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.     

Inhibition of N-methyl-D-aspartate receptor function appears to be one of the common actions for antidepressants

In order to explore the possible common action mechanisms of three kinds of classical antidepressants, inhibition of drugs on the N-methyl-D-aspartate (NMDA)-Ca(2)-nitric oxide synthase (NOS) signal pathway was observed. With 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay and lactic dehydrogenase (LDH) assay, classical antidepressants, desipramine (1, 10 microM), fluoxetine (0.625-10 microM) or moclobemide (2.5, 10 microM) antagonized NMDA 300 M induced-lesion in PC12 cells. Using fura-2/AM (acetoxymethyl ester) labelling assay, desipramine or fluoxetine at doses 1, 5 microM attenuated the intracellular Ca(2) overload induced by NMDA 200 microM for 24 h in PC12 cells. Meanwhile, using confocal microscope, it was also found that desipramine 5 microM, fluoxetine 2.5 microM or moclobemide 10 microM decreased the NMDA 20 microM induced intracellular Ca(2) overload in primarily cultured rat hippocampal neurons. Furthermore, desipramine (1, 5 microM), fluoxetine (1, 5 microM) or moclobemide (2.5, 10 microM) significantly inhibited NOS activity in NMDA (300 microM) treated PC12 cells for 4h. In summary, we suggest that inhibition on the function of NMDA-Ca(2) -NOS signal pathway appears to be one of the common actions for antidepressants despite their remarkably different structures, which is expected to have great implication for the evaluation and screening in vitro of new antidepressants.     

Direct enhancement of hippocampal dopamine or serotonin levels as a pharmacodynamic measure of combined antidepressant-anticonvulsant action

The neurobiological relationships between epilepsy and depression are receiving increased experimental attention. A key role for limbic monoamines in depression has been established and we recently showed the importance of hippocampal monoamines in limbic seizure control. We here studied whether anticonvulsant compounds are antidepressant and can elevate hippocampal dopamine (DA) or serotonin (5-HT) levels determined by in vivo microdialysis in rats. We used assessment of seizure severity in the focal pilocarpine model, antidepressant-like activity within the rat forced swim and the mouse tail suspension tests, and locomotor activity in an open field as behavioural tests. We studied the tricyclic antidepressant imipramine, the selective 5-HT reuptake inhibitor citalopram and the selective DA reuptake blocker GBR-12909. These compounds with combined antidepressant-anticonvulsant properties all directly enhanced extracellular hippocampal DA or 5-HT levels. Since glutamate-mediated hyperexcitability in temporal lobe regions seems to be involved in disturbed emotional behaviour, we next investigated possible antidepressant effects and hippocampal DA or 5-HT modulations exerted by selective ionotropic and metabotropic glutamate receptor ligands with anticonvulsant properties. Combined anticonvulsant-antidepressant activities of the NMDA antagonist MK-801 and the mGluR group I antagonists (AIDA, MPEP) were also associated with locally elicited increases in hippocampal DA and/or 5-HT levels. This study highlights that the hippocampus is an important site of action of combined anticonvulsant-antidepressant and monoamine enhancing effects.     

On the mechanism of antidepressant-like action of berberine chloride

Berberine, an alkaloid isolated from Berberis aristata Linn. has been used in the Indian system of medicines as a stomachic, bitter tonic, antiamoebic and also in the treatment of oriental sores. Evidences have demonstrated that berberine possesses central nervous system activities, particularly the ability to inhibit monoamine oxidase-A, an enzyme involved in the degradation of norepinephrine and serotonin (5-HT). With this background, the present study was carried out to elucidate the antidepressant-like effect of berberine chloride in different behavioural paradigms of despair. Berberine (5, 10, 20 mg/kg, i.p.) inhibited the immobility period in mice in both forced swim and tail-suspension test, however, the effect was not dose-dependent. Berberine (5 and 10 mg/kg, i.p.) also reversed the reserpine-induced behavioral despair. Berberine (5 mg/kg, i.p.) enhanced the anti-immobility effect of subeffective doses of various typical but not atypical antidepressant drugs in forced swim test. Berberine (5 mg/kg, i.p.) following its acute administration in mice resulted in increased levels of norepinephrine (31%), serotonin (47%) and dopamine (31%) in the whole brain. Chronic administration of berberine (5 mg/kg, i.p.) for 15 days significantly increased the levels of norepinephrine (29%), serotonin (19%) as well as dopamine (52%) but at higher dose (10 mg/kg, i.p.), there was no change in the norepinephrine (12%) levels but a significant increase in the serotonin (53%) and dopamine (31%) levels was found. The antidepressant-like effect of berberine (5 mg/kg, i.p.) in forced swim test was prevented by pretreatment with l-arginine (750 mg/kg, i.p.) or sildenafil (5 mg/kg, i.p.). On the contrary, pretreatment of mice with 7-nitroindazole (7-NI) (25 mg/kg, i.p.) or methylene blue (10 mg/kg, i.p.) potentiated the effect of berberine (2 mg/kg, i.p.) in the forced swim test. Pretreatment of mice with (+)-pentazocine (2.5 mg/kg, i.p.), a high-affinity sigma1 receptor agonist, produced synergism with subeffective dose of berberine (2 mg/kg, i.p.). Pretreatment with various sigma receptor antagonists viz. progesterone (10 mg/kg, s.c.), rimcazole (5 mg/kg, i.p.) and N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine (BD1047; 1 mg/kg, i.p.) reversed the anti-immobility effects of berberine (5 mg/kg, i.p.). Berberine at lower dose did not affect the locomotor activity and barbiturate-induced sleep time. It produced mild hypothermic action in rats and displayed analgesic effect in mice. Taken together, theses findings demonstrate that berberine exerted antidepressant-like effect in various behavioural paradigms of despair possibly by modulating brain biogenic amines (norepinephrine, serotonin and dopamine). Further, nitric oxide pathway and/or sigma receptors are involved in mediating its antidepressant-like activity in mouse forced swim test.     

脑5-HT1A/1B、α2受体及腺苷酸环化酶参与了胍丁胺的抗抑郁作用

为了在单胺受体及受体后腺苷酸环化酶(adenylate cyclase,AC)水平探讨胍丁胺(agmatine,AGM)抗抑郁作用的精细机制,采用小鼠悬尾实验和强迫游泳实验观察AGM抗抑郁行为改变。采用放射免疫方法测定大鼠前额皮层突触膜蛋白AC活性。结果表明,AGM(5～40 mg·kg^-1,ig)在小鼠悬尾实验和强迫游泳实验模型上均有显著抗抑郁活性。同时伍用β受体/5-HT_1A/1B受体阻断剂吲哚洛尔(pindolol, PIN, 20 mg·kg^-1, ip)、 α₂肾上腺素受体拮抗剂育亨宾(yohimbine, YOH, 5～10 mg·kg^-1, ip)或咪唑克生(idazoxan, IDA, 4 mg·kg^-1, ip)对AGM(40 mg·kg^-1, ig)的抗抑郁活性具有显著拮抗效应; 而β受体阻断剂普萘洛尔(propranolol, PRO, 5～20 mg·kg^-1, ip)或5-HT₃受体拮抗剂曲匹西隆(tropisetron, TRO, 5～40 mg·kg^-1, ip)对AGM(40 mg·kg^-1, ig)的抗抑郁活性无显著影响。AGM(0.1～6.4 μmol·L^-1)与大鼠前额皮层提取的突触膜共孵可剂量依赖地激活AC活性, 而PIN(1 μmol·L^-1)或YOH(0.25～1 μmol·L^-1)均显著拮抗AGM(6.4 μmol·L^-1)对AC的激活作用; 慢性给予大鼠AGM(10 mg·kg^-1, ig, bid)或氟西汀(fluoxetine, FLU, 10 mg·kg^-1, ig, bid) 2 w也显著增强大鼠前额皮层基础及Gpp(NH)p 预激活的AC活性。本研究表明, 调节脑内5-HT_1A/1B和α₂等受体功能, 并激活前额皮层AC可能是AGM抗抑郁活性的重要机制之一。     

Augmentation effect of combination therapy of aripiprazole and antidepressants on forced swimming test in mice

Rationale  A deficiency in brain monoamine systems (serotonin, dopamine, and/or norepinephrine) have long been hypothesized for the pathogenesis of depression. Drugs enhancing neurotransmission of those monoamines have been proven to have antidepressant effects. We hypothesized that aripiprazole, a partial D₂ agonist, could increase the activity of various antidepressants in the mice forced swimming test (FST), an animal model of depression. Objectives  The scope of this study was to investigate the antidepressant-like effect of aripiprazole, when combined with conventional antidepressants drugs. Materials and methods  This study assessed the effects of co-administration of aripiprazole with selective serotonin reuptake inhibitors (SSRIs; sertraline, paroxetine, and citalopram), selective serotonin–norepinephrine reuptake inhibitors (SNRIs; venlafaxine and minalcipran), selective norepinephrine reuptake inhibitor (NRI; desipramine), and the dual dopamine and norepinephrine reuptake inhibitor (bupropion), using the FST in mice. Subactive doses of aripiprazole and antidepressants sertraline, paroxetine, citalopram, venlafaxine, minalcipran, bupropion (4 and 8 mg/kg), and desipramine (2 and 4 mg/kg) were given i.p. 30 and 45 min, respectively, before the test. Results  Aripiprazole (0.03 and 0.06 mg/kg) combined with inactive doses of antidepressants, increased the activity of all antidepressants with the exception of bupropion and desipramine. Conclusion  The augmentation effects of aripiprazole, in the present study, are in agreement with clinical evidence suggesting that aripiprazole may enhance the efficacy of therapeutic effect of SSRIs and SNRIs but not of NRI. These results suggest that augmentation effect of aripiprazole only appears when 5-HT system is activated and might implicate complex regulation between dopamine and 5-HT_1A and 5-HT_2A receptors.     

Depletion of serotonin and catecholamines block the acute behavioral response to different classes of antidepressant drugs in the mouse tail suspension test

RATIONALE: Few studies have investigated whether the behavioral effects elicited by different types of antidepressant drugs are mediated by either serotonin (5-HT) or the catecholamines norepinephrine (NE) and dopamine (DA). OBJECTIVES: By depleting 5-HT, or NE and DA, the present study investigated the contributions of these monoamines to the acute behavioral effects of selective serotonin reuptake inhibitors (SSRIs; fluoxetine and citalopram) and norepinephrine reuptake inhibitors (NRIs; desipramine and reboxetine) in the mouse tail suspension test (TST). RESULTS: Depletion of 5-HT tissue content by para-chlorophenylalanine (PCPA), an inhibitor of tryptophan hydroxylase, completely blocked reductions of immobility by the SSRIs in the TST. In contrast, PCPA did not alter the behavioral effects of the NRIs. Inhibition of catecholamine synthesis by alpha-methyl-para-tyrosine (AMPT) reduced brain NE and DA tissue content, whereas disruption of vesicular storage with reserpine decreased brain NE, DA and 5-HT tissue content. However, neither treatment completely prevented responses to desipramine, fluoxetine, or citalopram in the TST. Depleting both newly synthesized and vesicular components of NE and DA transmission with a combination of reserpine and AMPT completely prevented the behavioral effects of desipramine, reboxetine, and fluoxetine and attenuated those of citalopram. Although PCPA did not alter baseline immobility, AMPT and reserpine increased baseline values in the TST. CONCLUSIONS: These studies demonstrated that endogenous 5-HT synthesis mediates the behavioral effects of SSRIs, but not NRIs, in the TST. In contrast, disruption of the behavioral effects of NRI and SSRI antidepressants required disruption of both catecholamine synthesis and vesicular storage and release mechanisms.     

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

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

Antidepressant-like effects of serotonin type-3 antagonist, ondansetron: an investigation in behaviour-based rodent models

The present behavioural investigation evaluates the antidepressant potential of ondansetron (OND), a widely used (in management of cancer chemotherapy-induced nausea and emesis) 5-HT3 receptor antagonist. Separate groups of mice received acute or chronic treatment of OND (0.005-1000 microg/kg), and were subjected to spontaneous locomotor activity test or antidepressant assays, namely, the forced swim and tail suspension tests. Interaction studies with fluoxetine, venlafaxine, desipramine and 8-hydroxy-2-(di-n-propylamino) tetralin were conducted in the forced swim test. The effect of OND (0.01-1000 microg/kg) in combination with paroxetine (10 mg/kg, for 14 days) on the behaviour of male bulbectomized or sham-operated rats was also assessed. The postbulbectomy behavioural analysis included exploration in the open field and elevated plus maze. OND exhibited a biphasic dose-response profile, with antidepressant-like effects peaking at 0.1 microg/kg, in the forced swim and tail suspension tests. None of the tested doses influenced spontaneous locomotor activity. Chronic OND pretreatment augmented the antidepressant effects of fluoxetine and venlafaxine but did not influence the effects of desipramine or 8-hydroxy-2-(di-n-propylamino) tetralin. Chronic OND (10 microg/kg) reversed hyperactivity in the open field, and decreased the percentage entry and time spent in open arms in the elevated plus maze. Summing up, it is observed that OND exhibits antidepressant-like effects, possibly mediated through postsynaptic 5-HT3 receptors.     

Effects of desipramine on neuronal responses to dopamine, noradrenaline, 5-hydroxytryptamine and acetylcholine in the caudate nucleus of the rat.

1 The sensitivity of single neurones to microelectrophoretically applied dopamine, noradrenaline (NA), 5-hydroxytryptamine (5-HT) and acetylcholing (ACh) was investigated in the caudate nucleus of the rat, anaesthetized with halothane. Both excitatory and depressant responses could be observed to each of the agonists. There was a high correlation between the direction of responses to dopamine and noradrenaline, whereas there was no significant correlation between the direction of responses to dopamine and ACh. 2 The effect of desipramine was studied on both excitatory and depressant responses to dopamine, NA and 5-HT, and on excitatory responses to ACh. Both potentiation and antagonism of neuronal responses to monoamines and ACh could be observed after a brief application of desipramine. 3 Excitatory responses to glutamate were not affected by desipramine. 4 The observation that responses to dopamine and NA can be potentiated by desipramine in the caudate nucleus suggests that uptake blockade is not a prerequisite for potentiation. 5 It is suggested that the potentiation of neuronal responses to dopamine by desipramine may be responsible for the therapeutic efficacy of desipramine in Parkinson's disease.     

Alteration of monoamine concentrations in the brain of medaka, Oryzias latipes, exposed to tributyltin

We measured the concentrations of monoamines in the brain of Japanese medaka, Oryzias latipes, exposed to tributyltin (TBT). Fish were exposed to 0, 1, 5, 25, or 125 microg g(-1) of TBT via the diet for 21 days. After the administration period, six males and six females in each treatment group were dissected and their brains were collected. The following monoamines were analyzed: dopamine (DA), norepinephrine (NE), epinephrine (E), and 5-hydroxytryptamine (5-HT). The metabolites of DA, 3,4-dihydroxyphenylacetic acid and homovanilic acid, and the metabolite of 5-HT, 5-hydroxyindolacetic acid were also analyzed. The concentration of DA in the brain of male medaka and the concentrations of 5-HT and NE in the female brains were significantly decreased by exposure to 125 microg TBT g(-1). The concentrations of 5-HT and NE in males and of DA in females were slightly decreased by 125 micrg g(-1) of TBT, although the differences were not statistically significant. The present study demonstrates that TBT alters monoamine concentrations in the brain of medaka.     

Diurnal variation in 5-HT1B autoreceptor function in the anterior hypothalamus in vivo: effect of chronic antidepressant drug treatment.

1. Intracerebral microdialysis was used to examine the function of the terminal 5-hydroxytryptamine (5-HT) autoreceptor in the anterior hypothalamus of anaesthetized rats at two points in the light phase of the light-dark cycle. 2. Infusion of the 5-HT1A/1B agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU24969) 0.1, 1.0 and 10 microM through the microdialysis probe led to a concentration-dependent decrease (49, 56 and 65% respectively) in 5-HT output. The effect of RU24969 (1 and 5 microM) was prevented by concurrent infusion of methiothepin (1 and 10 microM) into the anterior hypothalamus via the microdialysis probe. Infusion of methiothepin alone (1.0 and 10 microM) increased (15 and 142% respectively) 5-HT output. 3. Infusion of RU24969 (5 microM) through the probe at mid-light and end-light resulted in a quantitatively greater decrease in 5-HT output at end-light compared with mid-light. 4. Following treatment with either paroxetine hydrochloride (10 mg kg(-1) i.p.) or desipramine hydrochloride (10 mg kg)(-1) i.p.) for 21 days the function of the terminal 5-HT1B autoreceptor was more markedly attenuated at end-light. 5. The data show that, as defined by the response to RU24969, the function of the 5-HT1B receptors that control 5-HT output in the anterior hypothalamus is attenuated following chronic desipramine or paroxetine treatment in a time-of-day-dependent manner.     

Mechanisms involved in the antinociception caused by agmatine in mice

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

Psychopharmacological study of agmatine in behavioral tests of schizophrenia in rodents

The effect of agmatine in preclinical behavioral tests of schizophrenia has been examined in rodents. Agmatine at the doses of 40 and 80 mg/kg blocked conditioned avoidance responding, attenuated apomorphine induced climbing, diminished amphetamine and ketamine hyperlocomotor activity and augmented plasma prolactin levels. Pretreatment of animals with 20 mg/kg of agmatine potentiated the inhibitory effect of haloperidol (0.1 mg/kg, ip) and olanzepine (0.5 mg/kg, ip) in conditioned avoidance response test and apomorphine induced climbing. Agmatine alone at the doses tested here did not induce any cataleptic behavior in mice. However significant catalepsy was exhibited when agmatine (80 mg/kg, ip) was injected to mice pretreated with 5-HT1A receptor antagonist, WAY100, 635. These results indicate that agmatine via regulation of brain dopaminergic signaling modulates dopamine mediated behaviors. The alteration in the levels of endogenous agmatine may contribute to the genesis of psychosis and development of drugs that enhance endogenous agmatine content may be better therapeutic approach to treat schizophrenia with low incidences of extra pyramidal side effects.     

Acute increases in monoamine release in the rat prefrontal cortex by the mGlu2/3 agonist LY379268 are similar in profile to risperidone, not locally mediated, and can be elicited in the presence of uptake blockade

Our recent work (Cartmell et al., Journal of Neurochemistry, 75 (2000) 1147-1154) demonstrated that systemic injection of the potent, selective mGlu2/3 receptor agonist, LY379268, acutely increased extracellular levels of dopamine, its metabolites DOPAC and HVA, and the 5-HT metabolite, 5-HIAA, in rat medial prefrontal cortex (mPFC). Here, we compared the acute effects of LY379268 with those of clozapine and risperidone (atypical antipsychotics) on extracellular levels of both dopamine and 5-HT in the mPFC of freely-moving rats. Uptake blockers were included to minimize metabolism of monoamines near the probe area. One hour after injection, LY379268 (10 mg/kg s.c.), clozapine (10 mg/kg s.c.) or risperidone (1 mg/kg s.c.) maximally increased dopamine by 224, 257 and 234% of basal levels. These effects were followed by maximal increases in DOPAC and HVA levels 2 to 3.5 hours after administration. LY379268, at 3 and 10 mg/kg s.c., and risperidone (1 mg/kg s.c.) also increased dialysate 5-HT to 169, 179 and 140% of basal levels and 5-HIAA to 144, 154 and 121% of basal levels, respectively. These neurochemical changes in the mPFC could not be mimicked when LY379268 (3 or 30 microM) was administered locally via the microdialysis probe. These data demonstrate that increases in extracellular monoamines in the rat prefrontal cortex evoked acutely by the mGlu2/3 agonist, LY379268, are similar in profile to risperidone, not locally mediated, and can be elicited in the presence of uptake blockade.     

胍丁胺对NMDA受体药理作用的研究进展

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

IRAS, a candidate for I1-imidazoline receptor, mediates inhibitory effect of agmatine on cellular morphine dependence

Agmatine, an endogenous ligand for the I1-imidazoline receptor, has previously been shown to prevent morphine dependence in rats and mice. To investigate the role of imidazoline receptor antisera-selected protein (IRAS), a strong candidate for I1R, in morphine dependence, two CHO cell lines were created, in which mu opioid receptor (MOR) was stably expressed alone (CHO-mu) or MOR and IRAS were stably co-expressed (CHO-mu/IRAS). After 48 h administration of morphine (10 microM), naloxone induced a cAMP overshoot in both cell lines, suggesting cellular morphine dependence had been produced. Agmatine (0.1-2.5 microM) concentration-dependently inhibited the naloxone-precipitated cAMP overshoot when co-pretreated with morphine in CHO-mu/IRAS, but not in CHO-mu. Agmatine at 5-100 microM also inhibited the cAMP overshoot in CHO/mu and CHO-mu/IRAS. Efaroxan, an I1R-preferential antagonist, completely blocked the effect of agmatine on the cAMP overshoot at 0.1-2.5 microM in CHO-mu/IRAS, while partially reversing the effects of agmatine at 5-100 microM. L-type calcium channel blocker nifedipine entirely mimicked the effects of agmatine at high concentrations on forskolin-stimulated cAMP formation in CHO-mu and naloxone-precipitated cAMP overshoot in morphine-pretreated CHO-mu. Therefore, IRAS, in the co-transfected CHO-mu/IRAS cell line, appears necessary for low concentrations of agmatine to cause attenuation of cellular morphine dependence. An additional effect of agmatine at higher concentrations seems to relate to both transfected IRAS and some naive elements in CHO cells, and L-type voltage-gated calcium channels are not ruled out. This study suggests that IRAS mediates agmatine's high affinity effects on cellular morphine dependence and may play a role in opioid dependence.     

胍丁胺对体外培养海马神经前体细胞增殖的影响及作用机制

目的前期证明胍丁胺(agmatine,Agm)作为一种新型神经递质具有抗抑郁作用,能够促进慢性应激小鼠海马神经元再生。本实验通过观察胍丁胺对体外培养海马神经前体细胞增殖的影响及作用机制,探讨其抗抑郁作用的细胞分子机制。方法培养扩增新生大鼠海马神经前体细胞,通过3H-TdR参入法和CCK-8试剂盒检测胍丁胺对其增殖的影响;培养体系内加入H89(PKA特异性抑制剂)或PD98059(MEK特异性抑制剂),观察这两种抑制剂对胍丁胺作用的影响。结果胍丁胺在0.1~10μmol·L-1浓度范围内,浓度依赖性地促进神经前体细胞的增殖,这种促增殖作用可以被H89(10μmol·L-1)和PD98059(10μmol·L-1)取消。结论胍丁胺可以促进体外培养的海马神经前体细胞增殖,此作用与cAMP-PKA-CREB通路和MEK-ERK-CREB通路相关。     

Ginsenoside Rb1 Modulates Level of Monoamine Neurotransmitters in Mice Frontal Cortex and Cerebellum in Response to Immobilization Stress

Cerebral monoamines play important roles as neurotransmitters that are associated with various stressful stimuli. Some components such as ginsenosides (triterpenoidal glycosides derived from the Ginseng Radix) may interact with monoamine systems. The aim of this study was to determine whether ginsenoside Rb1 can modulate levels of the monoamines such as dihydroxyphenylalanine (DOPA), dopamine (DA), norepinephrine (NE), epinephrine (EP), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydorxytryptamine (5-HT), 5-hydroxindole-3-acetic acid (5-HIAA), and 5-hydroxytryptophan (5-HTP) in mice frontal cortex and cerebellum in response to immobilization stress. Mice were treated with ginsenoside Rb1 (10 mg/kg, oral) before a single 30 min immobilization stress. Acute immobilization stress resulted in elevation of monoamine levels in frontal cortex and cerebellum. Pretreatment with ginsenoside Rb1 attenuated the stress-induced changes in the levels of monoamines in each region. The present findings showed the anti-stress potential of ginsenoside Rb1 in relation to regulation effects on the cerebral monoaminergic systems. Therefore, the ginsenoside Rb1 may be a useful candidate for treating several brain symptoms related with stress.     

Evaluation of sigma (σ) receptors in the antidepressant-like effects of ketamine in vitro and in vivo

Ketamine is an NMDA antagonist and dissociative anesthetic that has been shown to display rapid acting and prolonged antidepressant activity in small-scale human clinical trials. Ketamine also binds to σ receptors, which are believed to be protein targets for a potential new class of antidepressant medications. The purpose of this study was to determine the involvement of σ receptors in the antidepressant-like actions of ketamine. Competition binding assays were performed to assess the affinity of ketamine for σ(1) and σ(2) receptors. The antidepressant-like effects of ketamine were assessed in vitro using a neurite outgrowth model and PC12 cells, and in vivo using the forced swim test. The σ receptor antagonists, NE-100 and BD1047, were evaluated in conjunction with ketamine in these assays to determine the involvement of σ receptors in the antidepressant-like effects of ketamine. Ketamine bound to both σ(1) and σ(2) receptors with μM affinities. Additionally, ketamine potentiated NGF-induced neurite outgrowth in PC12 cells and this effect was attenuated in the presence of NE-100. Ketamine also displayed antidepressant-like effects in the forced swim test; however, these effects were not attenuated by pretreatment with NE-100 or BD1047. Taken together, these data suggest that σ receptor-mediated neuronal remodeling may contribute to the antidepressant effects of ketamine.