Differences in results from in vivo and in vitro studies on the use-dependency of N-methylaspartate antagonism by MK-801 and other phencyclidine receptor ligands

Antimelanogenic Effects of Curcumin and Its Dimethoxy Derivatives: Mechanistic Investigation Using B16F10 Melanoma Cells and Zebrafish (Danio rerio) Embryos

Regulation of melanin production via the MC1R signaling pathway is a protective mechanism of the skin of living organisms against exposure to ultraviolet rays. The discovery of human skin-whitening agents has been one of the most intense pursuits of the cosmetic industry. The MC1R signaling pathway is activated by its agonist, alpha-melanocyte stimulating hormone (α-MSH), and mainly regulates melanogenesis. Here, we evaluated the antimelanogenic activities of curcumin (CUR) and its two derivatives, dimethoxycurcumin (DMC) and bisdemethoxycurcumin (BDMC), in B16F10 mouse melanoma cells and zebrafish embryos. CUR and BDMC reduced the α-MSH-induced melanin production in B16F10 cells and also downregulated the expression of the melanin-production-related genes Tyr, Mitf, Trp-1, and Trp-2. Moreover, the biological activity of these two compounds against melanogenesis was confirmed in in vivo experiments using zebrafish embryos. However, the highest concentration of CUR (5 µM) resulted in slight malformations in zebrafish embryos, as indicated by acute toxicity tests. In contrast, DMC did not show any biological activity in vitro or in vivo. Conclusively, BDMC is a strong candidate as a skin-whitening agent.

Drug repurposing strategy: An emerging approach to identify potential therapeutics for treatment of bovine mastitis

The current study was designed to characterize methicillin-resistant Staphylococcus aureus (MRSA) isolated from bovine milk, along with its response to antibiotics, and ultimately reverse its mechanism of resistance by modulation with non-antibiotics. The synergistic combination of antibiotics with NSAIDs were tested in-vivo by giving MRSA challenge to rabbits. The current study reported an overall 23.79% prevalence of MRSA. The BLAST alignment of current study sequences revealed 99% similarity with mecA gene of MRSA from NCBI database. The current study isolates were more similar to each other and also with reference sequences as compared to other mecA gene sequences from Turkey, India, and Russia. Antibiogram of MRSA isolates showed a highly resistant response to cefoxitin, amoxicillin, and gentamicin. Amoxicillin, gentamicin, tylosin, vancomycin, and ciprofloxacin elicited a significant response (p < 0.05) in combination with non-antibiotics against tested MRSA isolates. The highest zone of inhibition (ZOI) increase was noted for vancomycin in combination with flunixin meglumine (145.45%) and meloxicam (139.36%); gentamicin with flunixin meglumine (85.71%) and ciprofloxacin with ivermectin (71.13%). Synergistic behavior was observed in the combination of gentamicin with ketoprofen; sulfamethoxazole and oxytetracycline with meloxicam. Hematological analysis showed significant differences (p < 0.05) among lymphocyte count and bilirubin. On histopathological examination of skin tissue, hyperplasia of epithelium, sloughed off epidermis, hyperkeratosis, infiltration of inflammatory cells, and hemorrhages were observed. The highest cure rate was observed in case of gentamicin in combination with ketoprofen as compared to other treatment groups. The current study concluded antibiotics in combination with non-antibiotics as potential therapeutic agents for resistance modulation against MRSA. This study will help to devise treatment and control strategies against bovine mastitis. Although the prospect of using NSAIDs to manage infections caused by MRSA appears to be a promising direction, further studies should be conducted to test these medications using suitable in-vivo models in controlled clinical trials to justify their repurposing as a treatment for MRSA infections.

Synaptic memory requires CaMKII

Long-term potentiation (LTP) is arguably the most compelling cellular model for learning and memory. While the mechanisms underlying the induction of LTP ('learning') are well understood, the maintenance of LTP ('memory') has remained contentious over the last 20 years. Here, we find that Ca2+-calmodulin-dependent kinase II (CaMKII) contributes to synaptic transmission and is required LTP maintenance. Acute inhibition of CaMKII erases LTP and transient inhibition of CaMKII enhances subsequent LTP. These findings strongly support the role of CaMKII as a molecular storage device.

Evidence-Based Tracking of MDR E. coli from Bovine Endometritis and Its Elimination by Effective Novel Therapeutics

Antibiotic-resistant bacteria have become the predominant etiology of endometritis and thus require effective treatment approaches. We used ultrasonography coupled with clinical signs and presented complaints of reproductive issues to investigate the epidemiology, phylogenetic analysis, antimicrobial resistance, and development of novel therapeutics against Escherichia coli isolated from endometritis in bovine (n = 304 from 10 commercial dairy farms). The prevalence of bovine endometritis in this study was 43.75%, while among these, 72.18% samples were positive for E. coli. Nucleotide analysis performed through BLAST and MEGAX showed 98% similarity to the nucleotide sequence of the reference E. coli strain (accession number CP067311.1). The disk diffusion assay revealed pathogen resistance to most antibiotics. Pattern of MIC order of resistance was as follows: enrofloxacin < gentamicin < co-amoxiclav < streptomycin < amoxicillin < metronidazole < oxytetracycline. Field trials revealed the highest recovery rate (in terms of clearance of endometritis and establishment of pregnancy) in case of gentamicin + enrofloxacin (100%) and gentamicin alone (100%), followed by co-amoxiclav + gentamicin (84.61%), oxytetracycline alone (78.57%), and metronidazole + enrofloxacin (33.33%). Hence, the current study reported a higher prevalence of multidrug-resistant E. coli showing considerable similarity with reference strain, and finally, the effective response of novel antibiotics to treat cases.

Physiology and Therapeutic Potential of SK, H, and M Medium AfterHyperPolarization Ion Channels

SK, HCN, and M channels are medium afterhyperpolarization (mAHP)-mediating ion channels. The three channels co-express in various brain regions, and their collective action strongly influences cellular excitability. However, significant diversity exists in the expression of channel isoforms in distinct brain regions and various subcellular compartments, which contributes to an equally diverse set of specific neuronal functions. The current review emphasizes the collective behavior of the three classes of mAHP channels and discusses how these channels function together although they play specialized roles. We discuss the biophysical properties of these channels, signaling pathways that influence the activity of the three mAHP channels, various chemical modulators that alter channel activity and their therapeutic potential in treating various neurological anomalies. Additionally, we discuss the role of mAHP channels in the pathophysiology of various neurological diseases and how their modulation can alleviate some of the symptoms.

Modulation of H-reflex responses and frequency-dependent depression by repetitive spinal electromagnetic stimulation: From rats to humans and back to chronic spinal cord injured rats

The lack of propagation of signals through survived fibers is among the major reasons for functional loss after incomplete spinal cord injury (SCI). Our recent results of animal studies demonstrate that spinal electromagnetic stimulation (SEMS) can enhance transmission in damaged spinal cord, and this type of modulation depends on the function of NMDA receptors at the neuronal networks below the injury level. Here, our pilot human study revealed that administration of repetitive SEMS induced long‐lasting modulation of H‐responses in both healthy and participants with chronic SCI. In order to understand the mechanisms underlying these effects, we have used an animal model and examined effects of SEMS on H‐responses. Effects of SEMS on H‐responses, frequency‐dependent depression (FDD) of H‐reflex, and possible underlying mechanisms have been examined in both naïve and rats with SCI. Our results demonstrate that consistent with the effects of SEMS on H‐reflex seen in humans, repetitive SEMS induced similar modulation in excitability of peripheral nerve responses in both non‐injured and rats with SCI. Importantly, our results confirmed the reduced FDD of H‐reflex in SCI animals and revealed that SEMS was able to recover FDD in rats with chronic SCI. Using intraspinal injections of the NMDA receptor blocker MK‐801, we have identified NMDA receptors as an important contributor to these SEMS‐induced effects in rats with SCI. These results identify SEMS as a novel non‐invasive technique for modulation of neuro‐muscular circuits and, importantly, modulation of spinal networks after chronic SCI.

Ephenidine: A new psychoactive agent with ketamine-like NMDA receptor antagonist properties

To avoid legislation based on chemical structure, research chemicals, frequently used for recreational purposes, are continually being synthesized. N-Ethyl-1,2-diphenylethanamine (ephenidine) is a diarylethylamine that has recently become popular with recreational users searching for dissociative hallucinogenic effects.

In the present study, the pharmacological basis of its neural actions has been investigated, initially by assessing its profile in central nervous system receptor binding assays and subsequently in targeted electrophysiological studies. Ephenidine was a potent inhibitor of ³H-MK-801 binding (Ki: 66 nM), implying that it acts at the PCP site of the N-methyl-d-aspartate (NMDA) receptor. It also showed modest activity at dopamine (379 nM) and noradrenaline (841 nM) transporters and at sigma 1 (629 nM) and sigma 2 (722 nM) binding sites. In experiments of extracellular recording of field excitatory postsynaptic potentials (fEPSPs) from area CA1 of rat hippocampal slices, ephenidine, 1 and 10 μM, respectively, produced a 25% and a near maximal inhibition of the NMDA receptor mediated fEPSP after 4 h superfusion. By contrast, ephenidine (50 μM) did not affect the AMPA receptor mediated fEPSPs. In whole cell patch clamp recordings, from hippocampal pyramidal cells, ephenidine (10 μM) blocked NMDA receptor-mediated EPSCs in a highly voltage-dependent manner. Additionally, ephenidine, 10 μM, blocked the induction of long term potentiation (LTP) in CA1 induced by theta burst stimulation.

The present data show that the new psychoactive substance, ephenidine, is a selective NMDA receptor antagonist with a voltage-dependent profile similar to ketamine. Such properties help explain the dissociative, cognitive and hallucinogenic effects in man.

This article is part of the Special Issue entitled ‘Ionotropic glutamate receptors’.

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New ‘legal high’, the dissociative ephenidine, displaces MK-801 binding.

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Ephenidine, like ketamine, blocks NMDA receptor mediated synaptic potentials and plasticity.

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Ephenidine, like ketamine, blocks the NMDA receptor in a highly voltage-dependent manner.

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Ephenidine blocks long-term potentiation, LTP.

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NMDA receptor antagonism likely underlies the psychoactive effects of ephenidine.

The inhibition of Pseudomonas aeruginosa biofilm formation by micafungin and the enhancement of antimicrobial agent effectiveness in BALB/c mice

Micafungin inhibits biofilm formation by impeding 1,3-β-D-glucan synthesis in Candida albicans. Since Pseudomonas aeruginosa also has 1,3-β-D-glucan in its cell wall, this study assessed the effects of antibacterial agents in vitro and in vivo on micafungin-treated biofilm-forming P. aeruginosa isolates. After treatment with micafungin as well as with a panel of four antibacterial agents, biofilm production was significantly reduced as measured by spectrophotometry. The relative mRNA transcription levels for the genes encoding pellicles (pelC) and cell wall 1,3-β-D-glucan (ndvB), which were measured by quantitative reverse transcription PCR (qRT-PCR), significantly decreased with micafungin treatment. In vivo, the survival rates of P. aeruginosa-infected BALB/c mice significantly increased after combined treatment with micafungin and each of the antibacterial agents. Of these treatments, the combination of micafungin with levofloxacin had the highest survival rate; this combination was the most effective treatment against P. aeruginosa-induced infection.

Assessment of combination therapy in BALB/c mice injected with carbapenem-resistant Enterobacteriaceae strains

Monotherapeutic options for carbapenem resistant infections are limited. Studies suggest that combination therapy may be associated with better outcomes than monotherapies. However, this is still controversial. This study assessed, the efficacy of combination therapy against carbapenem resistant Enterobacteriaceae harboring singly various extended spectrum beta lactamase or carbapenemase encoding genes. Thus, four isolates harboring either bla_CTXM-15, bla_CTXM-15 and bla_OXA-48, bla_NDM-1, or bla_KPC-2 genes were selected for testing. Minimal inhibitory concentration was determined by broth dilution method. Gene transcript levels on single and combined treatments were done in vitro and in vivo by qRT-PCR. Assessment of treatments was done in BALB/c mice according to a specific protocol. As such, the qRT-PCR revealed a significant decrease of transcript levels in all isolates upon using rifampicin or tigecycline, singly or in combination with colistin. However, variable levels were obtained using colistin singly or in combination with meropenem or fosfomycin. In vivo assessment showed that all combinations used were effective against isolates harboring bla_CTXM-15, bla_OXA-48, and bla_NDM-1. Conversely, the most significant combination against the isolate harboring bla_KPC-2 gene was colistin with either carbapenem, fosfomycin, or kanamycin. As a conclusion, combination therapy selected based on the type of carbapenemase produced, appeared to be non-toxic and might be effective in BALB/c mice. Therefore, the use of a rationally optimized combination therapy might lead to better results than monotherapy, however, clinical trials are needed for human consumption.

Ketamine and phencyclidine: the good, the bad and the unexpected

The history of ketamine and phencyclidine from their development as potential clinical anaesthetics through drugs of abuse and animal models of schizophrenia to potential rapidly acting antidepressants is reviewed. The discovery in 1983 of the NMDA receptor antagonist property of ketamine and phencyclidine was a key step to understanding their pharmacology, including their psychotomimetic effects in man. This review describes the historical context and the course of that discovery and its expansion into other hallucinatory drugs. The relevance of these findings to modern hypotheses of schizophrenia and the implications for drug discovery are reviewed. The findings of the rapidly acting antidepressant effects of ketamine in man are discussed in relation to other glutamatergic mechanisms.

Patch clamp combined with voltage/concentration clamp to determine the kinetics and voltage dependency of N-methyl-D-aspartate (NMDA) receptor open channel blockers

Electrophysiological techniques can be used to great effect to help determine the mechanism of action of a compound. However, many factors can compromise the resulting data and their analysis, such as the speed of solution exchange, expression of additional ion channel populations including other ligand-gated receptors and voltage-gated channels, compounds having multiple binding sites, and current desensitization and rundown. In this chapter, such problems and their solutions are discussed and illustrated using data from experiments involving the uncompetitive NMDA receptor antagonist memantine. Memantine differs from many other NMDA receptor channel blockers in that it is well tolerated and does not cause psychotomimetic effects at therapeutic doses. Various electrophysiological parameters of NMDA-induced current blockade by memantine have been proposed to be important in determining therapeutic tolerability; potency, onset and offset kinetics, and voltage dependency. These were all measured using whole cell patch clamp techniques using hippocampal neurons. Full results are shown here for memantine, and these are summarized and compared to those from similar experiments with other NMDA channel blockers. The interpretation of these results is discussed, as are theories concerning the tolerability of NMDA channel blockers, with the aim of illustrating how electrophysiological data can be used to form and support a physiological hypothesis.

Repetitive spinal electromagnetic stimulation opens a window of synaptic plasticity in damaged spinal cord: role of NMDA receptors

As we reported previously, propagation of action potentials through surviving axons is impaired dramatically, resulting in reduced transmission to lumbar motoneurons after midthoracic lateral hemisection (HX) in rats. The aim of the present study was to evoke action potentials through the spared fibers using noninvasive electromagnetic stimulation (EMS) over intact T2 vertebrae in an attempt to activate synaptic inputs to lumbar motoneurons and thus to enhance plasticity of spinal neural circuits after HX. We found that EMS was able to activate synaptic inputs to lumbar motoneurons and motor-evoked potentials (MEP) in hindlimb muscles in adult anesthetized rats. Amplitude of MEP was attenuated in parallel with the decline of responses recorded from the motoneuron pool after HX. Repetitive EMS (50 min, 0.2 Hz) facilitated the amplitudes of responses elicited by electric stimulation of lateral white matter or dorsal corticospinal tracts in HX rats. Facilitation sustained for at least 1.5 h after termination of EMS. The N-methyl-d-aspartate (NMDA) receptor blocker MK-801, injected intraspinally close to the recording electrode prior to EMS, did not alter these responses but blocked the EMS-induced facilitation, suggesting that activation of NMDA receptors is required to initiate an EMS-evoked increase. When MK-801 was administered after EMS-induced facilitation was established, it induced depression of these elevated responses. Results suggest that repetitive EMS over intact vertebrae could be used as a therapeutic approach to open a window of synaptic plasticity after incomplete midthoracic injuries, i.e., to activate NMDA receptors in the lumbar motoneuron pool at synaptic inputs and to strengthen transmission in damaged spinal cord.

Ca2+ entry pathways in mouse spinal motor neurons in culture following in vitro exposure to methylmercury

Methylmercury (MeHg) is a widespread environmental toxicant with major actions on the central nervous system. Among the neurons reportedly affected in cases of Hg poisoning are motor neurons; however, the direct cellular effects of MeHg on motor neurons have not been reported. Ratiometric fluorescence imaging, using the Ca(2+)-sensitive fluorophore fura-2, was used to examine the effect of MeHg on Ca(2+) homeostasis in primary cultures of mouse spinal motor neurons. In vitro MeHg exposure at concentrations (0.1-2 μM) known to affect other neurons in culture differentially, induced a biphasic rise in fura-2 fluorescence ratio indicating an increase in [Ca(2+)](i). The time-to-onset of these fura-2 fluorescence ratio changes was inversely correlated with MeHg concentration. TPEN (20 μM), a non-Ca(2+), divalent cation chelator, reduced the amplitude of the increase in fura-2 fluorescence induced by MeHg in the first phase, indicating that both Ca(2+) and non-Ca(2+) divalent cations contribute to the MeHg-induced effect. When examining various Ca(2+) entry pathways as possible targets contributing to Ca(2+) influx, we found that excitatory amino acid receptor blockers MK-801 (15 μM), and AP-5 (100 μM)-both NMDA receptor-operated ion channel blockers, CNQX (20 μM), a non-NMDA receptor blocker, and the voltage-dependent Ca(2+) channel blockers nifedipine (1 μM) and ω-conotoxin-GVIA (1 μM) all significantly delayed the development of increased Ca(2+) caused by MeHg. The voltage-dependent Na(+) channel blocker tetrodotoxin (TTX, 1 μM) did not alter the MeHg-induced increases in fura-2 fluorescence ratio. Thus, MeHg alters Ca(2+) homeostasis in mouse spinal motor neurons through excitatory amino acid receptor-mediated pathways, and nifedipine and ω-conotoxin-GVIA-sensitive pathways. Spinal motor neurons are highly sensitive to this effect of acute exposure to MeHg.

Pharmacodynamics of memantine: an update

Memantine received marketing authorization from the European Agency for the Evaluation of Medicinal Products (EMEA) for the treatment of moderately severe to severe Alzheimer s disease (AD) in Europe on 17(th) May 2002 and shortly thereafter was also approved by the FDA for use in the same indication in the USA. Memantine is a moderate affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist with strong voltage-dependency and fast kinetics. Due to this mechanism of action (MOA), there is a wealth of other possible therapeutic indications for memantine and numerous preclinical data in animal models support this assumption. This review is intended to provide an update on preclinical studies on the pharmacodynamics of memantine, with an additional focus on animal models of diseases aside from the approved indication. For most studies prior to 1999, the reader is referred to a previous review [196].In general, since 1999, considerable additional preclinical evidence has accumulated supporting the use of memantine in AD (both symptomatic and neuroprotective). In addition, there has been further confirmation of the MOA of memantine as an uncompetitive NMDA receptor antagonist and essentially no data contradicting our understanding of the benign side effect profile of memantine.

Potency, voltage-dependency, agonist concentration-dependency, blocking kinetics and partial untrapping of the uncompetitive N-methyl-D-aspartate (NMDA) channel blocker memantine at human NMDA (GluN1/GluN2A) receptors

Both the clinical tolerability and the symptomatic effects of memantine in the treatment of Alzheimer's disease have been attributed to its moderate affinity (IC(50) around 1 microM at -70 mV) for NMDA receptor channels and associated fast, double exponential blocking/unblocking kinetics and strong voltage-dependency. Most of these biophysical data have been obtained from rodent receptors. Some substances show large species-specific differences, so using human rather than rodent receptors and tissue may highlight important differences in the effects of drugs. In the present study we compared the potency of memantine, ketamine and (+)MK-801 in binding to NMDA receptors in post-mortem human cortical tissue and to antagonize intracellular Ca(2+) responses of human GluN1/GluN2A receptors expressed in HEK-293 cells. In addition, the biophysical properties of memantine and ketamine were compared using patch clamp recordings from these cells. Memantine was confirmed to be a moderate affinity (IC(50) at -70 mV of 0.79+/-0.02 microM, Hill=0.92+/-0.02), strongly voltage-dependent (delta=0.90+/-0.09) uncompetitive antagonist of human GluN1/GluN2A receptors. Moreover, the rapid double exponential blocking kinetics (e.g. at 10 microM - onset tau(fast)=273+/-25 ms (weight 69%), onset tau(slow)=2756+/-296 ms, offset tau(fast)=415+/-82 ms (weight 38%) offset tau(slow)=5107+/-1204 ms) and partial untrapping (around 20%) previously reported for memantine on rodent receptors were confirmed for human receptors. Ketamine showed similar potency (IC(50) at -70 mV of 0.71+/-0.03 microM, Hill=0.84+/-0.02) but somewhat less pronounced voltage-dependency (delta=0.79+/-0.04), slower, single exponential kinetics (ketamine: k(on)=0.15+/-0.05 x 10(6)M(-1)s(-1), k(off)=0.22+/-0.05 s(-1)c.f. memantine following normalization k(on)=0.32+/-0.11 x 10(6)M(-1)s(-1), k(off)=0.53+/-0.10s(-1)) and was fully trapped. The present data closely match previously reported data from studies in rodent receptors and suggest that the proposed mechanism of action of memantine in Alzheimer's disease as a fast, voltage-dependent open-channel blocker of NMDA receptors can be confirmed for human NMDA receptors.

Memantine: a NMDA receptor antagonist that improves memory by restoration of homeostasis in the glutamatergic system--too little activation is bad, too much is even worse

The neurotransmitter glutamate activates several classes of metabotropic receptor and three major types of ionotropic receptor--alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA). The involvement of glutamate mediated neurotoxicity in the pathogenesis of Alzheimer's disease (AD) is finding increasing scientific acceptance. Central to this hypothesis is the assumption that glutamate receptors, in particular of the NMDA type, are overactivated in a tonic rather than a phasic manner. Such continuous, mild, chronic activation ultimately leads to neuronal damage/death. Additionally, impairment of synaptic plasticity (learning) may result not only from neuronal damage per se but may also be a direct consequence of this continuous, non-contingent NMDA receptor activation. Complete NMDA receptor blockade has also been shown to impair neuronal plasticity, thus, both hypo- and hyperactivity of the glutamatergic system leads to dysfunction. Memantine received marketing authorization from the EMEA (European Medicines Agency) for the treatment of moderate to severe AD in Europe and was subsequently also approved by the FDA (Food and Drug Administration) for use in the same indication in the USA. Memantine is a moderate affinity, uncompetitive NMDA receptor antagonist with strong voltage-dependency and fast kinetics. This review summarizes existing hypotheses on the mechanism of action (MOA) of memantine in an attempt to understand how the accepted interaction with NMDA receptors could allow memantine to provide both neuroprotection and reverse deficits in learning/memory by the same MOA.

Memantine as an example of a fast, voltage-dependent, open channel N-methyl-D-aspartate receptor blocker

Electrophysiological techniques can be used to great effect to help determine the mechanism of action of a compound. However, many factors can compromise the resulting data and their analysis, such as the speed of solution exchange, expression of additional ion channel populations including other ligand-gated receptors and voltage-gated channels, compounds having multiple binding sites, and current desensitization and rundown. In this chapter, such problems and their solutions are discussed and illustrated using data from experiments involving the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist memantine. Memantine differs from many other NMDA receptor channel blockers in that it is well tolerated and does not cause psychotomimetic effects at therapeutic doses. Various electrophysiological parameters of NMDA-induced current blockade by memantine have been proposed to be important in determining therapeutic tolerability, potency, onset and offset kinetics, and voltage dependency. These were all measured using whole cell patch-clamp techniques using hippocampal neurons. Full results are shown here for memantine, and these are summarized and compared with those from similar experiments with other NMDA channel blockers. The interpretation of these results is discussed, as are theories concerning the tolerability of NMDA channel blockers, with the aim of illustrating how electrophysiological data can be used to form and support a physiological hypothesis.

Effect of ketamine, dextromethorphan, and MK-801 on cochlear dysfunction induced by transient ischemia

Overstimulation of the N-methyl-D-aspartate (NMDA) glutamate receptor has been implicated as a factor in the pathogenesis of hypoxic-ischemic injury in the central nervous system. To evaluate the role played by NMDA antagonists in ischemia-reperfusion injury of the cochlea, 3 noncompetitive NMDA antagonists--ketamine, dextromethorphan, and MK-801--were administered to 53 albino guinea pigs subjected to transient ischemia of 30 minutes' duration, and the threshold shifts of the compound action potential were compared with those of nontreated animals 4 hours after the onset of recirculation. Ketamine and dextromethorphan moderately ameliorated the compound action potential threshold shifts, whereas MK-801, the most potent NMDA receptor antagonist among these 3 agents, did not show any protective effect. These results indicate that the action antagonizing the NMDA receptor has no protective effect against ischemia-reperfusion injury of the cochlea, and that ketamine and dextromethorphan act as protective agents for the cochlea via other pathways.

The in vivo relevance of the varied channel-blocking properties of uncompetitive NMDA antagonists: tests on spinal neurones

The voltage dependence and channel-blocking kinetics of uncompetitive NMDA receptor antagonists have been well-described using in vitro techniques, but there is little evidence concerning the functional significance of these properties in vivo. We have now compared the effects of NMDA antagonists that display varied profiles of voltage-dependent block in vitro, on responses of spinal neurones in anaesthetised rats. The compounds examined were the uncompetitive channel blockers memantine, ketamine and MK-801 and, for comparison, an antagonist that acts at the strychnine-insensitive glycine binding site (MRZ 2/502). Using frequency of spike discharge as an indicator of somatic depolarisation, we have compared the effects of these antagonists on responses evoked by iontophoretic NMDA application and on synaptic responses evoked by pinch or electrical stimulation (the latter eliciting "wind-up"). The effectiveness of the antagonists was directly but variably related to the discharge frequency of the test response. The rank order of dependence on firing rate matched the rank order of voltage dependence reported in vitro, namely: memantine > ketamine > MK-801> or = MRZ 2/502. Doses that reduced responses to iontophoretic application of NMDA were less effective at reducing responses to pinch, perhaps due to the major non-NMDA component of the synaptic response. Memantine preferentially reduced "wind-up" relative to responses to pinch, whereas ketamine and MK-801 reduced both types of synaptic responses in parallel. This "filtering" by low affinity, voltage-dependent NMDA antagonists such as memantine, of non-physiological activity whilst leaving normal synaptic events relatively untouched, may contribute to their more favourable clinical profile.

NT-3 evokes an LTP-like facilitation of AMPA/kainate receptor-mediated synaptic transmission in the neonatal rat spinal cord

Neurotrophin-3 (NT-3) is a neurotrophic factor required for survival of muscle spindle afferents during prenatal development. It also acts postsynaptically to enhance the monosynaptic excitatory postsynaptic potential (EPSP) produced by these fibers in motoneurons when applied over a period of weeks to the axotomized muscle nerve in adult cats. Similar increases in the amplitude of the monosynaptic EPSP in motoneurons are observed after periodic systemic treatment of neonatal rats with NT-3. Here we show an acute action of NT-3 in enhancing the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA/kainate) receptor-mediated fast monosynaptic EPSP elicited in motoneurons by dorsal root (DR) stimulation in the in vitro hemisected neonatal rat spinal cord. The receptor tyrosine kinase inhibitor K252a blocks this action of NT-3 as does the calcium chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) injected into the motoneuron. The effect of NT-3 resembles long-term potentiation (LTP) in that transient bath application of NT-3 to the isolated spinal cord produces a long-lasting increase in the amplitude of the monosynaptic EPSP. An additional similarity is that activation of N-methyl-D-aspartate (NMDA) receptors is required to initiate this increase but not to maintain it. The NMDA receptor blocker MK-801, introduced into the motoneuron through the recording microelectrode, blocks the effect of NT-3, indicating that NMDA receptors in the motoneuron membrane are crucial. The effect of NT-3 on motoneuron NMDA receptors is demonstrated by its enhancement of the depolarizing response of the motoneuron to bath-applied NMDA in the presence of tetrodotoxin (TTX). The potentiating effects of NT-3 do not persist beyond the first postnatal week. In addition, EPSPs with similar properties evoked in the same motoneurons by stimulation of descending fibers in the ventrolateral funiculus (VLF) are not modifiable by NT-3 even in the initial postnatal week. Thus, NT-3 produces synapse-specific and age-dependent LTP-like enhancement of AMPA/kainate receptor-mediated synaptic transmission in the spinal cord, and this action requires the availability of functional NMDA receptors in the motoneuron.

The postnatal development of spinal sensory processing

The mechanisms by which infants and children process pain should be viewed within the context of a developing sensory nervous system. The study of the neurophysiological properties and connectivity of sensory neurons in the developing spinal cord dorsal horn of the intact postnatal rat has shed light on the way in which the newborn central nervous system analyzes cutaneous innocuous and noxious stimuli. The receptive field properties and evoked activity of newborn dorsal horn cells to single repetitive and persistent innocuous and noxious inputs are developmentally regulated and reflect the maturation of excitatory transmission within the spinal cord. These changes will have an important influence on pain processing in the postnatal period.

Memantine is a clinically well tolerated N-methyl-D-aspartate (NMDA) receptor antagonist--a review of preclinical data

N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential in numerous CNS disorders ranging from acute neurodegeneration (e.g. stroke and trauma), chronic neurodegeneration (e.g. Parkinson's disease, Alzheimer's disease, Huntington's disease, ALS) to symptomatic treatment (e.g. epilepsy, Parkinson's disease, drug dependence, depression, anxiety and chronic pain). However, many NMDA receptor antagonists also produce highly undesirable side effects at doses within their putative therapeutic range. This has unfortunately led to the conclusion that NMDA receptor antagonism is not a valid therapeutic approach. However, memantine is clearly an uncompetitive NMDA receptor antagonist at therapeutic concentrations achieved in the treatment of dementia and is essentially devoid of such side effects at doses within the therapeutic range. This has been attributed to memantine's moderate potency and associated rapid, strongly voltage-dependent blocking kinetics. The aim of this review is to summarise preclinical data on memantine supporting its mechanism of action and promising profile in animal models of chronic neurodegenerative diseases. The ultimate purpose is to provide evidence that it is indeed possible to develop clinically well tolerated NMDA receptor antagonists, a fact reflected in the recent interest of several pharmaceutical companies in developing compounds with similar properties to memantine.

Effect of ketamine on hypoxic-ischemic brain damage in newborn rats

The present study tests the hypothesis that ketamine, a dissociative anesthetic known to be a non-competitive antagonist of the NMDA receptor, will attenuate hypoxic-ischemic damage in neonatal rat brain. Studies were performed in 7-day-old rat pups which were divided into four groups. Animals of the first group, neither ligated nor exposed to hypoxia, served as controls. The second group was exposed to hypoxic-ischemic conditions and sacrificed immediately afterwards. Animals of the third and fourth groups were treated either with saline or ketamine (20 mg/kg, i.p.) in four doses following hypoxia. Hypoxic-ischemic injury to the left cerebral hemisphere was induced by ligation of the left common carotid artery followed by 1 h of hypoxia with 8% oxygen. Measurements of high energy phosphates (ATP and phosphocreatine) and amino acids (glutamate and glutamine) and neuropathological evaluation of the hippocampal formation were used to assess the effects of hypoxia-ischemia. The combination of common carotid artery ligation and exposure to an hypoxic environment caused major alterations in the ipsilateral hemisphere. In contrast, minor alterations in amino acid concentrations were observed after the end of hypoxia in the contralateral hemisphere. These alterations were restored during the early recovery period. Post-treatment with ketamine was associated with partial restoration of energy stores and amino acid content of the left cerebral hemisphere. Limited attenuation of the damage to the hippocampal formation as demonstrated by a reduction in the number of damaged neurons was also observed. These findings demonstrate that systemically administered ketamine after hypoxia offers partial protection to the newborn rat brain against hypoxic-ischemic injury.

Electrophysiological study, biodistribution in mice, and preliminary PET evaluation in a rhesus monkey of 1-amino-3-[18F]fluoromethyl-5-methyl-adamantane (18F-MEM): a potential radioligand for mapping the NMDA-receptor complex

The effect of the fluorinated memantine derivative and NMDA receptor antagonist, 1-amino-3-fluoromethyl-5-methyl-adamantane (19F-MEM), at the NMDA receptor ion channel was studied by patch clamp recording. The results showed that 19F-MEM is a moderate NMDA receptor channel blocker. A procedure for the routine preparation of the 18F-labelled analog 18F-MEM has been developed using a two-step reaction sequence. This involves the no-carrier-added nucleophilic radiofluorination of 1-[N-(tert-butyloxy)carbamoyl]-3-(toluenesulfonyloxy)methyl- 5-methyl-adamantane and the subsequent cleavage of the BOC-protecting group using aqueous HCI. The 18F-MEM was obtained in 22 +/- 7% radiochemical yield (decay-corrected to EOB) in a total synthesis time including HPLC purification of 90 min. A biodistribution study after i.v. injection of 18F-MEM in mice showed a fast clearance of radioactivity from blood and relatively high initial uptake in the kidney and in the lung, which gradually decreased with time. The brain uptake was high (up to 3.6% ID/g, 60 min postinjection) with increasing brain-blood ratios: 2.40, 5.10, 6.33, and 9.27 at 5, 30, 60, and 120 min, respectively. The regional accumulation of the radioactivity in the mouse brain was consistent with the known distribution of the PCP recognition site. Preliminary PET evaluation of the radiotracer in a rhesus monkey demonstrated good uptake and prolonged retention in the brain, with a plateau from 35 min onwards p.i. in the NMDA receptor-rich regions (frontal cortex, striata, and temporal cortex). Delineation of the hippocampus, a region known to contain a high density of NMDA receptors, was not possible owing to the resolution of the PET tomograph. The regional brain uptake of 18F-MEM was changed by memantine and by a pharmacological dose of (+)-MK-801, indicating competition for the same binding sites. In a preliminary experiment, haloperidol, a dopamine D2 and sigma receptor antagonist, decreased the binding of 18F-MEM from the brain regions examined, suggesting that binding was also occurring to the sigma recognition sites.

Weak blockade of AMPA receptor-mediated depolarisations in the rat cortical wedge by phenytoin but not lamotrigine or carbamazepine

The effects of the anticonvulsants, lamotrigine, phenytoin and carbamazepine, were investigated on NMDA and non-NMDA receptor agonist-evoked responses and against spontaneous epileptiform discharges, in the in vitro rat cortical wedge. Lamotrigine weakly attenuated responses to (RS)-alpha-amino-3-hydroxy-5-methoxy-4-isoxazole propionic acid (AMPA) and quisqualate (IC50 values >> 100 microM), but was without effect on responses to NMDA. Phenytoin weakly, but concentration-dependently, attenuated responses to AMPA and quisqualate, but much less potently attenuated responses to NMDA (IC50 values 163, 248 and >> 300 microM, respectively). Carbamazepine (3-100 microM) significantly attenuated responses to NMDA and at 100 microM attenuated responses to AMPA and quisqualate. These effects were not concentration dependent, with the IC50 values >> 100 microM. Lamotrigine and phenytoin weakly, but concentration-dependently, reduced the frequency (IC50 values 254 and > 300 microM, respectively) and amplitude (IC50 values 141 and > 300 microM, respectively) of spontaneous epileptiform discharges, whereas carbamazepine had no effect. The results show that the anticonvulsant effects of these antiepileptics are unlikely to involve antagonism of ionotropic glutamate receptors, although blockade of non-NMDA responses may play a role in the anticonvulsant profile of phenytoin. Furthermore, the data show that clinically effective anticonvulsants do not necessarily attenuate spontaneous epileptiform discharges in the rat cortical wedge.

Discriminative stimulus effects of the NMDA receptor antagonists MK-801 and CGP 37849 in rats

Rats were trained to discriminate MK-801 (0.05 mg/kg, IP), an uncompetitive, or CGP 37849 (3 mg/kg, IP), a competitive NMDA receptor antagonist from saline, using a two-lever, operant drug discrimination paradigm. In generalization tests the role of dopaminergic and serotonergic systems in the discriminative stimulus effects produced by both NMDA receptor antagonists was studied with amphetamine (0.5 mg/kg), cocaine (5.0 and 7.5 mg/kg), and fenfluramine (2.5 and 5.0 mg/kg). Additionally, memantine (5.0, 7.5 and 10.0 mg/kg), an uncompetitive NMDA receptor antagonist, was tested. The discriminative stimuli produced by MK-801 and CGP 37849 were not generalized to each other. Among the tested drugs only memantine generalized to the MK-801 discriminative stimulus. None of the tested drugs showed CGP 37849-like discriminative stimulus properties. The different mechanisms underlying NMDA antagonism by MK-801 and GP 37849 might explain the observed lack of cross-generalization. The results suggest that dopaminergic and serotonergic systems are not of major importance in the discriminative stimulus effects produced by both MK-801 and CGP 37849.

Effects of memantine and MK-801 on NMDA-induced currents in cultured neurones and on synaptic transmission and LTP in area CA1 of rat hippocampal slices

The effects of the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists, memantine (1-amino-3,5-dimethyladamantane) and MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzocyclo-hepten-5,10-imin e maleate) were compared on synaptic transmission and long-term potentiation (LTP) in hippocampal slices and on NMDA-induced currents in cultured superior collicular neurones. 2. Memantine (10-100 microM) reversibly reduced, but did not abolish, NMDA receptor-mediated secondary population spikes recorded in area CA1 of hippocampal slices bathed in Mg(2+)-free artificial cerebrospinal fluid. 3. Memantine (100 microM) antagonized NMDA receptor-mediated excitatory postsynaptic currents recorded in area CA1 in a strongly voltage-dependent manner i.e. depressed to 11 +/- 4% of control at -35 mV and 95 +/- 5% of control at +40 mV (n = 9), with no apparent effect on response kinetics. 4. The effects of MK-801 and memantine on the induction of LTP were assessed after prolonged pre-incubations with these antagonists. When present for 6.6 +/- 0.4 h prior to tetanic stimulation, memantine blocked the induction of LTP with an IC50 of 11.6 +/- 0.53 microM. By comparison, similar long pre-incubations with MK-801 (6.4 +/- 0.4 h) blocked the induction of LTP with an IC50 of 0.13 +/- 0.02 microM. 5. Memantine and MK-801 reduced NMDA-induced currents in cultured superior colliculus neurones recorded at -70 mV with IC50s of 2.2 +/- 0.2 microM and 0.14 +/- 0.04 microM respectively. The effects of memantine were highly voltage-dependent and behaved as though the affinity decreased epsilon fold per 50 mV of depolarization (apparent delta = 0.71). In contrast, under the conditions used, MK-801 appeared to be much less voltage-dependent i.e. affinity decreased epsilon fold per 329 mV of depolarization (apparent delta = 0.15). 6. Depolarizing steps from -70 mV to +50 mV in the continuous presence of memantine (10 microM) caused a rapid relief of blockade of NMDA-induced currents from 83.7 +/- 1.9% to 21.8 +/- 1.8% (n = 5). This relief was best fitted by a double exponential function (17.2 +/- 11.7 and 698 +/- 204 ms), the faster component of which was most pronounced. 7. In conclusion, whereas MK-801 is equipotent in blocking NMDA-induced currents (at - 70 mV) and the induction of LTP, memantine is relatively less potent in blocking the induction of LTP. This is due to its rapid relief of blockade upon depolarization; a property which might explain its promising clinical profile in the treatment of chronic neurodegenerative diseases.

WIN 63480, a hydrophilic TCP-site ligand, has reduced agonist-independent NMDA ion channel access compared to MK-801 and phencyclidine

NMDA channel blockers are potentially advantageous therapeutic agents for the treatment of ischemia and head trauma, which greatly elevate extracellular glutamate, because they should most effectively inhibit high levels of receptor activation. A novel high affinity TCP site ligand, WIN 63480, does not produce MK-801- or PCP-like behavioral activation at anti-ischemic doses. While WIN 63480, MK-801 and PCP were all observed to be effective blockers of open NMDA channels, WIN 63480 had much less access to closed NMDA channels. This difference may be due to the fact that WIN 63480 is hydrophilic (logD = -4.1) while MK-801 and PCP are lipophilic (logD = +1.8). In vivo, closed channel access may result in a non-competitive profile of antagonism for MK-801 and PCP compared to a more uncompetitive profile for WIN 63480. Release of glutamate, and depolarization, are likely to produce a high level of NMDA receptor activation in ischemic areas compared to normal tissue. Consequently, at anti-ischemic doses, WIN 63480 may produce less inhibition of physiological NMDA-mediated processes in neural systems involved in behavioral regulation than MK-801 or PCP, leading to an improved side effect profile.

Pharmacological heterogeneity of NMDA receptors in cerebellar granule cells in immature rat slices. A microfluorimetric study with the [Ca2+]i sensitive dye Indo-1

We have developed a technique for monitoring the internal calcium concentration--[Ca2+]i--in a single selected cell in cerebellar slices of 8-day-old rats. In post-migratory granule cells NMDA induced a rapid, reversible and concentration-dependent increase in [Ca2+]i (+22% at 10 microM and +210% at 100 microM). This effect was dependent on the presence of Ca2+ in the extracellular medium. When Mg2+ was absent from the perfusion buffer the sensitivity to NMDA was greatly increased (+108% at 10 microM). The NMDA response was not affected by glycine site agonists or by tetrodotoxin, a sodium channel blocker. In the absence of magnesium, the NMDA (10 microM)-induced increase in [Ca2+]i was prevented in a monophasic manner by the recognition site antagonist 2-amino-phosphonovalerate (2-APV; IC50 = 13 microM), and in a biphasic manner by the glycine site antagonist 7-chlorokynurenate (IC50s = 25 nM and 5.9 microM) and by the channel blocker dizocilpine (IC50s = 5 nM and 3 microM). In contrast, this NMDA response was only partially antagonized by the polyamine site antagonists ifenprodil and eliprodil (maximal inhibition approximately 50% at concentrations > or = 10 microM, IC50 1 and 2 microM, respectively). These results demonstrate the presence in granule cells from immature rat cerebellum of at least two NMDA receptor populations which can be differentiated by their sensitivity to inhibitors acting upon the different sites of the NMDA receptor complex.

Behavioral studies on FR115427, a novel selective N-methyl-D-aspartate antagonist

Behavioral and in vitro receptor binding methods were used to evaluate and compare the effects of FR115427 ((+)-l-methyl-1-phenyl-1,2,3,4-tetrahydroisoquinoline hydrochloride) with those of MK801, a non-competitive NMDA antagonist. FR115427 inhibited NMDA-induced convulsions in mice by intracerebroventrical(ICV) and systematic injection. FR115427 was found to be about ten times less potent than MK801. Furthermore, the inhibitory effect of FR115427 and MK801 on NMDA-induced convulsions was evaluated in time course studies in mice. MK801 exhibited a more sustained anticonvulsive activity than FR115427. In addition, PCP-like behaviors were examined in mice after ICV injection of these compounds. At the lowest dose FR115427 significantly increased locomotor activity, although the effect of this compound was about hundred times less potent than that of MK801. At higher dose a more complex pattern of behavior, e.g. head-movement and eventually ataxia was observed. In binding assays with rat brain membranes, FR115427 inhibited the binding of (3H)TCP (IC50 = 0.249 microM) and (3H)MK801 (IC50 = 0.312 microM) but did not inhibit the binding of (3H)CPP or (3H)glycine. These results suggest that FR115427 is a novel non-competitive NMDA antagonist that acts on a binding site located within the NMDA receptor associated ion channel.

Chronic embryonic MK-801 exposure disrupts the somatotopic organization of cutaneous nerve projections in the chick spinal cord

The effect of altering neural activity on the development of the central projections of cutaneous and muscle sensory neurons was studied in the embryonic chick spinal cord. Animals were treated chronically with MK-801, a non-competitive N-methyl-D-aspartate receptor antagonist, during the period when both cutaneous and muscle sensory afferents form connections in the spinal cord. Daily applications of MK-801 began on embryonic day 5, 1 day before sensory collaterals penetrate the spinal cord gray matter, and continued until the animals were analyzed (at embryonic day 14). The patterns of cutaneous and muscle sensory nerve projections were determined by applying fluorescent tracers to individual, identified peripheral nerves. MK-801 treatment did not overtly alter the pattern of muscle afferent projections. However, in the MK-801-treated embryos, the somatotopic organization of cutaneous afferent projections was dramatically altered. Normally, the projections formed by the lateral femoral cutaneous and the medial femoral cutaneous nerves are located immediately adjacent to one another in the lumbar dorsal horn, with little overlap. In the MK-801-treated embryos, the projections from these two cutaneous nerves both expanded significantly within dorsal horn laminae to become almost completely superimposed. These data suggest that MK-801 disrupts the development of the somatotopic organization of cutaneous afferent projections in the spinal cord.

Glutamate antagonists have different effects on spontaneous locomotor activity in rats

Locomotor activity, ataxia, and stereotypy were assessed in the open field after administration of NMDA and AMPA antagonists acting by different mechanisms. The interaction with glutamatergic receptors was confirmed in the binding assay. (+)MK-801 and phencyclidine (PCP) produced similar changes in horizontal activity, i.e., a strong increase from the beginning of the test. Ketamine, and to a lesser extent, memantine, enhanced horizontal activity at the later observation periods only. Amantadine and NBQX produced a slight inhibition, while GYKI-52466, d-cycloserine, (+R)-HA-966, CGP-37849, and dextromethorphan were ineffective. Vertical activity (rearings) were inhibited by most agents except GYKI-52466 and gly-B partial agonists. At higher doses ataxia was seen after: MK-801, PCP, ketamine, memantine, amantadine, CGP-37849, dextromethorphan, and GYKI-52466. Hence, the inhibition of NMDA and AMPA receptors by agents acting at different recognition sites produces qualitatively different behavioral consequences.

Prevention by NMDA receptor antagonists of the centrally-evoked increases of cardiac inotropic responses in rabbits

1. The purpose of this study was to investigate further the role of the excitatory amino acid (EAA) system of neurotransmission, particularly of the NMDA receptor, in the central regulation of cardiac function. 2. Electrical stimulation of the paraventricular nucleus of the hypothalamus (PVN) in pentobarbitone anaesthetized rabbits induced a cardiovascular response mainly characterized by a positive inotropic effect, hypertension and a marked increase in the myocardial oxygen demand index. 3. The intracerebroventricular (i.c.v.) or intravenous (i.v.) injection of different EAA antagonists acting on different sites of the NMDA receptor/channel complex dose-dependently blunted the excitatory cardiovascular effects of PVN stimulation. 4. 5,7 Dichlorokynurenic acid was used as a specific glycine site antagonist and 2-amino-5-phosphonovaleric acid was used to block the agonist recognition site; ketamine was used as a channel blocker site antagonist and ifenprodil as a blocker of the polyamine binding site. 5. 5,7 Dichlorokynurenic acid (125 and 250 micrograms kg-1, i.c.v.) virtually abolished the cardiovascular responses, inducing only haemodynamic depression at the highest dose used. 2-Amino-5-phosphonovaleric acid (0.1 to 1.0 mg kg-1, i.c.v.) elicited a reduction of the peak values observed during PVN stimulation which was accompanied by a decrease of the basal cardiovascular parameters. Ketamine (2.5 and 10 mg kg-1) and ifenprodil (1 mg kg-1), injected intravenously, blocked the haemodynamic response induced by PVN stimulation without marked reduction of the basal haemodynamics. 6. It is concluded that glutamate neurotransmission is not only involved in vasomotor tone control but also in the central control of cardiac function and can therefore modulate the myocardial oxygen demand.

Excitatory amino acids: implications for psychiatric disorders research

The hyperdopaminergic theory of schizophrenia may account for some types of schizophrenia, but schizophrenia with negative symptoms or resulting in a chronic state of deterioration after repeated relapses cannot be explained by this theory. This minireview first discusses the interactions between dopamine and excitatory amino acid (EAA) neurons to produce abnormal behavior. Secondly, it deals with the influence of the psychotropic drugs on EAA, such as the relationship between phencyclidine and the hypoglutamate theory, the involvement of EAA in behavioral sensitization induced by amphetamines, the interactions between antipsychotic, antidepressant and antianxiety drugs and EAA, considering the possibility of developing newer psychotropic drugs related with EAA. Finally, glutamate receptors measured in postmortem schizophrenic brains are tabulated and the bases of the hypoglutamate hypothesis are discussed.

Patch clamp studies on the kinetics and selectivity of N-methyl-D-aspartate receptor antagonism by memantine (1-amino-3,5-dimethyladamantan)

Memantine (1-amino-3,5-dimethyladamantan) was tested as an antagonist of N-methyl-D-aspartate (NMDA) receptors on cultured superior collicular and hippocampal neurones using the patch clamp technique and its actions were compared to those of Mg2+ ions, ketamine, dextrorphan, dextromethorphan, phencyclidine and dizocilpine (MK-801). Memantine (2-33 microM) concentration-dependently antagonized responses to NMDA 100 microM with an IC50 of 2.92 +/- 0.05 microM. In contrast, current responses to (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (L-AMPA 50-100 microM) and gamma-amino butyric acid (GABA 10 microM) were unaffected by Memantine 8 microM. Memantine 8 microM caused a non-parallel shift of the NMDA concentration-response curve to the right in a manner indicative of uncompetitive open channel block. The effects of memantine were similar to ketamine in that both antagonists were weakly use- and strongly voltage-dependent. In contrast, MK-801, phencyclidine and dextrorphan showed much slower kinetics that was reflected in their marked use- and weaker voltage-dependency. The antagonistic effects of memantine were not reversed by increasing concentrations of glycine (0.1-100 microM) ruling out the possibility of an interaction of memantine with the strychnine-insensitive glycine modulatory site associated with the NMDA receptor-channel complex. Memantine (1-100 microM) also selectively antagonized responses to NMDA (40 microM) in the cortical wedge preparation with IC50 of 12.9 +/- 1.5 microM.

Polyamine modulation of excitatory amino acid responses in the rat cortical wedge

Rat neocortex slices in Mg2+ free buffer show spontaneous discharges which have a constant frequency and are dependent on N-methyl-D-aspartate (NMDA) receptor activation. Spermine increased the frequency of discharges at concentrations below 1 mM, had biphasic effects at 1 mM, and only decreased the frequency of discharges at 3 mM. In contrast, the amplitude of these discharges was only reduced by spermine in a concentration dependent manner (300 microM to 3 mM). Spermidine produced similar effects, but was a less potent inhibitor of discharge frequency and amplitude than spermine. Diethylenetriamine (DET) 300 microM did not significantly inhibit polyamine-induced increases in the discharge frequency. Polyamines inhibited direct depolarizations induced by the glutamate agonists NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) in a concentration and time dependent manner. The data are consistent with two sites of action for polyamines, one enhancing the frequency of NMDA-mediated spontaneous epileptiform discharges and the other inhibiting direct glutamate responses in rat cortex. The slow onset (time to maximal enhancement or inhibition by polyamines greater than 30 min) and lack of reversibility with polyamine removal suggest that these sites are intracellular and/or presynaptic.

Cerebral cortex injury: effect of blockers of re-uptake of catecholamines

In rats, surgical injury of the neocortex enhances the level of procholecystokinin-mRNA in the ipsilateral cortex. This increase in procholecystokinin gene expression was significantly reduced by the blockers of catecholamine re-uptake nomifensine (4 mg/kg), cocaine (5 mg/kg) and (+)-oxaprotiline (1.5 mg/kg) given i.m. 30 min before the injury. The ganglionic blocking agent hexamethonium (3 mg/kg) prevented this effect of (+)-oxaprotiline and nomifensine. Also the alpha 1-adrenoceptor antagonists corynanthine (2 mg/kg) and prazosin (2 mg/kg) blocked this effect of (+/-)-oxaprotiline (1.5 mg/kg). It is concluded that catecholamines acting on alpha 1-adrenoceptors can reduce the increase in procholecystokinin-mRNA caused by cortex injury. The catecholamines may be released from the sympathetic nervous system.

Effects of pH on the actions of dizocilpine at the N-methyl-D-aspartate receptor complex

1. The effects of varying pH from 6.5 to 7.4 and 8.0 on the actions of dizocilpine (MK801) on the N-methyl-D-aspartate (NMDA) receptor were investigated by use of a [3H]-dizocilpine binding assay and NMDA-induced intracellular free Ca2+ ([Ca2+]i) increases in cultured forebrain neurones. 2. Increasing pH from 6.5 to 8.0 significantly increased the rate of association and dissociation of [3H]-dizocilpine. The association process was better described by two rate constants under each condition, while only dissociation of [3H]-dizocilpine at pH 8.0 required two rate constants adequately to describe the curve. Equilibrium affinity of [3H]-dizocilpine was not altered by changing pH from 6.5 to 8.0. 3. NMDA and glycine together increased [Ca2+]i measured by fura-2 microspectrofluorimetry in single cultured neurones from rat forebrain. Compared to control response measured at pH 7.4, the combined effects of NMDA and glycine were reduced to 38.9% of control values by lowering pH to 6.5 and increased to 148.9% by raising pH to 8.0. 4. Dizocilpine (200 nM) effectively reversed increases in [Ca2+]i produced by NMDA together with glycine. The rate of reversal produced by this concentration of dizocilpine was considerably slower than that required for cells to recover to baseline following agonist removal. The rate at which dizocilpine reversed NMDA- and glycine-induced [Ca2+]i increases was not altered by raising pH from 6.5 to 8.0. 5. These data support the hypothesis that the rates associated with [3H]-dizocilpine binding are controlled by the level of activation of the receptor. In addition, these data confirm previous findings that NMDA responses are sensitive to small changes in pH.However, the lack of sensitivity to pH changes of dizocilpine inhibition of NMDA responses show that other factors are also important in controlling the action of channel-blocking NMDA receptor antagonists.

Combination therapy with nimodipine and dizocilpine in a rat model of transient forebrain ischemia

BACKGROUND AND PURPOSE

We explored the effectiveness of dual blockade of calcium channels in preventing ischemic necrosis in a rat model of transient forebrain ischemia.

METHODS

To assess all the major brain regions, the entire brain was subserially sectioned and examined histologically 1 week after ischemia in 44 male Wistar rats. Brain temperature was monitored and controlled to avoid hypothermia or intergroup temperature differences at the time drugs were administered. All regimens were begun 20 minutes after ischemia. Treated animals received either the L-type calcium channel blocker nimodipine (0.25 microgram/min x 24-hour i.v. infusion), the noncompetitive N-methyl-D-aspartate receptor antagonist MK-801 (dizocilpine; 5 mg/kg i.v.), or both regimens in combination.

RESULTS

In the neocortex (p less than 0.05) and striatum (p less than 0.05), only double-treated animals showed a statistically significant reduction in neuronal necrosis. Dual therapy eliminated neuronal necrosis in the caudate nucleus entirely. In the septal (densely ischemic) hippocampus, protection was weak and inconsistent (0.012 less than p less than 0.788), but in the temporal (incompletely ischemic) hippocampus, the dual-treated group showed the most significant reduction (p less than 0.006).

CONCLUSIONS

We conclude that the combination of nimodipine and MK-801, if begun 20 minutes after ischemia, may offer a neuroprotective effect against neuronal necrosis in transient forebrain ischemia and that protection is maximal in the major extrahippocampal brain regions.

Kinetic mechanisms of glycine requirement for N-methyl-D-aspartate channel activation

Glycine potentiates N-methyl-D-aspartate (NMDA) receptor-mediated responses via its interaction with a strychnine-insensitive glycine recognition site. We have previously shown that the potent glycine receptor antagonist 7-chlorokynurenic acid (7Cl-KYN) dose-dependently inhibits [3H]MK-801 binding to the PCP receptor and that this effect is reversed by glycine. [3H]MK-801 binding to the PCP receptor within the NMDA receptor-gated ion channel is a measure of channel activation. Association of PCP receptor ligands is biexponential with the fast component of binding serving as a marker of activated NMDA channels. In the present study we utilize 7Cl-KYN as a probe of the kinetic mechanism of the glycine effect upon NMDA receptor functioning. In the presence of L-glutamate, incubation with 7Cl-KYN completely abolished the fast component of [3H]MK-801 association in 4 out of 5 experiments. In the fifth experiment where the fast component was detected, it accounted for less than half of that seen in the presence of L-glutamate alone. 7Cl-KYN-induced inhibition of the fast component of [3H]MK-801 association was reversed by the addition of glycine. Since the fast component represents ligand binding to the PCP receptor via the open NMDA channel, selective reduction of this component by 7Cl-KYN indicates that glycine receptor antagonists reduce the probability of channel opening, and also that the selective reduction in the component of [3H]MK-801 binding that manifests fast kinetics can serve as a marker for glycine antagonists.

NMDA Actions on Rat Abducens Motoneurons

Intracellular recordings were made from rat abducens motoneurons in vivo during local extracellular micro-ionophoretic application of N-methyl-d-aspartate (NMDA) and NMDA receptor antagonists. Typical NMDA responses, at a resting potential of -60 mV, consisted of a slow depolarization with an apparent increase in membrane resistance, bursts of action potentials followed by stable repetitive firing. Ionophoretic applications of aminophosphonovalerate (APV), kynurenate or MK801 reduced or blocked the NMDA-induced responses. The NMDA responses were voltage-dependent. NMDA responses induced by short (< 30 s) NMDA application pulses were blocked by hyperpolarizing the neuron. Long duration (> 30 s) NMDA applications induced rhythmic plateau potentials in hyperpolarized abducens motoneurons. The rhythmic depolarizations (15 - 30 mV) were modulated in both frequency and duration by current injection. They were abolished by further hyperpolarization or replaced by stable repetitive firing when hyperpolarization was removed. Our data show that NMDA receptors are present in rat abducens motoneurons and may be involved in the induction of rhythmic activities. The voltage-dependent blockade of somatic NMDA receptor-associated ion channels by cell hyperpolarization may be important for these oscillations. It is suggested that the rhythmic behaviour is due to the activation of dendritic NMDA receptors.

Interaction between free radicals and excitatory amino acids in the formation of ischemic brain edema in rats

Both oxygen free radicals and excitatory amino acids have been implicated as important cellular toxins in ischemic brain. Recent in vitro studies suggest that there may be a mutual interaction between these two mediators. We explored the relation between oxygen free radicals and excitatory amino acids in the development of ischemic brain edema in vivo. Male Sprague-Dawley rats were treated with the free radical scavenger dimethylthiourea 1 hour before ischemia or with the excitotoxin antagonist MK-801 30 minutes before ischemia produced by occlusion of the middle cerebral artery. Groups of seven or eight animals were treated with vehicle, low-dose (375 mg/kg) dimethylthiourea, high-dose (750 mg/kg) dimethylthiourea, low-dose (0.5 mg/kg) MK-801, high-dose (2.0 mg/kg) MK-801, or both high-dose dimethylthiourea and low-dose MK-801. After 4 hours of ischemia, brain water content was determined. In eight vehicle-treated controls, mean +/- SEM water content of tissue in the center of the ischemic zone was 83.29 +/- 0.18%. A significant reduction of brain edema was observed in all drug-treated groups: for example, 50.2% (p less than 0.001) in the high-dose dimethylthiourea group, 53.7% (p less than 0.001) in the low-dose MK-801 group, and 66.4% (p less than 0.001) in the combined dimethylthiourea and MK-801 group. Combined treatment with dimethylthiourea and MK-801 provided no significant additive effect over that resulting from treatment with MK-801 alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory amino acid receptors in normal and abnormal vestibular function

Although excitatory amino acid (EAA) receptors have been investigated extensively in the limbic system and neocortex, less is known of the function of EAA receptors in the brainstem. A number of biochemical and electrophysiological studies suggest that the synapse between the ipsilateral vestibular (VIIIth) nerve and the brainstem vestibular nucleus (VN) is mediated by an EAA acting predominantly on kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. In addition, there is electrophysiological evidence that input from the contralateral vestibular nerve via the contralateral VN is partially mediated by N-methyl-D-aspartate (NMDA) receptors. Input to the VN from the spinal cord may also be partially mediated by NMDA receptors. All of the electrophysiological studies conducted so far have used in vitro preparations, and it is possible that denervation of the VN during the preparation of an explant or slice causes changes in EAA receptor function. Nonetheless, these results suggest that EAA receptors may be important in many different parts of the vestibular reflex pathways. Studies of the peripheral vestibular system have also shown that EAAs are involved in transmission between the receptor hair cells and the vestibular nerve fibers. A number of recent studies in the area of vestibular plasticity have reported that antagonists for the NMDA receptor subtype disrupt the behavioral recovery that occurs following unilateral deafferentation of the vestibular nerve fibers (vestibular compensation). It has been suggested that vestibular compensation may be owing to an upregulation or increased affinity of NMDA receptors in the VN ipsilateral to the peripheral deafferentation; however; at present, there is no clear evidence to support this hypothesis.

The induction and maintenance of central sensitization is dependent on N-methyl-D-aspartic acid receptor activation; implications for the treatment of post-injury pain hypersensitivity states

Repetitive stimulation of small diameter primary afferent fibres produces a progressive increase in action potential discharge (windup) and a prolonged increase in the excitability of neurones in the spinal cord following the stimulus. Previous studies have demonstrated that windup is the consequence of the temporal summation of slow synaptic potentials and that the slow potentials and windup are reduced by pretreatment with N-methyl-D-aspartic acid (NMDA) antagonists. We have now examined whether primary afferent induced hypersensitivity states in flexor motoneurones are also dependent on the activation of NMDA receptors and whether windup is a possible trigger for the production of the central hypersensitivity. Both a non-competitive (MK-801) and a competitive (D-CPP) NMDA antagonist, at doses that did not modify the baseline reflex, reduced the facilitation of the flexor reflex produced by either brief electrical stimulation of the sural nerve (1 Hz for 20 sec at C-fibre strength), or by the cutaneous application of the chemical irritant mustard oil. These antagonists also prevented windup from occurring in the motoneurones. When the the MK-801 and the D-CPP were administered once a state of central facilitation had been induced by prior treatment with mustard oil, they returned the facilitated reflex to its pretreatment level. These results indicate that NMDA receptors are involved in the induction and maintenance of the central sensitization produced by high threshold primary afferent inputs. Because central sensitization is likely to contribute to the post-injury pain hypersensitivity states in man, these data have a bearing both on the potential role of NMDA antagonists for pre-emptive analgesia and for treating established pain states.