Neurochemical aspects of the N-methyl-D-aspartate receptor complex

Animal toxins: As an alternative therapeutic target following ischemic stroke condition

Globally, Ischemic stroke (IS) has become the second leading cause of mortality and chronic disability. The process of IS has triggered by the blockages of blood vessels to form clots in the brain which initiates multiple interactions with the key signaling pathways, counting excitotoxicity, acidosis, ionic imbalance, inflammation, oxidative stress, and neuronal dysfunction of cells, and ultimately cells going under apoptosis. Currently, FDA has approved only tissue plasminogen activator therapy, which is effective against IS with few limitations. However, the mechanism of excitotoxicity and acidosis has spurred the investigation of a potential candidate for IS therapy. Acid-sensing ion channels (ASICs) and Voltage-gated Ca²⁺ channels (VDCCs) get activated and disturb the brain's normal physiology. Animal toxins are novel inhibitors of ASICs and VDCCs channels and have provided neuroprotective insights into the pathophysiology of IS. This review will discuss the potential directions of translational ASICs and VDCCs inhibitors research for clinical therapies.

Affinity of 1-aryl-1,2,3,4-tetrahydroisoquinoline derivatives to the ion channel binding site of the NMDA receptor complex

A series of 1-aryl-1,2,3,4-tetrahydroisoquinoline and 8-methyl-1-aryl-1,2,3,4-tetrahydroisoquinoline derivatives was evaluated for affinity to the PCP binding site of the NMDA receptor complex. The (S)-configured tetrahydroisoquinoline derivative (S)-4 e x HCl bearing a 2-methylphenyl substituent in position 1 of the heterocyclic ring system and a methyl group in position 8 was found to exhibit the highest affinity among the derivatives with a K(i)-value of 0.0374 microM. In addition, this compound shows a remarkable enantioselectivity of binding by being almost 90 times more potent than the corresponding (R)-enantiomer (R)-4 e x HCl. Additionally, a convenient and efficient synthetic approach to racemic 1-aryl-1,2,3,4-tetrahydroisoquinoline derivatives is described.

NMDA-NR2B subtype selectivity of stereoisomeric 2-(1,2,3,4-tetrahydro-1-isoquinolyl)ethanol derivatives

Enantiopure 2-(1,2,3,4-tetrahydro-1-isoquinolyl)ethanol derivatives were tested for their affinity to the ifenprodil binding site of the NMDA receptor, their potency to inhibit [3H]MK801 binding and their NMDA-NR2B subtype selectivity. The (1S,1'S)-configurated series displayed the highest affinity to the ifenprodil binding site. A reasonable potency and NMDA-NR2B subtype selectivity was found for (1S,1'S)-4c (R1=Me, R2=OMe). A high affinity to HERG K+ channels, however, suggests that (1S,1'S)-4c may involve an increased risk of cardiovascular side effects.

The effects of alanine-substituted conantokin-G and ifenprodil on the human spermine-activated N-methyl-D-aspartate receptor

We evaluated the effects of Ala-7-conantokin-G (Con-G(A7)) and ifenprodil on the modulation by spermine of [(3)H]MK801 binding to human cortical membranes. Human cortical tissue was obtained at autopsy and stored at -80 degrees C until assay. Both Con-G(A7) and ifenprodil inhibited [(3)H]MK801 binding, but spermine affected these inhibitions differently. Con-G(A7) IC(50) changed little with spermine concentration, indicative of a non-competitive interaction, whereas the rightward shift in ifenprodil IC(50) with increasing spermine concentration suggested partial competition. When the two agents were tested against the biphasic activation of [(3)H]MK801 binding by spermine, they again differed in their effects. In the activation phase Con-G(A7) was a non-competitive inhibitor of spermine activation, and may even enhance the spermine EC(50), while the ifenprodil data indicated a partially competitive interaction. Both agents were non-competitive in the inhibitory phase. Overall, the data suggest that Con-G(A7) and ifenprodil interact differently with the polyamine modulation of the glutamate-N-methyl-D-aspartate receptor.

Oxidative metabolites are involved in polyamine-induced microglial cell death

Pathological activation of microglia, which reside quiescently in physiological CNS, is associated with various neurodegenerative diseases. Endogenous polyamines, spermidine and spermine, are known to be activators of cell proliferation and differentiation. We previously reported that both spermidine and spermine induce dose-dependent cell death in cultured rat brain microglia at a submicromolar concentration range via apoptotic process, whereas cultured astrocytes were less sensitive to these polyamines [Neuroscience 120 (2003) 961]. These polyamine effects were observed only in the presence of fetal bovine serum. In the present study we examined further the mechanism of polyamine-induced microglial cell death. Amine oxidase in fetal bovine serum produces hydrogen peroxide and an aminoaldehyde from spermine, and the latter generates acrolein spontaneously. Acrolein was found to be much more toxic to microglia than to astrocytes and the effective concentration of acrolein was similar to that of spermine, whereas hydrogen peroxide was marginally toxic. Aminoguanidine, an inhibitor of amine oxidase, blocked the toxic effects of spermine on microglia. Spermine cytotoxicity was also prevented by antioxidant reagents; glutathione (reduced form), cysteine, and N-acetylcysteine. These results suggest that polyamine-induced apoptotic cell death of microglia is triggered by an oxidative stress with acrolein, which is produced by amine oxidase from polyamine. The different toxicities of polyamine between two glial cells may regulate the balance of glial activation in some pathological conditions of CNS.

Synthesis, radiosynthesis and In vivo evaluation of 5-[3-(4-Benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-[11C]one, as a potent NR1A/2B subtype selective NMDA PET radiotracer

Recently, a new series of potent and highly subtype-selective 1-(heteroarylalkynyl)-4-benzylpiperidine antagonists of the NMDA receptors has been described by Pfizer Laboratories. In this series, 5-[3-(4-benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-one (1) was identified as a selective antagonist for the NR1(A)/2B subtype, displaying IC(50) values for inhibition of the NMDA responses of 5.3 nM for this subtype (compared to NR1(A)/2A: 35 microM and NR1(A)/2C>100 microM) and was active in rat at a relatively low dosage (10mg/kg po). Derivative 1 has been synthesized in four chemical steps in good overall yield and labelled with carbon-11 at its benzoimidazolone ring using [(11)C]phosgene. The pharmacological profile of [(11)C]-1 was evaluated in vivo in rats with biodistribution studies and brain radioactivity monitored with intracerebral radiosensitive beta-microprobes. The brain uptake of [(11)C]-1 was extremely low (0.07% I.D./mL on average at 30 min) and rather uniform across the different brain structures. This in vivo brain regional distribution of [(11)C]-1 did not match with autoradiographic or binding data obtained with other NR2B subtype-selective NMDA ligands. Competition studies with ifenprodil (20 mg/kg, ip, 30 min before the radiotracer injection) failed to demonstrate specific binding of the radiotracer in the brain. In view of these results, and especially considering the low brain penetration of the radiotracer, [(11)C]-1 does not have the required properties for imaging NMDA receptors using positron emission tomography.

Microglial cell death induced by a low concentration of polyamines

Pathological activation of microglia, which reside quiescently in physiological CNS, contributes various neurodegenerative diseases. Endogenous polyamines, spermidine (SPD) and spermine (SPM) are known to be activators of cell proliferation and differentiation. We examined the effect of polyamines on microglial activation in culture. Cultured microglia prepared from the whole brains of newborn rats produced nitric oxide (NO) by the stimulation with lipopolysaccharide (LPS). LPS-induced NO production was markedly inhibited by SPD and SPM; half effective concentrations (EC(50)) of SPD and SPM were about 3 and 1 microM, respectively. Cell viability assessed by total mitochondrial activity decreased by the incubation with SPD and SPM for 24 h at similar concentration ranges. After the treatment with SPM for 24 h, the cells changed into small round morphology, and were strongly stained with propidium iodide. By the staining with bis-benzimide trihydrochloride, condensation and fragmentation of the nucleus were often observed. Semiquantitative analysis of fragmented DNA with enzyme-linked immunosorbent assay technique revealed that a large amount of fragmented DNA appeared in cytosol prior to disruption of the cell membrane. Fragmented DNA in the cytosol increased dose dependently with SPM; EC(50) was less than 10 microM. Furthermore, most of the cells after 24 h incubation with 10 microM SPD and SPM were positive for terminal deoxyribonucleotidyl transferase-mediated dUTP-biotin nick end labeling. These results suggest that microglial cell death is induced by a low concentration of polyamines via an apoptotic process rather than necrotic one.

Inorganics and hormesis

The article is a comprehensive review of the occurrence of hormetic dose-response relationships induced by inorganic agents, including toxic agents, of significant environmental and public health interest (e.g., arsenic, cadmium, lead, mercury, selenium, and zinc). Hormetic responses occurred in a wide range of biological models (i.e., plants, invertebrate and vertebrate animals) for a large and diverse array of endpoints. Particular attention was given to providing an assessment of the quantitative features of the dose-response relationships and underlying mechanisms that could account for the biphasic nature of the hormetic response. These findings indicate that hormetic responses commonly occur in appropriately designed experiments and are highly generalizeable with respect to biological model responses. The hormetic dose response should be seen as a reliable feature of the dose response for inorganic agents and will have an important impact on the estimated effects of such agents on environmental and human receptors.

A role for glycine in the gating of plant NMDA-like receptors

The amino acid glycine has a well-established role in signalling in the mammalian central nervous system. For example, glycine acts synergistically with the major excitatory neurotransmitter, glutamate, to regulate the influx of ions such as calcium, through N-methyl-d-aspartate (NMDA) receptors. Plants possess NMDA-like receptors, generically referred to as glutamate receptors (GLRs), named on the basis of their presumed ligand, glutamate. Previously, glycine has not been implicated in plant GLR activity or any other aspect of plant signalling. Using transgenic Arabidopsis seedlings expressing aequorin to monitor ligand-mediated changes in the cytosolic concentration of Ca2+ ([Ca2+]cyt), the data presented herein show that glutamate and glycine act synergistically to control ligand-mediated gating of calcium in plants. Glutamate and glycine synergism also regulates hypocotyl elongation. Transient increases in [Ca2+]cyt mediated by glutamate and glycine, as well as hypocotyl elongation, were inhibited by 6,7-dinitroquinoxaline-2,3 dione (DNQX), a competitive inhibitor of animal GLRs. Using a multiscale docking algorithm in combination with a molecular model of the ligand-binding domain of plant GLRs, evidence is provided indicating that glycine, and not glutamate, is likely to be the natural ligand for most plant GLR subunits. These findings uncover a hitherto unconsidered role for glycine signalling in plants, and suggest that the synergistic action of glutamate and glycine at NMDA-like receptors predates the divergence of plants and animals.

In vivo evaluation of [11C]-3-[2-[(3-methoxyphenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxylic acid ([11C]3MPICA) as a PET radiotracer for the glycine site of the NMDA ion channel

Alterations in normal NMDA receptor composition, densities and function have been implicated in the pathophysiology of certain neurological and neuropsychiatric disorders such as Parkinson's Disease, Huntington's Chorea, schizophrenia, alcoholism and stroke. In our first effort to provide PET ligands for the NMDA/glycine site, we reported the synthesis of a novel high affinity glycine site ligand, 3-[2-[(3-methoxyphenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxylic acid ((3MPICA), Ki = 4.8 +/- 0.9 nM) and the corresponding carbon-11 labeled PET ligand, [11C]3MPICA. We report here the in vivo evaluation of [11C]3MPICA in rats. Biodistribution analysis revealed that [11C]3MPICA exhibited low degree of brain penetration and high blood concentration. The average uptake at two minutes was highest in the cerebellum (0.19 +/- 0.04 %ID/g) and thalamus (0.18 +/- 0.05 %ID/g) and lower in the hippocampus (0.13 +/- 0.03) and frontal cortex (0.11 +/- 0.04 %ID/g). The radioactivity cleared quickly from all brain regions examined. Administration of unlabeled 3MPICA (1 mg/kg, i.v.) revealed at 60 minutes a small general reduction in regional brain radioactivity concentrations in treated animals versus controls, however, the blood radioactivity concentration was also lowered, confounding the assessment of the degree of saturable binding. Warfarin co-administration (100 mg/kg, i.v.) significantly lowered blood activity at 5 minutes post-injection (-27%, P < 0.01) but failed to significantly increase the brain uptake of the radiotracer. In view of these results, and especially considering the low brain penetration of this tracer, [11C]3MPICA does not appear to be a promising PET radiotracer for in vivo use.

Constitutive expression of heterologous N-methyl-D-aspartate receptor subunits in rat adrenal medulla

An RT-PCR analysis revealed constitutive expression of mRNA for N-methyl-D-aspartate (NMDA) receptor (NR)-1, NR-2C, and NR-2D subunits in rat adrenal and pituitary glands, in addition to brain and retina. Constitutive expression of mRNA was detected for the NR-2A subunit in pituitary but not adrenal gland. Although on Western blotting assays adrenal medulla exhibited expression of NR-1 subunit protein without expression of NR-2C and NR-2D subunit proteins, an immunohistochemical investigation clearly showed selective localization of proteins for NR-1, NR-2C, and NR-2D subunits in adrenal medulla but not in adrenal cortex. Prior treatment with different glycosidases invariably resulted in a marked increase in immunoreactivity to the anti-NR-1 antibody in both hippocampus and adrenal medulla. An intraperitoneal injection of a blocker of NMDA receptor channel, but not NMDA itself, led to marked potentiation of DNA binding activity of the transcription factor activator protein-1 in adrenal nuclear extracts 2 hr after administration. These results suggest that heteromeric NMDA receptor channels may be constitutively and functionally expressed with glycosylation of NR-1 subunit under the influence of tonic stimulation by circulating agonists such as L-glutamate in rat adrenal medulla.

Consolidation of transient ionotropic glutamate signals through nuclear transcription factors in the brain

Long-lasting alterations of neuronal functions could involve mechanisms associated with consolidation of transient extracellular signals through modulation of de novo synthesis of particular functional proteins in the brain. In eukaryotes, protein de novo synthesis is mainly under the control at the level of gene transcription by transcription factors in the cell nucleus. Transcription factors are nuclear proteins with an ability to recognize particular core nucleotides at the upstream and/or downstream of target genes, and thereby to modulate the activity of RNA polymerase II that is responsible for the formation of mRNA from double stranded DNA. Gel retardation electrophoresis is widely employed for conventional detection of DNA binding activities of a variety of transcription factors with different protein motifs. Extracellular ionotropic glutamate (Glu) signals lead to rapid and selective potentiation of DNA binding of the nuclear transcription factor activator protein-1 (AP1) that is a homo- and heterodimeric complex between Jun and Fos family members, in addition to inducing expression of the corresponding proteins, in a manner unique to each Glu signal in murine hippocampus. Therefore, extracellular Glu signals may be differentially transduced into the nucleus to express AP1 with different assemblies between Jun and Fos family members, and thereby to modulate de novo synthesis of the individual target proteins at the level of gene transcription in the hippocampus. Such mechanisms may be operative on synaptic plasticity as well as delayed neuronal death through consolidation of alterations of a variety of cellular functions induced by transient extracellular signals in the brain.

Targeting the N-methyl-D-aspartate receptor for chronic pain management. Preclinical animal studies, recent clinical experience and future research directions

A 1967-1999 MEDLINE search of published reports evaluating the role of the glutamate N-methyl-D-aspartate (NMDA) receptor in pain identified 378 animal studies and 132 human studies. There is convincing evidence in these studies that the NMDA receptor mediates prolonged nociceptive behaviors in animal models and various chronic pain symptoms in the clinical population. Administration of older compounds, such as ketamine, dextromethorphan, and amantadine, which are now known to act as NMDA receptor antagonists, have recently been shown to alleviate chronic pain. For years, the pharmaceutical industry has been attempting to produce novel compounds that modulate NMDA receptor activity; however, the adverse effects associated with this class of drugs have prevented their widespread clinical use. Collaborative studies between basic researchers, clinical scientists, and clinicians are needed to delineate characteristics of NMDA receptor antagonism that predict optimal analgesic activity and an acceptable toxicity profile in patients with chronic pain.

Effects of an adenosine analogue administration on the striatal NMDA receptors in an experimental model of epilepsy

Specific [3H]-MK801 binding to rat NMDA receptors following the administration of the convulsant drug 3-mercaptopropionic acid (MP) and the adenosine analogue cyclopentyladenosine (CPA) was studied in striatal membrane fractions. MP administration (150 mg/kg, i.p.) caused an increase of 53% and 82% in [3H]-MK801 binding during seizure and the postseizure period respectively. Administration of CPA (2 mg/kg, i.p.) raised [3H]-MK801 binding by 72%. When CPA was administered 30 min before MP and rats sacrificed at seizure (CPA + MPc), an increase of 64%, was observed. Saturation results indicate that receptor sites increased their maximal binding capacity (Bmax) in all treatments while the apparent dissociation constant (Kd) remained unchanged. MP administration brought about an increase of 52% and 42% in [3H]-MK801 binding sites during seizure and postseizure respectively. Administration of CPA raised receptor density by 75%. When CPA was administered 30 min before MP and rats sacrificed at seizure (CPA + MPc), an increase of 62%, was observed. These results show that striatal NMDA receptors have a selective role in seizure activity in the basal ganglia and that the adenosine analogue administration may modify [3H]-MK801 binding in a way similar to that of the convulsant drug.

Effects of spermidine treatment on neurobehavioral development in intrauterine growth retarded (IUGR) rats

It was previously shown that polyamine treatment could induce precocious development of several somatic and neurobehavioral functions in newborn rats. This study investigates the effects of daily injections of spermidine (SPMD) 50 microl/10 g s.c. on neurobehavioral development of newborn rats experiencing undernutrition. Neurobehavioral development was assessed by measurements of gripping and righting reflexes. SPMD treated intrauterine growth retarded (IUGR) rats reached righting reflex control values at 30 days postnatal (1.87 +/- 0.78 s vs 1.75 +/- 0.66 s). Beginning from 7 days postnatal, gripping reflex values of SPMD treated IUGR rats declined, reaching that of controls at 30 days postnatal (1.77 +/- 91 degrees vs 1.82 +/- 65 degrees). These results suggest the utility of exogenous SPMD in rats experiencing undernutrition, thus indicating a clinical relevance.

Modulation by L- and D-isoforms of amino acids of the L-glutamate response of N-methyl-D-aspartate receptors

N-Methyl-D-aspartate (NMDA) receptor subtypes epsilon 1 and zeta 1 were coexpressed in Xenopus oocytes for the investigation of the magnitude of augmentation of the L-glutamate response by 20 common L-amino acids and their 19 D-isoforms. Simultaneous application of L- and D-alanine, -cysteine, and -serine, or glycine and L-glutamate potentiated the glutamate-induced current. Other amino acids produced only marginal effects. Analysis of the relationship between the response and amino acid size revealed that the critical threshold size is between those of cysteine and aspartate. No amino acid alone induced a current. The effects of L- and D-alanine, -cysteine, and -serine applied with L-glutamate were concentration-dependent. Molecular modeling of these three amino acids revealed a positive relationship between the charge at an atom of the side chain and the receptor sensitivity, which may explain the efficacies of these amino acids.

Modification of [3H]MK801 binding to rat brain NMDA receptors after the administration of a convulsant drug and an adenosine analogue: a quantitative autoradiographic study

Specific [3H]MK801 binding to rat brain NMDA receptors after the administration of the convulsant drug 3-mercaptopropionic acid (MP) and the adenosine analogue cyclopentyladenosine (CPA) was studied by means of a quantitative autoradiographic method. MP administration (150 mg/kg, i.p.) caused significant decreases in [3H]MK801 binding in several hippocampus subareas and layers, mainly in CA1 and CA3 at seizure (11-27%) and postseizure (8-16%) and in cerebral occipital cortex at seizure (18-22%). In nucleus accumbens, a rise was observed at postseizure (44%) and a tendency to increase at seizure (24%). CPA (2mg/kg, i.p.) decreased ligand binding in hippocampus (CAI, CA2, CA3) (17-22%) and in occipital cerebral cortex (18-24%). When CPA was administered 30 minutes before MP (which delayed seizure onset) and rats were sacrificed at seizure, decreases in [3H]MK801 binding in several layers of CA1 and CA3 of hippocampus (11-27%) and in CA1, CA2, CA3 (24-35%) after CPA+MP postseizure, and an increase in CA2 after CPA and CPA+MP postseizure (20-34%), were observed. A drop was found in the occipital subarea (18-24%) after CPA and in the frontal and occipital subarea after CPA+MP postseizure (24-34%) while no changes were observed in any treatment involving the other cerebral cortex regions, thalamic nuclei, caudate putamen and olfactory tubercle. These results show that [3H]MK801 binding changes according to drug treatment and the area being studied, thus indicating a different role in seizure activity.

NMDA-coupled periaqueductal gray glycine receptors modulate anxioselective drug effects on plus-maze performance

The present study was carried out to investigate a possible interaction between the effects of anxiety modulating drugs which act at the GABA-A receptor complex and selective N-methyl-D-aspartic acid (NMDA) coupled glycine receptor (GLY-B receptor) ligands within the dorsal periaqueductal gray (DPAG). The plus-maze performance of rats pretreated with diazepam (0.37 and 0.75 mg/kg, i.p.) or pentylenetetrazole (15 and 30 mg/kg, i.p.), standard anxiolytic and anxiogenic drugs respectively, was assessed following intra-periaqueductal injections of either glycine (0.2 M, 0.4 microl/30 s, i.c.) or its competitive antagonist, 7-chlorokynurenic acid (7ClKYN, 0.02 M, 0.4 microl/30 s, i.c.). Whilst diazepam produced a typical anxiolytic effect in intracranially-injected CSF rats, increasing open arm exploration, pentylenetetrazole displayed an opposite anxiogenic profile. Either anxiogenic or anxiolytic effects were seen in peripherally-injected vehicle rats following intra-periaqueductal injections of glycine or 7ClKYN, respectively. Intra-periaqueductal injection of glycine markedly attenuated the anxiolytic effect of diazepam. Moreover, while the anxiogenic effects of pentylenetetrazole were barely changed by glycine, they were markedly attenuated by intra-periaqueductal injection of 7ClKYN. Interaction of diazepam and 7ClKYN produced non-selective sedative-like effects which masked any possible anxiolytic action. Accordingly, the present results suggest that the NMDA-coupled glycine receptors located in the DPAG interfere with anxioselective effects of GABA-A acting drugs on the elevated plus-maze. In spite of the prevailing notion that the NMDA coupled glycine receptor is saturated at in vivo brain concentrations of glycine, our results also suggest that either unoccupied or low-affinity GLY-B receptors are likely to be activated by glycine injection into DPAG.

Uptake of D- and L-serine in C6 glioma cells

Cultured C6 glioma cells were able to accumulate [3H]D- and [3H]L-serine in a temperature- and Na+-dependent and saturable manner. The kinetic analysis of these accumulation phenomena indicates that the D- or L-serine uptake into the glioma cells might occur by a single-component system with an apparent Km value around 2480 microM (for D-serine) or 110 microM (for L-serine) and a Vmax value around 7 nmol/min per mg protein. The ratio of the L- to D-serine uptake affinity was similar to that previously reported in the astrocytes of rat cerebrum. The inhibition profiles of D- and L-serine uptake by various amino acid and related compounds resembled each other and those of the ASCT-like neutral amino acid transporter. The present findings therefore suggest that C6 glioma cells may accumulate D- and L-serine through a common ASCT-like transporter and provide a useful model for the glial uptake of extracellular D- and L-serine in the brain.

Selective depression of the spinal polysynaptic reflex by the NMDA receptor antagonists in an isolated spinal cord in vitro

1. The effects of N-methyl-D-aspartate (NMDA) receptor glycine-binding site antagonists 7-chlorokynurenate (7-Clkyn) and (+/-)-3-amino-1-hydroxy-2-pyrrolidone (HA-966) on spinal reflexes in an isolated spinal cord that was maintained in Mg(2+)-free medium in vitro were examined. The actions of 7-Clkyn and HA-966 were compared with those of the channel-site antagonist (i.e., dizocilpine) and NMDA-binding site antagonists--that is, 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonate (CPP) and DL-2-amino-5-phosphonovalerate (APV). 2. 7-Clkyn and HA-966 produced a selective depression of the polysynaptic reflex (PSR) while negligibly affecting the activity of the monosynaptic reflex (MSR). The PSR was also differentially suppressed by dizocilpine, CPP and APV. The PSR inhibitory activity of the NMDA antagonists was in the following order: dizocilpine > CPP > APV = 7-Clkyn > HA-966. 3. The inhibitory effects of 7-Clkyn on PSR were markedly antagonized by the simultaneous application of D-serine, an agonist for the NMDA receptor glycine-binding sites. However, PSR inhibition by dizocilpine and CPP was unaffected. 4. Inhibition of the PSR by 7-Clkyn persisted in the presence of strychnine, which markedly increased the PSR activity by itself. 5. These findings suggest that the NMDA receptor glycine-binding sites play a role in generating the NMDA receptor-mediated PSR in the spinal cord in vitro.

Zn2+ inhibition of [3H]MK-801 binding is different in mouse brain and spinal cord: effect of glycine and glutamate

Zn2+ inhibits NMDA-type excitatory amino acid activity by a non-competitive action. Based on regional differences in the central nervous system (CNS) in binding characteristics of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate ([3H]MK-801) and other non-competitive antagonists of NMDA used to label open channels in the receptor complex, we compared the inhibitory influence of Zn2+ on [3H]MK-801 binding in whole mouse brain and spinal cord membranes. Radioligand binding techniques were used in the presence and absence of maximally effective concentrations of glycine and glutamate. Using extensively washed membranes without exogenous glycine and glutamate, Zn2+ was found to be a weaker inhibitor of the [3H]MK-801-labeled site in the spinal cord than in the whole brain. In contrast, exogenous glycine and glutamate decreased the inhibitory effect of Zn2+ in the brain but dramatically increased the inhibitory effect of Zn2+ in the spinal cord. Thus the inhibitory effect of Zn2+ in the spinal cord appears to be magnified by glutamatergic and glycinergic activity while that in the brain is not. The different actions of Zn2+ may be attributable to the differential distribution of NMDA receptor subunits in the mouse brain and spinal cord.

Age-related increase in nitric oxide synthase activity in senescence accelerated mouse brain and the effect of long-term administration of superoxide radical scavenger

The levels of nitric oxide (NO) and NO synthase (NOS) activities were compared in the brains of young adult (3 months old), aged (11 months old) and TJ-960 administered (11 months old) senescence accelerated mice (SAM), of which the SAMP8 substrain is inferior in acquisition of learning due to the abnormality of glutamatergic neurotransmission in the cerebral cortex. TJ-960, which is based on the Kampo (Japanese traditional herbal medicine) prescription Sho-saiko-to-go-keishi-ka-shakuyaku-to, acts as a superoxide radical scavenger and attenuates the deterioration of neuronal activity associated with aging. We administered TJ-960 orally for 5 months. In the cerebral cortex of aged SAMP8, NOS activity was increased compared with that of young adult SAMP8. Though TJ-960 did not alter the contents of NO in any brain region compared with those in aged SAMP8, it did prevent the increase in NOS activity in the aged cerebral cortex. Our data suggest that NOS activity may increase to compensate for the reduced sensitivity of the NO reaction system in the aging process, and that TJ-960 may normalize this increased NOS activity in the cerebral cortex, although further work is clearly needed to ascertain maintenance in the acquisition of learning.

The stimulatory effect of L-glutamate and related agents on inositol 1,4,5-trisphosphate production in the cestode Hymenolepis diminuta

The effects of L-glutamate, acetylcholine, and serotonin (5HT) were examined on generation of inositol 1,4,5-triphosphate [Ins(1,4,5)P3], in membrane preparations of the cestode Hymenolepis diminuta. Only L-glutamate and acetylcholine stimulated a significant elevation in Ins(1,4,5)P3. The response to L-glutamate was stereospecific; D-glutamate or L-aspartate were not as potent. A role for G-protein(s) was supported by the observations that sodium fluoride stimulated Ins(1,4,5)P3 generation, and the L-glutamate response was potentiated by GTP and GTP-S and was suppressed by GDPS. However, studies with pertussis and cholera toxins indicated that the putative G-protein(s) was not pertussis or cholera toxin sensitive. The pharmacological profile of the L-glutamate response was examined partially. Trans-ACPD was a very effective agonist at 10(-5)M. While 10(-3)M L-glutamate, NMDA, and AMPA significantly elevated Ins(1,4,5)P3 levels, quisqualate and kainate did not. The elevation of Ins(1,4,5)P3 levels by L-glutamate and NMDA was antagonized by the specific glutamatergic antagonists AP-5, AP-7, CNQX, and CPP. While the response to ACPD was antagonized by AP5, CPP and CPG, CNQX was without effect. Collectively, the data support the hypothesis that in the cestode H. diminuta, L-glutamate activation of a metabotropic (ACPD) and/or ionotropic-like AMPA/NMDA receptor subtypes proceeds via a G protein(s) to enhance phospholipase C activity, ultimately resulting in the elevation of Ins(1,4,5)P3 levels in the tissues.

Dizocilpine binding to cerebral cortical membranes from developing and ageing mice

The binding of [3H]dizocilpine (MK-801) to the N-methyl-D-aspartate (NMDA)-gated ion channel was characterized in cerebral cortical membranes during the major portion of the mouse life-span (from 7-day- to 22-month-olds). The binding was saturable, consisting of only one component at all ages studied. The maximal binding capacity Bmax was very substantial in 14-day-old mice when compared to adults (3-month-olds), decreasing thereafter during ageing. The binding constant KD remained unchanged during development and increased only slightly in aged mice. Glutamate and glycine potentiated dizocilpine binding concentration-dependently. Their efficacy varied markedly with age. Both glutamate and glycine had considerably less effect on the immature cerebral cortex and in the oldest group of mice (22-month-old) than in young adults. The marked increase in dizocilpine binding sites at the age of 2 weeks coincides with the previously reported transient increase in NMDA binding sites in the cerebral cortex. The weak potentiation of dizocilpine binding by glutamate and glycine in the immature brain could be a factor which protects neurons during this period from excitotoxicity and increased susceptibility to seizures induced by acidic amino acids. The decrease in the number of dizocilpine binding sites during ageing could result partly from the loss of cortical neurons.

An information processing theory of anaesthesia

A theory of anaesthesia is presented. It consists of four hypotheses: (1) The occurrence of states of consciousness causally depends on the formation of transient higher-order, self-referential mental representations. The occurrence of such states is identical with the appearance of conscious phenomena. Loss of consciousness will occur, if and only if the brain's representational activity falls below a critical threshold. (2) Mental representations are instantiated by neural cell assemblies. (3) The formation of assemblies involves the activation of the NMDA receptor channel complex. The activation state of this receptor determines the rate at which assemblies are generated. (4) General anaesthetics have a common operative mechanism: they directly or indirectly affect the function of the NMDA system.

Biochemical and pharmacological studies on a lethal neurotoxic polypeptide from Phoneutria nigriventer spider venom

Fractionation of Phoneutria nigriventer spider venom by gel filtration and HPLC yielded a few fractions that induced different effects when administered intraperitoneally in mice. One of these fractions, PF3, was chemically characterized as a cysteine-rich polypeptide of approximately 8360 MW. Administered at 0.1 mg/kg, i.p., PF3 induced a progressive paralysis and death of mice within 30 minutes. Partial sequence analysis of PF3 revealed certain homologies with other spider toxins already described, particularly omega-AGAIIA (60%) from Agelenopsis aperta. Pharmacological characterization carried out in superfused chopped rat striatal tissues preloaded with [3H]-Dopamine ([3H]-DA) showed that PF3 (0.1 microgram/ml) decreased the [3H]-DA release induced by 20 mM K+ or 100 microM glutamate without changing the basal release. At 1 microgram/ml, PF3 inhibited 33% of the basal release of [3H]-DA; the transmitter release stimulated by K+ or by glutamate was reduced by respectively, 87% and 77% of corresponding control values. PF3 (0.1 micrograms/ml) altered the dose-response curves of glutamate (1 microM-10 mM), by reducing by 36% of its maximal effect. Naloxone (1 microM) did not influence the effect of PF3. The results indicate that PF3 inhibits the [3H]-DA release induced by membrane depolarization or that mediated by NMDA glutamate receptors. These data suggest that the mechanism of action of PF3 may involve a blockade of Ca2+ channels as well as a direct effect on the exocytotic machinery.

The role of excitatory amino acids in experimental models of Parkinson's disease

The aim of this article was to review the recent literature on the role of excitatory amino acids in Parkinson's disease and in animal equivalents of parkinsonian symptoms. Effects of NMDA and AMPA antagonists on the reserpine-induced akinesia, catalepsy and rigidity, on the neuroleptic-induced catalepsy, on the turning behaviour of 6-OHDA-lesioned rats, as well as on the parkinsonian symptoms evoked by MPTP in monkeys were analysed. Moreover, the role of NMDA antagonists in Parkinson's disease was discussed. Data concerning the protective influence of these drugs on degenerative properties of methamphetamine, MPTP and 6-OHDOPA were also presented. On the basis of the above findings, the following conclusions may be drawn: (1) disturbances in the glutamatergic transmission in various brain structures seem to play a significant role in the development of symptoms of Parkinson's disease; (2) the NMDA-receptor blocking component may make a substantial contribution to the therapeutic effect of antiparkinsonian drugs; a similar contribution of AMPA-receptor blocking component has not been sufficiently documented, so far; (3) compounds blocking NMDA receptors may possibly prevent the development of Parkinson's disease; this presumption needs, however further studies; (4) side effects of NMDA receptor antagonists may be a limiting factor in the use of these compounds in humans.

Metabolic modulation by amino acid stimulation of the paraventricular nucleus of the hypothalamus

The role of the paraventricular nucleus of the hypothalamus (PVN) in the regulation of energy expenditure and energy substrate utilization was investigated after the injection of the excitatory amino acid D,L-homocysteic acid (DLH) or its vehicle. Male Wistar rats with chronic PVN cannulae were tested for 1 h with no food available in an open-circuit calorimeter. Whereas low (0.5 nmol), excitatory doses of DLH increased energy expenditure, the thermogenic effect became smaller and then vanished as the DLH dose was increased to inhibitory levels (7 and 50 nmol). None of these doses affected motor activity, indicating a primary thermogenic effect. The highest dose (100 nmol) increased energy expenditure, but this appeared to be secondary to increased locomotor activity. The increased locomotor activity produced by the highest dose of DLH constitutes the first demonstration of an activity effect induced by stimulating the PVN. However, this effect likely reflects the activation of neighboring areas. Only the 50 nmol dose of DLH increased respiratory quotient, indicating a shift toward the preferential utilization of carbohydrates as an energy substrate. These data complement our findings with neuropeptide Y and insulin in showing that different doses of the same substance injected into the PVN may produce qualitatively different effects. Furthermore, the present study demonstrates that exciting PVN neurons activates catabolic forces, whereas inhibiting them activates anabolic forces.

Neuronal protection and preservation of calcium/calmodulin-dependent protein kinase II and protein kinase C activity by dextrorphan treatment in global ischemia

This study analyzed the ability of the N-methyl-D-aspartate receptor antagonist dextrorphan (DX) to prevent neuronal degeneration (analyzed by light microscopy), calmodulin (CaM) redistribution (analyzed by immunocytochemistry) and changes in activity of two major Ca(2+)-dependent protein kinases--calcium/calmodulin-dependent protein kinase II (CaM-KII) and protein kinase C (PKC) (analyzed by specific substrate phosphorylation) after 20 min of global ischemia (four-vessel occlusion model) in rats. DX treatment before and after ischemia significantly protected hippocampal and cortical neurons from neurodegeneration whereas DX posttreatment alone did not have any effect on preservation of neuronal morphology as compared with placebo treatment analyzed 72 h after 20 min of ischemia. Similarly to histological changes, DX exhibited protection against redistribution of CaM observed after ischemia. These changes were detected both in hippocampus as well as in cerebral cortex. Finally, DX administered before ligation of the carotid arteries reduced loss in both CaM-KII and PKC activity evoked by ischemia.

Excitatory amino acid receptor binding in hippocampus of gerbils with transient global brain ischemia

Binding of a variety of ligands for brain excitatory amino acid receptors was examined in membrane preparations extensively washed and treated with Triton X-100 that were obtained from the hippocampus and cerebral cortex of gerbils that survived for different periods after transient global brain ischemia. Bilateral occlusion of the carotid arteries for 5 min did not affect the binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) to an open ion channel associated with the N-methyl-D-aspartate (NMDA)-sensitive subclass in both central structures of gerbils that survived for 1 to 4 weeks after the injury when determined at equilibrium in the presence of 3 different endogenous agonists including L-glutamic acid (Glu), glycine (Gly) and spermidine at maximally effective concentrations. In contrast, the ischemic occlusion significantly diminished [3H]MK-801 binding when determined before equilibrium in the presence of the 3 stimulants in hippocampal membranes without altering that in cortical membrane 2 weeks after the insult, so that the initial association rates were invariably reduced by more than 60%. Moreover, the occlusion not only reduced the binding of both [3H]Glu and [3H]D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid to the NMDA domain on the NMDA receptor ionophore complex, but also decreased the binding of both [3H]Gly and [3H]5,7-dichlorokynurenic acid to the Gly domain. However, the insult did not induce any detectable changes under the experimental conditions employed in either the binding of [3H]DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) to the AMPA-sensitive subclass or the binding of kainic acid (KA) to the KA-sensitive subclass in both central regions of animals that survived for 2 weeks. These results suggest that transient global brain ischemia may predominantly impair neuronal and/or glial cells enriched of the NMDA receptor ionophore complex in gerbil hippocampus.

Phencyclidine-binding sites in mouse cerebral cortex during development and ageing: effects of inhibitory amino acids

The binding of N-[1-(2-thienyl)cyclohexyl]-[3H]piperidine ([3H]TCP) to the phencyclidine-binding sites in the N-methyl-D-aspartate (NMDA) receptor complex-associated ion channel was characterized in cerebral cortical membranes from 3-day-old to 24-month-old mice. The binding was saturable, exhibiting only one binding component during the whole life-span studied. The maximal binding capacity Bmax, calculated per protein content, decreased during postnatal development until 3 months of age, remaining thereafter constant in ageing mice, thus indicating the greatest availability of phencyclidine-binding sites in the immature cerebral cortex. The binding constant KD increased during the first postnatal week, remained thereafter unchanged and increased again during the second year of life, indicating a decreased affinity of the receptor sites for the ligand. The general properties of the binding; potentiation by glutamate and NMDA, as well as by glycine in a strychnine-insensitive manner, prevailed during development and ageing, certain of these effects being however less pronounced in the immature brain. Taurine and beta-alanine stimulated TCP binding, acting probably at the glycine modulatory site. The actions of these inhibitory amino acids were weak and inconsistent when compared to that of glycine. Since NMDA receptors have been suggested to be involved in neuronal plasticity and learning and memory processes, these modifications in the properties of cortical phencyclidine-binding sites might be of importance in the regulation of excitatory amino acid functions during development and ageing.

Differential potentiation by spermidine of abilities of a variety of displacers for [3H]MK-801 binding in hippocampal synaptic membranes

Binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) to an ion channel associated with the N-methyl-D-aspartate (NMDA)-sensitive subtype of brain excitatory amino acid receptors was studied in Triton-treated preparations of synaptic membranes of rat brain. The initial association rate of the binding measured at 30 min after onset of incubation was markedly potentiated by the addition of either L-glutamic acid (Glu) alone or both Glu and glycine (Gly) in a concentration-dependent manner at 10 nM to 0.1 mM. Potentiation occurred to a significantly greater extent in the hippocampus and cerebral cortex than in the cerebellum. In the presence of both Glu and Gly, the endogenous polyamine spermidine (SPD) further potentiated binding in hippocampal and cortical membranes at concentrations above 10 microM without significantly affecting that in cerebellar membranes. The binding of [3H]MK-801 was slowly equilibrated in 16 h. When examined in hippocampal synaptic membranes, the binding at equilibrium was markedly displaced by numerous noncompetitive antagonists for the NMDA receptor. The addition of SPD markedly enhanced potencies of those displacers having a high affinity to [3H]MK-801 binding sites, without affecting other displacers having a low affinity. These results suggest that SPD promotes transition of sites responsible for mediating NMDA responses within the channel to a state with higher affinity for noncompetitive blockers.

MK-801 protects against neuronal injury induced by electrical stimulation

The ability of MK-801, a non-competitive N-methyl-D-aspartate receptor antagonist, to protect neurons in the cerebral cortex from injury induced by prolonged electrical stimulation was assessed in cats. Platinum disc electrodes 8.0 mm in diameter and with a surface area of 0.5 cm2 were implanted in the subdural space over the parietal cortex. Ten days after implantation of the electrodes, all animals received continuous stimulation for 7 h using charge-balanced, cathodic-first, controlled current pulses with a charge density of 20 microC/cm2 and a charge/phase of 10 microC/phase. They received either no MK-801, or 0.33 or 5.0 mg/kg (i.v.) administered intravenously, just before the start of the stimulation. Immediately following the stimulation, the animals were perfused and the cerebral cortex examined by light microscopy at eight sites beneath the electrodes. Neuronal damage in the form of shrunken, hyperchromic neurons and perineuronal halos was present only beneath the stimulating electrodes; damage was moderate to severe in stimulated animals that had not received MK-801, slight in animals receiving 0.33 mg/kg, and none to slight in animals receiving 5.0 mg/kg. These results indicate that MK-801, in an apparently dose-dependent fashion, provides substantial but not complete protection against neuronal injury induced by prolonged electrical stimulation. Thus prolonged electrical stimulation can be added to the list of neuropathologic conditions which involve glutamate-induced excitotoxic damage via the N-methyl-D-aspartate receptor. The results also support the hypothesis of neuronal hyperactivity as a principal cause of electrically-induced injury in the central nervous system. The implications for design of protocols for functional electrical stimulation are discussed.

Inhibition by calmodulin antagonists of [3H]MK-801 binding in brain synaptic membranes

In brain synaptic membranes not extensively washed, (+)-5-[3H]methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5, 10-imine ([3H]MK-801) binding was markedly inhibited in a concentration-dependent manner (at concentrations above 1 microM) by several compounds having antagonistic activity at the Ca(2+)-binding protein calmodulin. Scatchard analysis revealed that N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) inhibited the binding through a significant decrease in the density of binding sites without affecting the affinity at 10 microM. In membranes extensively washed and treated with a low concentration of Triton X-100, L-glutamic acid (Glu) drastically accelerated the initial association rate of [3H]MK-801 binding with glycine (Gly), almost doubling the initial association rate found in the presence of Glu alone. The addition of W-7 invariably reduced the initial association rate observed in the presence of either Glu alone or both Glu and Gly, without significantly altering the dissociation rate of bound [3H]-MK-801, irrespective of the presence of the two stimulatory amino acids. The maximal potencies of Glu, Gly, and spermidine in potentiating the binding were all attenuated by W-7. These results suggest that calmodulin antagonists may interfere with opening processes of an ion channel associated with an N-methyl-D-aspartate-sensitive subclass of excitatory amino acid receptors in rat brain.

Differential modulation by divalent cations of [3H]MK-801 binding in brain synaptic membranes

Endogenous divalent cations, such as Mg2+, Ca2+, and Zn2+, differentially affected the binding of (+)-[3H]5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate ([3H]MK-801) to an ion channel associated with an N-methyl-D-aspartate-sensitive subclass of excitatory amino acid receptors in different preparations of brain synaptic membranes. Both Mg2+ and Ca2+ were weak inhibitors of the binding in membranes which had not been extensively washed (nonwashed membranes), over a concentration range effective in markedly potentiating the binding in the absence of any added stimulants in membranes which had been extensively washed, but not treated with a detergent (untreated membranes). In membranes extensively washed and treated with Triton X-100 (Triton-treated membranes), both cations significantly potentiated the binding in the presence of added glutamate alone. In contrast, Zn2+ was invariably active as a potent inhibitor of the binding irrespective of the membrane preparations used. In untreated membranes, Ca2+ markedly accelerated the initial association rate of [3H]MK-801 binding without affecting the binding at equilibrium in a manner similar to that found with glycine, as well as with glutamate; Mg2+, however, facilitated the initial association rate with a concomitant reduction of the binding at equilibrium. Zn2+ was effective in accelerating the initial rapid phase of association, with the initial slow phase being delayed, and in markedly reducing the binding at equilibrium. Both Mg2+ and Ca2+ also facilitated dissociation of the bound [3H]MK-801 and Zn2+ slowed the dissociation in untreated membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Screening of thiol compounds: depolarization-induced release of glutathione and cysteine from rat brain slices

Superfusates from rat brain slices were screened for thiol compounds after derivatization with monobromobimane by reversed-phase HPLC. Only glutathione and cysteine were detected. The Ca(2+)-dependent release of these compounds from slices of different regions of rat brain was investigated, applying a highly sensitive and reproducible quantification method, based on reduction of superfusates with dithiothreitol, reaction of thiols with iodoacetic acid, precolumn derivatization with o-phthalaldehyde reagent solution, and analysis with reversed-phase HPLC. This methodology allowed determination of reduced and total thiols in aliquots of the same superfusates. Mostly reduced glutathione and cysteine were released upon K+ depolarization and the Ca2+ dependency suggests that they originate from a neuronal compartment. The GSH release was most prominent in the mesodiencephalon, cortex, hippocampus, and striatum and lowest in the pons-medulla and cerebellum. This underscores a physiologically significant role for glutathione in CNS neurotransmission.

Preferential location of N-methyl-D-aspartate (NMDA) receptors on postsynaptic membranes and on non-noradrenergic nerve terminals of the rat brain cortex

The effect of DSP4-induced destruction of noradrenergic neurones on 3H-3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (3H-CPP) binding to N-methyl-D-aspartate (NMDA) receptors and on 3H-desipramine (3H-DMI) binding to the neuronal noradrenaline carrier was investigated in rat brain cortex buffy coat membranes. 3H-DMI bound with high affinity to a single site at the neuronal noradrenaline carrier (KD = 5.26 +/- 1.67 nmol/l) whereas the binding of 3H-CPP to the NMDA receptor was of intermediate affinity (KD = 274 +/- 45 nmol/l). Fourteen days after a single-dose treatment with DSP4 (1) the Bmax value for 3H-DMI binding was reduced by 74%, (2) the Bmax value for 3H-CPP binding only tended to be decreased (by 24%; not statistically significant), (3) the endogenous noradrenaline content was reduced by 70% compared to untreated controls and, (4) the absolute amount of the NMDA-evoked 3H-noradrenaline overflow but not the fractional release was reduced by 55%. It is concluded that in the rat cerebral cortex presynaptic NMDA-receptors on noradrenergic nerve endings, which have previously been detected in release experiments with NMDA on cortical synaptosomes preincubated with 3H-noradrenaline, cannot be identified in radioligand binding experiments. Obviously, the cerebral cortical NMDA receptors are predominantly located on postsynaptic neuronal membranes and potentially on non-noradrenergic nerve terminals as well.

Presence of the binding of a variety of ligands related to ionotropic excitatory amino acid receptors in rat retina

The binding of [3H]L-glutamic (Glu), [3H](+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic and [3H]D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acids was detected in rat retinal membranes extensively washed and treated with a low concentration of Triton X-100, in addition to the binding of both [3H]glycine (Gly) and [3H]5,7-dichlorokynurenic acid. Furthermore, retinal membranes exhibited the binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne in the presence of Glu, Gly and spermidine irrespective of the incubation period employed. Rat retina also contained the binding of [3H]kainic acid as well as the binding of [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid which was sensitive to potentiation by potassium thiocyanate. In addition, the binding of both [3H](+)-3-(hydroxyphenyl)-N-(l-propyl)piperidine and [3H]1,3-di-o-tolyl-guanidine was found in rat retinal membranes extensively washed but not treated with Triton X-100. These results give support for the proposal that the rodent retina contains subclasses of ionotropic brain excitatory amino acid receptors including the N-methyl-D-aspartate receptor ionophore complex as well as sigma sites.

Effect of CGS 19755, a competitive N-methyl-D-aspartate antagonist, on general anesthetic potency

CGS 19755 is a competitive N-methyl-D-aspartate (NMDA) receptor antagonist which penetrates the blood-brain barrier. The effect of pretreatment with subanesthetic doses of CGS 19755 on general anesthetic potency was determined in mice. Mice were pretreated with saline or CGS 19755 by intraperitoneal (IP) administration 30 min before IP administration of an anesthetic dose of ethanol or pentobarbital or measurement of the volatile anesthetic minimum alveolar concentration (MAC). CGS 19755 increased the duration of ethanol- and pentobarbital-induced loss of righting reflex in a dose-dependent manner. The highest dose of CGS 19755 tested, 50 mg/kg, increased duration of loss of righting reflex by about four- and twofold for ethanol and pentobarbital, respectively. CGS 19755 also decreased the MAC for halothane. However, CGS 19755 pretreatment had no effect on the MAC for diethyl ether. These results suggest that the potency of certain general anesthetic agents can be increased by antagonism of brain NMDA receptors.

Identification and characterization of specific binding sites of [3H]spermidine in synaptic membranes of rat brain

Synaptic membranes of rat brain contained specific binding sites of [3H]spermidine (SPD) that exhibited an inverse temperature dependency, structure selectivity, reversibility, saturability, low affinity and high density with an uneven distribution profile. The affinities were not significantly different from each other in the rodent brain, while the highest density was found in the medulla-pons among the central structures examined with progressively lower densities in the midbrain, striatum, cerebellum, hypothalamus, hippocampus and cerebral cortex. The binding was insensitive to digestion by various proteases and glycosidases but sensitive to potentiation by phospholipases. A clear correlation was seen between the abilities of several natural and synthetic polyamines to displace [3H]SPD binding and to potentiate [3H] (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine binding to open cation channels associated with an N-methyl-D-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors. Treatment of brain membranes with deoxycholic acid resulted in a significant solubilization of [3H]SPD binding sites. Furthermore, [3H]SPD markedly associated with the acidic phospholipid phosphatidylserine irrespective of the presence of synaptic membranes in a manner sensitive to inhibition by a variety of calmodulin antagonists. These results suggest that endogenous polyamines may play a stimulatory role in neuronal responses mediated by the NMDA receptor ionophore complex through an interaction between their positive charges and negative charges of membranous phosphatidylserine in rat brain.