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

Preventive putative effect of agmatine on cognitive and molecular outcomes in ventral tegmental area of male offspring following physical and psychological prenatal stress

Prenatal stress (PS) results from a maternal experience of stressful events during pregnancy, which has been associated with an increased risk of behavioral disorders including substance abuse and anxiety in the offspring. PS is known to result in heightened dopamine release in the ventral tegmental area (VTA), in part through the effects of corticotropin-releasing hormone, which directly excites dopaminergic cells. It has recently been suggested that agmatine plays a role in modulating anxiety-like behaviors. In this study, we investigated whether agmatine could reduce negative cognitive outcomes in male mice prenatally exposed to psychological/physical stress, and whether this could be associated with molecular changes in VTA. Agmatine (37.5 mg/kg) was administrated 30 min prior to PS induction in pregnant Swiss mice. Male offspring were evaluated in a series of behavioral and molecular assays. Findings demonstrated that agmatine reduced the impairment in locomotor activity induced by both psychological and physical PS. Agmatine also decreased heightened conditioned place preference to morphine seen in PS offspring. Moreover, agmatine ameliorated the anxiety-like behavior and drug-seeking behavior induced by PS in the male offspring. Molecular effects were seen in VTA as the enhanced brain-derived neurotrophic factor (BDNF) induced by PS in the VTA was reduced by agmatine. Behavioral tests indicate that agmatine exerts a protective effect on PS-induced impairments in male offspring, which could be due in part to agmatine-associated molecular alterations in the VTA. Taken together, our data suggest that prenatal treatment with agmatine exerts protective effect against negative consequences of PS on the development of affective circuits in the offspring.

The expression of agmatinase manipulates the affective state of rats subjected to chronic restraint stress

Agmatine is an endogenous polyamine produced from l-arginine and degraded by agmatinase (AGMAT). Studies in humans and animals have shown that agmatine has neuroprotective, anxiolytic, and antidepressant-like actions. However, little is known about the role of AGMAT in the action of agmatine or in the pathophysiology of psychiatric disorders. Therefore, this study aimed to investigate the role of AGMAT in the pathophysiology of MDD. In this study, we observed that AGMAT expression increased in the ventral hippocampus rather than in the medial prefrontal cortex in the chronic restraint stress (CRS) animal model of depression. Furthermore, we found that AGMAT overexpression in the ventral hippocampus elicited depressive- and anxiety-like behaviors, whereas knockdown of AGMAT exhibited antidepressant and anxiolytic effects in CRS animals. Field and whole-cell recordings of hippocampal CA1 revealed that AGMAT blockage increased Schaffer collateral-CA1 excitatory synaptic transmission, which was expressed both pre- and post-synaptically and was probably due to the inhibition of AGMAT-expressing local interneurons. Therefore, our results suggest that dysregulation of AGMAT is involved in the pathophysiology of depression and is a potential target for designing more effective antidepressants with fewer adverse effects to offer a better therapy for depression.

Effects of imidazoline agents in a rat conditioned place preference model of addiction

Agmatine (AG), idazoxan (IDZ), and efaroxan (EFR) are imidazoline receptor ligands with beneficial effects in central nervous system disorders. The present study aimed to evaluate the interaction between AG, IDZ, and EFR with an opiate, tramadol (TR), in a conditioned place preference (CPP) paradigm. In the experiment, we used five groups with 8 adult male Wistar rats each. During the condition session, on days 2, 4, 6, and 8, the rats received the drugs (saline, or TR, or IDZ and TR, or EFR and TR, or AG and TR) and were placed in their least preferred compartment. On days 1, 3, 5, and 7, the rats received saline in the preferred compartment. In the preconditioning, the preferred compartment was determined. In the postconditioning, the preference for one of the compartments was reevaluated. TR increased the time spent in the non-preferred compartment. AG decreased time spent in the TR-paired compartment. EFR, more than IDZ, reduced the time spent in the TR-paired compartment, but without statistical significance. AG reversed the TR-induced CPP, while EFR and IDZ only decreased the time spent in the TR-paired compartment, without statistical significance.

Inhibition of NMDA receptors by agmatine is followed by GABA/glutamate balance in benzodiazepine withdrawal syndrome

Background

Drug withdrawal syndrome occurs due to abrupt cessation of an addictive substance. Dependence to diazepam can be manifested by withdrawal syndrome which may include symptoms such as irritability, psychosis, sleep disturbance, seizures, mood disturbance, and anxiety. Studies have described the therapeutic role of agmatine in various neurological disorders such as depressive mood, learning deficits, anxiety, memory impairment, and psychosis. Various studies have also validated agmatine as a putant neuromodulator and revealed its mechanism of action with other neurotransmitters. The study was designed to reveal the potentials of agmatine in benzodiazepine withdrawal syndrome by maintaining GABA/glutamate balance. The study aimed to determine the underlying mechanism of action of agmatine at synaptic level using behavioral and biochemical evaluations.

Results

Agmatine significantly enhanced locomotion in open filed test and decreased anxiety as observed in elevated plus maze test (p < 0.01). Agmatine also reduced withdrawal symptoms scores along with compulsive behaviors in marble burying test and improved muscular strength by decreasing latency to fall in inverted screen test (p < 0.01). Moreover, agmatine established GABA/glutamate balance by increasing GABA levels and decreased glutamate concentration significantly (p < 0.01).

Conclusion

The present study reveals the possible mechanism of action of agmatine on NMDA receptor at GABA interneurons and glutamate post synaptic neuron that may lead to GABA/glutamate balance during withdrawal syndrome.

Neural processing of the reward value of pleasant odorants

Pleasant odorants are represented in the posterior olfactory bulb (pOB) in mice. How does this hedonic information generate odor-motivated behaviors? Using optogenetics, we report here that stimulating the representation of pleasant odorants in a sensory structure, the pOB, can be rewarding, self-motivating, and is accompanied by ventral tegmental area activation. To explore the underlying neural circuitry downstream of the olfactory bulb (OB), we use 3D high-resolution imaging and optogenetics and determine that the pOB preferentially projects to the olfactory tubercle, whose increased activity is related to odorant attraction. We further show that attractive odorants act as reinforcers in dopamine-dependent place preference learning. Finally, we extend those findings to humans, who exhibit place preference learning and an increase BOLD signal in the olfactory tubercle in response to attractive odorants. Thus, strong and persistent attraction induced by some odorants is due to a direct gateway from the pOB to the reward system.

Opioid Preconditioning Modulates Repair Responses to Prevent Renal Ischemia-Reperfusion Injury

Progression to renal damage by ischemia-reperfusion injury (IRI) is the result of the dysregulation of various tissue damage repair mechanisms. Anesthetic preconditioning with opioids has been shown to be beneficial in myocardial IRI models. Our main objective was to analyze the influence of pharmacological preconditioning with opioids in renal function and expression of molecules involved in tissue repair and angiogenesis. Experimental protocol includes male rats with 45 min ischemia occluding the left renal hilum followed by 24 h of reperfusion with or without 60 min preconditioning with morphine/fentanyl. We analyzed serum creatinine and renal KIM-1 expression. We measured circulating and intrarenal VEGF. Immunohistochemistry for HIF-1 and Cathepsin D (CTD) and real-time PCR for angiogenic genes HIF-1α, VEGF, VEGF Receptor 2 (VEGF-R2), CTD, CD31 and IL-6 were performed. These molecules are considered important effectors of tissue repair responses mediated by the development of new blood vessels. We observed a decrease in acute renal injury mediated by pharmacological preconditioning with opioids. Renal function in opioid preconditioning groups was like in the sham control group. Both anesthetics modulated the expression of HIF-1, VEGF, VEGF-R2 and CD31. Preconditioning negatively regulated CTD. Opioid preconditioning decreased injury through modulation of angiogenic molecule expression. These are factors to consider when establishing strategies in pathophysiological and surgical processes.

Modulation of the anticonvulsant effect of swim stress by agmatine

Agmatine is an endogenous l-arginine metabolite with neuroprotective effects in the stress-response system. It exerts anticonvulsant effects against several seizure paradigms. Swim stress induces an anticonvulsant effect by activation of endogenous antiseizure mechanisms. In this study, we investigated the interaction of agmatine with the anticonvulsant effect of swim stress in mice on pentylenetetrazole (PTZ)-induced seizure threshold. Then we studied the involvement of nitric oxide (NO) pathway and endogenous opioid system in that interaction. Swim stress induced an anticonvulsant effect on PTZ seizures which was opioid-independent in shorter than 1-min swim durations and opioid-dependent with longer swims, as it was completely reversed by pretreatment with naltrexone (NTX) (10mg/kg), an opioid receptor antagonist. Agmatine significantly enhanced the anticonvulsant effect of opioid-independent shorter swim stress, in which a combination of subthreshold swim stress duration (45s) and subeffective dose of agmatine (1mg/kg) revealed a significantly higher seizure threshold compared with either one. This effect was significantly reversed by NO synthase inhibitor N^G-nitro-l-arginine (L-NAME (Nω-Nitro-L-arginine methyl ester), 5mg/kg), suggesting an NO-dependent mechanism, and was unaffected by NTX (10mg/kg), proving little role for endogenous opioids in the interaction. Our data suggest that pretreatment of animals with agmatine acts additively with short swim stress to exert anticonvulsant responses, possibly by mediating NO pathway.

Agmatine Reduces Lipopolysaccharide-Mediated Oxidant Response via Activating PI3K/Akt Pathway and Up-Regulating Nrf2 and HO-1 Expression in Macrophages

Macrophages are key responders of inflammation and are closely related with oxidative stress. Activated macrophages can enhance oxygen depletion, which causes an overproduction of reactive oxygen species (ROS) and leads to further excessive inflammatory response and tissue damage. Agmatine, an endogenous metabolite of L-arginine, has recently been shown to have neuroprotective effects based on its antioxidant properties. However, the antioxidant effects of agmatine in peripheral tissues and cells, especially macrophages, remain unclear. In this study we explored the role of agmatine in mediating antioxidant effects in RAW 264.7 cells and studied its antioxidant mechanism. Our data demonstrate that agmatine is an activator of Nrf2 signaling that markedly enhances Nrf2 nuclear translocation, increases nuclear Nrf2 protein level, up-regulates the expression of the Nrf2 downstream effector HO-1, and attenuates ROS generation induced by Lipopolysaccharide (LPS). We further demonstrated that the agmatine-induced activation of Nrf2 is likely through the PI3K/Akt pathway. LY294002, a specific PI3K/Akt inhibitor, abolished agmatine-induced HO-1 up-regulation and ROS suppression significantly. Inhibiting HO-1 pathway significantly attenuated the antioxidant effect of agmatine which the products of HO-1 enzymatic activity contributed to. Furthermore, the common membrane receptors of agmatine were evaluated, revealing that α2-adrenoceptor, I1-imidazoline receptor or I2-imidazoline receptor are not required by the antioxidant properties of agmatine. Taken together, our findings revealed that agmatine has antioxidant activity against LPS-induced ROS accumulation in RAW 264.7 cells involving HO-1 expression induced by Nrf2 via PI3K/Akt pathway activation.

A Nonrewarding NMDA Receptor Antagonist Impairs the Acquisition, Consolidation, and Expression of Morphine Conditioned Place Preference in Mice

N-methyl-D-aspartate (NMDA) receptor antagonists block morphine-induced conditioned place preference (CPP). Although polyamines are endogenous modulators of the NMDA receptor, it is not known whether polyaminergic agents induce CPP or modulate morphine-induced CPP. Here, we examined whether polyamine ligands modify morphine CPP acquisition, consolidation, and expression. Adult male albino Swiss mice received saline (0.9 % NaCl, intraperitoneally (i.p.)) or morphine (5 mg/kg, i.p.) and were respectively confined to a black or a white compartment for 30 min for four consecutive days for CPP induction. The effect of arcaine (3 mg/kg, i.p.) or spermidine (30 mg/kg, i.p.), respectively, an antagonist and an agonist of the polyamine-binding site at the NMDA receptor, on the acquisition, consolidation, and expression of morphine CPP was studied. In those experiments designed to investigate whether spermidine prevented or reversed the effect of arcaine, spermidine (30 mg/kg, i.p.) was administered 15 min before or 15 min after arcaine, respectively. Arcaine and spermidine did not induce CPP or aversion per se. Arcaine (3 mg/kg, i.p.) impaired the acquisition, consolidation, and expression of morphine CPP. Spermidine prevented the impairing effect of arcaine on the acquisition of morphine CPP but not the impairing effect of arcaine on consolidation or expression of morphine CPP. These results suggest that arcaine may impair morphine CPP acquisition by modulating the polyamine-binding site at the NMDA receptor. However, the arcaine-induced impairment of consolidation and expression of morphine CPP seems to involve other mechanisms.

Cloning of two LIMCH1 isoforms: characterization of their distribution in rat brain and their agmatinase activity

Agmatine, a precursor for polyamine biosynthesis, is also associated with neurotransmitter, anticonvulsant, antineurotoxic and antidepressant actions in the brain. This molecule results from the decarboxylation of L-arginine by arginine decarboxylase, and it is hydrolyzed to urea and putrescine by agmatinase. Recently, we have described a new protein that also hydrolyzes agmatine, agmatinase-like protein (ALP), which was identified through immunohistochemical analysis in the hypothalamus and hippocampus of rats. However, its sequence differs greatly from all known agmatinases and does not contain the typical Mn(2+) ligands associated with the urea hydrolase family of proteins. ALP has a LIM-like domain close to its carboxyl terminus, and the removal of which results in a truncated variant with a tenfold increased k cat value and a threefold decreased K m value for agmatine. Analysis of the gene database revealed several transcripts, denominated LIMCH1 isoforms, with extreme 3' sequences identical to ALP. Limch1 gene products have been described as members of a multi-domain family of proteins with the biggest isoform containing a calponin homology (CH) domain at its N-terminus. Here, we cloned two LIMCH1 transcripts, one of 3177 bp and the other of 2709 bp (ALP contains 1569 bp) and analyzed LIMCH1 expression and distribution in rat brain using RT-PCR, Western blot and immunohistochemical analyses. LIMCH1 was detected mainly in the hypothalamic and hippocampal regions, which is similar to the distribution of ALP and agmatine in brain. In addition, we cloned and expressed both isoforms in E. coli and confirmed that they were catalytically active on agmatine with kinetic parameters similar to ALP. LIM domain-truncated variants of both isoforms moderately increased the k cat and catalytic efficiency. Thus, we propose that LIMCH1 is useful to regulate the intracellular concentrations of the neurotransmitter/neuromodulator, agmatine.

Agmatine attenuates methamphetamine-induced hyperlocomotion and stereotyped behavior in mice

We investigated whether pretreatment with the neurotransmitter/neuromodulator agmatine (decarboxylated L-arginine) affected methamphetamine (METH)-induced hyperlocomotion and stereotypy in male ICR mice. Agmatine pretreatment alone had no effects on locomotion or stereotypy, but it produced a dose-dependent attenuation of locomotion and the total incidence of stereotyped behavior induced by a low dose of METH (5 mg/kg). The stereotypy induced by this dose was predominantly characterized by stereotyped sniffing. By contrast, agmatine did not affect the total incidence of stereotypy induced by a higher dose of METH (10 mg/kg). However, the nature of stereotypy induced by this dose of METH was substantially altered; agmatine pretreatment significantly reduced stereotyped biting but significantly increased stereotyped sniffing and persistent locomotion. Agmatine pretreatment therefore appears to produce a rightward shift in the dose-response curve for METH. Pretreatment of mice with piperazine-1-carboxamidine (a putative agmatinase inhibitor) had no effect on locomotion or stereotypy induced by a low dose of METH, suggesting that endogenous agmatine may not regulate the METH action.

Adrenaline rush: the role of adrenergic receptors in stimulant-induced behaviors

Psychostimulants, such as cocaine and amphetamines, act primarily through the monoamine neurotransmitters dopamine (DA), norepinephrine, and serotonin. Although stimulant addiction research has largely focused on DA, medication development efforts targeting the dopaminergic system have thus far been unsuccessful, leading to alternative strategies aimed at abating stimulant abuse. Noradrenergic compounds have shown promise in altering the behavioral effects of stimulants in rodents, nonhuman primates, and humans. In this review, we discuss the contribution of each adrenergic receptor (AR) subtype (α1, α2, and β) to five stimulant-induced behaviors relevant to addiction: locomotor activity, conditioned place preference, anxiety, discrimination, and self-administration. AR manipulation has diverse effects on these behaviors; each subtype profoundly influences outcomes in some paradigms but is inconsequential in others. The functional neuroanatomy and intracellular signaling mechanisms underlying the impact of AR activation/blockade on these behaviors remain largely unknown, presenting a new frontier for research on psychostimulant-AR interactions.

Agmatine attenuates nicotine induced conditioned place preference in mice through modulation of neuropeptide Y system

The purpose of the present study was to examine the effect of agmatine on nicotine induced conditioned place preference (CPP) in male albino mice. Intra-peritoneal (ip) administration of nicotine (1mg/kg) significantly increased time spent in drug-paired compartment. Agmatine (20 and 40 mg/kg, ip) co-administered with nicotine during the 6 days conditioning sessions completely abolished the acquisition of nicotine-induced CPP in mice. Concomitant administration of neuropeptide Y (NPY) (1 pg/mouse, icv) or [Leu(31), Pro(34)]-NPY (0.1 pg/mouse, icv), selective NPY Y1 receptor agonist potentiated the inhibitory effect of agmatine (10 mg/kg, ip) on nicotine CPP. Conversely, pretreatment with NPY Y1 receptor antagonist, BIBP3226 (0.01 ng/mouse, icv) blocked the effect of agmatine (20 mg/kg, ip) on nicotine induced CPP. In immunohistochemical study, nicotine decreased NPY-immunoreactivity in nucleus accumbens shell (AcbSh), bed nucleus of stria terminalis, lateral part (BNSTl), arcuate nucleus (ARC) and paraventricular nucleus (PVN). Conversely, administration of agmatine prior to the nicotine significantly reversed the effect of nicotine on NPY-immunoreactivity in the above brain nuclei. This data indicate that agmatine attenuate nicotine induced CPP via modulation of NPYergic neurotransmission in brain.

Agmatine attenuates acquisition but not the expression of ethanol conditioned place preference in mice: a role for imidazoline receptors

The present study investigated the effect of agmatine on acquisition and expression of ethanol conditioned place preference (CPP) and its modulation by imidazoline agents. Swiss albino mice were treated intraperitoneally with saline or agmatine (20-40 mg/kg) before injection of ethanol (1.25 mg/kg) during conditioning days or on a test day (20-120 mg/kg), to observe the effect on acquisition or expression of CPP, respectively. Agmatine inhibited the acquisition but not the expression of ethanol CPP. Furthermore, both the I₁ receptor antagonist, efaroxan (9 mg/kg) and the I₂ receptor antagonist, BU224 (5 mg/kg) attenuated the agmatine-induced inhibition of the ethanol CPP acquisition. In contrast, the I₂ receptor agonist, 2-BFI (5 mg/kg) and I₁ receptor agonist, moxonidine (0.4 mg/kg) alone, or a combination of their subeffective doses, significantly attenuated the effect of agmatine (20 mg/kg) on acquisition of ethanol CPP. Agmatine or imidazoline agents alone produced neither place preference nor aversion, and at the doses used in the present study did not affect locomotor activity. Thus, agmatine attenuates the acquisition of ethanol CPP at least in part by imidazoline (I₁ or I₂) receptors. In future studies, agmatine or agents acting at the imidazoline receptors could be explored for their therapeutic potential in ethanol dependence.

Protective effects of agmatine on lipopolysaccharide-injured microglia and inducible nitric oxide synthase activity

AIMS

Proinflammatory factors released from activated microglia contribute to maintaining homeostasis against various noxious stimuli in the central nervous system. If excessive, however, they may initiate a pathologic neuroinflammatory process. In this investigation, we evaluated whether agmatine, a primary polyamine known to protect neurons, reduces lipopolysaccharide (LPS)-induced damage to microglia in vitro and in vivo.

MAIN METHODS

For in vitro study, BV2-immortalized murine microglia were exposed to LPS with agmatine treatment. After 24hours, cell viability and the amount of nitrite generated were determined. For in vivo study, LPS was microinjected into the corpus callosum of adult male albino mice. Agmatine was intraperitoneally administered at the time of injury. Brains were evaluated 24hours after LPS microinjection to check for immunoreactivity with a microglial marker of ionized calcium binding adaptor molecule 1 (Iba1) and inducible nitric oxide synthase (iNOS). Using western blot analysis, protein expression of iNOS as well as that of the proinflammatory cytokines, tumor necrosis factor (TNF)-α and interleukin (IL)-1β, was determined.

KEY FINDINGS

Agmatine significantly reduced the LPS-induced BV2 microglial cytotoxicity from over 80% to less than 60% (p<0.001), as determined by lactate dehydrogenase assay. It suppressed the nitrite production from 16.4±3.14μM to 5.5±1.27μM (p<0.001), as measured using the Griess reaction. Agmatine also decreased the activities of microglia and iNOS induced by LPS microinjection into corpus callosum.

SIGNIFICANCE

Our findings reveal that agmatine attenuates LPS-induced microglial damage and suggest that agmatine may serve as a novel therapeutic strategy for neuroinflammatory diseases.

Imidazoline receptor antisera-selected/Nischarin regulates the effect of agmatine on the development of morphine dependence

Agmatine, an endogenous ligand for imidazoline receptor, has been shown to prevent opioid dependence, but not much is known about the mechanisms of the effect of agmatine. In the present study, we investigated the function of I1 imidazoline receptor and its candidate protein imidazoline receptor antisera-selected (IRAS)/Nischarin in morphine dependence as well as in the effect of agmatine inhibiting morphine dependence by pharmacological and molecular approaches. Results showed that inhibition of IRAS or Nischarin did not change the development of morphine dependence in vitro and in vivo under the basal condition. Agmatine could reduce the cyclic 3', 5' adenosine monophosphate (cAMP) overshoot at the concentration of 0.01-10 µM in the primary cultured rat hippocampal neurons and attenuated the withdrawal signals and the elevation of FosB and ΔFosB at the dose of 5 mg/kg in the morphine-dependent mice. The effect of agmatine was inhibited by efaroxan (I1 imidazoline receptor non-specific antagonist) and the RNA interference against IRAS or Nischarin. These findings indicate that I1 imidazoline receptor or IRAS/Nischarin mediates the effect of agmatine on morphine dependence and provide evidence that I1 imidazoline receptor may be a new target for treating morphine dependence.

Agmatine attenuates methamphetamine-induced conditioned place preference in rats

The polyamine agmatine modulates a variety of behavioral effects including the abuse-related effects of opioids and has been proposed as a potential medication candidate for the treatment of opioid abuse. However, little is known of the effects of agmatine on the abuse-related effects of other drugs of abuse. This study examined the effects of agmatine on the rewarding effects of methamphetamine in rats using a conditioned place preference paradigm. Methamphetamine (0.1-1.0mg/kg) dose-dependently increased the time spent in methamphetamine-paired side (place preference). Agmatine, at doses that did not produce place preference or aversion (10-32mg/kg), significantly decreased the development of methamphetamine-induced place preference when agmatine was administered in combination with methamphetamine during place conditioning. Agmatine also significantly decreased the expression of methamphetamine-induced place preference when an acute injection of agmatine was given immediately before test session. These doses of agmatine do not alter the motor activity in rats, suggesting that the observed attenuation of methamphetamine-induced place preference was not due to general behavioral disruption. Together, these data suggests that agmatine attenuates the rewarding effects of methamphetamine and may be able to modulate the abuse liability of methamphetamine.

Agmatine effects on mitochondrial membrane potential and NF-κB activation protect against rotenone-induced cell damage in human neuronal-like SH-SY5Y cells

Agmatine, an endogenous arginine metabolite, has been proposed as a novel neuromodulator that plays protective roles in the CNS in several models of cellular damage. However, the mechanisms involved in these protective effects in neurodegenerative diseases are poorly understood. The present study was undertaken to investigate the effects of agmatine on cell injury induced by rotenone, commonly used in establishing in vivo and in vitro models of Parkinson's disease, in human-derived dopaminergic neuroblastoma cell line (SH-SY5Y). We report that agmatine dose-dependently suppressed rotenone-induced cellular injury through a reduction of oxidative stress. Similar effects were obtained by spermine, suggesting a scavenging effect for these compounds. However, unlike spermine, agmatine also prevented rotenone-induced nuclear factor-κB nuclear translocation and mitochondrial membrane potential dissipation. Furthermore, rotenone-induced increase in apoptotic markers, such as caspase 3 activity, Bax expression and cytochrome c release, was significantly attenuated with agmatine treatment. These findings demonstrate mitochondrial preservation with agmatine in a rotenone model of apoptotic cell death, and that the neuroprotective action of agmatine appears because of suppressing apoptotic signalling mechanisms. Thus, agmatine may have therapeutic potential in the treatment of Parkinson's disease by protecting dopaminergic neurons.

A role for alpha-adrenergic receptors in extinction of conditioned fear and cocaine conditioned place preference

Previous work has demonstrated an important role for adrenergic receptors in memory processes in fear and drug conditioning paradigms. Recent studies have also demonstrated alterations in extinction in these paradigms using drug treatments targeting beta- and alpha2-adrenergic receptors, but little is known about the role of alpha-adrenergic receptors in extinction. The current study examined whether antagonism of alpha-adrenergic receptors would impair the consolidation of extinction in fear and cocaine conditioned place preference paradigms. After contextual fear conditioning, injections of the alpha-adrenergic receptor antagonist prazosin (1.0 or 3.0 mg/kg) following nonreinforced context exposures slowed the loss of conditioned freezing over the course of 5 extinction sessions (Experiment 1). After cocaine place conditioning, prazosin had no effect on the rate of extinction over 8 nonreinforced test sessions. Following postextinction reconditioning, however, prazosin-treated mice showed a robust place preference, but vehicle-treated mice did not, suggesting that prazosin reduced the persistent effects of extinction (Experiment 2). These results confirm the involvement of the alpha-adrenergic receptor in extinction processes in both appetitive and aversive preparations.

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

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

Racemic gamma vinyl-GABA (R,S-GVG) blocks methamphetamine-triggered reinstatement of conditioned place preference

Preventing relapse poses a significant challenge to the successful management of methamphetamine (METH) dependence. Although no effective medication currently exists for its treatment, racemic gamma vinyl-GABA (R,S-GVG, vigabatrin) shows enormous potential as it blocks both the neurochemical and behavioral effects of a variety of drugs, including METH, heroin, morphine, ethanol, nicotine, and cocaine. Using the reinstatement of a conditioned place preference (CPP) as an animal model of relapse, the present study specifically investigated the ability of an acute dose of R,S-GVG to block METH-triggered reinstatement of a METH-induced CPP. Animals acquired a METH CPP following a 20-day-period of conditioning, in which they received 10 pairings of alternating METH and saline injections. During conditioning, rats were assigned to one of four METH dosage groups: 1.0, 2.5, 5.0, or 10.0 mg/kg (i.p., n = 8/group). Animals in all dosage groups demonstrated a robust and consistent CPP. This CPP was subsequently extinguished in each dosage group with repeated saline administration. Upon extinction, all groups reinstated following an acute METH challenge. On the following day, an acute dose of R,S-GVG (300 mg/kg, i.p.) was administered 2.5 h prior to an identical METH challenge. R,S-GVG blocked METH-triggered reinstatement in all four groups. Given that drug re-exposure may potentiate relapse to drug-seeking behavior, the ability of R,S-GVG to block METH-triggered reinstatement offers further support for its use in the successful management of METH dependence.

Antagonism of muscarinic M1 receptors by dicyclomine inhibits the consolidation of morphine-associated contextual memory

M1 muscarinic receptor has been shown to be involved in cognitive functions of the brain. Conditioned place preference (CPP) paradigm involves memory for the association between environmental stimuli and the rewarding properties produced by a treatment. Using a balanced CPP design, we studied the possible involvement of M1 muscarinic receptors on the acquisition, expression and consolidation of morphine place conditioning in male mice. Subcutaneous administration of morphine sulphate-induced CPP in a dose-dependent manner. Using a 6-day schedule of conditioning, it was found that dicyclomine, an M1 muscarinic antagonist, significantly reduced the time spent by mice in the morphine compartment when given immediately, but not 6h, after each conditioning session (consolidation). It had no effect when administered 30 min before each conditioning session during CPP training period (acquisition) or 30 min before testing for place preference in the absence of morphine (expression). It is concluded that M1 muscarinic receptors may play a time-dependent role in the consolidation of reward-related memory of morphine.

Different time schedules affect conditioned place preference after morphine and morphine-6-glucuronide administration

A number of studies have investigated the reward potential of morphine, using the Conditioned Place Preference (CPP) procedure. The morphine-metabolite morphine-6-glucuronide (M6G) is known to have analgesic activity comparable to morphine, but its reward properties are unclear. An unbiased two compartment counterbalanced procedure was used to investigate the induction of CPP by morphine or M6G in C57BL/6J-Bom mice using different conditioning schedules. The conditioning sessions took place either immediately after the injections and lasted either 20 or 40 min, or were delayed until 15 min after the injections and lasted for 20 min. Locomotor activity was recorded during the conditioning sessions. Morphine induced CPP when the 20-minute conditioning sessions were conducted directly after the injections, but not when they were delayed. M6G induced CPP when the 20-minute conditioning sessions were delayed, but not when the animals were conditioned directly after the injections. Neither morphine nor M6G induced CPP after 40-minute direct conditioning sessions. M6G had a biphasic effect on locomotor activity, with an initial decrease followed by excitation. This study indicates that M6G has rewarding effects, and might contribute to the development of addiction after heroin or morphine administration. However, in any attempts to explore the reward properties of M6G, the choice of time schedule should be carefully considered.

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.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

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.

Melatonin enhances the rewarding properties of morphine: involvement of the nitric oxidergic pathway

Melatonin has different interactions with opioids including the enhancement of the analgesic effects of morphine and also reversal of tolerance and dependence to morphine. The present study assessed the effect of melatonin on morphine reward in mice using a conditioned place preference (CPP) paradigm. Our data showed that subcutaneous administration of morphine (1-7.5 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. Intraperitoneal (i.p.) administration of melatonin (1-40 mg/kg) alone did not induce either CPP or conditioned place aversion (CPA), while the combination of melatonin (5-20 mg/kg) and sub-effective dose of morphine (0.5 mg/kg) led to rewarding effect. We further investigated the involvement of the nitric oxidergic pathway in the enhancing effect of melatonin on morphine CPP, by a general nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME). L-NAME (1 and 5 mg/kg, i.p.) alone or in combination with morphine (0.5 mg/kg) did not show any significant CPP or CPA. Co-administration of L-NAME (5 mg/kg) with an ineffective combination of melatonin (1 mg/kg) plus morphine (0.5 mg/kg) produced significant CPP that may imply the similarity of action of melatonin and L-NAME and involvement of the nitric oxidergic pathway in this regard. Our results indicate that pretreatment of animals with melatonin enhances the rewarding properties of morphine via a mechanism which may involve the nitric oxidergic pathway.