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

Dynamics of arginase gene evolution in metazoans

In the present study, a comprehensive analysis of the arginase gene family in metazoans was performed. A total of 126 arginase genes have been identified in 44 species. Phylogenetic analyses indicate that arginase genes consist of four groups. Conservative and divergent gene structures are found among the groups. The syntenies also exist in distantly related genomes among multiple species. Adaptive evolution shows that, while purifying selection may have been the main force driving the evolution of the arginases, some of critical sites responsible for the functional divergence may have been under positive selection. Overall, the data obtained from our investigation contribute to a better understanding of the complexity of the arginase gene family and of the function and evolution of this family in metazoans.

Amino acid profile of plasma and cerebrospinal fluid in preeclampsia

OBJECTIVES

To determine patterns in amino acids (arginine, GABA, glutamate and glutamine) and the diamine (agmatine) in plasma and cerebrospinal fluid (CSF) of mild and severe preeclampsia compared to control patients, using capillary zone electrophoresis to generate methods for refining diagnosis and prognosis and shed light on the pathophysiological mechanisms of preeclampsia.

STUDY DESIGN

This is an observational case-control study in pregnant women that attended the emergency ward of the University Hospital, Mérida, Venezuela, during the period April, 2009-April 2010.

MAIN OUTCOME MEASURES

Molar concentration of amino acids and diamine in plasma and CSF in control, mild and severe preeclampsia patients.

RESULTS

An increase in glutamate plasma levels was observed in mild preeclampsia and even higher in severe patients, while a biphasic response occurred in the CSF samples with a significant increment in mild preeclampsia patients and a decrease in severe preeclampsia patients. GABA significantly decreased both in plasma and CSF in mild preeclampsia with a tendency to return to normal levels in severe preeclampsia patients. Arginine CSF and plasma levels decreased in mild preeclampsia patients and even more in severe preeclampsia while agmatine significantly increased in plasma levels with no changes in CSF.

CONCLUSIONS

The results are discussed in terms of molecules that could be used as biomarkers of the severity of the disease and the possible involvement of these substances in the pathophysiology of preeclampsia.

A series of case studies: practical methodology for identifying antinociceptive multi-target drugs

Since the introduction of drug discovery based on single targets, the number of newly developed drugs has steadily declined, and the reliablility of the current drug-discovery paradigm has been unceasingly questioned. As an alternative, an emerging approach pursuing multi-targeting drugs has arisen to reflect multifactorial diseases caused by the complex networks of various mechanisms. The purpose of this paper is to review multi-target drugs and introduce our progress in establishing a practical methodology for identifying antinociceptive multi-target drugs. We have adopted a system of ex vivo efficacy screening using long-term potentiation in rat spinal cord as a surrogate biomarker for neuropathic pain. A bait-target approach is also adopted to lure an unknown target combination that induces synergistic mechanisms.

The interaction of melatonin and agmatine on pentylenetetrazole-induced seizure threshold in mice

Melatonin, the major hormone produced by the pineal gland, has a number of functions in mammals, for example, its function as an anticonvulsant. Agmatine, a biogenic amine formed by decarboxylation of L-arginine by arginine decarboxylase, also has anticonvulsant effects. This study investigated the effect of the interaction of melatonin and agmatine on seizure susceptibility in the mouse model of pentylenetetrazole (PTZ)-induced clonic seizures. Further, the researchers investigated the involvement of melatonin receptors in this interaction using luzindole, a ML(1/2) receptor antagonist and prazosin, a ML(3) receptor antagonist. Melatonin, at 40 and 80 mg/kg, and agmatine, at 10 and 20mg/kg, exerted anticonvulsant effects. Luzindole, at 1.25 and 2.5mg/kg, or prazosin, at 0.5mg/kg, did not change the seizure threshold as compared with that of vehicle-treated mice. The anticonvulsant effect of melatonin (40 and 80 mg/kg) was prevented by luzindole (2.5mg/kg) (P<0.001) but not prazosin (0.5mg/kg), indicating the possible involvement of ML(1/2) receptors in the anticonvulsant effect of melatonin. Agmatine (5mg/kg) significantly increased the anticonvulsant effect of both the noneffective dose (20mg/kg) (P<0.05) and the effective dose (80 mg/kg) (P<0.001) of melatonin. Luzindole (2.5mg/kg), but not prazosin (0.5mg/kg), decreased the anticonvulsant effect of agmatine (20mg/kg) (P<0.05). Luzindole (2.5mg/kg), but not prazosin (0.5mg/kg), also decreased the seizure threshold when agmatine (5mg/kg) was administered before melatonin (20mg/kg); the decrease was significant compared with that of the group that received only agmatine and melatonin (P<0.001). In conclusion, melatonin and agmatine exhibit an additive effect in decreasing pentylenetetrazole-induced seizure threshold in mice, probably through ML(1/2) receptors.

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.

Icilin-evoked behavioral stimulation is attenuated by alpha₂-adrenoceptor activation

Icilin is a transient receptor potential cation channel subfamily M (TRPM8) agonist that produces behavioral activation in rats and mice. Its hallmark overt pharmacological effect is wet-dog shakes (WDS) in rats. The vigorous shaking associated with icilin is dependent on NMDA receptor activation and nitric oxide production, but little else is known about the biological systems that modulate the behavioral phenomenon. The present study investigated the hypothesis that alpha(2)-adrenoceptor activation inhibits icilin-induced WDS. Rats injected with icilin (0.5, 1, 2.5, 5mg/kg, i.p.) displayed dose-related WDS that were inhibited by pretreatment with a fixed dose of clonidine (0.15 mg/kg, s.c.). Shaking behavior caused by a fixed dose (2.5mg/kg) of icilin was also inhibited in a dose-related manner by clonidine pretreatment (0.03-0.15 mg/kg, s.c.) and reduced by clonidine posttreatment (0.15 mg/kg, s.c.). Pretreatment with a peripherally restricted alpha(2)-adrenoceptor agonist, ST91 (0.075, 0.15 mg/kg), also decreased the incidence of shaking elicited by 2.5mg/kg of icilin. Pretreatment with yohimbine (2mg/kg, i.p.) enhanced the shaking induced by a low dose of icilin (0.5mg/kg). The imidazoline site agonists, agmatine (150mg/kg, i.p.) and 2-BFI (7 mg/kg, i.p.), did not affect icilin-evoked shaking. These results suggest that alpha(2)-adrenoceptor activation inhibits shaking induced by icilin and that increases in peripheral, as well as central, alpha(2)-adrenoceptor signaling oppose the behavioral stimulant effect of icilin.

Analgesic efficacy of CR4056, a novel imidazoline-2 receptor ligand, in rat models of inflammatory and neuropathic pain

Two decades of investigations have failed to unequivocally clarify the functions and the molecular nature of imidazoline-2 receptors (I2R). However, there is robust pharmacological evidence for the functional modulation of monoamino oxidase (MAO) and other important enzyme activities by I2 site ligands. Some compounds of this class proved to be active experimental tools in preventing both experimental pain and opioid tolerance and dependence. Unfortunately, even though these compounds bind with high potency to central I2 sites, they fail to represent a valid clinical opportunity due to their pharmacokinetic, selectivity or side-effects profile. This paper presents the preclinical profile of a novel I2 ligand (2-phenyl-6-(1H-imidazol-1yl) quinazoline; [CR4056]) that selectively inhibits the activity of human recombinant MAO-A in a concentration-dependent manner. A sub-chronic four day oral treatment of CR4056 increased norepinephrine (NE) tissue levels both in the rat cerebral cortex (63.1% ±4.2%; P < 0.05) and lumbar spinal cord (51.3% ± 6.7%; P < 0.05). In the complete Freund’s adjuvant (CFA) rat model of inflammatory pain, CR4056 was found to be orally active (ED50 = 5.8 mg/kg, by mouth [p.o.]). In the acute capsaicin model, CR4056 completely blocked mechanical hyperalgesia in the injured hind paw (ED50 = 4.1 mg/kg, p.o.; ED100 = 17.9 mg/kg, p.o.). This effect was dose-dependently antagonized by the non-selective imidazoline I2/α2 antagonist idazoxan. In rat models of neuropathic pain, oral administration of CR4056 significantly attenuated mechanical hyperalgesia and allodynia. In summary, the present study suggests a novel pharmacological opportunity for inflammatory and/or neuropathic pain treatment based on selective interaction with central imidazoline-2 receptors.

The animal model of spinal cord injury as an experimental pain model

Pain, which remains largely unsolved, is one of the most crucial problems for spinal cord injury patients. Due to sensory problems, as well as motor dysfunctions, spinal cord injury research has proven to be complex and difficult. Furthermore, many types of pain are associated with spinal cord injury, such as neuropathic, visceral, and musculoskeletal pain. Many animal models of spinal cord injury exist to emulate clinical situations, which could help to determine common mechanisms of pathology. However, results can be easily misunderstood and falsely interpreted. Therefore, it is important to fully understand the symptoms of human spinal cord injury, as well as the various spinal cord injury models and the possible pathologies. The present paper summarizes results from animal models of spinal cord injury, as well as the most effective use of these models.

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.

OCT2 and MATE1 provide bidirectional agmatine transport

Agmatine is a biogenic amine (l-arginine metabolite) of potential relevance to several central nervous system (CNS) conditions. The identities of transporters underlying agmatine and polyamine disposition in mammalian systems are not well-defined. The SLC-family organic cation transporters (OCT) OCT1 and OCT2 and multidrug and toxin extrusion transporter-1 (MATE1) are transport systems that may be of importance for the cellular disposition of agmatine and putrescine. We investigated the transport of [(3)H]agmatine and [(3)H]putrescine in human embryonic kidney (HEK293) cells stably transfected with hOCT1, hOCT2, and hMATE1. Agmatine transport by hOCT1 and hOCT2 was concentration-dependent, whereas only hOCT2 demonstrated pH-dependent transport. hOCT2 exhibited a greater affinity for agmatine (K(m) = 1.84 ± 0.38 mM) than did hOCT1 (K(m) = 18.73 ± 4.86 mM). Putrescine accumulation was pH- and concentration-dependent in hOCT2-HEK cells (K(m) = 11.29 ± 4.26 mM) but not hOCT1-HEK cells. Agmatine accumulation, in contrast to putrescine, was significantly enhanced by hMATE1 overexpression, and was saturable (K(m) = 240 ± 31 μM; V(max) = 192 ± 10 pmol/min/mg of protein). Intracellular agmatine was also trans-stimulated (effluxed) from hMATE1-HEK cells in the presence of an inward proton-gradient. The hMATE1-mediated transport of agmatine was inhibited by polyamines, the prototypical substrates MPP+ and paraquat, as well as guanidine and arcaine, but not l-arginine. These results suggest that agmatine disposition may be influenced by hOCT2 and hMATE1, two transporters critical in the renal elimination of xenobiotic compounds.

Agmatine prevents LPS-induced spatial memory impairment and hippocampal apoptosis

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

Sensory stimulation prior to spinal cord injury induces post-injury dysesthesia in mice

Chronic pain and dysesthesias are debilitating conditions that can arise following spinal cord injury (SCI). Research studies frequently employ rodent models of SCI to better understand the underlying mechanisms and develop better treatments for these phenomena. While evoked withdrawal tests can assess hypersensitivity in these SCI models, there is little consensus over how to evaluate spontaneous sensory abnormalities that are seen in clinical SCI subjects. Overgrooming (OG) and biting after peripheral nerve injury or spinal cord excitotoxic lesions are thought to be one behavioral demonstration of spontaneous neuropathic pain or dysesthesia. However, reports of OG after contusion SCI are largely anecdotal and conditions causing this response are poorly understood. The present study investigated whether repeated application of sensory stimuli to the trunk prior to mid-thoracic contusion SCI would induce OG after SCI in mice. One week prior to SCI or laminectomy, mice were subjected either to nociceptive and mechanical stimulation, mechanical stimulation only, the testing situation without stimulation, or no treatment. They were then examined for 14 days after surgery and the sizes and locations of OG sites were recorded on anatomical maps. Mice subjected to either stimulus paradigm showed increased OG compared with unstimulated or uninjured mice. Histological analysis showed no difference in spinal cord lesion size due to sensory stimulation, or between mice that overgroomed or did not overgroom. The relationship between prior stimulation and contusion injury in mice that display OG indicates a critical interaction that may underlie one facet of spontaneous neuropathic symptoms after SCI.

Current approaches with the glutamatergic system as targets in the treatment of neuropathic pain

Glutamate is the most widely distributed and a major excitatory neurotransmitter in the CNS. It has been found to play a critical role in various physiological functions in which increased glutamate or its subsequent stimulation is thought to have a role in pathophysiological mechanism of various CNS diseases like epilepsy, stroke, depression and pain. Early attempts to develop glutamatergic antagonists failed in clinical studies due to nonselective or competitive antagonism and have a lot of safety issues like loss of cognitive functions, psychomimetic effect and sedation. Neuropathic pain can be described as pain associated with damage or permanent alteration of the peripheral or central nervous system. At present, there are very few effective therapies for neuropathic pain. The current approach includes targeting specific or alternate binding sites of glutamate receptors, resulting in reduced CNS liabilities. Targeting the glutamatergic system shows a better efficacy and fewer side effects, compared with classical drugs for the treatment of neuropathic pain. This review discusses the various targets on glutamatergic system, which includes the receptors, transporters and enzymes, for the treatment of neuropathic pain and their advantages over classical glutamatergic antagonists. The review also highlights the newer drugs in clinical trials for neuropathic pain.

Disruption of nNOS-PSD95 protein-protein interaction inhibits acute thermal hyperalgesia and chronic mechanical allodynia in rodents

BACKGROUND AND PURPOSE

Post-synaptic density protein 95 (PSD95) contains three PSD95/Dosophilia disc large/ZO-1 homology domains and links neuronal nitric oxide synthase (nNOS) with the N-methyl-D-aspartic acid (NMDA) receptor. This report assesses the effects of disruption of the protein-protein interaction between nNOS and PSD95 on pain sensitivity in rodent models of hyperalgesia and neuropathic pain.

EXPERIMENTAL APPROACH

We generated two molecules that interfered with the nNOS-PSD95 interaction: IC87201, a small molecule inhibitor; and tat-nNOS (residues 1-299), a cell permeable fusion protein containing the PSD95 binding domain of nNOS. We then characterized these inhibitors using in vitro and in vivo models of acute hyperalgesia and chronic allodynia, both of which are thought to require nNOS activation.

KEY RESULTS

IC87201 and tat-nNOS (1-299) inhibited the in vitro binding of nNOS with PSD95, without inhibiting nNOS catalytic activity. Both inhibitors also blocked NMDA-induced 3',5'-cyclic guanosine monophosphate (cGMP) production in primary hippocampal cultures. Intrathecal administration of either inhibitor potently reversed NMDA-induced thermal hyperalgesia in mice. At anti-hyperalgesic doses, there was no effect on acute pain thresholds or motor coordination. Intrathecal administration of IC87201 and tat-nNOS also reversed mechanical allodynia induced by chronic constriction of the sciatic nerve.

CONCLUSIONS AND IMPLICATIONS

nNOS-PSD95 interaction is important in maintaining hypersensitivity in acute and chronic pain. Disruption of the nNOS-PSD95 interaction provides a novel approach to obtain selective anti-hyperalgesic compounds.

Agmatine reduces ultrasonic vocalization deficits in female rat pups exposed neonatally to ethanol

Rat pups, in isolation, produce ultrasonic vocalizations (USVs). These USVs have been used as a diagnostic tool for developmental toxicity. We have shown that neonatal ethanol (ETOH) exposure produces deficits in this behavior. The current study was designed to examine whether agmatine (AG), which binds to imidazoline receptors and modulates n-methyl-d-aspartate receptors (NMDAR), could reduce these deficits. In addition, this study examined critical periods for ETOH's effects on USVs by administering ETOH during either the 1st or 2nd postnatal week. Neonatal rats received intragastric intubations of either ETOH (6g/kg/day), ETOH and AG (6g/kg/day and 20mg/kg/day), AG (20mg/kg/day), or maltose on postnatal days (PND) 1-7 or 8-14. A non-intubated control was also included. Subjects were tested on PND 15. Neonatal ETOH exposure significantly increased the latency to vocalize for females and reduced the rate of USVs in both males and females exposed to ETOH on PND 1-7. Agmatine reduced these deficits, in female but not male pups. Subjects exposed to ETOH on PND 8-14 showed no evidence of abnormal USVs. These findings suggest that there may be gender differences in response to AG following neonatal ETOH exposure and also provide further support that the first neonatal week is a particularly sensitive time for the developmentally toxic effects of ETOH in rodents.

Immunoneutralization of agmatine sensitizes mice to micro-opioid receptor tolerance

Systemically or centrally administered agmatine (decarboxylated arginine) prevents, moderates, or reverses opioid-induced tolerance and self-administration, inflammatory and neuropathic pain, and sequelae associated with ischemia and spinal cord injury in rodents. These behavioral models invoke the N-methyl-D-aspartate (NMDA) receptor/nitric-oxide synthase cascade. Agmatine (AG) antagonizes the NMDA receptor and inhibits nitric-oxide synthase in vitro and in vivo, which may explain its effect in models of neural plasticity. Agmatine has been detected biochemically and immunohistochemically in the central nervous system. Consequently, it is conceivable that agmatine operates in an anti-glutamatergic manner in vivo; the role of endogenous agmatine in the central nervous system remains minimally defined. The current study used an immunoneutralization strategy to evaluate the effect of sequestration of endogenous agmatine in acute opioid analgesic tolerance in mice. First, intrathecal pretreatment with an anti-AG IgG (but not normal IgG) reversed an established pharmacological effect of intrathecal agmatine: antagonism of NMDA-evoked behavior. This result justified the use of anti-AG IgG to sequester endogenous agmatine in vivo. Second, intrathecal pretreatment with the anti-AG IgG sensitized mice to induction of acute spinal tolerance of two micro-opioid receptor-selective agonists, [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin and endomorphin-2. A lower dose of either agonist that, under normal conditions, produces moderate or no tolerance was tolerance-inducing after intrathecal pretreatment of anti-AG IgG (but not normal IgG). The effect of the anti-AG IgG lasted for at least 24 h in both NMDA-evoked behavior and the acute opioid tolerance. These results suggest that endogenous spinal agmatine may moderate glutamate-dependent neuroplasticity.

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

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

Agmatine blocks acquisition and re-acquisition of intravenous morphine self-administration in rats

Our previous studies showed that agmatine inhibits morphine-induced conditioned place preference, locomotor sensitization and drug discrimination in rats. In the present study, we investigated the effects of agmatine on intravenous morphine self-administration in rats. At a dose of 80 mg/kg/infusion, agmatine did not substitute for intravenous morphine (0.5 mg/kg/infusion) self-administration, suggesting that agmatine itself has no reinforcing effect. However, pretreatment with agmatine (40 or 80 mg/kg, i.g.) significantly inhibited the acquisition of intravenous morphine self-administration as assessed by the nose-poke response and morphine intake. The mean number of days required to meet the acquisition criteria for intravenous morphine self-administration was significantly prolonged. After acquisition of intravenous morphine self-administration, chronic administration of agmatine (40 or 80 mg/kg x 30 days, bid, i.g.) during the extinction period significantly prevented the re-acquisition of intravenous morphine self-administration. The ability of agmatine to inhibit the acquisition and re-acquisition of intravenous morphine self-administration suggests a possible use of agmatine in the treatment of opioid dependence.

Postnatal roles of glial cell line-derived neurotrophic factor family members in nociceptors plasticity

The neurotrophin and glial cell line-derived neurotrophic factor (GDNF) family of growth factors have been extensively studied because of their proven ability to regulate development of the peripheral nervous system. The neurotrophin family, which includes nerve growth factor (NGF), NT-3, NT4/5 and BDNF, is also known for its ability to regulate the function of adult sensory neurons. Until recently, little was known concerning the role of the GNDF-family (that includes GDNF, artemin, neurturin and persephin) in adult sensory neuron function. Here we describe recent data that indicates that the GDNF family can regulate sensory neuron function, that some of its members are elevated in inflammatory pain models and that application of these growth factors produces pain in vivo. Finally we discuss how these two families of growth factors may converge on a single membrane receptor, TRPV1, to produce long-lasting hyperalgesia.

Supraspinally-administered agmatine attenuates the development of oral fentanyl self-administration

The decarboxylation product of arginine, agmatine, has effectively reduced or prevented opioid-induced tolerance and dependence when given either systemically (intraperitoneally or subcutaneously) or centrally (intrathecally or intracerebroventricularly). Systemically administered agmatine also reduces the escalation phase of intravenous fentanyl self-administration in rats. The present study assessed whether centrally (intracerebroventricular, i.c.v.) delivered agmatine could prevent the development of fentanyl self-administration in mice. Mice were trained to respond under a fixed-ratio 1 (FR1) schedule for either fentanyl (0.7 microg/70 microl, p.o.) or food reinforcement. Agmatine (10 nmol/5 microl), injected i.c.v. 12-14 h before the first session and every other evening (12-14 h before session) for 2 weeks, completely attenuated oral fentanyl self-administration (but not food-maintained responding) compared to saline-injected controls. When agmatine was administered after fentanyl self-administration had been established (day 8) it had no attenuating effects on bar pressing. This dose of agmatine does not decrease locomotor activity as assessed by rotarod. The present findings significantly extend the previous observation that agmatine prevents opioid-maintained behavior to a chronic model of oral fentanyl self-administration as well as identifying a supraspinal site of action for agmatine inhibition of drug addiction.

Thermal nociception and TRPV1 function are attenuated in mice lacking the nucleotide receptor P2Y2

Recent studies indicate that ATP and UTP act at G protein-coupled (P2Y) nucleotide receptors to excite nociceptive sensory neurons; nucleotides also potentiate signaling through the pro-nociceptive capsaicin receptor, TRPV1. We demonstrate here that P2Y(2) is the principal UTP receptor in somatosensory neurons: P2Y(2) is highly expressed in dorsal root ganglia and P2Y(2)-/- mice showed profound deficits in UTP-evoked calcium transients and potentiation of capsaicin responses. P2Y(2)-/- mice were also deficient in the detection of painful heat: baseline thermal response latencies were increased and mutant mice failed to develop thermal hypersensitivity in response to inflammatory injury (injection of complete Freund's adjuvant into the hindpaw). P2Y(2) was the only Gq-coupled P2Y receptor examined that showed an increase in DRG mRNA levels in response to inflammation. Surprisingly, TRPV1 function was also attenuated in P2Y(2)-/- mice, as measured by the frequency and magnitude of capsaicin responses in vitro and behavioral responses to capsaicin administration in vivo. However, TRPV1 mRNA levels and immunoreactivity were not reduced, and behavioral sensitivity to capsaicin could be largely restored in P2Y(2)-/- mice by pretreatment with bradykinin, suggesting that normal function of TRPV1 requires ongoing modulation by G protein-coupled receptors. These results indicate that nucleotide signaling through P2Y(2) plays a key role in thermal nociception.

Agmatine : metabolic pathway and spectrum of activity in brain

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

Synergistic depression of NMDA receptor-mediated transmission by ketamine, ketoprofen and L-NAME combinations in neonatal rat spinal cords in vitro

BACKGROUND AND PURPOSE

Spinal N-methyl-D-aspartate (NMDA) receptor/cyclooxygenase (COX) and nitric oxide synthase (NOS) pathways play a major role in nociceptive processing, and influencing them simultaneously may induce synergistic analgesia. This study determined the spinal antinociceptive interactions between ketamine (NMDA receptor channel blocker), ketoprofen (COX inhibitor) and L-NAME (NOS inhibitor) combinations.

EXPERIMENTAL APPROACH

Using an in vitro neonatal rat spinal cord preparation, two A-fibre-mediated reflexes, the monosynaptic reflex (MSR) and the low-intensity excitatory postsynaptic potential (epsp), and one C-fibre-mediated reflex, the high-intensity epsp, were evoked electrically. The effect of drugs and drug combinations on these reflexes was assessed and the type of interaction determined by isobolographic analysis.

KEY RESULTS

Infusion of ketamine alone decreased all three reflexes. That of ketoprofen decreased both the low and the high-intensity epsp only. Infusion of L-NAME alone produced no significant effects. Co-infusion of fixed ratios of IC(40) fractions of both (ketamine+ketoprofen) and (ketamine+L-NAME) were synergistic for depressing the low and the high-intensity epsps. The interaction was sub-additive for both combinations on the MSR. The only significant effect for the (ketoprofen+L-NAME) combination was synergism on the high-intensity epsp.

CONCLUSIONS AND IMPLICATIONS

All three combinations synergistically depressed nociceptive spinal transmission, and both ketamine and ketoprofen and ketamine and L-NAME combinations did so with potentially decreased motor side effects. If such combination profiles also occur in vivo, the present findings raise the possibility of ultimate therapeutic exploitation of increased analgesia with fewer side effects.

Physiological polyamines: simple primordial stress molecules

Physiological polyamines are ubiquitous polycations with pleiotropic biochemical activities, including regulation of gene expression, cell proliferation and modulation of cell signalling. Reports that the polyamines with cytoprotective activities were induced by diverse stresses raised the hypothesis that physiological polyamines may play a role in inducing stress response. In a wide range of organisms, physiological polyamines were not only induced by diverse stresses, such as reactive oxygen species (ROS), heat, ultraviolet (UV) and psychiatric stress but were able to confer beneficial effects for survival. Recent biochemical and genetic evidences show that polyamines can function as an ROS scavenger, acid tolerance factor and chemical chaperone, and positive regulators for expression of stress response genes which may explain their protective functions against diverse stresses. Taken together, these data suggest that physiological polyamines can function as primordial stress molecules in bacteria, plants and mammals, and may play an essential role in regulation of pathogen-host interactions.

Agmatine and imidazoline receptors: their role in opioid analgesia, tolerance and dependence

Agmatine is an endogenous amine that is synthesized following the decarboxylation of L-arginine by arginine decarboxylase. Agmatine exists in mammalian brain and has been proposed as a neurotransmitter and/or neurotransmodulator. Agmatine binds to several targets and is considered as an endogenous ligand for imidazoline receptors. This review, mainly based on our research work in the past decade, focused on the modulations by agmatine action on imidazoline receptors to opioid analgesia, tolerance and dependence, and its possible neurochemical mechanisms. We went on to propose that agmatine and imidazoline receptors constitute a novel system of modulating opioid functions.

Resuscitation from experimental traumatic brain injury by agmatine therapy

Both nitric oxide and glutamate contribute to ischaemic brain injury. Agmatine inhibits all isoforms of nitric oxide synthase and blocks N-methyl-d-aspartate receptors. In this study, we gave agmatine intraperitoneally and assessed its effect on fluid percussion brain injury in rats. Anaesthetised rats, immediately after the onset of fluid percussion traumatic brain injury (TBI), were divided into two major groups and given the vehicle solution (1mL/kg) or agmatine (50mg/kg) intraperitoneally. Mean arterial pressure, intracranial pressure, cerebral perfusion pressure, and levels of glutamate, nitric oxide, lactate/pyruvate ratio, and glycerol in hippocampus were monitored continuously within 120min after TBI. The weight loss was determined by the difference between the first and third day of body weight after TBI. The maximal grip angle in an inclined plane was measured to determine motor performance whereas the percent of maximal possible effect was used to measure blockade of proprioception. The triphenyltetrazolium chloride staining procedures were used for cerebral infarction assay. Compared to those of the sham-operated controls, the animals with TBI had higher values of extracellular levels of glutamate, nitric oxide, lactate-to-pyruvate ratio, and glycerol in hippocampus and intracranial pressure, but lower values of cerebral perfusion pressure. Agmatine administered immediately after TBI significantly attenuated the TBI-induced increased hippocampal levels of glutamate, nitric oxide, lactate-to-pyruvate ratio, and glycerol, intracranial hypertension, and cerebral hypoperfusion. In addition, the TBI-induced cerebral infarction, motor and proprioception deficits, and body weight loss evaluated 3 days after TBI were significantly attenuated by agmatine therapy. The present data indicate that agmatine may attenuate TBI by reducing the excessive accumulation of both glutamate and nitric oxide in the brain.

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.

Potassium- and capsaicin-induced release of agmatine from spinal nerve terminals

Agmatine (decarboxylated arginine) was originally identified in the CNS as an imidazoline receptor ligand. Further studies demonstrated that agmatine antagonizes NMDA receptors and inhibits nitric oxide synthase. Intrathecally administered agmatine inhibits opioid tolerance and hyperalgesia evoked by inflammation, nerve injury, and intrathecally administered NMDA. These actions suggest an anti-glutamatergic role for agmatine in the spinal cord. We have previously reported that radiolabeled agmatine is transported into spinal synaptosomes in an energy- and temperature-dependent manner. In the present study, we demonstrate that agmatine is releasable from purified spinal nerve terminals upon depolarization. When exposed to either elevated potassium or capsaicin, tritiated agmatine (but not its precursor L-arginine or its metabolite putrescine) is released in a calcium-dependent manner. Control experiments confirmed that the observed release was specific to depolarization and not due to permeabilization of or degradation of synaptosomes. That capsaicin-evoked stimulation results in agmatine release implicates the participation of primary afferent nerve terminals. Radiolabeled agmatine also accumulates in purified spinal synaptosomal vesicles in a temperature-dependent manner, suggesting that the source of releasable agmatine may be vesicular in origin. These results support the proposal that agmatine may serve as a spinal neuromodulator involved in pain processing.

Agmatine blocks morphine-evoked hyperthermia in rats

The present study investigated the effect of agmatine on morphine-evoked hyperthermia in rats. Morphine (4 mg/kg, s.c.) produced hyperthermia by activating mu opioid receptors. Agmatine (10 and 50 mg/kg, i.p.) was ineffective. For combined administration, agmatine decreased morphine-evoked hyperthermia. The effect was prevented by idazoxan (5 mg/kg, i.p.), an imidazoline/alpha(2)-adrenoeceptor receptor antagonist. Yohimbine, an alpha(2)-adrenoeceptor antagonist, did not prevent the attenuation of morphine-evoked hyperthermia by agmatine. The present data provide pharmacological evidence that agmatine blocks the hyperthermic effect of morphine by activating imidazoline receptors.

Agmatine transport into spinal nerve terminals is modulated by polyamine analogs

Agmatine (decarboxylated arginine) is an endogenous amine found in the CNS that antagonizes NMDA receptors and inhibits nitric oxide synthase. Intrathecally administered agmatine inhibits hyperalgesia evoked by inflammation, nerve injury and intrathecally administered NMDA. These actions suggest an antiglutamatergic neuromodulatory role for agmatine in the spinal cord. Such a function would require a mechanism of regulated clearance of agmatine such as neuronal or glial uptake. Consistent with this concept, radiolabeled agmatine has been shown to accumulate in synaptosomes, but the mechanism of this transport has not been fully characterized. The present study describes an agmatine uptake system in spinal synaptosomes that appears driven by a polyamine transporter. [(3)H]Agmatine uptake was Ca(2+), energy and temperature dependent. [(3)H]Agmatine transport was not moderated by L-arginine, L-glutamate, glycine, GABA, norepinephrine or serotonin. In contrast, [(3)H]agmatine uptake was concentration dependently inhibited by unlabeled putrescine and by unlabeled spermidine (at significantly higher concentrations). Similarly, [(3)H]putrescine uptake was inhibited in a concentration-dependent manner by unlabeled agmatine and spermidine. The polyamine analogs paraquat and methylglyoxal bis (guanylhydrazone) inhibited, whereas the polyamine transport enhancer difluoromethylornithine increased, [(3)H]agmatine transport. Taken together, these results suggest that agmatine transport into spinal synaptosomes may be governed by a polyamine transport mechanism.