in planarians. This suggests that a change in agmatine signaling is a common mechanism in the withdrawal caused by these drugs, at least in planarians.

Agmatine : metabolic pathway and spectrum of activity in brain

Agmatine is an endogenous neuromodulator that, based on animal studies, has the potential for new drug development. As an endogenous aminoguanidine compound (1-amino-4-guanidinobutane), it is structurally unique compared with other monoamines. Agmatine was long thought to be synthesised only in lower life forms, until its biosynthetic pathway (decarboxylation of arginine) was described in the mammalian brain in 1994. Human arginine decarboxylase has been cloned and shown to have 48% identity to ornithine decarboxylase. In neurons of the brain and spinal cord, agmatine is packaged into synaptic vesicles and released upon neuronal depolarisation. Other evidence of a neuromodulation role for agmatine is the presence of a specific cellular uptake mechanism and a specific metabolic enzyme (agmatinase; which forms putrescine).Initially, agmatine was conceptualised as an endogenous clonidine-displacing substance of imidazoline receptors; however, it has now been established to have affinity for several transmembrane receptors, such as alpha(2)-adrenergic, imidazoline I(1) and glutamatergic NMDA receptors. In addition to activity at these receptors, agmatine irreversibly inhibits neuronal nitric oxide synthase and downregulates inducible nitric oxide synthase. Endogenous agmatine is induced in response to stress and/or inflammation. Stressful conditions that induce agmatine include hypoxic-ischaemia and cold-restraint stress of ulcerogenic proportion. Induction of agmatine in the brain seems to occur in astrocytes, although neurons also synthesise agmatine. The effects of injected agmatine in animals include anticonvulsant-, antineurotoxic- and antidepressant-like actions. Intraperitoneal or intracerebroventricular injections of agmatine rapidly elicit antidepressant-like behavioural changes in the rodent forced swim test and tail suspension test. Intraperitoneal injections of agmatine into rats and mice also elicit acute anxiolytic-like behavioural changes in the elevated plus-maze stress test. In an animal model of acute stress disorder, intraperitoneal agmatine injections diminish contextual fear learning. Furthermore, intraperitoneal injections of agmatine reduce alcohol and opioid dependence by diminishing behaviour in a rat conditioned place preference paradigm. Based on these findings, agmatine appears to be an endogenous neuromodulator of mental stress. The possible roles and/or beneficial effects of agmatine in stress-related disorders, such as depression, anxiety and post-traumatic stress disorder, merit further investigation.

Agmatine protects retinal ganglion cells from hypoxia-induced apoptosis in transformed rat retinal ganglion cell line

BACKGROUND

Agmatine is an endogenous polyamine formed by the decarboxylation of L-arginine. We investigated the protective effects of agmatine against hypoxia-induced apoptosis of immortalized rat retinal ganglion cells (RGC-5). RGC-5 cells were cultured in a closed hypoxic chamber (5% O2) with or without agmatine. Cell viability was determined by lactate dehydrogenase (LDH) assay and apoptosis was examined by annexin V and caspase-3 assays. Expression and phosphorylation of mitogen-activated protein kinases (MAPKs; JNK, ERK p44/42, and p38) and nuclear factor-kappa B (NF-kappaB) were investigated by Western immunoblot analysis. The effects of agmatine were compared to those of brain-derived neurotrophic factor (BDNF), a well-known protective neurotrophin for retinal ganglion cells.

RESULTS

After 48 hours of hypoxic culture, the LDH assay showed 52.3% cell loss, which was reduced to 25.6% and 30.1% when agmatine and BDNF were administered, respectively. This observed cell loss was due to apoptotic cell death, as established by annexin V and caspase-3 assays. Although total expression of MAPKs and NF-kappaB was not influenced by hypoxic injury, phosphorylation of these two proteins was increased. Agmatine reduced phosphorylation of JNK and NF-kappaB, while BDNF suppressed phosphorylation of ERK and p38.

CONCLUSION

Our results show that agmatine has neuroprotective effects against hypoxia-induced retinal ganglion cell damage in RGC-5 cells and that its effects may act through the JNK and NF-kappaB signaling pathways. Our data suggest that agmatine may lead to a novel therapeutic strategy to reduce retinal ganglion cell injury related to hypoxia.

Effect of agmatine on brain l-citrulline production during morphine withdrawal in rats: A microdialysis study in nucleus accumbens

Agmatine, an endogenous nitric oxide (NO) synthase inhibitor and ligand for imidazoline receptors, has been previously shown to prevent morphine dependence in rats. The present study was designed to investigate NO formation in nucleus accumbens core region (NAcc) during naloxone (NL)-precipitated morphine withdrawal in rats treated with agmatine or l-NAME by using intracerebral microdialysis in freely moving rats, through measuring extracellular l-citrulline concentrations, an indirect sign of NO production since equal amounts of l-citrulline and NO are produced from l-arginine. l-Citrulline levels in the NAcc core did not change following administration of agmatine (40 mg/kg i.p.) or l-NAME (100 mg/kg i.p.) in control rats. Both agmatine and l-NAME attenuated withdrawal symptoms of morphine in NL (2 mg/kg i.p.)-precipitated withdrawal. l-Citrulline levels showing the release of NO increased in morphine-dependent rats during NL-precipitated withdrawal. Agmatine and l-NAME treatments significantly suppressed the increase in l-citrulline levels compared to physiological saline-treated rats in this setting. The results suggest that the release of l-citrulline in NAcc may be involved in the processes of morphine withdrawal and agmatine as an endogenous inhibitor of NO synthase may be one of the factors involved in the changes in the physiology and behavioral state during opioid withdrawal and may have pharmacological importance.

Cloning and functional expression of a rodent brain cDNA encoding a novel protein with agmatinase activity, but not belonging to the arginase family

A rat brain cDNA encoding for a novel protein with agmatinase activity was cloned and functionally expressed. The protein was expressed as a histidine-tagged fusion product with a molecular weight of about 63 kDa. Agmatine hydrolysis was strictly dependent on Mn(2+); K(m) and k(cat) values were 2.5+/-0.2 mM and 0.8+/-0.2 s(-1), respectively. The product putrescine was a linear competitive inhibitor (K(i)=5+/-0.5 mM). The substrate specificity, metal ion requirement and pH optimum (9.5) coincide with those reported for Escherichia coli agmatinase, the best characterized of the agmatinases. However, as indicated by the k(cat)/K(m) (320 M(-1)s(-1)), the recombinant protein was about 290-fold less efficient than the bacterial enzyme. The deduced amino sequence revealed great differences with all known agmatinases, thus excluding the protein from the arginase family. It was, however, highly identical (>85%) to the predicted sequences for fragments of hypothetical or unnamed LIM domain-containing proteins. As a suggestion, the agmatinase activity is adscribed to a protein with an active site that promiscuously catalyze a reaction other than the one it evolved to catalyze.

Agmatine and a cannabinoid agonist, WIN 55212-2, interact to produce a hypothermic synergy

Agmatine blocks morphine withdrawal symptoms and enhances morphine analgesia in rats. Yet, the role of agmatine in the pharmacological effects of other abused drugs has not been investigated. The present study investigates the effect of agmatine administration on the hypothermic response to cannabinoids. Hypothermia is an effective endpoint because cannabinoid agonists produce a rapid, reproducible, and significant decrease in body temperature that is abolished by cannabinoid CB(1) receptor antagonists. WIN 55212-2, a cannabinoid agonist, was administered to rats by itself and with agmatine. WIN 55212-2 (1, 2.5, 5 and 10 mg/kg, i.m.) caused a significant hypothermia. Agmatine (10, 25 and 50 mg/kg, i.p.) was ineffective. For combined administration, agmatine (50 mg/kg, i.p.) enhanced the hypothermic effect of WIN 55212-2 (1, 2.5, 5 and 10 mg/kg, i.m.). The enhancement was strongly synergistic, indicated by a 2.7-fold increase in the relative potency of WIN 55212-2. The central administration of agmatine (25 and 50 mug/rat, i.c.v.) significantly increased the hypothermic effect of WIN 55212-2 (2.5 mg/kg, i.m.). This indicates that agmatine acts through a central mechanism to augment cannabinoid-evoked hypothermia. Idazoxan (2 mg/kg, i.p.), an imidazoline antagonist, blocked the enhancement by agmatine, thus suggesting that imidazoline receptor activation is required for agmatine to enhance cannabinoid-evoked hypothermia. The present data reveal that agmatine and a cannabinoid agonist interact to produce a hypothermic synergy in rats. These results show that agmatine acts in the brain and via imidazoline receptors to enhance cannabinoid-evoked hypothermia.

Determination of agmatine in biological samples by capillary electrophoresis with optical fiber light-emitting-diode-induced fluorescence detection

A capillary electrophoresis (CE)/optical fiber light-emitting diode (LED)-induced fluorescence detection method is developed for the determination of agmatine in biological samples. The agmatine was precolumn-derivatized with fluorescence tagging reagent, fluorescein isothiocyanate (FITC). Optimal separation and determination for agmatine were obtained with an electrophoretic buffer of 20 mM sodium borate (pH 9.2). Under the optimal conditions, the determination of agmatine was achieved in less than 4 min, and the detection limit was 4.1x10(-9) M (S/N = 3). The relative standard deviation (RSD) for 11 parallel determination of agmatine was less than 3.0%. The present CE-LED induced fluorescence detection method has been applied to detect agmatine in rat brain tissue, rat stomach tissue, human serum, and human urine. The level of agmatine in human urine was quantified by CE for the first time and found to be in the range 2.5-4.1x10(-7) M.

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.

Expression of arginine decarboxylase in brain regions and neuronal cells

After our initial report of a mammalian gene for arginine decarboxylase, an enzyme for the synthesis of agmatine from arginine, we have determined the regional expression of ADC in rat. We have analyzed the expression of ADC in rat brain regions by activity, protein and mRNA levels, and the regulation of expression in neuronal cells by RNA interference. In rat brain, ADC was widely expressed in major brain regions, with a substantial amount in hypothalamus, followed by cortex, and with least amounts in locus coeruleus and medulla. ADC mRNA was detected in primary astrocytes and C6 glioma cells. While no ADC message was detected in fresh neurons (3 days old), significant message appeared in differentiated neurons (3 weeks old). PC12 cells, treated with nerve growth factor, had higher ADC mRNA compared with naive cells. The siRNA mixture directed towards the N-terminal regions of ADC cDNA down-regulated the levels of mRNA and protein in cultured neurons/C6 glioma cells and these cells produced lower agmatine. Thus, this study demonstrates that ADC message is expressed in rat brain regions, that it is regulated in neuronal cells and that the down-regulation of ADC activity by specific siRNA leads to lower agmatine production.

Determination of agmatine in biological samples by capillary electrophoresis with chemiluminescence detection

A fast and simple method based on capillary electrophoresis (CE) with chemiluminescence (CL) detection has been developed for the determination of agmatine, a recently identified neurotransmitter/modulator. The CE run time was approximately 2 min for each sample injected. CL detection employed a lab-built reaction flow cell and a photon counter. The CL reagents used were luminol and NaBrO. The optimized conditions for the CL detection were 5 x 10(-4)M luminol added to the CE running buffer and 5.0 x 10(-4)M NaBrO in 100 mM NaCO3-NaOH buffer solution at pH 12.5 introduced post column. Detection limit for agmatine was 4.3 x 10(-6)M (S/N=3). The precision (R.S.D.) on peak height (at 1 x 10(-5)M agmatine) and migration time were 3.7 and 2.5%, respectively. The present CE-CL method was evaluated with the determination of agmatine in tissue samples taken from rat brain, and rat and monkey stomachs. Samples were directly injected into the CE-CL system after the removal of proteins. A higher level of agmatine was detected in the stomach samples. Agmatine concentrations in the tissue samples taken from rat and monkey stomachs were similar at approximately 1950 ng/g wet tissue.

Effect of agmatine on the development of morphine dependence in rats: potential role of cAMP system

Agmatine is an endogenous amine derived from arginine that potentiates morphine analgesia and blocks symptoms of naloxone-precipitated morphine withdrawal in rats. In this study, we sought to determine whether treatment with agmatine during the development of morphine dependence inhibits the withdrawal symptoms and that the effect is mediated by cAMP system. Exposure of rats to morphine for 7 days resulted in marked naloxone-induced withdrawal symptoms and agmatine treatment along with morphine significantly decreasing the withdrawal symptoms. The levels of cAMP were markedly increased in morphine-treated rat brain slices when incubated with naloxone and this increase was significantly reduced in rats treated with morphine and agmatine. The induction of tyrosine hydroxylase after morphine exposure was also reduced in locus coeruleus when agmatine was administered along with morphine. We conclude that agmatine reduces the development of dependence to morphine and that this effect is probably mediated by the inhibition of cAMP signaling pathway during chronic morphine exposure.

Agmatine reduces only peripheral-related behavioral signs, not the central signs, of morphine withdrawal in nNOS deficient transgenic mice

Agmatine inhibits morphine tolerance/dependence and potentiates morphine analgesia. This study was designed to investigate whether neuronal nitric oxide mediates the actions of agmatine in morphine dependence by using mice lacking a functional form of this enzyme. Mice received agmatine just after the morphine pellet implantation for 3 days twice daily or single injection 30 min before naloxone. In both genotypes treated for 3 days with morphine pellets, naloxone administration precipitated clear signs of withdrawal. Both acute and chronic administration of agmatine reduced withdrawal signs in wild type mice and reduced only peripheral signs of morphine dependence in neuronal nitric oxide synthase knockout mice. Withdrawal signs, that are related to central nervous system activity were not affected. These findings indicate that neuronal nitric oxide synthase partly mediates the effects of agmatine in morphine physical dependence.

Spinal antinociceptive effect of agmatine and tentative analysis of involved receptors: study in an electrophysiological model of rats

The present study was designed to evaluate the antinociceptive effect of agmatine and the receptors involved at the spinal level by using an experimental model in which agmatine was intrathecally (i.t.) administered while the changes of nociceptively-evoked discharges in thalamic parafascicular (PF) neurons were monitored in anesthetized Wistar rats. The results showed that: (1) i.t. agmatine dose-dependently suppressed the nociceptive discharges of PF neurons induced by the tail pinch; (2) i.t. yohimbine did not block the agmatine-induced suppressive effect of nociceptive discharges in these neurons; and (3) the agmatine-induced suppression could be blocked significantly by i.t. idazoxan. The results suggest that agmatine suppresses the transmission of nociceptive inputs at the spinal level mainly through the activation of imidazoline receptors other than alpha(2)-adrenoceptors.

Agmatine suppresses nitric oxide production and attenuates hypoxic-ischemic brain injury in neonatal rats

Nitric oxide and excitatory amino acids contribute to hypoxic-ischemic brain injury. Agmatine, an endogenous neurotransmitter or neuromodulator, is an inhibitor of nitric oxide synthase and an antagonist of N-methyl-D-aspartate receptors. Does agmatine reduce brain injury in the rat pup hypoxic-ischemic model? Seven-day old rat pups had right carotid arteries ligated followed by 2.5 h of hypoxia (8% oxygen). Agmatine or vehicle was administered by i.p. injection at 5 min after reoxygenation and once daily thereafter for 3 d. Brain damage was evaluated by weight deficit of the right hemisphere at 22 d after hypoxia by a blinded observer. Agmatine treatments significantly reduced weight loss in the right hemisphere from -30.5 +/- 3.6% in vehicle-treated pups (n = 22) to -15.6 +/- 4.4% in the group treated with 50 mg/kg (n = 18, p < 0.05) and to -15.0 +/- 3.7% in the group treated with 100 mg/kg (n = 18, p < 0.05), but the group treated with 150 mg/kg showed no reduction. Other pups received agmatine or vehicle at 5 min after reoxygenation, and brain biochemistry was assessed. Levels of endogenous brain agmatine rose 2- to 3-fold owing to hypoxic-ischemic (3 h), whereas pups treated with agmatine (100 mg/kg) showed 50-fold higher brain agmatine levels (3 h). Agmatine (100 mg/kg) blocked a hypoxia-induced increase in brain nitric oxide metabolites at 6 h (vehicle-treated, +60.2 +/- 15.2%; agmatine-treated, +4.2 +/- 8.4%; p < 0.05). Agmatine thus reduces brain injury in the neonatal rat hypoxic-ischemic model, probably by blunting the rise in nitric oxide metabolites normally seen after hypoxia.

Effect of chronic morphine treatment on the biosynthesis of agmatine in rat brain and other tissues

Agmatine is an endogenous amine derived from the decarboxylation of arginine by arginine decarboxylase (ADC), and metabolized to putrescine by agmatinase. Exogenously administered agmatine has several biological actions including its ability to potentiate morphine analgesia and block symptoms of morphine tolerance/withdrawal in rats. To investigate the role of endogenous agmatine in this action, we sought to determine whether chronic exposure to morphine and induction of withdrawal modulate the synthesis of agmatine in rat brain and other tissues. Exposure of rats to morphine for three days significantly decreases the activity of ADC and the levels of agmatine in rat liver, kidney, brain, aorta and intestine with no changes in agmatinase activity. The precipitation of withdrawal syndrome by injecting naloxone further decreases ADC activity and agmatine levels in these tissues. We conclude that endogenous agmatine may play an important role in regulating morphine tolerance/dependence and withdrawal symptoms.

Evidence for endogenous agmatine in hypothalamo-neurohypophysial tract and its modulation on vasopressin release and Ca2+ channels

Agmatine, decarboxylated from arginine by arginine decarboxylase, is particularly prominent in the hypothalamus. The present study utilized the rat hypothalamo-neurohypophysial system to determine expression and "pre-synaptic" modulation of agmatine in the central nervous system (CNS). Under confocal-laser scanning, agmatine-like immunoreactivity (Agm-LI) was found enriched in arginine-vasopressin (AVP)-containing magnocellular neurons of the supraoptic nuclei (SON) and paraventricular nuclei (PVN). In addition, using electron microscopy, Agm-LI was found closely associated with large neurosecretory-like vesicles in neurohypophysial nerve terminals of posterior pituitary gland. Radioimmunoassay revealed that 10 and 30 microM agmatine concentration-dependently inhibited the depolarization-evoked AVP release from isolated neurohypophysial terminals. Using perforated patch-clamp, effects of agmatine on whole-terminal voltage-gated ion currents in the isolated neurohypophysial nerve terminals were examined. While it did not significantly affect either tetrodotoxin (TTX)-sensitive Na(+) or sustained Ca(2+)-activated K(+) channel currents, agmatine (1-40 microM) inhibited Ca(2+) channel currents in approximately 53% of the total nerve terminals investigated. The onset of inhibitory effect was immediate, and the inhibition was reversible and concentration-dependent with an IC(50)=4.6 microM. In the remaining (approximately 47%) neurohypophysial nerve terminals, only a higher (120 microM) concentration of agmatine could moderately inhibit Ca(2+) channel currents. The results suggest that: (1) endogenous agmatine is co-expressed in AVP-containing, hypothalamic magnocellular neurons of the SON/PVN and in neurohypophysial nerve terminals of posterior pituitary gland; (2) agmatine may serve as a physiological neuromodulator by regulating the voltage-gated Ca(2+) channel and, as a result, the release of AVP from neurohypophysial nerve terminals.

Effects of polyamines on the release of gonadotropin-releasing hormone and gonadotropins in developing female rats

Polyamines, putrescine (PUT), spermidine (SPD), spermine (SPM), and agmatine (AGM), are polycationic amines related to multiple cell functions found in high concentrations during the development of hypothalamus and pituitary. In previous works, we demonstrated that alpha-difluoromethylornithine (DFMO), an inhibitor of polyamines biosynthesis, induced a delay in puberty of female rats, accompanied by high, sustained follicle-stimulating hormone (FSH) levels during the infantile period. Also, DFMO treatment induced changes in polyamine concentration both in hypothalamus and pituitary of rats, mainly a decrease of PUT and SPD, an increase in SPM, and no change in AGM. In the present work, we investigated the direct effects of polyamines on the secretion of hypothalamic GnRH and pituitary gonadotropins in 6- and 15-day-old female rats. In 6-day-old animals, in vitro incubations with PUT, SPD, and AGM of hypothalami or anterior pituitaries were able to inhibit GnRH, FSH, and leutinizing hormone (LH) secretion, respectively. SPM showed a nonspecific transient inhibitory effect on FSH. When challenged with either high K(+) (hypothami) or GnRH (pituitaries), the tissues incubated in the presence of polyamines showed no differences when compared with their controls. No effects of polyamines in 15-day-old rats in either tissue were observed. Pituitary cell cultures of 6-day-old animals incubated with DFMO for 4 days showed a significant increase in FSH, but not in LH. We conclude that high PUT, SPD, and AGM levels during the first 10 days of life are important for the development of the hypothalamic-hypophyseal unit, probably related to an inhibitory effect on GnRH and gonadotropins. Therefore, polyamine participation, especially PUT and SPD, is of importance in the regulation of GnRH and gonadotropin secretion in the neonatal and infantile periods, critical stages in the establishment of sexual differentiation.

Effect of agmatine on locus coeruleus neuron activity: possible involvement of nitric oxide

1. To investigate whether agmatine (the proposed endogenous ligand for imidazoline receptors) controls locus coeruleus neuron activity and to elucidate its mechanism of action, we used single-unit extracellular recording techniques in anaesthetized rats. 2. Agmatine (10, 20 and 40 microg, i.c.v.) increased in a dose-related manner the firing rate of locus coeruleus neurons (maximal increase: 95 +/- 13% at 40 microg). 3. I(1)-imidazoline receptor ligands stimulate locus coeruleus neuron activity through an indirect mechanism originated in the paragigantocellularis nucleus via excitatory amino acids. However, neither electrolytic lesions of the paragigantocellularis nucleus nor pretreatment with the excitatory amino acid antagonist kynurenic acid (1 micromol, i.c.v.) modified agmatine effect (10 microg, i.c.v.). 4. After agmatine administration (20 microg, i.c.v.), dose-response curves for the effect of clonidine (0.625 - 10 microg kg(-1) i.v.) or morphine (0.3 - 4.8 mg kg(-1) i.v.) on locus coeruleus neurons were not different from those obtained in the control groups. 5. Pretreatment with the nitric oxide synthase inhibitors N(omega)-nitro-L-arginine (10 microg, i.c.v.) or N(omega)-nitro-L-arginine methyl ester (100 microg, i.c.v.) but not with the less active stereoisomer N(omega)-nitro-D-arginine methyl ester (100 microg, i.c.v.) completely blocked agmatine effect (10 and 40 microg, i.c.v.). 6. Similarly, when agmatine (20 pmoles) was applied into the locus coeruleus there was an increase that was blocked by N(omega)-nitro-L-arginine methyl ester (100 microg, i.c.v.) in the firing rate of the locus coeruleus neurons (maximal increase 53 +/- 11% and 14 +/- 10% before and after nitric oxide synthase inhibition, respectively). 7. This study demonstrates that agmatine stimulates the firing rate of locus coeruleus neurons via a nitric oxide synthase-dependent mechanism located in this nucleus.

Localization of agmatine in vasopressin and oxytocin neurons of the rat hypothalamic paraventricular and supraoptic nuclei

Agmatine (decarboxylated l-arginine), an endogenous ligand of imidazoline and alpha(2) adrenoreceptors, is particularly enriched in the rat hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. The present study utilized light and electron microscopic immunocytochemical methods to determine the distribution and extent of colocalization of agmatine relative to subpopulations of vasopressin- (VP) and oxytocin- (OT) producing neurons in PVN and SON nuclei. By light microscopy, agmatine-immunoreactive perikarya were found in both the magnocellular and the parvocellular neuronal subdivisions of PVN and SON. Confocal and electron microscopy revealed that agmatine-immunoreactivity (I) within neuronal perikarya was associated with the nuclear membrane as well as mitochondria, Golgi complexes, endoplasmic reticula, and plasmalemma. Additionally, agmatine-I was identified in both axons and axonal terminals, which were enriched in large dense-core vesicles. Dual and triple immunocytochemical labeling experiments also demonstrated that agmatine coexists with VP or OT in most PVN and SON magnocellular neurons. Combinations of iontophoretic injections of Fluorogold into the dorsomedullary complex with immunocytochemical labeling revealed that many retrogradely labeled neurons in the parvocellular region of the PVN contained agmatine-I and either VP or OT. These findings provide evidence that agmatine may function as a modulator of both hypothalamically mediated neuroendocrine and autonomic responses.

Is agmatine a novel neurotransmitter in brain?

Recent evidence suggests that agmatine, which is an intermediate in polyamine biosynthesis, might be an important neurotransmitter in mammals. Agmatine is synthesized in the brain, stored in synaptic vesicles in regionally selective neurons, accumulated by uptake, released by depolarization, and inactivated by agmatinase. Agmatine binds to alpha2-adrenoceptors and imidazoline binding sites, and blocks NMDA receptor channels and other ligand-gated cationic channels. Furthermore, agmatine inhibits nitric oxide synthase, and induces the release of some peptide hormones. As a result of its ability to inhibit both hyperalgesia and tolerance to, and withdrawal from, morphine, and its neuroprotective activity, agmatine has potential as a treatment of chronic pain, addictive states and brain injury.

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.

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.

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.

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.

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.

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.

Determination of agmatine in brain and plasma using high-performance liquid chromatography with fluorescence detection

Decarboxylated arginine, agmatine, is a neurotransmitter candidate for imidazoline receptors. A method is described to measure agmatine in rat brain and human plasma by isocratic high-performance liquid chromatography (HPLC) with fluorescence detection and o-phthalaldehyde derivatization. Quantitation is based on the method of additions of internal agmatine spikes. This assay has sensitivity in the low picomole range and a detection limit of 100 fmol. The correlation coefficient for the agmatine standard curve was 0.999+/-0.001 S.D., and intra- and inter-assay C.V.s were less than 8%. The accuracy of our isocratic method compared favorably with a gradient HPLC protocol, originally developed for bacterial agmatine, which we modified for use with tissues. Agmatine concentrations in rat brain were proportioned similarly to the regional distribution of imidazoline-1 receptors. These methods can be used as reliable research tools in various biological samples.

Inhibition of mammalian nitric oxide synthases by agmatine, an endogenous polyamine formed by decarboxylation of arginine

Agmatine, decarboxylated arginine, is a metabolic product of mammalian cells. Considering the close structural similarity between L-arginine and agmatine, we investigated the interaction of agmatine and nitric oxide synthases (NOSs), which use L-arginine to generate nitric oxide (NO) and citrulline. Brain, macrophages and endothelial cells were respectively used as sources for NOS isoforms I, II and III. Enzyme activity was measured by the production of nitrites or L-citrulline. Agmatine was a competitive NOS inhibitor but not an NO precursor. Ki values were approx. 660 microM (NOS I), 220 microM (NOS II) and 7.5 mM (NOS III). Structurally related polyamines did not inhibit NOS activity. Agmatine, therefore, may be an endogenous regulator of NO production in mammals.