Agmatine reverses pain induced by inflammation, neuropathy, and spinal cord injury

Agmatine increases proliferation of cultured hippocampal progenitor cells and hippocampal neurogenesis in chronically stressed mice

AIM

To explore the mechanism of agmatine's antidepressant action.

METHODS

Male mice were subjected to a variety of unpredictable stressors on a daily basis over a 24-d period. The open-field behaviors of the mice were displayed and recorded using a Videomex-V image analytic system automatically. For bromodeoxyuridine (BrdU; thymidine analog as a marker for dividing cells) labeling, the mice were injected with BrdU (100 mg/kg, ip, twice per d for 2 d), and the hippocampal neurogenesis in stressed mice was measured by immunohistochemistry. The proliferation of cultured hippocampal progenitor cells from neonatal rats was determined by colorimetric assay (cell counting kit-8) and 3H-thymidine incorporation assay.

RESULTS

After the onset of chronic stress, the locomotor activity of the mice in the open field significantly decreased, while coadministration of agmatine 10 mg/kg (po) blocked it. Furthermore, the number of BrdU-labeled cells in the hippocampal dentate gyrus significantly decreased in chronically stressed mice, which was also blocked by chronic coadministration with agmatine 10 mg/kg (po). Four weeks after the BrdU injection, some of the new born cells matured and became neurons, as determined by double labeling for BrdU and neuron specific enolase (NSE), a marker for mature neurons. In vitro treatment with agmatine 0.1-10 micromol/L for 3 d significantly increased the proliferation of the cultured hippocampal progenitor cells in a dose-dependent manner.

CONCLUSION

We have found that agmatine increases proliferation of hippocampal progenitor cells in vitro and the hippocampal neurogenesis in vivo in chronically stressed mice. This may be one of the important mechanisms involved in agmatine's antidepressant action.

Anxiolytic effect of agmatine in rats and mice

In mammalian brain, agmatine is an endogenous neurotransmitter and/or neuromodulator. In this study, the anxiolytic action of agmatine (p.o. or s.c.) was evaluated in three animal behavioral models in mice or rats. In the light-dark transition test, agmatine in a single dose (80 mg/kg, s.c) or repeated administration (20 mg/kg, s.c. or 10 mg/kg, p.o., once a day for 3 days) significantly increased the number of light-dark transitions in mice. Furthermore, treatment with agmatine (20-80 mg/kg, s.c or 10-40 mg/kg, p.o) three times in 24 h significantly increased the number of licks in the Vogel's drinking conflict test in rats. In the social interaction test, agmatine (10-40 mg/kg, p.o, three times in 24 h prior to test) increased the active social interaction of rats. All these results indicate that agmatine exerts a significant anxiolytic effect in both rats and mice.

Glial cell line-derived neurotrophic factor family members sensitize nociceptors in vitro and produce thermal hyperalgesia in vivo

Nerve growth factor (NGF) has been implicated as an effector of inflammatory pain because it sensitizes primary afferents to noxious thermal, mechanical, and chemical [e.g., capsaicin, a transient receptor potential vanilloid receptor 1 (TRPV1) agonist] stimuli and because NGF levels increase during inflammation. Here, we report the ability of glial cell line-derived neurotrophic factor (GDNF) family members artemin, neurturin and GDNF to potentiate TRPV1 signaling and to induce behavioral hyperalgesia. Analysis of capsaicin-evoked Ca2+ transients in dissociated mouse dorsal root ganglion (DRG) neurons revealed that a 7 min exposure to GDNF, neurturin, or artemin potentiated TRPV1 function at doses 10-100 times lower than NGF. Moreover, GDNF family members induced capsaicin responses in a subset of neurons that were previously insensitive to capsaicin. Using reverse transcriptase-PCR, we found that artemin mRNA was profoundly upregulated in response to inflammation induced by hindpaw injection of complete Freund's adjuvant (CFA): artemin expression increased 10-fold 1 d after CFA injection, whereas NGF expression doubled by day 7. No increase was seen in neurturin or GDNF. A corresponding increase in mRNA for the artemin coreceptor GFRalpha3 (for GDNF family receptor alpha) was seen in DRG, and GFRalpha3 immunoreactivity was widely colocalized with TRPV1 in epidermal afferents. Finally, hindpaw injection of artemin, neurturin, GDNF, or NGF produced acute thermal hyperalgesia that lasted up to 4 h; combined injection of artemin and NGF produced hyperalgesia that lasted for 6 d. These results indicate that GDNF family members regulate the sensitivity of thermal nociceptors and implicate artemin in particular as an important effector in inflammatory hyperalgesia.

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.

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.

Interaction of endogenous ligands mediating antinociception

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

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.

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.

Bronchial epilepsy or broncho-pulmonary hyper-excitability as a model of asthma pathogenesis

Over the last 20 years, the prevalence of asthma has nearly doubled in industrialized countries. A similar increase has been predicted for the next two decades. Asthma is major illness in terms of morbidity and suffering, asthma is the leading cause of hospitalizations in children under 15 years of age. According to many top experts, asthma is correctly characterized as a syndrome rather than disease. This lack of definition for asthma makes the search for a cause, prevention and potential cure elusive. Episodic airway obstruction and reversible bronchial hyperresponsiveness to non-specific irritants are the major symptoms of asthma. Airway inflammation is now widely accepted as the key factor underlying the pathogenesis of asthma. However, many patients show no signs of inflammation, yet they still have severe airflow limitation and asthma symptoms. The primary clinical symptoms of asthma are attacks of shortness of breath, wheezing, and coughing resulting from excessive and inappropriate constriction of the airway smooth muscle. Our research suggests a possible epileptic or hyper-excitatory condition of bronchial system in asthma pathogenesis. The paroxysmal, spasmodic character of asthma attacks may be similar to seizures. We propose a unified pathogenetic mechanism of asthma as a syndrome of inducible or genetically predisposed membrane hyper-excitability (bronchial epilepsy).

Neuroprotective effects of agmatine against cell damage caused by glucocorticoids in cultured rat hippocampal neurons

In the present study the neuroprotective effects of agmatine against neuronal damage caused by glucocorticoids were examined in cultured rat hippocampal neurons. Spectrophotometric measurements of lactate dehydrogenase activities, beta-tubulin III immunocytochemical staining, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick-end-labeling assay (TUNEL) labeling and caspase-3 assays were carried out to detect cell damage or possible involved mechanisms. Our results show that dexamethasone and corticosterone produced a concentration-dependent increase of lactate dehydrogenase release in 12-day hippocampal cultures. Addition of 100 microM agmatine into media prevented the glucocorticoid-induced increase of lactate dehydrogenase release, an effect also shared with the specific N-methyl-D-aspartate receptor antagonist MK801 and glucocorticoid receptor antagonists mifepristone and spironolactone. Arcaine, an analog of agmatine with similar structure as agmatine, also blocked glucocorticoid-induced increase of lactate dehydrogenase release. Spermine and putrescine, the endogenous polyamine and metabolic products of agmatine without the guanidino moiety of agmatine, have no appreciable effect on glucocorticoid-induced injuries, indicating a structural relevance for this neuroprotection. Immunocytochemical staining with beta-tubulin III confirmed the substantial neuronal injuries caused by glucocorticoids and the neuroprotective effects of agmatine against these neuronal injuries. TUNEL labeling demonstrated that agmatine significantly reduced TUNEL-positive cell numbers induced by exposure of cultured neurons to dexamethasone. Moreover, exposure of hippocampal neurons to dexamethasone significantly increased caspase-3 activity, which was inhibited by co-treatment with agmatine. Taken together, these results demonstrate that agmatine can protect cultured hippocampal neurons from glucocorticoid-induced neurotoxicity, through a possible blockade of the N-methyl-D-aspartate receptor channels or a potential anti-apoptotic property.

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.

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.

Nociceptive plasticity inhibits adaptive learning in the spinal cord

Spinal plasticity is known to play a role in central neurogenic pain. Over the last 100 years researchers have found that the spinal cord is also capable of supporting other forms of plasticity including several forms of learning. To study instrumental (response-outcome) learning in the spinal cord, we use a preparation in which spinally transected rats are given shock to the hind leg when the leg is extended. The spinal cord rapidly learns to hold the leg in a flexed position when given this controllable shock. However, if shock is independent of leg position (uncontrollable shock), subjects fail to learn. Uncontrollable shock also impairs future learning. As little as 6 min of uncontrollable shock to either the leg or the tail generates a learning deficit that lasts up to 48 h. Recent data suggest links between the learning deficit and the sensitization of pain circuits associated with inflammation or injury (central sensitization). Here, we explored whether central sensitization and the spinal learning deficit share pharmacological and behavioral features. Central sensitization enhances reactivity to mechanical stimulation (allodynia) and depends on the N-methyl-d-aspartate receptor (NMDAR). The uncontrollable shock stimulus that generates a learning deficit produced a tactile allodynia (Exp. 1) and administration of the NMDAR antagonist MK-801 blocked induction of the learning deficit (Exp. 2). Finally, a treatment known to induce central sensitization, intradermal carrageenan, produced a spinal learning deficit (Exp. 3). The findings suggest that the induction of central sensitization inhibits selective response modifications.

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.

Supraspinally administered agmatine prevents the development of supraspinal morphine analgesic tolerance

We have determined the effect of intracerebroventricularly (i.c.v.) administered decarboxylated arginine (agmatine) on supraspinally induced chronic morphine analgesic tolerance. Mice pre-treated with a schedule of chronic i.c.v administration of morphine (10 nmol, b.i.d. 3 days) show a 12-fold reduction in the potency of acutely administered i.c.v morphine compared to saline injected controls. Co-administration of agmatine (10 nmol) with one of the two daily morphine injections completely prevents the reduction in i.c.v morphine analgesia. Mice injected with agmatine once daily (but no morphine) do not show a increase in morphine analgesic potency relative to saline controls, indicating that a mere potentiation of acute morphine analgesia cannot account for the agmatine-mediated anti-tolerance effect in those mice subjected to the morphine tolerance induction schedule. These observations agree with previous reports that systemically and intrathecally administered agmatine prevent opioid tolerance, and extend these results to include a supraspinal site of action.

Agmatine protects against cell damage induced by NMDA and glutamate in cultured hippocampal neurons

Agmatine is a polyamine and has been considered as a novel neurotransmitter or neuromodulator in the central nervous system. In the present study, the neuroprotective effect of agmatine against cell damage caused by N-methyl-D-aspartate (NMDA) and glutamate was investigated in cultured rat hippocampal neurons. Lactate dehydrogenase (LDH) activity assay, beta-tubulin III immunocytochemical staining and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick end-labeling (TUNEL) assay were conducted to detect cell damage. Exposure of 12-day neuronal cultures of rat hippocampus to NMDA or glutamate for 1 h caused a concentration-dependent neurotoxicity, as indicated by the significant increase in released LDH activities. Addition of 100 microM agmatine into media ablated the neurotoxicity induced by NMDA or glutamate, an effect also produced by the specific NMDA receptor antagonist dizocilpine hydrogen maleate (MK801). Arcaine, an analog of agmatine with similar structure as agmatine, fully prevented the NMDA- or glutamate-induced neuronal damage. Spermine and putrescine, the endogenous polyamine and metabolic products of agmatine without the guanidine moiety of agmatine, failed to show this effect, indicating a structural relevance for this neuroprotection. Immunocytochemical staining and TUNEL assay confirmed the findings in the LDH measurement. That is, agmatine and MK801 markedly attenuated NMDA-induced neuronal death and significantly reduced TUNEL-positive cell numbers induced by exposure of cultured hippocampal neurons to NMDA. Taken together, these results demonstrate that agmatine can protect cultured hippocampal neurons from NMDA- or glutamate-induced excitotoxicity, through a possible blockade of the NMDA receptor channels or a potential anti-apoptotic property.

Agmatine treatment and vein graft reconstruction enhance recovery after experimental facial nerve injury

The rate of nerve regeneration is a critical determinant of the degree of functional recovery after injury. Here, we sought to determine whether treatment with the neuroprotective compound, agmatine, with or without nerve reconstruction utilizing a regional autogenous vein graft would accelerate the rate of facial nerve regeneration. Experiments compared the following seven groups of adult male rats: (A) Intact untreated controls. (B) Sham operation with interruption of the nerve blood supply (controls). (C) Transection of the mandibular branch of the facial nerve (generating a gap of 3 mm) followed by saline treatment. (D) Nerve transection with unsutured autogenous vein (external jugular) graft reconstruction plus saline treatment. (E) Nerve transection with sutured vein graft approximation (coaptation of the proximal and distal nerve stumps) plus saline. (F) Nerve transection with sutured vein graft followed by agmatine treatment (four daily intraperitoneal injections of 100 mg/kg agmatine sulfate). (G) Nerve transection with unsutured vein graft followed by agmatine treatment. Functional recovery, as assessed by grading vibrissae movements and by recording nerve conduction velocity and numbers of regenerated axons, indicated that either vein reconstruction or agmatine treatment resulted in accelerated and more complete recovery as compared with controls. But best results were observed in animals that underwent combined treatment, i.e., vein reconstruction plus agmatine injection. We conclude that agmatine treatment can accelerate facial nerve regeneration and that agmatine treatment together with autogenous vein graft offers an advantageous alternative to other facial nerve reconstruction procedures.

Release of tritiated agmatine from spinal synaptosomes

Intrathecal agmatine (decarboxylated arginine) moderates induction of neuropathic pain, spinal cord injury, and opioid tolerance in rodents. An endogenous central nervous system molecule and N-methyl-D-aspartate receptor antagonist/nitric oxide synthase inhibitor, agmatine may be a neuromodulator. We evaluated depolarization-induced release of agmatine from purified spinal nerve terminals (synaptosomes). Agmatine immunoreactivity was observed colocalized or closely apposed to some synaptophysin- and/or synaptotagmin-labeled structures. A temperature- and concentration-dependent uptake of [3H]-agmatine into synaptosomes was observed, consistent with an uptake mechanism. Potassium-induced depolarization resulted in release of [3H]-agmatine from the synaptosomes in a Ca2+-dependent manner, consistent with a neuromodulatory function. These results agree with previous reports of agmatine uptake into synaptosomes of the brain and extend those results to include stimulated release and a spinal site of activity.

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.

Excitation mapping with the organic cation AGB2+

Excitation mapping is a method of visualizing the signaling history of neurons with permeant organic cations. It is compatible with high-resolution imaging, allowing concurrent visualization of all neuronal classes and their glutamate-gated excitation histories. Excitation mapping documents the stability and precision of neuronal signaling within a given neuronal class, arguing that single unit electrophysiological sampling accurately reflects neuronal diversity. We here review the theory of excitation mapping, provide methods and protocol links; outline imaging concepts; provide parametric data on the temporal range and physiological sensitivity of excitation mapping; and show that immunocytochemical methods for macromolecules are compatible with excitation mapping.

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.

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 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.

Molecular targets in spinal cord injury

The spinal cord can be compared to a highway connecting the brain with the different body levels lying underneath, with the axons being the ultimate carriers of the electrical impulse. After spinal cord injury (SCI), many cells are lost because of the injury. To reconstitute function, damaged axons from surviving neurons have to grow through the lesion site to their initial targets. However, the territory they have to traverse has changed: the highway is full of inhibitory signals (myelin and scar components); the pavement itself has become bumpy (demyelination); and specialized cells are recruited to clear the way (inflammatory cells). Thus, actual strategies to treat spinal injuries aim at providing a permissive environment for regenerating axons and boosting the endogenous potential of axons to regenerate while limiting progression of secondary damage. Here we review some of the strategies currently under consideration to treat spinal injuries.

Activation of the ERK1/2 signaling cascade by excitotoxic spinal cord injury

The role of the ERK1/2 signal transduction pathway and related transcription factors in the regulation of gene expression and pain behavior following excitotoxic spinal cord injury (SCI) was examined. Specifically, phosphorylation of ERK1/2, activation of transcription factors NF-kB, ELK-1, and CREB, and gene expression of the neurokinin-1 receptor and NMDA receptor subunits NR1 and NR-2A were investigated. Excitotoxic injury was produced by intraspinal injection of quisqualic acid (QUIS) in male Sprague-Dawley rats. Western blots were used to evaluate phosphorylation and activation of ERK1/2 and transcription factors using phospho-specific or total antibodies. Real-time PCR was used to evaluate gene expression of NK-1R, NR-1, and NR-2A. Assessment of excessive grooming behavior was used to evaluate the presence of spontaneous pain behavior. Excitotoxic spinal injury resulted in: (1) increased phosphorylation of ERK1/2; (2) increased activation of NF-kB and phosphorylation of ELK-1; and (3) increased gene expression for the NK-1 receptor and NR1 and NR-2A subunits of the NMDA receptor. Blockade of the ERK cascade with the MEK inhibitor PD98059 inhibited phosphorylation of ELK-1, activation of NF-kB and gene expression of NR1, NR-2A and NK-1R, and prevented the development of excessive grooming behavior. The results have shown that excitotoxic spinal injury leads to the injury-induced activation of the ERK-->ELK-1 and NF-kB signaling cascades and transcriptional regulation of receptors important in the development of chronic pain. Blockade of this intracellular kinase cascade prevented the onset of injury-induced pain behavior.

Effect of agmatine on heteromeric N-methyl-d-aspartate receptor channels

Endogenous polyamines like spermine are known to have four distinct effects on recombinant N-methyl-d-aspartate (NMDA) receptor channels: voltage-dependent inhibition, glycine-dependent stimulation, glycine-independent stimulation and decreased affinity to the agonist (l-glutamate). These effects are highly dependent on the constituting epsilon subunits (epsilon1-epsilon4) of the recombinant NMDA receptor channels. Agmatine reportedly inhibits native NMDA receptor channels in cultured hippocampal neurons. In the present investigation, the effects of agmatine on the epsilon/zeta heteromeric NMDA receptor channels expressed in Xenopus laevis oocytes were examined using the two-electrode voltage clamp method. Agmatine inhibited the four epsilon/zeta (epsilon1/zeta1, epsilon2/zeta1, epsilon3/zeta1 and epsilon4/zeta1) channels with similar sensitivity (an IC50 value of about 300microM at -70mV). This effect was dependent on the membrane potential and was more pronounced at hyperpolarized membrane potentials (voltage-dependent inhibition). Agmatine did not exhibit other stimulatory (glycine-dependent and -independent effects) or inhibitory (decreased affinity to l-glutamate) effects. These properties are similar to the pharmacological profile of well-characterized NMDA receptor channel blockers like phencyclidine and ketamine. Thus, regarding the effects on the NMDA receptor channels, agmatine is not like other endogenous polyamines rather it acts as a channel blocker.

Tumor-induced mechanical hyperalgesia involves CGRP receptors and altered innervation and vascularization of DsRed2 fluorescent hindpaw tumors

Functional and anatomical relationships among primary afferent fibers, blood vessels, and cancers are poorly understood. However, recent evidence suggests that physical and biochemical interactions between these peripheral components are important to both tumor biology and cancer-associated pain. To determine the role of these peripheral components in a mouse model of cancer pain, we quantified the change in nerve and blood vessel density within a fibrosarcoma tumor mass using stereological analysis of serial confocal optical sections of immunostained hind paw. To this end we introduced the Discoma coral-derived red fluorescent protein (DsRed2) into the NCTC 2472 fibrosarcoma line using the Sleeping Beauty transposon methodology, thus providing a unique opportunity to visualize tumor-nerve-vessel associations in context with behavioral assessment of tumor-associated hyperalgesia. Tumors from hyperalgesic mice are more densely innervated with calcitonin gene related peptide (CGRP)-immunoreactive nerve fibers and less densely vascularized than tumors from non-hyperalgesic mice. As hyperalgesia increased from Day 5 to 12 post-implantation, the density of protein gene product 9.5 (PGP9.5)-immunoreactive nerves and CD31-immunoreactive blood vessels in tumors decreased, whereas CGRP-immunoreactive nerve density remained unchanged. Importantly, intra-tumor injection of a CGRP1 receptor antagonist (CGRP 8-37) partially blocked the tumor-associated mechanical hyperalgesia, indicating that local production of CGRP may contribute to tumor-induced nociception through a receptor-mediated process. The results describe for the first time the interaction among sensory nerves, blood vessels and tumor cells in otherwise healthy tissue, and our assessment supports the hypothesis that direct tumor cell-axon communication may underlie, at least in part, the occurrence of cancer pain.

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.

Neurochemical Evidence for Agmatine Modulation of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine (MPTP) Neurotoxicity

Agmatine treatment is known to exert neuroprotective effects in several models of neurotoxic and ischemic brain and spinal cord injuries. Here we sought to find out whether agmatine treatment would also prove to be neuroprotective in the mouse 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease. Concomitant daily treatment (intraperitoneal injections) with agmatine (100 mg/kg for 5 days) and MPTP (40 mg/kg for 2 days) exacerbated MPTP-related toxicity as evidenced by a larger reduction in dopamine uptake into striatal synaptosomes (42.4% as compared to 58.3% of control, respectively). In contrast, agmatine treatment commencing after MPTP, produced partial protection (31%) against MPTP dopaminergic toxicity. The findings implicate agmatine in mechanisms regulating MPTP neurotoxicity, but underscore the characteristic neuroprotective efficacy of agmatine when applied after the insult.

nNOS expression following inferior alveolar nerve injury in the ferret

Damage to the inferior alveolar nerve (IAN) may result in permanent painful dysaesthesia, and there is compelling evidence to suggest that ectopic activity from the injury site plays a crucial role in the initiation of this disorder. The aim of this study was to determine whether neuronal nitric oxide synthase (nNOS), a regulator of neuronal excitability, could be involved in the development of the abnormal activity. In seven ferrets, the left IAN was exposed and a retrograde tracer, fluorogold, was applied to the nerve for the identification of cell bodies in the trigeminal ganglion with axons in the IAN. In four animals, the nerve was sectioned distal to the injection site, and three served as controls. After 3 days, the animals were perfused with fixative, and the left and right IANs and trigeminal ganglia were processed using indirect immunofluorescence for nNOS. Image analysis was used to quantify the percentage area of staining (PAS) at the injury site. In the ganglia, counts were made of positively labelled cells in the fluorogold population. At the injury site, PAS was significantly greater in injured nerves than in either contralateral or control nerves, and contralateral PAS was elevated compared to control. In the ganglia, the proportion of nNOS-labelled cells was significantly reduced following injury. These results indicate a possible translocation of the nNOS protein from the cell body to the site of nerve injury, where it accumulates. Thus, nNOS could play a role in the development of ectopic activity at a site of trigeminal nerve injury.

Transgenic mice possessing increased numbers of nociceptors do not exhibit increased behavioral sensitivity in models of inflammatory and neuropathic pain

At least two classes of neciceptors can be distinguished based on their growth factor requirements: glial cell-line derived neurotrophic factor (GDNF)- and nerve growth factor (NGF)-dependent primary afferent neurons. Based on numerous anatomical and biochemical differences, GDNF- and NGF-dependent neurons have been proposed to be involved in the development of different types of persistent pain. To examine this hypothesis we used two lines of transgenic mice that contained a supernormal number of either NGF- or GDNF-dependent neurons (referred to as NGF-OE and GDNF-OE mice, respectively). These mice were tested in a model of inflammatory pain (induced by injection of complete Freund's adjuvant) and neuropathic pain (using a spinal nerve ligation protocol). Contrary to expectations, neither line of transgenic mice became more hyperalgesic following induction of persistent pain. In fact, NGF-OE mice recovered more rapidly and became hypoalgesic despite extensive paw swelling in the inflammatory pain model. In the neuropathic pain model, only wildtype mice became hyperalgesic. Real-time PCR analysis showed that the NGF-OE and GDNF-OE mice exhibited changes in neuronal-specific mRNAs in the dorsal root ganglia but not the spinal cord dorsal horn. These results indicate that increasing the number of nociceptors results in potent compensatory mechanisms that may begin with changes in the sensory neurons themselves.

The Antinociceptive Potencies and Interactions of Endogenous Ligands During Continuous Intrathecal Administration: Adenosine, Agmatine, and Endomorphin-1

Recently, a series of endogenous ligands related to inhibition of sensory transduction of noxious stimuli at the spinal level has been described, including endomorphins, agmatine, and adenosine, which act on different receptors; however, little data exist concerning their effect during continuous administration or their interactions. In this study, we investigated the antinociceptive properties of continuously administered (for 60 min) adenosine and agmatine on carrageenan-induced thermal hyperalgesia by means of a thermal paw withdrawal test in awake rats. The possible interaction between endomorphin-1 and adenosine or agmatine was also determined. Continuous administration of adenosine (0.3-3 microg/min) did not influence the paw withdrawal latencies of the normal or inflamed paws during the infusion but in larger doses it resulted in a significant increase in latencies after the cessation of the infusion. Agmatine (0.3-3 microg/min) dose-dependently decreased the hyperalgesia, but the largest dose caused a temporary excitation in 50% of animals. The continuous administration of adenosine or agmatine (3 micro g/min) potentiated and prolonged the antinociceptive effect of endomorphin-1 (1 microg/min). Our results revealed that adenosine and agmatine have a small antinociceptive efficacy during continuous intrathecal administration but that both potentiate the effect of endomorphin-1. These data suggest that the combination of these endogenous ligands might represent novel targets for the therapeutic modulation of pain; however, the systematic examination of side effects is essential.

IMPLICATIONS

Adenosine and agmatine have little antinociceptive efficacy during continuous intrathecal administration, as shown by the inflammatory pain test in rats, but both potentiate the effect of endomorphin-1. These data suggest that the combination of these endogenous ligands might represent novel targets for the therapeutic modulation of pain; however, the systematic examination of side effects is essential.

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.

Conditions affecting the onset, severity, and progression of a spontaneous pain-like behavior after excitotoxic spinal cord injury

Intraspinal injection of quisqualic acid (QUIS) is associated with the development of spontaneous excessive grooming behavior in male Sprague Dawley rats. To further characterize this pain-like behavior we evaluated the relationship between the onset of this behavior and the rostrocaudal spread of injury-induced neuronal loss in 3 different strains of male rats. The severity and progression of this behavior also were evaluated. Unilateral intraspinal injections of 125 mmol/L QUIS were made in the following groups: Sprague Dawley males (SDMs, n = 21); Long Evans males (LEMs, n = 17); and Wistar Furth males (WFMs, n = 11). Because of differences in grooming characteristics between male and female rats, the modulatory effects of female gonadal hormones also were evaluated in Sprague Dawley females (SDFs, n = 17); bilaterally ovariectomized Sprague Dawley females (OVXs, n = 11); and SDMs treated with either 17-beta-estradiol (50 microg/kg; SDM-Est, n = 9) or progesterone (5 mg/kg; SDM-Pro, n = 11). The results showed that the development of excessive grooming behavior in males of all strains and ovariectomized females is related to the rostrocaudal spread of a specific pattern of neuronal loss in the dorsal horn. Excessive grooming behavior in SDFs was similar in many respects to that found in SDMs; however, SDFs did not show a dependence on the longitudinal extent of injury for the onset of this behavior. The onset, severity, and progression of excessive grooming in OVX females were similar to that found in SDMs. Furthermore, 8 of 9 estradiol-treated SDMs developed severe grooming characterized by an early onset and progressive time course, whereas progesterone treatment delayed the onset of grooming and attenuated its severity and progression. Strain-related differences in some, but not all, grooming characteristics also were observed, eg, WFMs exhibited more aggressive grooming than SDMs or LEMs. In conclusion, the results showed gender, strain, and gonadal hormones influence the onset and progression of injury-induced excessive grooming behavior. A causal relationship also was found between the onset of this behavior and the longitudinal extent of injury.

N-methyl-D-aspartate receptor subunit expression and phosphorylation following excitotoxic spinal cord injury in rats

The role of NMDA receptor expression and post-translational modification in the pathological and behavioral consequences of injury were examined in rats receiving spinal injections of quisqualate. Spinal cords were removed 3 days following the development of excessive grooming behavior or, if the spontaneous pain-like behavior was not observed, 13 days following injections. Western blots from the spinal tissue demonstrated that non-grooming animals had elevated protein levels of the NR1 subunit of the NMDA receptor. These subunits did not demonstrate an enhanced level of phosphorylation. NR1 protein in grooming rats was not elevated, but there was a significant increase in NR1 serine phosphorylation. These findings suggest that excitotoxic lesions of the spinal cord induce both NR1 expression and NR1 serine phosphorylation. However, the injury-induced excessive grooming behavior is only associated with phosphorylation of the NR1 subunit.

Impaired NMDA receptor-mediated postsynaptic function and blunted NMDA receptor-dependent persistent pain in mice lacking postsynaptic density-93 protein

Modification of synaptic NMDA receptor (NMDAR) expression influences NMDAR-mediated synaptic function and associated persistent pain. NMDARs directly bind to a family of membrane-associated guanylate kinases (MAGUKs) that regulate surface and synaptic NMDAR trafficking in the CNS. We report here that postsynaptic density-93 protein (PSD-93), a postsynaptic neuronal MAGUK, is expressed abundantly in spinal dorsal horn and forebrain, where it colocalizes and interacts with NMDAR subunits NR2A and NR2B. Targeted disruption of the PSD-93 gene reduces not only surface NR2A and NR2B expression but also NMDAR-mediated excitatory postsynaptic currents and potentials, without affecting surface AMPA receptor expression or its synaptic function, in the regions mentioned above. Furthermore, mice lacking PSD-93 exhibit blunted NMDAR-dependent persistent pain induced by peripheral nerve injury or injection of Complete Freund's Adjuvant, although they display intact nociceptive responsiveness to acute pain. PSD-93 appears to be important for NMDAR synaptic targeting and function and to be a potential biochemical target for the treatment of persistent pain.

Animal and cellular models of chronic pain

Chronic pain, especially neuropathic pain and cancer pain, is often not adequately treated by currently available analgesics. Animal models provide pivotal systems for preclinical study of pain. This article reviews some of the most widely used or promising new models for chronic pain. Partial spinal ligation, chronic constriction injury, and L5/L6 spinal nerve ligation represent three of the best characterized rodent models of peripheral neuropathy. Recently, several mouse and rat bone cancer pain models have been reported. Primary or permanent cultures of sensory neurons have been established to study the molecular mechanism of pain, especially for neurotransmitter release and signal transduction. The emerging gene microarray, genomics and proteomics methods may be applied to throughly characterize these cells. Each model is uniquely created with distinct mechanisms, it is therefore essential to report and interpret results in the context of a specific model.

Spinal delivery of analgesics in experimental models of pain and analgesia

Systemic administration of analgesics can lead to serious adverse side effects compromising therapeutic benefit in some patients. Information coding pain transmits along an afferent neuronal network, the first synapses of which reside principally in the spinal cord. Delivery of compounds to spinal cord, the intended site of action for some analgesics, is potentially a more efficient and precise method for inhibiting the pain signal. Activation of specific proteins that reside in spinal neuronal membranes can result in hyperpolarization of secondary neurons, which can prevent transmission of the pain signal. This is one of the mechanisms by which opioids induce analgesia. The spinal cord is enriched in such molecular targets, the activation of which inhibit the transmission of the pain signal early in the afferent neuronal network. This review describes the pre-clinical models that enable new target discovery and development of novel analgesics for site-directed pain management.

Impaired NMDA receptor-mediated postsynaptic function and blunted NMDA receptor-dependent persistent pain in mice lacking postsynaptic density-93 protein

Modification of synaptic NMDA receptor (NMDAR) expression influences NMDAR-mediated synaptic function and associated persistent pain. NMDARs directly bind to a family of membrane-associated guanylate kinases (MAGUKs) that regulate surface and synaptic NMDAR trafficking in the CNS. We report here that postsynaptic density-93 protein (PSD-93), a postsynaptic neuronal MAGUK, is expressed abundantly in spinal dorsal horn and forebrain, where it colocalizes and interacts with NMDAR subunits NR2A and NR2B. Targeted disruption of the PSD-93 gene reduces not only surface NR2A and NR2B expression but also NMDAR-mediated excitatory postsynaptic currents and potentials, without affecting surface AMPA receptor expression or its synaptic function, in the regions mentioned above. Furthermore, mice lacking PSD-93 exhibit blunted NMDAR-dependent persistent pain induced by peripheral nerve injury or injection of Complete Freund's Adjuvant, although they display intact nociceptive responsiveness to acute pain. PSD-93 appears to be important for NMDAR synaptic targeting and function and to be a potential biochemical target for the treatment of persistent pain.

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.

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 induces anxiolysis in the elevated plus maze task in adult rats

Agmatine is an endogenous released polyamine recently proposed to be a putative neurotransmitter, however its physiological role is still to be determined. We investigated the hypothesis that agmatine, systemically administered to adult Wistar rats, might exert anxiolytic-like behavior in the elevated plus maze (EPM) and the open field. Agmatine (1, 10, 20, 40 and 100 mg/kg) and saline were administered i.p. 30 min before the EPM and the open field. Administration of agmatine (20 and 40 mg/kg) increased the time spent in the open arms of the EPM, as compared to the saline group, with no effect on locomotion activity in the open field. However, 100 mg/kg of agmatine significantly reduced the number of entries into enclosed arms of the EPM and the total number of crossings in the open field. We suggest that agmatine, in doses of 20 and 40 mg/kg, causes a mild anxiolytic-like behavior and discuss the possibility that this first reported effect could be caused either by the inhibition of nitric oxide synthase, the blockage of NMDA receptors or by the activation of alpha-2-adrenoceptors.

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.

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.

Differential efficacy of intrathecal NMDA receptor antagonists on inflammatory mechanical and thermal hyperalgesia in rats

Spinal cord dorsal horn N-methyl-D-aspartate (NMDA) receptors have been implicated in central sensitization, enhanced responsiveness to peripheral stimuli following peripheral injury. Since hyperalgesia is a behavioral consequence of central sensitization, it should be attenuated at the level of the dorsal horn with NMDA receptor antagonists. However, responsiveness to thermal and mechanical hyperalgesia may be distinct, and have thus far not been directly compared in chronic inflammatory pain models. In the present study, inflammation was induced with complete Freund's adjuvant (CFA) injected into the rat hind paw and NMDA receptor antagonists dizocilpine (MK-801) or 2-amino-5-phosphonovaleric acid (AP-5) were intrathecally injected in rats to determine the effects on both mechanical and thermal hyperalgesia. Locomotor tests and reflexes were also conducted to evaluate potential motor side effects. The NMDA receptor antagonists dose-dependently ameliorated mechanical hyperalgesia, but had marginal effects on thermal hyperalgesia. In ranges near antihyperalgesic doses, significant disruption of motor coordination was observed for both antagonists. These results suggest that, depending on the stimulus, NMDA receptors may have variable significance for central sensitization-mediated hyperalgesia, and that NMDA receptor antagonists may have therapeutic potential for some, but not all components in the clinical manifestation of inflammatory pain.

Powerful antinociceptive effects of the cone snail venom-derived subtype-selective NMDA receptor antagonists conantokins G and T

Subunit non-selective N-methyl-D-aspartate (NMDA) receptor antagonists reduce injury-induced pain behavior, but generally produce unacceptable side effects. In this study, we examined the antinociceptive and motor effects of cone snail venom-derived peptides, conantokins G and T (conG and conT), which are selective inhibitors of the NR2B or NR2A and NR2B subtypes of the NMDA receptor, respectively. We tested the effects of conG and conT in models of tissue (formalin test), nerve injury (partial sciatic nerve ligation) and inflammation-induced (intraplantar Complete Freund's Adjuvant; CFA) pain in mice. In the formalin test, intrathecal (i.t.) conG or conT suppressed the ongoing pain behavior (ED(50) and 95% confidence intervals (CI), 11 (7-19) and 19 (11-33), respectively) at doses that were 17-27 times lower than those required to impair motor function (accelerating rotarod treadmill test: ED(50) and 95% CI, 300 (120-730) and 320 (190-540) pmol, respectively). By comparison, SNX-111, an N-type voltage-sensitive calcium channel antagonist that is also derived from cone snail venom, produced significant motor impairment at a dose (3.0 pmol, i.t.) that was only partially efficacious in the formalin test. Furthermore, conG reversed the allodynia produced by nerve injury, with greater potency on thermal (ED50 and 95% CI, 24 (10-55) pmol) than on mechanical allodynia (59 (33-105) pmol). Finally, a single dose of conG (100 pmol, i.t.) also reduced CFA-evoked thermal and mechanical allodynia. Taken together, these results demonstrate that conantokins exhibit potent antinociceptive effects in several models of injury-induced pain. The study supports the notion that drugs directed against subtypes of the NMDA receptor, by virtue of their reduced side-effect profile, hold promise as novel therapeutic agents for the control of pain.

Tumor implantation in mouse humerus evokes movement-related hyperalgesia exceeding that evoked by intramuscular carrageenan

In this paper we compare two innovative models of movement-related pain: tumor-induced nociception following implantation of fibrosarcoma cells into bone and muscle inflammation-induced nociception following injection of the irritant carrageenan into muscle. Importantly, using the grip force test, an assay of movement-related hyperalgesia, both non-malignant and malignant pain are examined in parallel. Movement-related hyperalgesia, known clinically as a specific type of 'breakthrough pain', is a common feature of bone cancer and is thought to be a predictor of poor response to conventional analgesic pharmacotherapy (Bruera et al., 1995, J. Pain Symptom. Manage. 10 (1995) 348; Mercadaute et al., 1992, Pain 50 (1992) 151; Pain 81 (1999) 129). Implantation of NCTC 2472 sarcoma cells in both humeri or injection of carrageenan (4%) in both triceps of C3H/He mice produced apparent forelimb hyperalgesia that was not associated with mechanical hyperalgesia in the forepaw, whereas carrageenan at 6 and 8% did evoke significant cutaneous hyperalgesia of the forepaw as well. Control groups receiving implants of vehicle or no treatment at all did not manifest this forelimb hyperalgesia. B6C3/F1 mice implanted with non-lysis-inducing G3.26 melanoma cells or vehicle did not manifest significant hyperalgesia when compared to B6C3/F1 mice receiving fibrosarcoma cells, indicating a dependence on bone involvement for induction of hyperalgesia in this model. Histological examination at days 3, 7, and 10 post-implantation showed a clear correlation of tumor growth-induced bone destruction with behavioral hyperalgesia. Morphine was more potent in decreasing the maximal hyperalgesia induced by carrageenan than that induced by tumor implantation. Acutely administered morphine (3-100mg/kg, i.p.) attenuated peak hyperalgesia of carrageenan-injected mice (ED(50) 6.9 mg/kg) and tumor-bearing mice (ED(50) 23.9 mg/kg) in a dose-related manner with a difference in potency of 3.5. Tumor-implanted mice with a level of hyperalgesia comparable to that induced by carrageenan required almost three times more morphine (ED(50) 18.5mg/kg) for equivalent attenuation of forelimb hyperalgesia. These animal models of movement-related hyperalgesia may aid in discerning the peripheral and central mechanisms underlying pain that accompanies bone metastases and distinguishing it from the pain associated with muscular inflammation. Importantly, they may also aid in predicting differences in analgesic efficacy in different types of musculoskeletal pain.