Use Of NMDA Receptor Antagonists Research Articles

Spontaneous fibrinolysis and possibilities of its acceleration in pulmonary embolism

This review contains the data concerning the mechanisms of spontaneous fibrinolysis in pulmonary vessels and possibilities of its acceleration in pulmonary embolism. The spontaneous fibrinolysis system is known to be sequential and multifactorial, with the interaction of accelerators (t-PA and u-PA) and inhibitors (alpha-2-antiplasmin, PAI-1, TAFI). The fibrinolytic processes take place in case of prevailing reactions of accelerating factors over inhibiting ones. The endothelium of pulmonary vessels possesses pronounced antithrombogenic and profibrinolytic properties, therefore, the processes of fibrinolysis in the pulmonary vascular bed normally occur more intensively than in the vessels of the systemic circulation. The membrane proteins of the endothelium annexins A2 activate plasminogen, whereas thrombomodulin inhibits the activity of PAI-1. The main approaches to increase the fibrinolysis intensity in conditions of pulmonary embolism may be aimed at elevating the activity of fibrinolytic enzymes (enhancing the synthesis of annexins A2, the use of NMDA-receptor antagonists) and suppressing its inhibitors (the use of monoclonal antibodies to alpha-2-antiplasmin, as well as plasminogen activator inhibitor-1 (PAI-1) and thrombin-activatable fibrinolysis inhibitor (TAFI). Promising directions for future research can be the synthesis of a new generation of tissue-type plasminogen activators, and investigations of the possibility of clinical application of antithrombin and thrombomodulin, angiotensin converting enzyme inhibitors and cortisol antagonists. To meet these challenges, it is necessary to develop new models of venous thrombosis and acute pulmonary embolism in different animal species, with the assessment of the changes in the venous haemodynamics and pulmonary microcirculation on the background of administration of a new class of fibrinolytic agents.

Влияние антагонистов NMDA рецепторов разных типов на вызванные ответы пирамидных нейронов в срезах коры мозга крысы

The use of NMDA receptor antagonists for the treatment of CNS diseases is currently limited. This may be due to the lack of knowledge about the effects of antagonists on the functional properties of neurons since in vitro models do not fully reproduce the real pathophysiological processes. We studied the effects of NMDA receptor antagonists on properties of pyramidal neuron responses in the rat prefrontal cortex. A competitive antagonist APV (50 µM) and an ion channel blocker memantine (100 µM) did not produce significant effects on evoked EPSP. When the inhibitory transmission was suppressed by picrotoxin (50 µM), the extracellular stimulation caused prolonged membrane depolarization and generation a series of action potentials. APV decreased duration and amplitude of such responses. Memantine was active only when magnesium was excluded from the extracellular environment. The lack of effects of memantine can be explained by its competition with magnesium for the binding site within the ion channel. This work demonstrates that in order to predict the systemic effects of blockers on the functions of neurons and their excitability, it is necessary to use in vitro models in which the activation of NMDA receptors occurs in the presence of physiological concentrations of magnesium and without clamping of membrane potential.

Male rats treated with subchronic PCP show intact olfaction and enhanced interest for a social odour in the olfactory habituation/dishabituation test

The olfactory system participates in many sensory processes, and olfactory endophenotypes appear in a variety of neurological disorders such as Alzheimer’s and Parkinson’s disease, depression and schizophrenia. Social withdrawal is a core negative symptom of schizophrenia and animal models have proven to be invaluable for studying the neurobiological mechanisms and cognitive processes behind the formation of social relationships. The subchronic phencyclidine (PCP) rat model is a validated model for negative symptoms of schizophrenia, such as impaired sociability. However, the complete range of social behaviour and deficits in the model are still not fully understood. Intact rodent olfaction is essential for a wide range of social behaviour and disrupted olfactory function could have severe effects on social communication and recognition. In order to examine the olfactory ability of male rats treated with subchronic PCP, we conducted an olfactory habituation/dishabituation test including both non-social and social odours. The subchronic PCP-treated rats successfully recognized and discriminated among the odours, indicative of intact olfaction. Interestingly, the subchronic PCP-treated rats showed greater interest for a novel social odour compared to the saline-treated rats and the rationale remains to be elucidated. Our data indicate that subchronic PCP treatment does not disrupt olfactory function in male rats. By ruling out impaired olfaction as cause for the poor social interaction performance in subchronic PCP-treated rats, our data supports the use of NMDA receptor antagonists to model the negative symptoms of schizophrenia.

Is Forced Swimming Immobility a Good Endpoint for Modeling Negative Symptoms of Schizophrenia? - Study of Sub-Anesthetic Ketamine Repeated Administration Effects.

Immobility time in the forced swimming has been described as analogous to emotional blunting or apathy and has been used for characterizing schizophrenia animal models. Several clinical studies support the use of NMDA receptor antagonists to model schizophrenia in rodents. Some works describe the effects of ketamine on immobility behavior but there is variability in the experimental design used leading to controversial results. In this study, we evaluated the effects of repeated administration of ketamine sub-anesthetic doses in forced swimming, locomotion in response to novelty and novel object recognition, aiming a broader evaluation of the usefulness of this experimental approach for modeling schizophrenia in mice. Ketamine (30 mg/kg/day i.p. for 14 days) induced a not persistent decrease in immobility time, detected 24h but not 72h after treatment. This same administration protocol induced a deficit in novel object recognition. No change was observed in mice locomotion. Our results confirm that repeated administration of sub-anesthetic doses of ketamine is useful in modeling schizophrenia-related behavioral changes in mice. However, the immobility time during forced swimming does not seem to be a good endpoint to evaluate the modeling of negative symptoms in NMDAR antagonist animal models of schizophrenia.

A role for inflammatory metabolites as modulators of the glutamate N-methyl-D-aspartate receptor in depression and suicidality.

Patients with depression and suicidality suffer from low-grade neuroinflammation. Pro-inflammatory cytokines activate indoleamine 2,3-dioxygenase, an initial enzyme of the kynurenine pathway. This pathway produces neuroactive metabolites, including quinolinic- and kynurenic acid, binding to the glutamate N-methyl-d-aspartate-receptor, which is hypothesized to be part of the neural mechanisms underlying symptoms of depression. We therefore hypothesized that symptoms of depression and suicidality would fluctuate over time in patients prone to suicidal behavior, depending on the degree of inflammation and kynurenine metabolite levels in the cerebrospinal fluid (CSF). We measured cytokines and kynurenine metabolites in CSF, collected from suicide attempters at repeated occasions over 2 years (total patient samples n=143, individuals n=30) and healthy controls (n=36). The association between the markers and psychiatric symptoms was assessed using the Montgomery Asberg Depression Rating Scale and the Suicide Assessment Scale. Quinolinic acid was increased and kynurenic acid decreased over time in suicidal patients versus healthy controls. Furthermore, we found a significant association between low kynurenic acid and severe depressive symptoms, as well as between high interleukin-6 levels and more severe suicidal symptoms. We demonstrate a long-term dysregulation of the kynurenine pathway in the central nervous system of suicide attempters. An increased load of inflammatory cytokines was coupled to more severe symptoms. We therefore suggest that patients with a dysregulated kynurenine pathway are vulnerable to develop depressive symptoms upon inflammatory conditions, as a result the excess production of the NMDA-receptor agonist quinolinic acid. This study provides a neurobiological framework supporting the use of NMDA-receptor antagonists in the treatment of suicidality and depression.

Ammonia toxicity to the brain

Hyperammonemia can be caused by various acquired or inherited disorders such as urea cycle defects. The brain is much more susceptible to the deleterious effects of ammonium in childhood than in adulthood. Hyperammonemia provokes irreversible damage to the developing central nervous system: cortical atrophy, ventricular enlargement and demyelination lead to cognitive impairment, seizures and cerebral palsy. The mechanisms leading to these severe brain lesions are still not well understood, but recent studies show that ammonium exposure alters several amino acid pathways and neurotransmitter systems, cerebral energy metabolism, nitric oxide synthesis, oxidative stress and signal transduction pathways. All in all, at the cellular level, these are associated with alterations in neuronal differentiation and patterns of cell death. Recent advances in imaging techniques are increasing our understanding of these processes through detailed in vivo longitudinal analysis of neurobiochemical changes associated with hyperammonemia. Further, several potential neuroprotective strategies have been put forward recently, including the use of NMDA receptor antagonists, nitric oxide inhibitors, creatine, acetyl-L-carnitine, CNTF or inhibitors of MAPKs and glutamine synthetase. Magnetic resonance imaging and spectroscopy will ultimately be a powerful tool to measure the effects of these neuroprotective approaches.

Role of Thalamic Projection in NMDA Receptor-Induced Disruption of Cortical Slow Oscillation and Short-Term Plasticity

NMDA receptor (NMDAR) antagonists, such as phencyclidine, ketamine, or dizocilpine (MK-801) are commonly used in psychiatric drug discovery in order to model several symptoms of schizophrenia, including psychosis and impairments in working memory. In spite of the widespread use of NMDAR antagonists in preclinical and clinical studies, our understanding of the mode of action of these drugs on brain circuits and neuronal networks is still limited. In the present study spontaneous local field potential (LFP), multi- (MUA) and single-unit activity, and evoked potential, including paired-pulse facilitation (PPF) in response to electrical stimulation of the ipsilateral subiculum were carried out in the medial prefrontal cortex (mPFC) in urethane anesthetized rats. Systemic administration of MK-801 (0.05 mg/kg, i.v.) decreased overall MUA, with a diverse effect on single-unit activity, including increased, decreased, or unchanged firing, and in line with our previous findings shifted delta-frequency power of the LFP and disrupted PPF (Kiss et al., 2011). In order to provide further insight to the mechanisms of action of NMDAR antagonists, MK-801 was administered intracranially into the mPFC and mediodorsal nucleus of the thalamus (MD). Microinjections of MK-801, but not physiological saline, localized into the MD evoked changes in both LFP parameters and PPF similar to the effects of systemically administered MK-801. Local microinjection of MK-801 into the mPFC was without effect on these parameters. Our findings indicate that the primary site of the action of systemic administration of NMDAR antagonists is unlikely to be the cortex. We presume that multiple neuronal networks, involving thalamic nuclei contribute to disrupted behavior and cognition following NMDAR blockade.

Erythropoietin protects the developing brain against N-methyl- d-aspartate receptor antagonist neurotoxicity

Pharmacological blockade of NMDA receptor function induces apoptotic neurodegeneration in the developing rat brain. However, the use of NMDA receptor antagonists as anesthetics and sedatives represents a difficult-to-avoid clinical practice in pediatrics. This warrants the search for adjunctive neuroprotective measures that will prevent or ameliorate neurotoxicity of NMDA receptor antagonists. The NMDA receptor antagonist MK801 triggered apoptosis in the neonatal rat forebrain, most notably in cortex and thalamus. MK801 exposure reduced mRNA levels of erythropoietin (EPO) and the EPO receptor, suggesting that loss of endogenous EPO activity may contribute to MK801-induced apoptosis. Coadministration of recombinant EPO (rEPO) conferred 50% neuroprotection, partially restored MK801-induced reduction of brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) mRNA, and prevented decreased phosphorylation levels of extracellular signal-regulated protein kinase-1/2 (ERK1/2) and Akt. These observations indicate that rEPO partly rescues newborn rats from MK801-mediated brain damage by enhancing neurotrophin-associated signaling pathways.

Reduced expression of astrocytic glycine transporter (Glyt-1) in acute liver failure.

A growing body of evidence suggests that alterations in N-methyl-D-asparate NMDA-mediated excitatory neurotransmission may be involved in the pathophysiology of hepatic encephalopathy (HE) in acute liver failure (ALF). The NMDA receptor requires glycine as a positive allosteric modulator. One of the glycine transporters Glyt-1 is expressed primarily in astrocytes of the cerebral cortex in association with regions of high NMDA receptor expression. As astrocytic transporters regulate the amino acid concentrations within excitatory synapses, the expression of Glyt-1 was studied in cortical preparations from rats with ischemic liver failure induced by portacaval anastomosis followed 24 hr later by hepatic artery ligation and from appropriate sham-operated controls. Expression of Glyt-1 mRNA, studied by reverse transcriptase-polymerase chain reaction, was significantly decreased in the brain at coma stages of encephalopathy (to approximately 50% of control) concomitant with a significant threefold increase of extracellular glycine, measured by in vivo cerebral microdialysis. These findings suggest that loss of expression of the Glyt-1 transporter may cause an impairment of regulation of glycine concentration at synaptic level and contribute to an overactivation of the NMDA receptor in ALF. The use of NMDA receptor antagonists, aimed specifically at the glycine modulatory site, could offer novel approaches to the prevention and treatment of HE in ALF.

NMDA receptor pharmacology: perspectives from molecular biology.

The NMDA receptor is an important target for drug development, with agents from many different classes acting on this receptor. While the severe side effects associated with complete NMDA receptor blockade have limited clinical usefulness of most antagonists, the understanding of the multiple forms of NMDA receptors provides an opportunity for development of subtype specific agents with potentially fewer side effects. Different NMDA receptor subtypes are assembled from combinations of NR1 and NR2 subunits with each subunit conveying distinct properties. The NRI subunit is the glycine binding subunit and exists as 8 splice variants of a single gene. The glutamate binding subunit is the NR2 subunit, which is generated as the product of four distinct genes, and provides most of the structural basis for heterogeneity in NMDA receptors. Pharmacological heterogeneity results from differences in the structure of ligand binding regions, as well as structural differences between subtypes in a modulatory region called the LIVBP-like domain. This region in NR1 and NR2B controls the action of NR2B-selective drugs like ifenprodil, while this domain in receptors containing the NR2A subunit controls the action of NR2A-selective drugs such as zinc. This suggests that NMDA receptor subtype selective drugs can be created, and further understanding of subtype specific mechanisms ultimately may allow successful use of NMDA receptor antagonists as therapeutic agents.

Glutamate receptor antagonists differentially affect the protective activity of conventional antiepileptics against amygdala-kindled seizures in rats

LY 300164 (5 mg/kg), a selective non-competitive antagonist of AMPA/kainate receptors, exerted a significant anticonvulsant effect in amygdala-kindled rats, being ineffective at 2 mg/kg. LY 235959 (1–5 mg/kg), a selective competitive antagonist of NMDA receptors, failed to modify behavioral and electrographic correlates of kindled seizures. Amygdala-kindled seizures were inhibited by conventional antiepileptics, their lowest effective doses were: 20 mg/kg for carbamazepine and phenobarbital, 50 mg/kg for diphenylhydantoin, and 100 mg/kg for valproate magnesium. The combined treatment of the AMPA/kainate antagonist (2 mg/kg) with valproate at sub-effective doses (25–75 mg/kg) resulted in the reduced severity and duration of kindled seizures. Also, a clear-cut protective effect was observed when LY 235959 was co-administered with diphenylhydantoin (40 mg/kg). Any interaction at the pharmacokinetic level can be excluded because neither LY 300164 nor LY 235959 interfered with the free plasma levels of valproate or diphenylhydantoin, respectively. The combination of the AMPA/kainate receptor antagonist (2 mg/kg) with valproate (75 mg/kg) did not impair performance of rats in the rotorod test (motor co-ordination) or passive-avoidance task (long-term memory). Conversely, the NMDA receptor antagonist alone or in combination with diphenylhydantoin, produced significant mnemonic deficits. The results indicate that AMPA/kainate receptor antagonists might be of importance as adjuvant antiepileptic drugs in patients treated with valproate. A possible use of NMDA receptor antagonists may be questionable.

The use of NMDA-receptor antagonists in the treatment of chronic pain.

Chronic pain can be maintained by a state of sensitization within the central nervous system that is mediated in part by the excitatory amino acids glutamate and aspartate binding to the N-methyl-D-aspartate (NMDA) receptor. A number of antagonists to the NMDA receptor are antinociceptive in animal models but are associated with significant dose-limiting side effects. Commercially available NMDA-receptor antagonists include ketamine, dextromethorphan, memantine, and amantadine. The opioids methadone, dextropropoxyphene, and ketobemidone are also antagonists at the NMDA receptor. The NMDA-receptor antagonists have a significant impact on the development of tolerance to opioid analgesics. Consequently, NMDA-receptor antagonists may represent a new class of analgesics and may have potential as coanalgesics when used in combination with opioids.

Decrement in Operant Performance Produced by NMDA Receptor Antagonists in the Rat: Tolerance and Crosstolerance

Current perspectives on the clinical use of NMDA receptor antagonists infer repeated administration schedules for the management of different pathological states. The development of tolerance and crosstolerance between different NMDA receptor antagonists may be an important factor contributing to the clinical efficacy of these drugs. The present study aimed to characterize the development of tolerance and crosstolerance to the ability of various site-selective NMDA receptor antagonists to produce a decrement of operant responding (multiple extinction 9 s fixed-interval 1-s schedule of water reinforcement). Acute administration of d-CPPen (SDZ EAA 494; 1–5.6 mg/kg), dizocilpine (MK-801; 0.03–0.3 mg/kg), memantine (0.3–17 mg/kg), ACEA-1021 (10–56 mg/kg), and eliprodil (1–30 mg/kg) differentially affected operant responding. Both increases and decreases in response rates and accuracy of responding were observed. Repeated preexposure to d-CPPen (5.6 mg/kg, once a day for 7 days) attenuated a behavioral disruption produced by an acute challenge with d-CPPen or ACEA-1021, but potentiated the effects of dizocilpine, memantine, and eliprodil. Based on the present results, one can suggest that the repeated administration of a competitive NMDA receptor antagonist differentially affects the functional activity of various sites on NMDA receptor complex.

State-dependent blockade of haloperidol-induced sensitization of catalepsy by MK-801.

NMDA receptor antagonists have been shown to block several forms of neural and behavioural plasticity. The prototypical and most widely-used noncompetitive NMDA receptor antagonist is dizocilpine (MK-801). Here we have examined the effect of MK-801 on the context-dependent augmentation ('sensitization') of catalepsy in rats which develops with repeated administration of haloperidol. It was found that over a 7-day treatment period animals receiving haloperidol (0.25 or 0.5 mg/kg) plus MK-801 (0.16 mg/kg) showed a context-dependent day-to-day increase in catalepsy similar to animals that received haloperidol alone. However, when all animals were treated with haloperidol alone on day 8 of the experiment, animals that had received haloperidol plus MK-801 before displayed a much smaller cataleptic response, similar to that observed in the haloperidol group on the first treatment day, i.e. the previously-established enhancement of catalepsy was no longer expressed. These results may be explained in terms of state-dependency effects induced by MK-801. Implications of these findings for the clinical use of NMDA receptor antagonists in the treatment of Parkinson's disease are discussed.

Ethanol withdrawal is associated with increased extracellular glutamate in the rat striatum

Extracellular glutamate was measured by microdialysis in the striatum of ethanol-dependent, freely behaving rats following withdrawal from chronic ethanol treatment. Within 12 h from withdrawal, extracellular glutamate rose to 255% of that in control, chronic sucrose-treated rats. Glutamate output remained elevated for the subsequent 12 h and returned to control levels within 36 h from the interruption of the treatment. The changes in glutamate were time-locked to the overt physical signs of withdrawal. In 12-h ethanol-withdrawn rats an ethanol challenge suppressed the withdrawal signs and reduced the extracellular glutamate. The NMDA receptor antagonist, dizocilpine, reduced both the physical signs of withdrawal and glutamate output. In contrast, diazepam reduced the withdrawal signs but failed to change the glutamate levels. These findings suggest that the increased extraneuronal glutamate reflects overactivity of excitatory neurotransmission during withdrawal. Furthermore, they provide a biochemical rationale for the use of NMDA receptor antagonists and ethanol itself in the treatment of ethanol withdrawal syndrome.