Non-competitive N-methyl-D-aspartate Receptor Research Articles

Preventive effects of dextromethorphan on methylmercury-induced glutamate dyshomeostasis and oxidative damage in rat cerebral cortex.

Methylmercury (MeHg) is a well-known environmental pollutant leading to neurotoxicant associated with aberrant central nervous system (CNS) functions, but its toxic mechanisms have not yet been fully recognized. In the present study, we tested the hypothesis that MeHg induces neuronal injury via glutamate (Glu) dyshomeostasis and oxidative damage mechanisms and that these effects are attenuated by dextromethorphan (DM), a low-affinity and noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist. Seventy-two rats were randomly divided into four groups of 18 animals in each group: control group, MeHg-treated group (4 and 12μmol/kg), and DM-pretreated group. After the 4-week treatment, we observed that the administration of MeHg at a dose of 12μmol/kg significantly increased in total mercury (Hg) levels, disrupted Glu metabolism, overexcited NMDARs, and led to intracellular calcium overload in the cerebral cortex. We also found that MeHg reduced nonenzymatic and enzymatic antioxidants, enhanced neurocyte apoptosis, induced reactive oxygen species (ROS), and caused lipid, protein, and DNA peroxidative damage in the cerebral cortex. Moreover, glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) appeared to be inhibited by MeHg exposure. These alterations were significantly prevented by the pretreatment with DM at a dose of 13.5μmol/kg. In conclusion, these findings strongly implicate that DM has potential to protect the brain from Glu dyshomeostasis and oxidative damage resulting from MeHg-induced neurotoxicity in rat.

Ketamine, but not MK-801, produces antidepressant-like effects in rats responding on a differential-reinforcement-of-low-rate operant schedule

The noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine has been shown to produce rapid and lasting antidepressant effects in treatment-resistant patients with major depressive disorder and in preclinical behavioral assays. The present study sought to extend the preclinical antidepressant-like effects of ketamine using the differential-reinforcement-of-low-rate 72 s operant task in rats, as well as to determine whether the more selective and higher affinity NMDA receptor antagonist MK-801 produced antidepressant-like effects similar to those of ketamine. Ketamine, the NMDA receptor agonist N-methyl-D-aspartic acid, the tricyclic antidepressant imipramine, and the selective serotonin reuptake inhibitor fluoxetine all produced antidepressant-like effects by increasing the number of reinforcers, decreasing the number of responses, and producing a rightward shift in the peak location of the inter-response time distributions. Conversely, MK-801 and the dopamine receptor agonist D-amphetamine produced a psychostimulant-like effect by decreasing the number of reinforcers, increasing the number of responses, and producing a leftward shift in the peak location of the inter-response time distributions. Although a subeffective dose of ketamine attenuated the antidepressant-like effects of NMDA, a subeffective dose of NMDA did not alter the antidepressant-like effects of ketamine. These data indicate that ketamine and MK-801 produced dissociable effects in the differential-reinforcement-of-low-rate 72 s task, and further suggest that the underlying mechanisms responsible for the antidepressant effects of ketamine may be unique to ketamine and not shared by all NMDA receptor antagonists.

Blockade of MK-801-Induced Heat Shock Protein 72/73 in Rat Brain by Antipsychotic and Monoaminergic Agents Targeting D2, 5-HT<sub>1A</sub>, 5-HT<sub>2A</sub> and α<sub>1</sub>-Adrenergic Receptors

Noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists can produce positive and negative symptomatology as well as impairment of cognitive function that closely resemble those present in schizophrenia. In rats, these drugs induce a behavioral syndrome (characterized by hyperlocomotion and stereotypies), an enhanced glutamatergic transmission in the medial prefrontal cortex, and damage to retrosplenial cortical neurons in adult rats, which was measured as the induction of the stress protein 72/73 kDa heat shock protein (Hsp72/73). In the present work, we have examined the existence of possible differences among different antipsychotic drugs in their capacity to block immunolabeling of Hsp72/73 in the retrosplenial cortex of the rat induced by the potent NMDA receptor antagonist, MK- 801. In addition, the effects of selective monoaminergic agents were also studied to delineate the particular receptors responsible for the actions of antipsychotic drugs. Pretreatment with clozapine, chlorpromazine, olanzapine, ziprasidone--and to a lesser extent haloperidol-reduced the formation of Hsp72/73 protein in the rat retrosplenial cortex after the administration of MK-801. In addition, antagonism at dopamine D2 (raclopride), 5-HT2 (M100907) and α1- adrenoceptors (prazosin) as well as agonism at 5-HT1A receptors (BAY x 3702) also diminished the MK-801-induced number of cells labeled with Hsp72/73. Each of these effects may contribute to antipsychotic action. The results suggest that the efficacy of atypical antipsychotic drugs in the clinic may result from a combined effect on 5-HT2, 5-HT1A and α1-adrenergic receptors added to the classical dopamine D2 receptor antagonism.

N-methyl-D-aspartate Receptors Are Involved in Caffeine-Induced Facilitation on Memory Retention of Passive Avoidance Learning in Rats

Background: Caffeine is an antagonist of adenosine receptors. It preserves cognition and reduces neurodegeneration. The MK-801, a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, has also shown amnesic properties in animal models. The effect of caffeine on MK-801-induced amnesia has not been the subject of any recent study. Methods: In this study, the effects of different doses (10 and 30 mg/kg) of caffeine on Wistar male rats (eight in each group), passive-avoidance (PA) response, and the impairment of this response by the NMDA receptor antagonist MK-801 (dizocilpine) were evaluated. The PA response was examined by measuring the step-through latency, 1 and 3 days after the animals received foot-shock training. Results: When given intraperitoneally before the training session, 10 mg/kg caffeine elongated the PA response, but 30 mg/kg did not, and it even reduced the step-through latency significantly. Before and after training, administration of 0.2 mg/kg MK-801 significantly elongated the PA response. This finding showed that MK-801 had a detrimental effect on learning, consolidation, and memory. We found the first evidence that pre-training and pre-testing administration of 10 mg/kg caffeine significantly reversed the learning and memory consolidation process disrupted by MK-801. Conclusion: These results show an interaction between caffeine and the glutamatergic system, suggesting that low-dose caffeine can prevent learning deficit and amnesia produced by a NMDA antagonist in rats (model of schizophrenia in human) when injected in pre- and post-training phase and may be clinically useful for treating cognitive impairments in schizophrenia.

Scopolamine and Ketamine: Evidence of Convergence?

Recent findings have demonstrated that the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine and the muscarinic acetylcholine receptor antagonist scopolamine have rapid antidepressant effects in depressed individuals (1–3). Patients resistant to other treatments reported alleviation of core major depression symptoms following infusion of low-dose ketamine or scopolamine, with effects lingering for longer than 2 weeks in some patients. The rapid efficacy of these compounds is in striking contrast to traditional antidepressants, which target the monoamine system and exert mood-elevating effects only after repeated administration (typically several weeks to months). Because there is an urgent need for antidepressants that exert therapeutic effects within hours or days after administration, preclinical studies have started to elucidate the mechanism of how ketamine can trigger a fast-acting antidepressant response (4,5). Despite initial progress, it remains unclear whether ketamine can exert long-term effects (beyond 1–2 weeks) and, if so, how such an effect can be maintained. It is also unknown how scopolamine triggers a rapid antidepressant effect and whether this effect has any commonalities with the effect of ketamine. In the current issue of Biological Psychiatry, two articles begin to address these questions. Acute ketamine treatment in patients gives rise to the rapid relief of depressive-like symptoms lasting for a short duration of 1 to 2 days to longer than 2 weeks (6). For routine use of fast-acting antidepressants, it is extremely desirable to be able to elicit longer-lasting effects; however, the consequences of repeated ketamine administration is unknown. The study by Parise et al. (7) shows that repeated ketamine treatment (20 mg/kg, twice/day for 15 consecutive days) in adolescent male rats can elicit longlasting antidepressant and anxiolytic-like effects up to 2 months that are not due to drug-seeking behavior. As expected, the authors show that an acute dose of ketamine does not elicit these long-term sustained effects. This study is of interest because it provides a proof of principle that it is possible to prolong ketamine’s beneficial response using repeated administration. However, the repeated twice-daily injections of ketamine and the relatively high dose used raise concerns regarding the potential undesirable side effects that may affect the patient population. Nevertheless, this study provides important evidence that repeated ketamine exposure may have long-term beneficial effects. One intriguing aspect of the study is that ketamine has a relatively short half-life of approximately 2 to 3 hours, suggesting the long-term effects at 2 months are not due to persistent blockade of NMDA receptors but rather to synaptic plasticity processes. Future studies will need to delineate the impact of

Memantine in the management of affective recurrences of bipolar disorders after the discontinuation of long-term lithium treatment: three case histories

Discontinuation of long-term lithium treatment leads to early and severe affective recurrences [Baldessarini et al. 1999], and to a bipolar disorder course more severe than that before lithium treatment with an increased risk of suicide [Post, 2012], which is often resistant not only to other mood stabilizers, but also to the reinstitution of lithium treatment at the prior effective serum lithium level [Post, 2012]. Unfortunately, the currently available lithium-alternative mood stabilizers are of limited (anticonvulsants) [Geddes et al. 2010; Kessing et al. 2011; Greil and Kleindiest, 1999], or questionable (atypical neuroleptics) [Goodwin et al. 2011] efficacy. We have recently provided clinical observations strongly suggesting that memantine, a noncompetitive N-methyl D-aspartate receptor antagonist, has a clinically relevant antimanic and a sustained mood-stabilizing effect in treatment-resistant bipolar disorder with excellent safety and tolerability [Koukopoulos et al. 2010, 2012; Sani et al. 2012; Serra et al. 2013]. More recently we have observed a long-lasting mood-stabilizing effect of memantine after lithium discontinuation in a bipolar I patient [Serra et al. 2013]. In order to evaluate further the effect of memantine in the prophylaxis of affective recurrences occurring after long-term lithium discontinuation, we administered the drug to three patients who had to discontinue lithium because of severe renal complications (two patients) or excessive tremor (one patient). These case histories confirm our previous observations, and suggest that memantine may be considered a useful lithium substitute to prevent the affective recurrences after lithium discontinuation. Case 1 Woman born in 1930, suffering from a bipolar II disorder with rapid cycling course. She has a family history of bipolar disorder. Her first affective episode was a depression in May 1979 (aged 49 years), followed by a hypomania until January 1980. She started lithium prophylaxis and had a very good response to lithium. In June 2009 lithium was gradually reduced to 150 mg every 2 days (serum lithium level 0.2 mmol/L) and then withdrawn because of renal impairment. After we had obtained the informed written consent, she was put on 20 mg/day memantine and lamotrigine (250 mg/day). She started with rapid cycling recurrences until May 2010. Since then she has been well and stable on memantine 20 mg/day, lamotrigine 250 mg/day and lithium 150 mg every 2 days (lithium serum level 0.2 mmol/L).

Crocins, the active constituents of Crocus Sativus L., counteracted ketamine–induced behavioural deficits in rats

Experimental evidence indicates that the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine impairs cognition and can mimic certain aspects of positive and negative symptoms of schizophrenia in rodents. Crocins are among the active components of the plant Crocus sativus L. and were found to be effective in different models of psychiatric disorders including anxiety and depression. The present study was designed to investigate the ability of crocins to counteract schizophrenia-like behavioural deficits produced by ketamine in rats. Crocin's ability to counteract hypermotility, stereotypies and ataxia induced by ketamine was evaluated in a motor activity cage. The ability of crocins to reverse ketamine-induced memory deficits was assessed using the novel object recognition task (NORT). The social interaction test was used in order to examine the effects of crocins on ketamine-induced social withdrawal. Crocins (50 but not 30 mg/kg, i.p.) attenuated ketamine (25 mg/kg, i.p.)-induced hypermotility, stereotypies and ataxia. In a subsequent study, post-training administration of crocins (15 and 30 mg/kg, i.p.) reversed ketamine (3 mg/kg, i.p.)-induced performance deficits in the NORT. Finally, crocins (50 but not 30 mg/kg, i.p.) counteracted the ketamine (8 mg/kg, i.p.)-induced social isolation in the social interaction test. Our findings show that crocins attenuated schizophrenia-like behavioural deficits induced by the non-competitive NMDA receptor antagonist ketamine in rats.

The therapeutic effect of adding dextromethorphan to clonidine for reducing symptoms of opioid withdrawal: a randomized clinical trial.

Background. Dextromethorphan is a noncompetitive N-methyl D-aspartate receptor antagonist that is clinically feasible for relieving the opioid withdrawal symptoms. This study compares the efficacy of a combination therapy with dextromethorphan and clonidine to treatment with clonidine alone. Methods and Materials. In this double-blind randomized clinical trial, patients were selected from inpatients of detox and rehabilitation ward of Razi Hospital, Tabriz, Iran. They were randomly allocated to two groups receiving either clonidine (0.4–1.2 mg/day) or clonidine and dextromethorphan (300 mg/day). Withdrawal symptoms were evaluated in the first day of admission and again 24, 48, and 72 hours later. Results. Thirty male patients completed the trial in each group. Withdrawal symptoms began to decrease in the second day in patients receiving dextromethorphan and clonidine while patients receiving clonidine experienced the more severe symptoms in 72 hours. Analysis of variance of the symptom severity score revealed a significant group × time interaction (F = 14.25; P < 0.001), so that patients receiving dextromethorphan plus clonidine had milder symptoms during three days in all of the measurements compared to clonidine group. Conclusion. Combination therapy of dextromethorphan and clonidine would result in milder opioid withdrawal symptoms compared to clonidine alone with a reduction beginning at the second day.

Temporally distinct cognitive effects following acute administration of ketamine and phencyclidine in the rat

Non-competitive N-methyl-d-aspartate receptor (NMDAR) antagonists such as phencyclidine (PCP) and ketamine are commonly and interchangeably used to model aspects of schizophrenia in animals. We compared here the effects of acute administration of these compounds over a range of pre-treatment times in tests of instrumental responding (VI 30s response schedule), simple reaction time (SRT) and cognitive flexibility (reversal learning and attentional set shifting digging task) in rats. At standard pre-treatment times (15–30min), both ketamine and PCP produced overall response suppression in VI 30 and increased reaction times in SRT suggesting that any concomitant cognitive performance deficits are likely to be confounded by motor and/or motivational changes. However, the use of extended pre-treatment times produced deficits in cognitive flexibility measured up to 4h after drug administration in the absence of motor/motivational impairment. Generally, PCP increased impulsive responding in the SRT indicating a possible loss of inhibitory response control that may have contributed to deficits observed in reversal learning and attentional set-shifting. In contrast to PCP, ketamine did not have the same effect on impulsive responding, and possibly as a consequence produced more subtle cognitive deficits in attentional set-shifting. In summary, acute treatment with NMDAR antagonists can produce cognitive deficits in rodents that are relevant to schizophrenia, provided that motor and/or motivational effects are allowed to dissipate. The use of longer pre-treatment times than commonly employed might be advantageous. Also, ketamine, which is more frequently used in clinical settings, did not produce as extensive cognitive deficits as PCP.

Protective Effects of Memantine Against Methylmercury-Induced Glutamate Dyshomeostasis and Oxidative Stress in Rat Cerebral Cortex

Methylmercury (MeHg) is one of the ubiquitous environmental toxicant that leads to long-lasting neurological deficits in animals and humans. The identification of the underlying mechanisms has been a main focus of research in the neurotoxicology field. Glutamate (Glu) dyshomeostasis and oxidative stress have been identified as two critical mechanisms mediating MeHg-induced neurotoxicity. However, little has been known of the interaction between these two mechanisms that play in MeHg poisoning in vivo. We, therefore, developed a rat model of MeHg subchronic poisoning to evaluate its neurotoxic effects and investigated the neuroprotective role of memantine, a low-affinity, noncompetitive N-methyl-D-aspartate receptors (NMDARs) antagonist, against MeHg-induced neurotoxicity. Ninety rats were randomly divided into five groups: control, memantine control, MeHg-treated (4 and 12 μmol/kg), and memantine pretreated. Administration of 12 μmol/kg MeHg for 4 weeks significantly elevated total Hg levels, disrupted Glu metabolism, overexcited NMDARs, and led to intracellular calcium overload, which might be critical to excessive reactive oxygen species (ROS) formation in cerebral cortex. Meanwhile, MeHg administration reduced non-enzymatic (non-protein sulfhydryl, NPSH) and enzymatic (superoxide dismutase, SOD and glutathione peroxidase, GSH-Px) antioxidants; caused lipid, protein, and DNA oxidative damage; and enhanced neurocyte apoptosis in cerebral cortex. Moreover, glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) appear to be inhibited by MeHg exposure. Pretreatment with memantine at a dose of 5 μmol/kg significantly prevented MeHg-induced alterations of Glu metabolism and oxidative stress, alleviated neurocyte apoptosis, and pathological injury. In conclusion, the results suggested that Glu dyshomeostasis and oxidative stress resulting from MeHg exposure contributed to neuronal injury. Memantine possesses the ability to attenuate MeHg-induced neurotoxicity through mechanisms involving its NMDARs-binding properties and indirect antioxidation.

Memantine attenuates 3,4-methylenedioxymethamphetamine-induced hyperthermia in rats

3,4-Methylenedioxymethamphetamine (MDMA) is an illegal drug that can induce life-threatening hyperthermia. No effective pharmacological treatment for MDMA-induced hyperthermia has yet been established. We investigated the effects of memantine, a non-competitive N-methyl-d-aspartate (NMDA)-type glutamate receptor antagonist and an α-7 nicotinic acetylcholine receptor (nAChR) antagonist, on MDMA-induced hyperthermia in rats. Treatment of animals with memantine (10 or 20mg/kg) either before or after MDMA (10mg/kg) administration significantly decreased the peak body temperature. Results from our microdialysis study indicated that pretreatment with memantine (20mg/kg) before MDMA administration had no effect on the MDMA-induced increase in serotonin (5-HT) and dopamine (DA) levels in the anterior hypothalamus. MDMA-induced hyperthermia was significantly suppressed by pretreatment with the non-competitive NMDA receptor antagonist MK-801 (0.5mg/kg) and the competitive NMDA antagonist CGS 19755 (5mg/kg), but not by the selective α-7 nAChR antagonist methyllycaconitine (6 or 10mg/kg). These results indicate that the inhibitory effect of memantine on MDMA-induced hyperthermia may be due to its activity as an NMDA receptor antagonist and not as a result of a direct effect on the 5-HT or DA systems. The present study suggests that moderate doses of memantine may be useful for the treatment of MDMA-induced hyperthermia in humans.

Ketamine-Induced Neuronal Damage and Altered N-Methyl-d-Aspartate Receptor Function in Rat Primary Forebrain Culture

Ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, is frequently used in pediatric general anesthesia. Accumulating evidence from animal experiments has demonstrated that ketamine causes neuronal cell death during the brain growth spurt. To elucidate the underlying mechanisms associated with ketamine-induced neuronal toxicity and search for approaches or agents to prevent ketamine's adverse effects on the developing brain, a primary nerve cell culture system was utilized. Neurons harvested from the forebrain of newborn rats were maintained under normal control conditions or exposed to either ketamine (10 µM) or ketamine plus L-carnitine (an antioxidant; 1-100 µM) for 24h, followed by a 24-h withdrawal period. Ketamine exposure resulted in elevated NMDA receptor (NR1) expression, increased generation of reactive oxygen species (ROS) as indicated by higher levels of 8-oxoguanine production, and enhanced neuronal damage. Coadministration of L-carnitine significantly diminished ROS generation and provided near complete protection of neurons from ketamine-induced cell death. NMDA receptors regulate channels that are highly permeable to calcium, and calcium imaging data demonstrated that neurons exposed to ketamine had a significantly elevated amplitude of calcium influx and higher intracellular free calcium concentrations ([Ca(2+)]i) evoked by NMDA (50 µM), compared with control neurons. These findings suggest that prolonged ketamine exposure produces an increase in NMDA receptor expression (compensatory upregulation), which allows for a higher/toxic influx of calcium into neurons once ketamine is removed from the system, leading to elevated ROS generation and neuronal cell death. L-Carnitine appears to be a promising agent in preventing or reversing ketamine's toxic effects on neurons at an early developmental stage.

Protective effects of MK-801 on methylmercury-induced neuronal injury in rat cerebral cortex: Involvement of oxidative stress and glutamate metabolism dysfunction

Methylmercury (MeHg) is one of the ubiquitous environmental toxicants, which can induce oxidative stress and an indirect excitotoxicity caused by altered glutamate (Glu) metabolism. However, little is known of the interaction between oxidative stress and Glu metabolism play in MeHg poisoning rats. We have investigated the neuroprotective role of MK-801, a non-competitive N-methyl-d-aspartate receptors (NMDAR) antagonist, against MeHg-induced neurotoxicity. Fifty rats were randomly divided into five groups of 10 animals in each group: control group, MK-801 control group, MeHg-treated group (4 and 12μmol/kg) and MK-801 pre-treated group. Administration of MeHg at a dose of 12μmol/kg for four weeks significantly increased in ROS and total Hg levels and that caused lipid, protein and DNA peroxidative damage in cerebral cortex. In addition, MeHg also reduced nonenzymic (reduced glutathione, GSH) and enzymic (glutathione peroxidase, GPx and superoxide dismutase, SOD) antioxidants and enhanced neurocyte apoptosis rate in cerebral cortex. MeHg-induced ROS production appears to inhibit the activity of the glutamine synthetase (GS), leading to Glu metabolism dysfunction. Pretreatment with MK-801 at a dose of 0.3μmol/kg prevented the alterations of the activities of PAG and GS and oxidative stress. In addition, pretreatment with MK-801 significantly alleviated the neurocyte apoptosis rate and histopathological damage. In conclusion, the results suggested ROS formation resulting from MeHg- and Glu-induced oxidative stress contributed to neuronal injury. MK-801 possesses the ability to attenuate MeHg-induced neurotoxicity in the cerebral cortex through mechanisms involving its NMDA receptor binding properties and antioxidation.

Role of the anterior thalamic nucleus in the motor hyperactivity induced by systemic MK-801 administration in rats

Non-competitive N-methyl-d-aspartate receptor (NMDA-R) antagonists have been extensively used in rodents to model psychotic symptoms of schizophrenia. Although the motor syndrome induced by acute and systemic administration of low doses of dizocilpine (MK-801) has been extensively characterized, its neurobiological basis is not fully understood. NMDA-R antagonists can disinhibit excitatory inputs in certain brain areas, but the precise circuitry is not fully known.We examined the involvement of the anterior thalamic nucleus (ATN) in hyperlocomotion and other related behaviors (stereotypies, ataxia signs) induced after acute systemic administration of MK-801. Since GABAergic neurons of the reticular thalamic nucleus (RTN) exert the main inhibitory control on thalamic projection neurons, we hypothesized that systemically injected MK-801 might block NMDA-R on RTN GABAergic neurons. This effect would subsequently result in disinhibition of GABAergic inputs onto ATN projections to cortical motor areas, thereby inducing behavioral effects. We evaluated the behavioral syndrome induced by the systemic administration MK-801 (0.2 mg/kg) in control rats and in rats subjected to a bilateral stereotaxic infusion of the GABAA agonist muscimol (0.2 μl of 2.5 and 5.0 mM; 0.5–1 nmol per application, respectively) into the ATN. As previously reported, MK-801-induced hyperlocomotion in parallel with disorganized movements (e.g. not guided by normal exploration) slight ataxia signs and stereotypies. All responses were antagonized by pre-infusion of muscimol but not saline into the ATN. According to our results we suggest that the ATN plays a role on hyperlocomotion evoked by MK-801 and could involve a thalamic GABAergic disinhibition mechanism.

The use of ketamine for perioperative pain management

Ketamine is a phencyclidine derivative that was introduced into clinical use in 1965. It is a noncompetitive N-methyl-Daspartate (NMDA) receptor antagonist, and has analgesic and antihyperalgesic properties. Today, it is unusual to use ketamine as the first-line drug in general anesthesia and the role of ketamine is changing in clinical practice. Ketamine may be a useful adjunct to improve the management of perioperative pain, because the mechanism of action differs from that of opioids. The antihyperalgesic mechanism of ketamine is not fully understood. Opioid-induced hyper algesia may be associated with the influence of excitatory neuro transmission [1]. Therefore, ketamine can prevent the develop ment of tolerance and hyperalgesia by inhibition of the NMDA receptors [2,3]. Various studies have reported on the potentiation of opioidinduced analgesia and the opioid-sparing effect of ketamine [4-6]. However, the results from several clinical trials are controversial. A subanesthetic dose, intravenous intraoperative ketamine reduced mechanical hyperalgesia and improved postoperative analgesia [7], and a small dose of ketamine given before skin incision decreased postoperative pain and reduced morphine consumption after open renal surgery [8]. A small intravenous dose of ketamine before the first incision followed by a 24-h infusion had a morphine-sparing effect after total hip arthroplasty and decreased postoperative chronic pain up to 6 months after surgery [9]. Intravenous patient-controlled analgesia (PCA) with a subanesthetic ketamine and morphine following transthoracic lung and heart surgery resulted in lower pain scores, reduced morphine consumption and shorter postoperative IV-PCA dependence, associated with cardiovascular stability and better respiratory parameters [10]. The potentiation of opioid-induced analgesia and the opioid-sparing effect of ketamine were observed in pediatric patients [6,11]. In this issue of the Korean Journal of Anesthesiology [12], the authors assessed the effectiveness of ketamine, when given intravenously via a PCA pump, as an alternative to nonsteroidal anti-inflammatory drugs towards the management of acute postoperative pain. The authors concluded that a small dose of ketamine (0.5-2.5 μg/kg/min) proportional to fentanyl was not only safe, but also lowered postoperative pain intensity in patients undergoing spinal fusion; however, the opioidsparing effects of ketamine were not demonstrated. However, there was no benefit provided to patients with either small-dose ketamine combined with morphine for PCA after major orthopedic surgery [13] or the addition of a low dose of ketamine to a multimodal analgesic regimen after gynecolo

Phencyclidine treatment increases NR2A and NR2B N-methyl-D-aspartate receptor subunit expression in rats

Administration of noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist phencyclidine to rats on postnatal days 7, 9, and 11 induces apoptosis in prefrontal cortex and hippocampus. In adulthood, these animals display cognitive impairment of working memory, reversal learning and attention that are similar to clinical observations in schizophrenia. In this study, expression of different NMDAR subunits, the postsynaptic mGlu5 receptor and the connecting NMDAR-mGluR5 intracellular postsynaptic density proteins have been measured in adult rats after treatment with phencyclidine on postnatal days 7, 9, and 11. We found that these animals exhibited elevated expression in medial prefrontal cortex of the NR2A and NR2B NMDA receptor subunits in adulthood. These results indicate how behavioral changes in a developmental model for cognitive dysfunction involve changes to specific molecular subsets of the cortical glutamate system.

Evaluation of brain pharmacokinetics of (+)MK-801 in relation to behaviour

The non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist (+)MK-801 is widely used in animal research (over 3000 publications), however its extracellular brain concentration has never been reported. Here, we show using in vivo microdialysis that systemic injection of (+)MK-801 at doses of 0.05, 0.1 or 0.2mg/kg resulted in peak brain ECF concentration of 6, 14 or 34nM, respectively. Moreover, (+)MK-801 resulted in a dose-dependent learning impairment in the Morris water maze as well as hyperactivity in the open field. These data demonstrate for the first time that (+)MK-801 at doses producing behavioural alterations expected from NMDA receptor blockade reaches extracellular brain concentrations corresponding to the affinity at NMDA receptors.

Differential effects of histamine H3 receptor inverse agonist thioperamide, given alone or in combination with the N-methyl-d-aspartate receptor antagonist dizocilpine, on reconsolidation and consolidation of a contextual fear memory in mice

Albeit there is no doubt that histamine and its H(3) receptors participate in several aspects of learning and memory, such as memory consolidation, nothing is known about their potential involvement in memory reconsolidation. On the basis of previous reports of pro-cognitive effects of histamine H(3) receptor inverse agonists (which augment histamine release), we investigated to what extent the most representative of them, thioperamide, is able to facilitate reconsolidation of a contextually-conditioned fear memory in C57BL/6J mice. We also examined the effects of thioperamide on the stark disruptive effect that the non-competitive N-methyl-d-aspartate (NMDA) antagonist dizocilpine (MK-801) typically exerts on both reconsolidation and consolidation. Post-training systemic injections (i.p.) of thioperamide facilitated consolidation at 10 and 20 mg/kg and reversed amnesia induced by an i.p. injection of 0.12 mg/kg dizocilpine at 5, 10 and 20 mg/kg. Importantly, none of the five thioperamide doses (2.5, 5, 10, 20 and 30 mg/kg) given right after reactivation (reexposure to the context in which training took place 48 h earlier) affected reconsolidation, whereas all similarly given doses of dizocilpine (0.03, 0.06 and 0.12 mg/kg) disrupted it more or less equally. By contrast, thioperamide was able to unambiguously reverse the deficit in reconsolidation induced by 0.12 mg/kg dizocilpine at 10 and 20, but not 5 mg/kg. This is the first demonstration of an involvement of the interactive articulation between histamine and NMDA receptors in the mechanisms of memory reconsolidation, which seems to be indifferent to an increase of brain histamine per se. The results suggest a qualitatively different participation of histaminergic signalling in the mechanisms of reconsolidation and consolidation. The precise circuits within which these interactions take place are yet to be identified.

Reduction of food intake by cholecystokinin requires activation of hindbrain NMDA-type glutamate receptors

Intraperitoneal injection of CCK reduces food intake and triggers a behavioral pattern similar to natural satiation. Reduction of food intake by CCK is mediated by vagal afferents that innervate the stomach and small intestine. These afferents synapse in the hindbrain nucleus of the solitary tract (NTS) where gastrointestinal satiation signals are processed. Previously, we demonstrated that intraperitoneal (IP) administration of either competitive or noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonists attenuates reduction of food intake by CCK. However, because vagal afferents themselves express NMDA receptors at both central and peripheral endings, our results did not speak to the question of whether NMDA receptors in the brain play an essential role in reduction of feeding by CCK. We hypothesized that activation of NMDA receptors in the NTS is necessary for reduction of food intake by CCK. To test this hypothesis, we measured food intake following IP CCK, subsequent to NMDA receptor antagonist injections into the fourth ventricle, directly into the NTS or subcutaneously. We found that either fourth-ventricle or NTS injection of the noncompetitive NMDA receptor antagonist MK-801 was sufficient to inhibit CCK-induced reduction of feeding, while the same antagonist doses injected subcutaneously did not. Similarly fourth ventricle injection of d-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphoric acid (d-CPPene), a competitive NMDA receptor antagonist, also blocked reduction of food intake following IP CCK. Finally, d-CPPene injected into the fourth ventricle attenuated CCK-induced expression of nuclear c-Fos immunoreactivity in the dorsal vagal complex. We conclude that activation of NMDA receptors in the hindbrain is necessary for the reduction of food intake by CCK. Hindbrain NMDA receptors could comprise a critical avenue for control and modulation of satiation signals to influence food intake and energy balance.

Delayed treatment with YM90K, an AMPA receptor antagonist, protects against ischaemic damage after middle cerebral artery occlusion in rats.

The neuroprotective effect of an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist YM90K [6-(1H-imidazol-1-yl)-7-nitro-2,3(1H,4H)-quinoxalinedione monohydrochloride] has been examined in a rat middle cerebral artery occlusion model. Intravenous infusion of YM90K (2.5-20mgkg(-l)h(-l) for 4h) starting immediately after occlusion of the middle cerebral artery significantly reduced the cortical infarct volume 24h after occlusion compared with the control group. The protection at the highest dose was 39% (P < 0.05). Similar protective effects were observed when YM90K (20mgkg(-1)h(-1) for 4h) was delayed up to 2h after middle cerebral artery occlusion (45% reduction, P < 0.05). CNS1102 [N-(1-naphthyl)-N'-(3-ethylphenyl)-N'-methylguanidine hydrochloride], a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, also reduced the cortical infarct volume when 1.13mgkg(-1) was administered by intravenous bolus injection immediately after middle cerebral artery occlusion, followed by intravenous infusion at 0.785mgkg(-l)h(-1) for 4h (35% reduction, P<0.05). This neuroprotective effect was not observed when administration was delayed lh after middle cerebral artery occlusion. These results suggest that AMPA receptors might play a more important role than NMDA receptors in the late development of neuronal cell damage after focal cerebral ischaemia and that AMPA receptor blockade would be one beneficial strategy in treating acute stroke.