Antidepressant-like effect of agmatine and its possible mechanism

In mammalian brain, agmatine is an endogenous neurotransmitter and/or neuromodulator, which is considered as an endogenous ligand for imidazoline receptors. In this study, the antidepressant-like action of agmatine administered p.o. or s.c. was evaluated in three behavioral models in mice or rats. Agmatine at doses 40 and 80 mg/kg (p.o.) reduced immobility time in the tail suspension test and forced swim test in mice or at dose 20 mg/kg (s.c.) in the forced swim test. Agmatine also reduced immobility time at 10 mg/kg (p.o.) or at 1.25, 2.5 and 5 mg/kg (s.c.) in the forced swim test in rats. These results firstly indicated that agmatine possessed an antidepressant-like action. With 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay and lactic dehydrogenase (LDH) assay, 1, 10 and 100 microM agmatine or a classical antidepressant, 2.5 and 10 microM desipramine, protected PC12 cells from the lesion induced by 300 microM N-methyl-D-aspartate (NMDA) treatment for 24 h. Using high-performance liquid chromatography with electrochemical detection (HPLC-ECD), it was found that the levels of monoamines including norepinephrine, epinephrine, dopamine or 5-hydroxytryptamine (5-HT) in PC12 cells decreased after the treatment with 200 microM NMDA for 24 h, while in the presence of 1 and 10 microM agmatine or 1 and 5 microM desipramine, the levels of norepinephrine, epinephrine or dopamine were elevated significantly while 5-HT did not change. Moreover, norepinephrine, 5-HT or dopamine had the same cytoprotective effect as agmatine at doses 0.1, 1 and 10 microM. In the fura-2/AM (acetoxymethyl ester) labeling assay, 1 and 10 microM agmatine, 1 and 5 microM desipramine or monoamines norepinephrine, 5-HT at doses 0.1 and 1 microM attenuated the intracellular Ca(2+) overloading induced by 200 microM NMDA treatment for 24 h in PC12 cells. In summary, we firstly demonstrated that agmatine has an antidepressant-like effect in mice and rats. A classical antidepressant, desipramine, as well as agmatine or monoamines protect the PC12 cells from the lesion induced by NMDA treatment. Agmatine reverses the NMDA-induced intracellular Ca(2+) overloading and the decrease of monoamines (including norepinephrine, epinephrine or dopamine) contents in PC12 cells, indicating that agmatine's antidepressant-like action may be related to its modulation of NMDA receptor activity and/or reversal of the decrease of monoamine contents and Ca(2+) overloading induced by NMDA.     

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.     

Agmatine: Biological role and therapeutic potentials in morphine analgesia and dependence

Agmatine is an amine that is formed by decarboxylation of L-arginine by the enzyme arginine decarboxylase (ADC) and hydrolyzed by the enzyme agmatinase to putrescine. Agmatine binds to several target receptors in the brain and has been proposed as a novel neuromodulator. In animal studies, agmatine potentiated morphine analgesia and reduced dependence/withdrawal. While the exact mechanism is not clear, the interactions with N-methyl-D-aspartate (NMDA) receptors, alpha2-adrenergic receptors, and intracellular cyclic adenosine monophosphate (cAMP) signaling have been proposed as possible targets. Like other monoamine transmitter molecules, agmatine is rapidly metabolized in the periphery and has poor penetration into the brain, which limits the use of agmatine itself as a therapeutic agent. However, the development of agmatinase inhibitors will offer a useful method to increase endogenous agmatine in the brain as a possible therapeutic approach to potentiate morphine analgesia and reduce dependence/withdrawal. This review provides a succinct discussion of the biological role/therapeutic potential of agmatine during morphine exposure/pain modulation, with an extensive amount of literature cited for further details.     

胍丁胺中枢作用的复杂性及多样性

1994年 ,人们发现胍丁胺在牛脑中具有可乐定替代物质的特性。随之的研究表明 ,胍丁胺的作用机制复杂 ,作用靶点多样 ,生物学效应广泛。最近的研究显示 ,胍丁胺是中枢的一种新的神经递质 ,胍丁胺可能是咪唑啉受体的内源性配体     

胍丁胺对NMDA受体药理作用的研究进展

胍丁胺是一种新的神经递质和/或神经调质,是咪唑啉受体的内源性配体。它作为一种阳离子胺类物质,除了咪唑啉受体外,在生物体内还存在多个作用靶点,NMDA受体是其中最重要的作用靶点之一。本文就胍丁胺与NMDA受体在中枢神经系统的分布、胍丁胺在NMDA受体上的作用位点以及胍丁胺通过NMDA受体介导的药理作用等几个方面进行了综述。     

胍丁胺抗阿片依赖研究进展

阿片成瘾,又称为阿片依赖,是一种慢性复发性脑疾病,可引起一系列严重的社会、经济和公共卫生问题。由于阿片依赖的神经生物学机制尚未阐明,目前仍缺乏有效的医学干预手段。研究阿片依赖的神经生物学机制、寻找有效的防复吸药物已成为阿片依赖研究领域的热点。胍丁胺是一种新发现的候选神经递质或调质,被认为是咪唑啉受体的内源性配体,本文主要以我们的研究工作为基础,对外源性胍丁胺的抗阿片依赖作用特点及其可能的作用机制加以综述。     

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.     

胍丁胺抑制小鼠吗啡戒断与其抑制一氧化氮合酶的关系(英文)

目的:观察胍丁胺抑制纳洛酮引起小鼠吗啡戒断跳跃与其抑制一氧化氮合酶(NOS)的关系,方法:用测定[~3H]胍氨酸浓度的方法确定NOS活性,结果:在体外胍丁胺底物竞争性抑制正常和吗啡依赖小鼠小脑、端脑和丘脑NOS活性,纳洛酮引起吗啡依赖小鼠戒断跳跃和小脑、端脑、丘脑NOS活性升高,用吗啡和胍丁胺共同处理小鼠显著抑制纳洛酮促使小鼠戒断跳跃和NOS活性升高的作用,咪唑克生抑制胍丁胺的此作用,结论:胍丁胺对纳洛酮引起戒断跳跃的抑制作用与其通过激活咪唑啉受体和底物竞争性抑制NOS活性相关。     

Agmatine blocks ethanol-induced locomotor hyperactivity in male mice

Ethanol-induced locomotor activity is associated to rewarding effects of ethanol and ethanol dependence. Agmatine is a novel endogenous ligand at α₂-adrenoceptors, imidazoline and N-methyl-d-aspartate (NMDA) receptors, as well as a nitric oxide synthase (NOS) inhibitor. There is no evidence presented for the relationship between the acute locomotor stimulating effect of ethanol and agmatine. Thus, the present study investigated the effects of agmatine on acute ethanol-induced locomotor hyperactivity in mice. Adult male Swiss–Webster mice (26–36 g) were used as subjects. Locomotor activity of the mice was recorded for 30 min immediately following intraperitoneal administration of ethanol (0.5, 1 and 2 g/kg) or saline (n = 8 for each group). Agmatine (5, 10 and 20 mg/kg) or saline was administered intraperitoneally to another four individual groups (n = 8 for each group) of the mice 20 min before the ethanol injection. In these groups, locomotor activity was also recorded immediately following ethanol (0.5 g/kg) injection for 30 min. Ethanol (0.5 g/kg) produced some significant increases in locomotor activity of the mice. Agmatine (5–20 mg/kg) significantly blocked the ethanol (0.5 g/kg)-induced locomotor hyperactivity. These doses of agmatine did not affect the locomotor activity in naive mice when they were administered alone. Our results suggest that agmatine has an important role in ethanol-induced locomotor hyperactivity in mice. There may be a relationship between the addictive psychostimulant effects of the ethanol and central agmatinergic system.     

胍丁胺抑制小鼠吗啡戒断与其抑制一氧化氮合酶的关系

AIM: To study correlation between inhibitions of naloxone-precipitated withdrawal jumps and nitric-oxide synthase (NOS) activity by agmatine. METHODS: NOS activities in mouse brain were measured by determination of concentration of [3H]citrulline, the product of [3H]arginine. RESULTS: Agmatine inhibited NOS activity in naive and morphine-dependent mouse cerebellum, forebrain, and thalamus in substrate-competitive manner in vitro. Naloxone induced withdrawal jumps and an increase in NOS activity in cerebellum, forebrain, and thalamus of abstinent mice. Pretreatment of mice with morphine plus agmatine inhibited the effect of naloxone to precipitate withdrawal jumps and increase in NOS activity. The effect of agmatine was blocked by idazoxan. CONCLUSION: The inhibitory effect of agmatine on naloxone-precipitated withdrawal jumps is related to its inhibition of NOS activity by substrate competitive manner and activation of imidazoline receptors.     

Determination of α(2)-adrenoceptor and imidazoline receptor involvement in augmentation of morphine and oxycodone analgesia by agmatine and BMS182874

Studies have demonstrated that clonidine (α(2)-adrenoceptor and imidazoline receptor agonist) and BMS182874 (endothelin ET(A) receptor antagonist) potentiate morphine and oxycodone analgesia. Agmatine, an endogenous clonidine-like substance, enhances morphine analgesia. However, its effect on oxycodone analgesia and its interaction with endothelin ET(A) receptor antagonists are not known. The present study was performed to determine the effect of agmatine on morphine and oxycodone analgesia and the involvement of α(2)-adrenoceptors, imidazoline receptors, opioid receptors, and endothelin receptors. Antinociception at various time intervals was determined by the tail-flick latency method in mice. Agmatine produced dose-dependent increase in tail-flick latency, while BMS182874 did not produce any change over the 360-min observation period. Agmatine significantly potentiated morphine as well as oxycodone analgesia which was not altered by BMS182874. BMS182874 pretreatment did not increase the analgesic effect produced by agmatine alone. Agmatine-induced potentiation of morphine and oxycodone analgesia was blocked by idazoxan (imidazoline receptor/α(2)-adrenoceptor antagonist) and yohimbine (α(2)-adrenoceptor antagonist). BMS182874-induced potentiation of morphine or oxycodone analgesia was not affected by yohimbine. However, idazoxan blocked BMS182874-induced potentiation of oxycodone but not morphine analgesia. This is the first report demonstrating that agmatine potentiates not only morphine but also oxycodone analgesia in mice. Potentiation of morphine and oxycodone analgesia by agmatine appears to involve α(2)-adrenoceptors, imidazoline receptors, and opioid receptors. In addition, imidazoline receptors may be involved in BMS182874-induced potentiation of oxycodone but not morphine analgesia. It is concluded that agmatine may be used as an adjuvant in opiate analgesia.     

Evidence for imidazoline receptors involvement in the agmatine antidepressant-like effect in the forced swimming test

This study investigated the involvement of the imidazoline receptors in the antidepressant-like effect of agmatine in the forced swimming test. The antidepressant-like effects of agmatine (10 mg/kg, i.p.) in the forced swimming test was blocked by pretreatment of mice with efaroxan (1 mg/kg, i.p., an imidazoline I1/alpha2-adrenoceptor antagonist), idazoxan (0.06 mg/kg, i.p., an imidazoline I2/alpha2-adrenoceptor antagonist) and antazoline (5 mg/kg, i.p., a ligand with high affinity for the I2 receptor). A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with clonidine (0.06 mg/kg, i.p, an imidazoline I1/alpha2-adrenoceptor agonist), moxonidine (0.5 mg/kg, i.p., an imidazoline I1/alpha2-adrenoceptor agonist), antazoline (1 mg/kg, i.p.) and MK-801 (0.001 mg/kg, i.p., a non-competitive NMDA receptor antagonist), but not with efaroxan (1 mg/kg, i.p.) and idazoxan (0.06 mg/kg, i.p.). Pretreatment of mice with yohimbine (1 mg/kg, i.p., an alpha2-adrenoceptor antagonist) blocked the synergistic antidepressant-like effect of agmatine (0.001 mg/kg, i.p.) with clonidine (0.06 mg/kg, i.p). A subeffective dose of MK-801 (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with antazoline (5 mg/kg, i.p.), but not with efaroxan (1 mg/kg, i.p.) or idazoxan (0.06 mg/kg, i.p.). In conclusion, this study suggests that the anti-immobility effect of agmatine in the forced swimming test is dependent on its interaction with imidazoline I1 and I2 receptors.     

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

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

Agmatine, an endogenous modulator of noradrenergic neurotransmission in the rat tail artery.

1. We investigated the vascular effects of agmatine (decarboxylated arginine), an endogenous ligand for alpha 2-adrenoceptors and non-adrenoceptor imidazoline (I-) receptors, present in endothelium, smooth muscle and plasma, using the rat tail artery as a model. 2. While by itself agmatine (10 nM-1 mM) was without effect on isolated arterial rings, at the highest concentration used (1 mM) it slightly increased EC50 values for contractions elicited respectively by the alpha 1- and alpha 2- adrenoceptor agonists methoxamine and clonidine. 3. Agmatine (0.03-1 mM) produced a concentration-dependent transient inhibition of the contractions induced by transmural nerve stimulation (TNS; 200 mA, 0.2 ms, 1 Hz, 10 s). This effect was abolished by the alpha 2-adrenoceptor antagonists, rawolscine and idazoxan. 4. In the presence of rawolscine or idazoxan, agmatine produced a concentration-dependent delayed facilitation of TNS-induced contractions, which was prevented by cocaine. 5. Neither inhibitory nor potentiating actions were produced by agmatine on contractions induced by noradrenaline (NA) administration. 6. Agmatine did not directly affect [3H]-NA uptake in bovine cultured chromaffin cells. 7. Agmatine can regulate vascular function by two opposing actions at sympathetic nerve terminals, with different latencies: a transient inhibition of NA release mediated by prejunctional alpha 2-adrenoceptors and a cocaine-sensitive delayed facilitation the mechanism of which is undetermined at present. 8. The results reveal the existence of a novel endogenous amine modulating NA release in the perivascular sympathetic terminals.     

Activation of imidazoline‐I3 receptors ameliorates pancreatic damage

Agmatine, an endogenous ligand of imidazoline receptors, is reported to exhibit anti‐hyperglycaemic and many other effects. It has been established that the imidazoline I3 receptor is involved in insulin secretion. The current study characterizes the role of the imidazoline I3 receptor in the protection of pancreatic islets. The activity effect of agmatine against on streptozotocin (STZ)‐induced (5 mmol/L) rat β cell apoptosis was examined by using ApoTox‐Glo triplex assay, live/dead cell double staining assay, flow cytometric analysis, and western blot. Imidazoline I3 receptors antagonist KU14R and the phospholipase C inhibitor named U73122 were treated in β cells to investigate the potential signalling pathways. The serum glucose and recovery of insulin secretion were measured in STZ‐treated rats after continuously injected agmatine. The apoptosis in rat β cells was reduced by agmatine in a dose‐dependent manner, cell viability was improved after treatment with agmatine and these effects were suppressed after the blockade of KU14R and U73122. Western blot analysis confirmed that agmatine could decrease caspase‐3 expression and increase the p‐BAD levels. In STZ‐treated rats, injection of agmatine for 4 weeks may significantly lower the serum glucose and recovery of insulin secretion. This improvement of pancreatic islets induced by agmatine was deleted by KU14R in vivo. Agmatine can activate the imidazoline I3 receptor linked with the phospholipase C pathway to induce cell protection against apoptosis induced by a low dose of STZ. This finding provides new insight into the prevention of early stage pancreatic islet damage.     

Agmatine Reverses Sub‐chronic Stress induced Nod‐like Receptor Protein 3 (NLRP3) Activation and Cytokine Response in Rats

The activation of Nod‐like receptor protein 3 (NLRP3) has lately been implicated in stress and depression as an initiator mechanism required for the production of interleukin (IL)‐1β and IL‐18. Agmatine, an endogenous polyamine widely distributed in mammalian brain, is a novel neurotransmitter/neuromodulator, with antistress, anxiolytic and antidepressant‐like effects. In this study, we examined the effect of exogenously administered agmatine on NLRP3 inflammasome pathway/cytokine responses in rats exposed to restraint stress for 7 days. The rats were divided into three groups: stress, stress+agmatine (40 mg/kg; i.p.) and control groups. Agmatine significantly down‐regulated the gene expressions of all stress‐induced NLRP3 inflammasome components (NLRP3, NF‐κB, PYCARD, caspase‐1, IL‐1β and IL‐18) in the hippocampus and prefrontal cortex (PFC) and reduced pro‐inflammatory cytokine levels not only in both brain regions, but also in serum. Stress‐reduced levels of IL‐4 and IL‐10, two major anti‐inflammatory cytokines, were restored back to normal by agmatine treatment in the PFC. The findings of the present study suggest that stress‐activated NLRP3 inflammasome and cytokine responses are reversed by an acute administration of agmatine. Whether antidepressant‐like effect of agmatine can somehow, at least partially, be mediated by the inhibition of NLRP3 inflammasome cascade and relevant inflammatory responses requires further studies in animal models of depression.     

胍丁胺对炎性疼痛的镇痛作用及对吗啡镇痛作用的影响

目的观察胍丁胺对炎性疼痛的镇痛作用及其对吗啡镇痛作用的影响,研究胍丁胺的镇痛作用是否与激动咪唑啉受体或影响受体前谷氨酸和γ-氨基丁酸(gamma-aminobutyr-icacid,GABA)释放有关。方法应用福尔马林致大鼠炎性疼痛模型,观察胍丁胺镇痛和增强吗啡镇痛的作用。应用高效液相色谱技术测定胍丁胺对脊髓切片孵育液中谷氨酸和GABA基础释放量及对高钾诱发神经元去极化引起神经递质释放的影响。结果单侧足底注射5%福尔马林使大鼠出现明显的双相伤害性行为反应。胍丁胺抑制福尔马林引起的第二相疼痛行为反应及痛觉过敏,并增强吗啡对第二相疼痛的镇痛作用,但在第一相疼痛过程中,无明显镇痛和增强吗啡镇痛的作用。咪唑啉受体拮抗剂咪唑克生不能拮抗胍丁胺镇痛及增强吗啡镇痛的作用。1~1000μmol.L-1胍丁胺对脊髓谷氨酸和GABA的基础释放量和高钾诱发谷氨酸和GABA释放量的升高均没有影响。结论胍丁胺对炎性疼痛具有明确的镇痛作用,并明显增强吗啡的镇痛效果,其镇痛机制可能与咪唑啉受体无关,也不是通过在受体前水平抑制谷氨酸或促进GABA释放来实现的。     

Effects of agmatine on the survival rate in rats bled to hemorrhage

Agmatine (CAS 2482-00-0), an amine formed by decarboxylation of L-arginine, interacts with several targets like alpha2-adrenergic, imidazoline and N-methyl-D-aspartic acid (NMDA) receptors and besides it is involved in the nitric oxide mediated effects. It has also been proposed that it possesses vasodilator effects and increases glomerular filtration rate in rats. The aim of this study was to supply evidence for the effects of agmatine in a rat model of hemorrhagic shock and explain the possible mechanisms of action. The iliac arteries and veins of Sprague-Dawley rats were catheterized under urethane anesthesia and around 2 ml/100 g blood was withdrawn within 20 min until the mean arterial blood pressure was stabilized around 25 mmHg. The rats were either pretreated with physiological saline, yohimbine (an alpha2-adrenergic receptor antagonist) or L-arginine (a nitric oxide donor) intravenously before administration of agmatine (300 microg/kg). Agmatine restored blood pressure in rats pretreated with physiological saline where all rats survived. Pretreatment with L-arginine abolished the increase in blood pressure produced by agmatine and the 1 h survival rate decreased to 67% (p < 0.01). Yohimbine pretreatment also suppressed agmatine induced restoration of blood pressure; however, the survival rate was found to be 17% for 3 min. No statistically significant effect was observed in the heart rate responses. These results may suggest that agmatine may increase survival through alpha2-adrenergic receptors and restores blood pressure through nitric oxide and adrenergic mechanisms in rats bled to hemorrhage.     

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.     

Blockade by agmatine of catecholamine release from chromaffin cells is unrelated to imidazoline receptors

The blockade of exocytosis induced by the putative endogenous ligand for imidazoline receptors, agmatine, was studied by using on-line measurement of catecholamine release in bovine adrenal medullary chromaffin cells. Agmatine inhibited the acetylcholine-evoked release of catecholamines in a concentration-dependent manner (IC(50)=366 microM); the K(+)-evoked release of catecholamines was unaffected. Clonidine (100 microM) and moxonidine (100 microM) also inhibited by 75% and 50%, respectively, the acetylcholine-evoked response. In cells voltage-clamped at -80 mV, the intermittent application of acetylcholine pulses elicited whole-cell inward currents (I(ACh)) that were blocked 63% by 1 mM agmatine. The onset of blockade was very fast (tau(on) = 31 ms); the recovery of the current after washout of agmatine also occurred very rapidly (tau(off = 39 ms). Efaroxan (10 microM) did not affect the inhibition of I(ACh) elicited by 1 mM agmatine. I(ACh) was blocked 90% by 100 microM clonidine and 50% by 100 microM moxonidine. The concentration-response curve for acetylcholine to elicit inward currents was shifted to the right in a non-parallel manner by 300 microM agmatine. The blockade of I(ACh) caused by agmatine (100 microM) was similar at various holding potentials, around 50%. When intracellularly applied, agmatine did not block I(ACh). At 1 mM, agmatine blocked I(Na) by 23%, I(Ba) by 14%, I(K(Ca)) by 16%, and I(K(VD)) by 18%. In conclusion, agmatine blocks exocytosis in chromaffin cells by blocking nicotinic acetylcholine receptor currents. In contrast to previous views, these effects seem to be unrelated to imidazoline receptors.