Agmatine produces antinociception in tonic pain in mice

Agmatine prevents the manifestation of impulsive burying and depression-like behaviour in progesterone withdrawn female rats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analysed the levels of agmatine, progesterone, and inflammatory cytokines in the hippocampal region of progesterone withdrawn rats.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in agmatine and progesterone levels and an elevation in cytokine levels in the hippocampal region of progesterone withdrawn rats compared to the control animals.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.

Alzheimer's disease as a chronic maladaptive polyamine stress response

Polyamines are nitrogen-rich polycationic ubiquitous bioactive molecules with diverse evolutionary-conserved functions. Their activity interferes with numerous genes' expression resulting in cell proliferation and signaling modulation. The intracellular levels of polyamines are precisely controlled by an evolutionary-conserved machinery. Their transient synthesis is induced by heat stress, radiation, and other traumatic stimuli in a process termed the polyamine stress response (PSR). Notably, polyamine levels decline gradually with age; and external supplementation improves lifespan in model organisms. This corresponds to cytoprotective and reactive oxygen species scavenging properties of polyamines. Paradoxically, age-associated neurodegenerative disorders are characterized by upsurge in polyamines levels, indicating polyamine pleiotropic, adaptive, and pathogenic roles. Specifically, arginase overactivation and arginine brain deprivation have been shown to play an important role in Alzheimer's disease (AD) pathogenesis. Here, we assert that a universal short-term PSR associated with acute stimuli is beneficial for survival. However, it becomes detrimental and maladaptive following chronic noxious stimuli, especially in an aging organism. Furthermore, we regard cellular senescence as an adaptive response to stress and suggest that PSR plays a central role in age-related neurodegenerative diseases' pathogenesis. Our perspective on AD proposes an inclusive reassessment of the causal relationships between the classical hallmarks and clinical manifestation. Consequently, we offer a novel treatment strategy predicated upon this view and suggest fine-tuning of arginase activity with natural inhibitors to preclude or halt the development of AD-related dementia.

Cannabinoids and agmatine as potential therapeutic alternatives for cisplatin-induced peripheral neuropathy

Cisplatin is a widely used antineoplastic agent in the treatment of various cancers. Peripheral neuropathy is a well-known side effect of cisplatin and has the potential to result in limiting and/or reducing the dose, decreasing the quality of life. Unfortunately, the mechanism for cisplatin-induced neuropathy has not been completely elucidated. Currently, available treatments for neuropathic pain (NP) are mostly symptomatic, insufficient and are often linked with several detrimental side effects; thus, effective treatments are needed. Cannabinoids and agmatine are endogenous modulators that are implicated in painful states. This review explains the cisplatin-induced neuropathy and antinociceptive effects of cannabinoids and agmatine in animal models of NP and their putative therapeutic potential in cisplatin-induced neuropathy.

Agmatine, a potential novel therapeutic strategy for depression

Major depressive disorder is the most common psychiatric disorder with lifetime prevalence of up to 20% worldwide. It is responsible for more years lost to disability than any other disorder. Despite the fact that current available antidepressant drugs are safe and effective, they are far from ideal. In addition to the need to administer the drugs for weeks or months to obtain clinical benefit, side effects are still a serious problem. Agmatine is an endogenous polyamine synthesized by the enzyme arginine decarboxylase. It modulates several receptors and is considered as a neuromodulator in the brain. In this review, studies demonstrating the antidepressant effects of agmatine are presented and discussed, as well as, the mechanisms of action related to these effects. Also, the potential beneficial effects of agmatine for the treatment of other neurological disorders are presented. In particular, we provide evidence to encourage future clinical studies investigating agmatine as a novel antidepressant drug.

Effects of imidazoline I₂ receptor ligands on morphine- and tramadol-induced antinociception in rats

Currently available analgesics cannot meet the increasing clinical needs and new analgesics with better therapeutic profiles are in great demand. The imidazoline I₂ receptor is an emerging drug target for analgesics. However, few studies have examined the effects of selective I₂ receptor ligands on the antinociceptive activity of opioids. This study examined the antinociceptive effects of the opioids morphine (0.1-10 mg/kg) and tramadol (3.2-56 mg/kg), the nonselective I₂ receptor ligand agmatine (10-100 mg/kg), and the selective I₂ receptor ligands 2-(2-benzofuranyl)-2-imidazoline hydrochloride (2-BFI; 1-10 mg/kg) and 2-(4, 5-dihydroimidazol-2-yl) quinoline hydrochloride (BU224; 1-10mg/kg), alone and in combination, in a warm water tail withdrawal procedure in rats. Morphine and tramadol but not agmatine, 2-BFI or BU224 increased tail withdrawal latency in a dose-related manner at 48°C water. Agmatine and 2-BFI but not BU224 dose-dependently enhanced the antinociceptive effects of morphine and tramadol, shifting the dose-effect curves of morphine and tramadol leftward. The enhancement of agmatine and 2-BFI on morphine and tramadol antinociception was prevented by BU224. These results, combined with the fact that BU224 and 2-BFI share similar behavioral effects under other conditions, suggest that BU224 has lower efficacy than 2-BFI at I₂ receptors, and that the enhancement of opioid antinociception by I₂ receptor ligands depends on their efficacies.

Imidazoline I2 receptors: target for new analgesics?

Pain remains a major clinical challenge because there are no effective analgesics for some pain conditions and the mainstay analgesics for severe pain, opioids, have serious unwanted effects. There is a dire need for novel analgesics in the clinic. Imidazoline receptors are a family of three receptors (I(1), I(2) and I(3)) that all can recognize compounds with an imidazoline structure. Accumulating evidence suggests that I(2) receptors are involved in pain modulation. Ligands acting at I(2) receptors are effective for tonic inflammatory and neuropathic pain but are much less effective for acute phasic pain. When studied in combination, I(2) receptor ligands enhance the analgesic effects of opioids in both acute phasic and chronic tonic pain. During chronic use, patients can develop tolerance to and dependence on opioids. Imidazoline I(2) receptor ligands can attenuate the development of tolerance to opioid analgesia and inhibit drug withdrawal or antagonist precipitation induced abstinence syndrome in animals. Taken together, drugs acting on I(2) receptors may be useful as a monotherapy or combined with opioids as an adjuvant for treating pain. Future studies should focus on understanding the relative efficacy of I(2) receptor ligands and developing new compounds to fill the gap in intrinsic efficacy continuum of I(2) receptors.

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.

Agmatine, an endogenous imidazoline receptor ligand modulates ethanol anxiolysis and withdrawal anxiety in rats

Present study investigated the role of agmatine in ethanol-induced anxiolysis and withdrawal anxiety using elevated plus maze (EPM) test in rats. The anxiolytic-like effect of ethanol was potentiated by pretreatment with imidazoline I(1)/I(2) receptor agonist agmatine (10-20 mg/kg, i.p.), imidazoline I(1) receptor agonists, moxonidine (0.25 mg/kg, i.p.) and clonidine (0.015 mg/kg, i.p.), imidazoline I(2) receptor agonist, 2-BFI (5 mg/kg, i.p.) as well as by the drugs known to increase endogenous agmatine levels in brain viz., L-arginine, an agmatine biosynthetic precursor (100 microg/rat, i.c.v.), ornithine decarboxylase inhibitor, DFMO (125 microg/rat, i.c.v.), diamine oxidase inhibitor, aminoguanidine (65 microg/rat, i.c.v.) and agmatinase inhibitor, arcaine (50 microg/rat, i.c.v.). Conversely, prior administration of I(1) receptor antagonist, efaroxan (1 mg/kg, i.p.), I(2) receptor antagonist, idazoxan (0.25mg/kg, i.p.) and arginine decarboxylase inhibitor, D-arginine (100 microg/rat, i.c.v.) blocked the anxiolytic-like effect of ethanol. Moreover, ethanol withdrawal anxiety was markedly attenuated by agmatine (10-20 mg/kg, i.p.), moxonidine (0.25 mg/kg, i.p.), clonidine (0.015 mg/kg, i.p.), 2-BFI (5 mg/kg, i.p.), L-arginine (100 microg/rat, i.c.v.), DFMO (125 microg/rat, i.c.v.), aminoguanidine (65 microg/rat, i.c.v.) and arcaine (50 microg/rat, i.c.v.). The anti-anxiety effect of agmatine in ethanol-withdrawn rats was completely blocked by efaroxan (1 mg/kg, i.p.) and idazoxan (0.25 mg/kg, i.p.). These results suggest that agmatine and imidazoline receptor system may be implicated in ethanol-induced anxiolysis and withdrawal anxiety and strongly support further investigation of agmatine in ethanol dependence mechanism. The data also project agmatine as a potential therapeutic target in overcoming alcohol withdrawal symptoms such as anxiety.

Anti-hypernociceptive properties of agmatine in persistent inflammatory and neuropathic models of pain in mice

The present study examined the anti-hypernociceptive effects of agmatine (AGM) in acute and chronic models of behavioural pain in mice. Agmatine (30 mg/kg, i.p. 30 min early), produced time-dependent inhibition of mechanical hypernociception induced by Complete Freund's Adjuvant (CFA) injected in the mice paw (inhibition of 52+/-7%) after 4 h. Given chronically (twice a day) during 10 days, AGM significantly reversed the mechanical hypernociception caused by CFA (inhibition of 43+/-6% to 67+/-5%). Moreover, AGM also significantly reduced the mechanical hypernociception caused by partial sciatic nerve ligation (PSNL) during 6 h, with inhibition of 81+/-8%. In thermal hypernociception (cold stimuli) caused by PSNL the antinociceptive effect of AGM was prolonged by 4 h with inhibition of 97+/-3% observed 1 h after the treatment. Nevertheless, AGM failed to inhibit the paw oedema caused by CFA and the myeloperoxidase enzyme activity. Of note, AGM (10-100 mg/kg, i.p., 30 min before) also elicited a pronounced inhibition of the biting response induced by TNF-alpha and IL-1beta in mice, with mean ID(50) values of 61.3 mg/kg (47.7-78.6 mg/kg) and 30.4 mg/kg (18.6-49.8 mg/kg) and inhibitions of 75+/-5% and 66+/-6%, respectively. Together, present and previous findings show that AGM given systemically is effective in inhibiting mechanical and thermal hypernociception present in chronic inflammatory processes caused by CFA and also the neuropathic pain caused by PSNL.

Agmatine Induces Antihyperalgesic Effects in Diabetic Rats and a Superadditive Interaction with R(–)-3-(2-Carboxypiperazine-4-yl)-propyl-1-phosphonic Acid, a N-Methyl-d-aspartate-Receptor Antagonist

Agmatine, an endogenous cationic amine resulting from the decarboxylation of L-arginine, produces antihyperalgesic and antiallodynic effects in animal models of chronic neuropathic and inflammatory pain. We examined the effect of agmatine on tactile and thermal allodynia and on mechanical hyperalgesia in streptozocin-induced diabetic rats. To determine its mechanism of action and the potential interest of some of its combinations, the antihyperalgesic effect of agmatine was challenged with alpha(2)-adrenergic imidazoline and opioid-receptor antagonists, and its interaction with the opioid-receptor agonist morphine, the competitive N-methyl-D-aspartate receptor antagonist D-CPP [R(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid], and the nitric-oxide synthase inhibitor L-NAME (L-N(G)-nitro-L-arginine methyl ester) were examined. When intrathecally (i.t.) injected (4.4 to 438 nmol/rat), agmatine was ineffective in normal rats but suppressed tactile allodynia (von Frey hair test), thermal allodynia (tail immersion test), and mechanical hyperalgesia (paw-pressure test) in diabetic rats. This spinal antihyperalgesic effect was suppressed by idazoxan (40 micromol/rat i.t.) but not by yohimbine (40 micromol/rat i.t.) or naloxone (0.69 micromol/rat i.v.). In diabetic rats, an isobolographic analysis showed that combinations of i.t. agmatine with i.v. L-NAME or with i.t. morphine resulted in an additive antihyperalgesic effect, whereas the agmatine/D-CPP i.t. combination was superadditive. In summary, the present findings reveal that spinal agmatine produces antiallodynic and antihyperalgesic effects in diabetic neuropathic pain involving, at least for its antihyperalgesic effect, the imidazoline receptors. Moreover, agmatine combined with D-CPP produces an antinociceptive synergy in experimental neuropathy, opening opportunities in the development of new strategies for pain therapy.

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.

Agmatine potentiates morphine-induced conditioned place preference in mice: modulation by alpha2-adrenoceptors

The effects of agmatine, an endogenous polyamine metabolite formed by decarboxylation of L-arginine, and its combination with morphine on conditioned place preference (CPP) has been investigated in male mice. Our data show that subcutaneous administration of morphine (1-7.5 mg/kg) significantly increases the time spent in the drug-paired compartment in a dose-dependent manner. Intraperitoneal administration of agmatine (1-40 mg/kg) alone does not induce either CPP or conditioned place aversion, while combination of agmatine and subeffective doses of morphine leads to potent rewarding effects. Lower doses of morphine (0.1, 0.05, and 0.01 mg/kg) are able to induce CPP in mice pretreated with agmatine 1, 5, and 10 mg/kg, respectively. Concomitant intraperitoneal administration of UK 14 304 (0.5 mg/kg), a highly selective alpha2-agonist, with per se noneffective dose of morphine (0.5 mg/kg) and also its combination with noneffective doses of agmatine (1 mg/kg) plus morphine (0.05 mg/kg) produces significant CPP. UK 14 304 (0.05, 0.5 mg/kg) alone, or in combination with agmatine (1, 5 mg/kg) have had no effect. We have further investigated the possible involvement of the alpha2-adrenoceptors in the potentiating effect of agmatine on morphine-induced place preference. Selective alpha2-antagonists, yohimbine (0.005 mg/kg) and RX821002 (0.1, 0.5 mg/kg), block the CPP induced by concomitant administration of agmatine (5 mg/kg) and morphine (0.05 mg/kg). Yohimbine (0.001-0.05 mg/kg) or RX821002 (0.05-0.5 mg/kg) alone or in combination with morphine (0.05 mg/kg) or agmatine (5 mg/kg) fail to show any significant place preference or aversion. Our results indicate that pretreatment of animals with agmatine enhances the rewarding properties of morphine via a mechanism which may involve alpha2-adrenergic receptors.

Contribution of spinal glutamatergic receptors to the antinociception caused by agmatine in mice

This study was designed to evaluate the role of spinal glutamatergic receptors in the antinociception elicited by agmatine in mice. Intraperitoneal (i.p.) administration of agmatine (1.0-100.0 mg/kg) dose dependently inhibited the nociceptive response induced by intrathecal (i.t.) injection of glutamate, N-methy-D-aspartate (NMDA) and (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), with mean ID(50) values of 16.7, 6.8 and 27.0 mg/kg, respectively. However, agmatine completely failed to affect the nociception induced by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or kainic acid (kainate). Agmatine injected by i.t. route (10-100 microg/site) also produced dose-related inhibition of NMDA- and trans-ACPD-induced biting response with mean ID(50) values of 29.6 and 36.0 mug/site, respectively. The nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (l-NOARG) (75.0 mg/kg, i.p.) also consistently inhibited glutamate-, NMDA- and trans-ACPD-induced nociception (41 +/- 13, 100 and 83 +/- 6%, of inhibition, respectively) but had no effect on the same response caused by AMPA and kainate agonists. The selective NMDA receptor antagonist (5S,10R)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d] (MK-801) at a low dose (0.05 mg/kg, i.p.) inhibited the nociceptive response caused by both glutamate and NMDA agonists (inhibitions of 35 +/- 1 and 72 +/- 2%, respectively). At a high dose, MK-801 (0.5 mg/kg, i.p.) significantly inhibited the biting response induced by i.t. administration of all the glutamatergic agonists tested: glutamate, AMPA, NMDA, kainate and trans-ACPD, with inhibitions of 49 +/- 8, 84 +/- 16, 84 +/- 3, 76 +/- 8 and 97 +/- 2%, respectively. Together, these results provide experimental evidence indicating that agmatine given systemically and spinally produce marked antinociception at spinal sites in mice. Furthermore, an interaction with glutamate receptors, namely NMDA and trans-ACPD, metabotropic and NMDA-ionotropic origin, by a mechanism similar to that of nitric oxide (NO) inhibitors, seems to account for the agmatine antinociceptive action.

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.

Agmatine attenuates stress- and lipopolysaccharide-induced fever in rats

Physiological stress evokes a number of responses, including a rise in body temperature, which has been suggested to be the result of an elevation in the thermoregulatory set point. This response seems to share similar mechanisms with infectious fever. The aim of the present study was to investigate the effect of agmatine on different models of stressors [(restraint and lipopolysaccharide (LPS)] on body temperature. Rats were either restrained for 4 h or injected with LPS, both of these stressors caused an increase in body temperature. While agmatine itself had no effect on body temperature, treatment with agmatine (20, 40, 80 mg/kg intraperitoneally) dose dependently inhibited stress- and LPS-induced hyperthermia. When agmatine (80 mg/kg) was administered 30 min later than LPS (500 microg/kg) it also inhibited LPS-induced hyperthermia although the effect became significant only at later time points and lower maximal response compared to simultaneous administration. To determine if the decrease in body temperature is associated with an anti-inflammatory effect of agmatine, the nitrite/nitrate levels in plasma was measured. Agmatine treatment inhibited LPS-induced production of nitrates dose dependently. As an endogenous molecule, agmatine has the capacity to inhibit stress- and LPS-induced increases in body temperature.

Agmatine reduces extracellular glutamate during pentylenetetrazole-induced seizures in rat brain: a potential mechanism for the anticonvulsive effects

Glutamate has been implicated in the initiation and spread of seizure activity. Agmatine, an endogenous neuromodulator, is an antagonist of NMDA receptors and has anticonvulsive effects. Whether agmatine regulate glutamate release, as measured by in vivo microdialysis, is not known. In this study, we used pentylenetetrazole (PTZ)-induced seizure model to determine the effect of agmatine on extracellular glutamate in rat brain. We also determined the time course and the amount of agmatine that reached brain after peripheral injection. After i.p. injection of agmatine (50 mg/kg), increase of agmatine in rat cortex and hippocampus was observed in 15 min with levels returning to baseline in one hour. Rats, naïve and implanted with microdialysis cannula into the cortex, were administered PTZ (60 mg/kg, i.p.) with prior injection of agmatine (100 mg/kg, i.p.) or saline. Seizure grades were recorded and microdialysis samples were collected every 15 min for 75 min. Agmatine pre-treatment significantly reduced the seizure grade and increased the onset time. The levels of extracellular glutamate in frontal cortex rose two- to three-fold after PTZ injection and agmatine significantly inhibited this increase. In conclusion, the present data suggest that the anticonvulsant activity of agmatine, in part, could be related to the inhibition glutamate release.

Comparative analysis of the neuronal activation and cardiovascular effects of nitroglycerin, sodium nitroprusside and l-arginine

In this study, we compare the biological effects, Fos expression and cardiovascular responses induced in the rat, of different nitric oxide modulators (nitroglycerin, sodium nitroprusside and L-arginine). Nitroglycerin and sodium nitroprusside induced a similar pattern of neuronal activation in several areas, which include the paraventricular and supraoptic nuclei of the hypothalamus, central nucleus of the amygdala, parabrachial nucleus, locus coeruleus, ventrolateral medulla and nucleus tractus solitarius. However, only nitroglycerin activated the periaqueductal grey and nucleus trigeminalis caudalis. L-arginine-induced neuronal activation was restricted to the paraventricular and supraoptic nuclei of the hypothalamus. As regards cardiovascular effect, both nitroglycerin and sodium nitroprusside induced moderate hypotension (nitroglycerin: -23.3%, sodium nitroprusside: -24.3%) that lasted 40 min in the case of sodium nitroprusside and 80 min in the case of nitroglycerin. L-arginine did not significantly influence blood pressure. These data suggest that nitroglycerin, sodium nitroprusside and L-arginine are associated with different biological effects on both the central nervous system and the cardiovascular system. Of the NO-related drugs tested in this study, only nitroglycerin confirmed its ability to activate brainstem areas implicated in nociception.

The synergistic anticonvulsant effect of agmatine and morphine: Possible role of alpha 2-adrenoceptors

Recent demonstrations of the anticonvulsant properties of agmatine suggest it may be considered as a potential adjunct for protection against seizure. We investigated the possibility of an additive anticonvulsant effect between low doses of agmatine and morphine. The thresholds for the clonic seizures induced by the intravenous administration of gamma-aminobutyric acid (GABA)-antagonist, pentylenetetrazole (PTZ) were assessed in mice. Morphine at lower doses (1-3mg/kg) increased and at higher doses (30, 60 mg/kg) decreased the seizure threshold. Pretreatment with a per se non-effective dose of agmatine (1mg/kg) potentiated the anticonvulsant effect of morphine. The combination of subeffective doses of agmatine and morphine led to potent anticonvulsant effects. The pro-convulsant effect of morphine was attenuated by agmatine. Yohimbine with a dose (1mg/kg) incapable of affecting seizure threshold reversed the effect of agmatine on both anticonvulsant and pro-convulsant effects of morphine. These results suggest that agmatine potentiates the anticonvulsant effect of morphine and alpha 2-adrenoceptors may be involved in this effect.

Mechanisms involved in the antinociception caused by agmatine in mice

The present study examined the antinociceptive effects of agmatine in chemical behavioural models of pain. Agmatine (1-30 mg/kg), given by i.p. route, 30 min earlier, produced dose-dependent inhibition of acetic acid-induced visceral pain, with mean ID50 value of 5.6 mg/kg. Given orally, 60 min earlier, agmatine (10-300 mg/kg) also produced dose-related inhibition of the visceral pain caused by acetic acid, with mean ID50 value of 147.3 mg/kg. Agmatine (3-100 mg/kg, i.p.) also caused significant and dose-dependent inhibition of capsaicin- and glutamate-induced pain, with mean ID50 values of 43.7 and 19.5 mg/kg, respectively. Moreover, agmatine (1-100 mg/kg, i.p.) caused marked inhibition of both phases of formalin-induced pain, with mean ID50 values for the neurogenic and the inflammatory phases of 13.7 and 5.6 mg/kg, respectively. The antinociception caused by agmatine in the acetic acid test was significantly attenuated by i.p. treatment of mice with L-arginine (precursor of nitric oxide, 600 mg/kg), naloxone (opioid receptor antagonist, 1 mg/kg), p-chlorophenylalanine methyl ester (PCPA, an inhibitor of serotonin synthesis, 100 mg/kg once a day for 4 consecutive days), ketanserin (a 5-HT2A receptor antagonist, 0.3 mg/kg), ondansetron (a 5-HT3 receptor antagonist, 0.5 mg/kg), yohimbine (an alpha2-adrenoceptor antagonist, 0.15 mg/kg) or by efaroxan (an I1 imidazoline/alpha2-adrenoceptor antagonist, 1 mg/kg). In contrast, agmatine antinociception was not affected by i.p. treatment of animals with pindolol (a 5-HT1A/1B receptor antagonist, 1 mg/kg) or idazoxan (an I2 imidazoline/alpha2-adrenoceptor antagonist, 3 mg/kg). Likewise, the antinociception caused by agmatine was not affected by neonatal pre-treatment with capsaicin. Together, these results indicate that agmatine produces dose-related antinociception in several models of chemical pain through mechanisms that involve an interaction with opioid, serotonergic (i.e., through 5-HT2A and 5-HT3 receptors) and nitrergic systems, as well as via an interaction with alpha2-adrenoceptors and imidazoline I1 receptors.

Agmatine attenuates neuropathic pain in rats: possible mediation of nitric oxide and noradrenergic activity in the brainstem and cerebellum

Effect of agmatine (10-400 mg/kg) on neuropathic pain in a rat model produced by loose ligatures around the common sciatic nerve was studied. The involvement of possible alterations in nitric oxide (NO) levels [measured as its stable metabolites nitrate + nitrite] and in noradrenergic activity [measured as norepinephrine and 3-methoxy-4-hydroxyphenylethylene glycol (MHPG) levels] in this effect was also investigated biochemically in the brainstem and cerebellum. Agmatine increased the neuropathic pain threshold at 300 and 400 mg/kg. There was almost a twofold increase in nitrate + nitrite levels in the brainstem and cerebellum of the rats with neuropathic pain and agmatine decreased the high nitrate + nitrite levels only in the brainstem at 300 mg/kg and both in the brainstem and cerebellum at 400 mg/kg. Ligation of sciatic nerve resulted in almost twofold increase in norepinephrine and MHPG levels only in the brainstem of the rats. Agmatine decreased MHPG levels at 300 and 400 mg/kg, however it decreased norepinephrine levels only at the higher dose. These findings indicate that agmatine decreases neuropathic pain, an effect which may involve the reduction of NO levels and noradrenergic activity in the brain.

Systemic agmatine attenuates tactile allodynia in two experimental neuropathic pain models in rats

Recent evidence suggests that agmatine, an endogenous polyamine metabolite, might be an important neurotransmitter in central nervous system and has potential as a treatment of pain. The aim of our study was to evaluate the effect of agmatine on allodynia in two experimental neuropathic pain models, the spinal nerve ligation (SNL) model and the streptozocin (STZ)-induced diabetic neuropathy in rats, and to determine if the N-methyl-D-aspartate (NMDA) receptor antagonists and the nitric oxide synthase (NOS) inhibitors influence this effect of agmatine. Nerve injury was produced by tight ligation of the left L5 and L6 spinal nerves, and diabetic neuropathy is induced with the injection of a single dose of STZ; these procedures resulted in tactile allodynia in the hindpaw. Tactile allodynia was detected by application of von Frey filaments to the plantar surface of the foot. Agmatine reduced mechanical allodynia with its higher doses. Dizocilpine maleate (MK-801), a NMDA receptor antagonist, and the NOS inhibitors, N(G)-nitro-L-arginine methyl ester and 7-nitroindazole, did not influence the antiallodynic effect of agmatine. These results suggest that agmatine has an antiallodynic effect in both spinal nerve ligation and diabetic models and may be a promising drug in the treatment of neuropathic pain.

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.

Effects of agmatine on the escalation of intravenous cocaine and fentanyl self-administration in rats

Escalation of drug intake reliably occurs when animals are allowed extended self-administration access. As a form of plasticity, escalation of drug intake may be accompanied by neuroadaptive changes that are related to the transition from controlled use to addiction. The purpose of the present experiment was to examine the effects of agmatine (decarboxylated L-arginine) on the escalation of intravenous (iv) fentanyl and cocaine self-administration in rats. Subjects were allowed 12 h of daily access to fentanyl (2.5 microg/kg) or cocaine (0.2 mg/kg) under a fixed-ratio (FR) 1 schedule of reinforcement for 30 days. Animals self-administering fentanyl were distributed into three groups: (1) low-dose agmatine (10 mg/kg) throughout self-administration; (2) high-dose agmatine (30 mg/kg) throughout self-administration; and (3) high-dose agmatine after significant escalation (Day 18) of drug intake had occurred. Animals in a fourth group were pretreated with a high dose of agmatine throughout 30 days of cocaine self-administration. Both doses of agmatine, when given throughout self-administration, significantly decreased the escalation of responding that occurred for fentanyl but not cocaine. In the group that received agmatine after significant escalation had occurred, fentanyl-maintained responding was not significantly altered. These data indicate that agmatine attenuates the escalation of fentanyl self-administration if administered before the escalation begins and may mediate neuroadaptive events related to chronic opioid self-administration.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Agmatine produces antidepressant-like effects in two models of depression in mice

Agmatine produces an antidepressant-like effect when assessed in the forced swimming test (FST) and in the tail suspension test (TST) in mice (dose range 0.01-50 mg/kg, i.p.), without accompanying changes in ambulation in an open-field. I.c.v. injection of agmatine (1-100 nmol/site) also reduced the immobility time in the FST. Agmatine significantly enhanced the anti-immobility effect of imipramine, but did not affect that of MK-801. The anti-immobility effect of agmatine assessed in the FST was not affected by pre-treatment with prazosin. In contrast, agmatine's antidepressant-like effect was completely prevented by pre-treatment of animals with yohimbine, GMP or L-arginine. Taken together these data demonstrate that agmatine elicited a significant antidepressant-like effect through a mechanism that seems to involve an interaction with NMDA receptors, the L-arginine-nitric oxide pathway and alpha2-adrenoceptors.